The Minoan World...

In the middle of the second millennium B.C. the island of Crete supported the most complex civilization in Europe. With elaborate palaces and well-developed towns, the Minoan civilization was the equal of many in the Near East and North Africa. With the collapse of this culture in the later part of the millennium, the world was left with faint glimpses of their achievements, limited to a few lines in certain Greek histories, such as that of Thucydi-des, and the references to Knossos and King Minos in such myths as that of Theseus and the Minotaur.

Modern knowledge of the Minoan people did not develop until the later part of the nineteenth century. Spurred on by the discoveries of Mycenae and Troy made by the German-American excavator Heinrich Schliemann, the British excavator Sir Arthur Evans began his remarkable excavation of the palace of Minos at Knossos. Archaeological work has continued on Crete until the present day, with excavations of palaces, villas, and towns and important archaeological surveys of much of the island. The portrait of this civilization that we can piece together is at the same time impressive and frustrating.

We now understand quite a bit about the architecture, diet, ceramic traditions, and so on of these people. It is not known, however, whether the Minoan world was a single culture with variations (similar to the ethnic distinctions that we observe today) or several cultures throughout the island of Crete, sharing in a common elite tradition. Our understanding of the process of cultural development and change is equally uncertain, mainly the product of conflicting arguments over chronology. Dated primarily through ceramic style, Minoan civilization presents problems when we note that some ceramic styles appear to be the result more of locational than of temporal differences. There is controversy concerning the correlation of the Minoan temporal stages to the eruption of the volcano on the ancient island of Thera (now Santorini) in the later seventeenth century B.C. Our dating could well be incorrect by at least a century. Rather than relying on the ceramic identification of Minoan time periods, it is better to refer to a chronology that focuses on large social developments:
Pre-palatial period: c. 3100/3000 to 1925/ 1900 B.C.
Proto-palatial period: c. 1925/1900 to 1750/ 1720 B.C.
Neo-palatial period: c. 1750/1720 to 1490/ 1470 B.C.
Post-palatial period: c. 1490/1470 to 1075/
1050 B.C.
The Neo-palatial period is most commonly considered the zenith of Minoan civilization. At this time there were four large palace centers—Knossos, Malia, Phaistos, and Kato Zakros—as well as large developed towns, such as Gournia, and numerous examples of small isolated farmsteads. Their economic base was a developed agricultural system that utilized wheat, barley, olives, grapes, sheep, goats, and cattle. But just how Minoan complexity fit into this agricultural background is only partially understood.

What we can determine of Minoan social structure derives basically from analysis of the palatial centers. Significant sections of the structure of all the palaces, with the exception of Kato Zakros, were devoted to the storage of large amounts of agricultural supplies. Knossos was by far the largest of the palaces and had the greatest storerooms. Within these rooms were stored massive amounts of olive oil, olives, wheat, and other agricultural items. The presence of these large storerooms gives a glimpse into the probable structure of the Minoan social hierarchy.
The storage and redistribution of agricultural goods are best paralleled in what anthropologists have identified as a social and economic construction in modern societies, the chiefdom. While a direct comparison between these modern social configurations and the ancient Minoans would be misleading, an analysis of just how cultures might use food storage in the development of their social and political structures gives insight into the possible basis for the Minoan political and social order.

Social storage of food often is a measure taken by cultures to moderate the risk of agricultural uncertainty. At times, this storage has been manipulated to afford the armature upon which social and political hierarchy first develops. Such was probably the case with the Minoans. The island is composed of a multitude of microenvironments, rather small isolated areas, that are locked in by topographical features, such as mountains. An important feature of these microenvironments in those times was that each had its own particular reaction to normal inter-annual fluctuations in rainfall. The result was that Crete often resembled a patchwork of distinct microenvironments with quite different agricultural yields every year throughout the island. Simply put, one microenvironment could have had a bumper crop of wheat while its near neighbors could have been experiencing a serious shortfall in that grain during the same summer.

Social and political hierarchy can develop when a person or a group begins to control agricultural storage within and between these different microenvironments. Often this is seen in the gathering of a certain percentage of the agricultural surplus and ensuring that some of it is redistributed to those people who live in areas with low productivity in a particular year. As one might surmise, therein lies the basis of social indebtedness and the platform for constructing social hierarchy.
The palace of Minos at Knossos best illustrates this economic system. The entire western basement was dedicated to food storage. The rulers of Knos-sos could either return some food to areas in need or, as can be seen from the plan of the palace, use much of it to support craft specialists, who occupied up to a fourth of the palace, in the production of luxury items for use by the ruling family. This system of centralized redistribution was probably in place throughout the island. Only the palace at Kato Zakros lacks such a distinctive storage capacity.

PRE-PALATIAL DEVELOPMENTS

We know too little about the development of this economic and political system. Our knowledge of Cretan culture before the rise of the palaces is scant, with much of our understanding limited to a few small villages. The most elaborate is Myrtos (c. 2600-2170 B.C.) on the southern coast of Crete. A small village, with up to sixty preserved rooms, Myrtos appears to have been settled by five or six family units, with no identifiable hierarchical relationship. The site was agriculturally based and displayed a range of artifacts, from storage jars to serving dishes. Within each family unit, we have been able identify different types of workrooms, such as kitchens. One unit apparently held a small pottery workshop.

Several common pottery types, most notably, a long-necked, almost bird-shaped teapot, were shared among these Pre-palatial communities, indicating a commonality of design and perhaps function. Regional differences, however, can be seen in distinct variations in tomb types. In the north they were burying the dead in "house tombs," rectangular structures subdivided into different spaces for burial. In the south, specifically the Messara, the common form of burial was the tholos, or circular tomb, which presumably was roofed. In general, it appears that both of these tomb types were collective burials, with the family unit or even a larger corporate group using individual tombs. Certain tombs appear to have been used for a millennium, highlighting their importance in the social construction of early Minoan civilization. With the ever increasing complexity of the later early Minoan and middle Minoan periods came an elaboration of tombs, with an emphasis on ancestry in the struggle to obtain and maintain social hierarchy.
Toward the end of the early Minoan period we see noticeable changes in Minoan culture. In addition to the emphasis on the importance of ancestry, there was a dramatic change in pottery types. The introduction of "Kamares ware," a new light-on-dark style of pottery, as well as the barbotine pottery style took place at this point of transition, marking social change, with a possible emphasis on the new social contexts—both political and religious— where these new pottery types were being used.

PROTO-PALATIAL AND NEO-PALATIAL PERIODS

The Proto-palatial and Neo-palatial periods combine to make the era of the construction of the major palaces of Minoan Crete. Knossos (the largest), Malia, and Phaistos were built shortly after the beginning of the second millennium, in the Proto-palatial period. These sites were to be rebuilt about three hundred years later, in the Neo-palatial period, along with the new construction of the easternmost major palace at Kato Zakros. These locales were the residences of Minoan elites or rulers, but other sites, such as the villa at Hagia Triadha, must equally have been homes to the leading families of Minoan Crete. During this period large towns, such as Gournia, developed around major elite residences. Sanctuaries on mountain peaks also make their appearance at this time.

The period was truly a high point in Minoan architecture. The palaces were often several stories high; that at Knossos, for example, probably was four stories in its domestic quarter. Minoan architects and craftsmen showed an attention to fine architectural detail in wall construction and a keen sense of overall design in layout and technical construction. Light wells were used with confidence to open up the interiors of several palaces. Monumen-tality was added by the use of grand staircases and imposing walls. Large courts were integrated into the rhythm of palatial construction. Minoans even had plumbing in the palaces and other elite residences.
Among the palaces there is a striking similarity in design and construction, which must have mirrored the similar lifestyles of most of the Minoan aristocracy. The likenesses are remarkable and, except for some differences at Kato Zakros, which was the latest of the palaces, are common features at all the sites. Perhaps the most impressive feature of all the palaces is the central court, a large, rectangular plaza, around which the other sections of the palaces were arranged. The east side of the central court appears to have had a religious character, as evidenced by cult rooms and pillar crypts (sacred rooms with recessed floors and a central post) at Knossos and Malia and the famous throne room— actually a religious installation—at Knossos. As mentioned, agricultural storage was important to the Minoan ruling power, and all the palaces, except Kato Zakros (which might have had storage structures in the form of outlying buildings), had large storage rooms. At Knossos, Malia, and Phaistos these storerooms lie on the ground floor in the wing just to the west of the central court. On the floor above these rooms were the public rooms, or piano nobile. These were large reception rooms, perhaps used for public ceremonies.

Each of the four palaces also had a large banquet hall, located on the upper floor, probably to take in a breeze. The hall was not necessarily attached to the public rooms and might have been meant for a more private gathering of elites for entertaining and meals. Residential quarters have been clearly identified at Knossos, Malia, and Phaistos. As we might expect in the layout of private quarters, there is a correspondence in the features of these rooms among similar groups in the same culture. The residential arrangement can be found in a large number of elaborate houses, not just the palaces. That at Knossos is the most elaborate, but it shows the overall regularity of design. Residential space there was composed of a long, triple-divided hall, consisting of a light well, an anteroom, and a back chamber. Running off this hall was access to a religious room, the lustral basin, and to toilet facilities. Within the triple-divided hall, folding doors and upper windows in the wall between the anteroom and the back chamber regulated the light and air coming from the light well.

The palaces themselves were decorated throughout with elaborate frescoes. Favorite themes in the wall paintings were scenes from nature, religious gatherings, palace or community events, and mythological landscapes. The most intricate pottery was used, and possibly manufactured, in the palaces. Several important examples show serving cups, amphorae (large standing containers for oils and water), stirrup jars for perfumed oil, and pithoi (storage vessels), decorated with detailed floral designs, geometric patterns, and marine creatures. In addition to this pottery, the palaces also used carved stone bowls, ritual drinking cups (rhyta) of carved stone and gold, and cut rock crystal ornaments.

An interesting point in relation to the palaces is the obvious lack of fortifications. We know that the Minoans were not without a military force, as seen in the military themes of their works of art and the chieftain's cup. But we are at a loss to explain why there was no need to fortify the different settlements. It may well have been that Knossos, the largest of the palaces, exercised control of the military, but reference to societies with such political central-ity shows that even the subordinate settlements had fortifications. It may well have been that military campaigns on Crete were limited to raiding, which often took place without elaborate fortifications.
Little is known concerning how the common Minoan lived. Perhaps the best-preserved site is that of Gournia. There a relatively large community surrounded what was an elite residence, with its identifiable central court. The town itself was composed of two- or three-room houses, some with upper floors, laid out on compact, paved streets. Unfortunately, the excavation data from Gournia was lost before it could be published.

It was during these palatial periods that the first writing in Europe arose. There is some evidence for a pictographic script, but by far the strongest evidence is for a script dubbed "Linear A," which was discovered in the Proto-palatial period at Phaistos. Large collections of this script, written on clay tablets, have been found at Hagia Triadha and Chania, on the northwest coast. Although it is recognized as a syllabary, attempts to decipher this form of writing have so far proved futile.
We know somewhat more about Minoan religion of this period. A great deal of the religious focus was centered in the palaces, with examples such as the tripartite shrine, the throne room complex, which had a religious function at Knossos. At this time there was a flowering of rituals on hilltops and in caves. The hilltop shrines, known as "peak sanctuaries," number at least fifty and appear along with the development of the first palaces, indicating the strong political function of these sanctuaries as well. Gournia supplies an example of a small town shrine. Figurines, found throughout the palaces, depict women who could have been goddesses or priestesses. One example of the most important figurines, the snake goddesses from the palace at Knos-sos, depicts women with snakes twirled around their arms and sacred animals, such as owls, on their heads. Male worshippers also seem to be featured, and there are ubiquitous representations of bulls, which have a long history of sacred male identification in the Mediterranean. These figures also appear in stylized form in Minoan culture, as horns of consecration.

Other artifacts indicate that the Minoans regarded trees and the double axe as sacred. We are fortunate to have a sarcophagus from Hagia Triad-ha, which, on its four sides, depicts events that took place during a funeral. We see worshipers, possible priestesses, and an offering table with a trussed bull waiting to be sacrificed. On a darker note, there is evidence from Knossos and elsewhere that the Mi-noans also practiced human sacrifice.
During the palatial period, Minoan culture had its greatest contacts with other contemporaneous civilizations in the eastern Mediterranean. The evidence indicates that the most contact Crete had outside its shores was with the Cyclades and Pelo-ponnesian Greece. Finds of Minoan pottery, domestic architecture using the Minoan pier and door hall system, and traces of Linear A script indicate a strong Minoan presence in the Cyclades. Signs of Minoan influence in Greece are directed largely toward the Peloponnese, with a concentration in the Argolid area. The famous grave circles of the elites at Mycenae show numerous works of art, such as sword scabbards and the famous Vapheio cups, that can arguably be attributed to Minoan artists in the employ of foreign elites.

The evidence for Minoan contacts in the rest of the Mediterranean is not as rich. Some Minoan pottery has been found at contemporary sites in western Asia Minor. Small amounts of Minoan goods have turned up in Near Eastern contexts, and tomb paintings from contemporary Egypt depict what appear to be Minoans, the Keftiu, presenting gifts. But we lack a full understanding of the structure of these contacts. While it could have been that Minoans were colonizing parts of the Aegean islands, as well as the Peloponnese, the evidence could just as well indicate that we are witnessing a strong Minoan cultural ascendancy, which foreign elites were copying.

POST-PALATIAL PERIOD

Exact dates may never be known, but sometime near the turn of the second millennium there was an abrupt collapse of a large section of Minoan culture. All the palaces, with the exception of Knossos, ceased to be occupied. Theories to explain this change vary from the devastating effect of the explosion of the volcano on the island of Thera around 1625 B.C. to the possibility of an invasion from overseas. Whatever the cause, most Minoan occupation on Crete was affected by some sort of catastrophe.
Alone of the palaces, Knossos remained occupied. But there is much to suggest that this survival was not Minoan in character. Evidence from burials around Knossos and from the palace itself points strongly to a foreign, Mycenaean presence on Crete. A rise in militarism, represented in artworks, is distinctly non-Minoan but closely parallels that of the Mycenaeans on the Greek mainland. Of great importance is the finding of Linear B writing tablets at Knossos. Linear B is a distinctively Greek script, which also has been found in the archives of Mycenaean palaces, such as Pylos and Mycenae.

While we are almost secure in seeing Mycenae-ans in control of parts of Crete at this point, the structure of this control is only vaguely understood. Decipherment of the Linear B tablets at Knossos shows that, economically at least, the palace at Knossos was operating within a structure very similar to that seen at the mainland Mycenaean palace of Pylos. Analysis of the Linear B tablets hints at a condition where Knossos controlled the major part of the island during this period, however.
In the early fourteenth century B.C., Knossos was subject to major destruction, and any Mycenaean presence at the palace disappeared. However, there is some evidence from other sites, such as the port of Kommos and Hagia Triadha, that occupation continued on Crete. Archaeological evidence indicates that at this period Crete was becoming more fragmented in terms of regional art styles as well as social and economic structures.

Galaxy formation and evolution

How the diverse array of galaxies that are now observed originated and evolved into their present form is a topic of intense speculation.Evidence from structural properties. Some clues can be discerned in certain structural properties of galaxies. The most plausible explanation for the smooth and round light distribution of an elliptical galaxy is that the stars formed out of a collapsing gas cloud.

The rapidly changing gravitational pull experienced by different stars as the collapse proceeds has been shown by computer simulations to rearrange the stars into the observed shape.
The highly flattened disks of spiral galaxies must have formed during a similar collapse, but it is believed that most star formation did not occur until the rotating gas cloud had already flattened into a pancakelike shape. Had the stars formed at an earlier stage of the collapse, their rapid motions would have led to the formation of an elliptical galaxy. However, if the cloud stays gaseous until it flattens, much of the kinetic energy of the collapse is radiated away by gas atoms. Subsequent star formation is found to maintain a highly flattened, disklike shape, characteristic of spiral galaxies.

The flattening occurs in part because of the centrifugal forces in the rotating cloud. A confirmation of this picture has come from the discovery that elliptical galaxies rotate much less rapidly than spiral galaxies. This raises the question of the origin of the rotation itself. A natural explanation seems to lie in the action of the gravitational torques exerted by neighboring protogalaxies.
Evidence from composition. Another aspect of galaxies that has evolutionary significance is their composition, and, in particular, the distribution of heavy elements. The amount of heavy-element enrichment can be inferred from the color of the starlight, blue stars being metal-poor. Galaxies are found to be significantly bluer in their outermost regions and redder toward their central nuclei. The explanation seems to be that galaxies formed out of collapsing gas clouds that formed stars in a piecemeal fashion. As stars formed, they evolved, underwent nuclear reactions, produced heavy elements, and eventually shed enriched material (some stars even exploding as supernovae). Successive generations of stars formed out of the debris of earlier stars, and in this way the stellar content of galaxies systematically became enriched. The greatest enrichment would naturally occur toward the center of a galaxy, where the gaseous stellar debris tended to collect.

Interactions. Observations reveal many systems of interacting galaxies and close pairs ofgalaxies, which are possible candidates for later interactions. As galaxies move about within clusters, they will occasionally pass very near one another or even collide directly. Possible outcomes include loss of material from the galaxies' outer regions, transfer of material from one galaxy to another, merger of the two galaxies, modification of the galaxies' forms by tidal perturbations, and loss of gas and dust due to collisional heating. Whether a merger occurs mainly depends on the relative velocity difference of the two galaxies. If they pass each other too fast, the gravitational drag between them will not be efficient enough to change their trajectory and the passage does not result in merging.

Observations show an increase of interactions in the last few 109 years. The random velocity of a galaxy inside an association of galaxies increases with the mass connected to that association. Once the association forms and the galaxies within it start moving under its gravitational influence, the relative velocities in encounters of galaxies will become too high for mergers to occur and the number of mergers decreases. Looking backin time, the increase ofinter-actions is stronger in clusters of galaxies than in environments with few galaxies. This is a consequence of the higher number density of galaxies, which increases the probability of having an encounter.
Galaxies undergoing mergers experience dramatic morphological changes. Due to tidal forces the merging galaxies start deforming and develop very prominent features, most notably the so-called tidal arms. Mergers can trigger periods of intense star formation. Gas which was available in the disks of the progenitor spiral galaxies will be driven to the centers of the remnant galaxies and start forming stars in a starburst. It has been proposed that ultraluminous infrared galaxies like the Antennae galaxies are just galaxies undergoing mergers in which an extensive starburst occurs. Theoretical considerations suggest that the outcome of the interaction between two galaxies of similar size will be an elliptical galaxy. This is one of the most favored formation mechanisms for elliptical galaxies.

Origin. An outstanding and unresolved issue concerns the origin of the primordial gas clouds out of which the galaxies evolved. The cosmic background radiation yields a glimpse of the universe prior to the epoch of galaxy formation. The universe is now completely transparent to this radiation. However, about 500,000 years after the big bang, the radiation was sufficiently hot that matter was ionized, and the matter was also sufficiently dense to render the universe completely opaque to the radiation. To observe the background radiation now (some 1010 years later) is to see back to this early epoch, known as the decoupling epoch: at earlier times, matter and radiation were intimately linked, and subsequently the radiation propagated freely until the present time.

Theory of formation from fluctuations. The cosmic background radiation is very uniform, but fluctuations were discovered in 1992 by the Cosmic Background Explorer (COBE) satellite. These are at a level of 1 part in 105, and are on angular scales of several degrees. The precursor fluctuations of the primordial inhomogeneities that gave rise to the observed structures in the universe would be on scales of degrees, for the largest superclusters and voids, to arc-minutes, for galaxy clusters. A definitive measurement is not yet available for these angular scales, but without such primordial fluctuations galaxies could not have formed. The mutual action of gravity exerted between these infinitesimal fluctuations results in their gradual enhancement. Eventually, great gas clouds develop that will collapse to form galaxies. The required amplitude for these primordial seed fluctuations must be of the order of 1 part in 105, precisely what is measured on larger scales, to within uncertainties of at most a factor of 2.

Numerical simulations of galaxy clustering have enabled the spectrum of fluctuation length scales and amplitudes to be inferred. Galaxies are not randomly distributed, as would be "white" noise; rather, they are correlated. Given a galaxy at an arbitrary position, at a distance away, there is an excess probability, above random, of finding another galaxy. These correlations are large on scales less than 5 mega-parsecs (1.0 x 1020 mi or 1.5 x 1020 km), and are measured out to 20 Mpc (4 x 1020mior6 x 1020km). The parent fluctuations that gave rise to the galaxies must be similarly correlated, although the amplitude of the effect was much less.
A theory of the very early universe, first proposed in 1980, provides an explanation of the distribution of amplitudes of the fluctuations with scale, but accounts only qualitatively for their strength. According to this theory, the initial stages of the big bang were characterized by a period of rapid inflation during the first 10-35 sof the expansion. One consequence of an inflationary epoch is that quantum-statistical fluctuations are amplified up to scales of galaxies and of clusters of galaxies.

The predicted distribution of fluctuations is initially the same, on all scales, from that of galaxy clusters to the observable universe. On the largest scales, where there has been little time to develop deviation from the initial conditions, the distribution ofcosmic microwave background fluctuations measured by COBE in 1992 and in many subsequent experiments, especially the Wilkinson Microwave AnisotropyProbe (WMAP),is approximately consistent with that predicted by the simplest inflationary cosmological model. Thus, the most simple of cosmologies may contain the nascent seeds of future galaxies.
Isothermal and adiabatic fluctuations. The possible fluctuations in the early universe can be categorized into distinct varieties. Of particular importance for galaxy formation are density fluctuations that are found to generally be a combination of two basic types: adia-batic and isothermal.

Primordial adiabatic fluctuations are analogous to a compression of both matter and radiation. They are generic to almost all models of the early universe. In the absence of weakly interacting dark matter, the diffusive tendency of the radiation tends to smooth out the smaller adiabatic fluctuations. This process remains effective until the decoupling epoch, and galaxy formation occurs only relatively recently. However, the dominant presence of dark matter that does not interact with the radiation other than by gravity allows fluctuations to survive on all scales in the weakly interacting dark matter. The theory of fluctuation origin does not specify the strength of the fluctuations. However, the observations of the cosmic microwave background demonstrate that some 300,000 years after the big bang when the radiation was last scattered by the matter, the amplitude of the density fluctuations amounted to only a few parts in 104 on galaxy cluster scales. This means that massive galaxies and galaxy clusters formed relatively recently, although small galaxies could have formed when the universe was just a tenth of its present size. Galaxies form when the gaseous matter cools and condenses in the gravity field of the dark matter, forming gas clouds that subsequently fragment into stars.

In some variations of the standard model for structure formation, primordial isothermal fluctuations were also present in the very early universe. These consist of variations in the matter density, without any corresponding enhancement in the radiation density. Consequently, in the radiation-dominated early phase of the big bang, isothermal fluctuations neither grow nor decay, as the uniform radiation field prevents any motion. Once the universe becomes transparent, the matter fluctuations respond freely to gravity and grow if they are above a certain critical size. The smallest isothermal fluctuations that can become enhanced and form gas clouds contain about 106 solar masses. Galaxy formation occurs very early in this case.
Role of dark matter. The presence of weakly interacting dark matter is almost universally accepted by astronomers in order to account for the rotation curves of galaxies. The baryonic component of matter in the universe is known from calculations of the abundances of the light elements to amount to about 3% of the critical density for closing the universe. There is at least 10 times as much dark matter, which constitutes at least 90% of the mass of the universe. Consequently, dark matter dominates the growth of the primordial density fluctuations. The gravitational influence of this dark matter greatly aids this growth.

Baryon fluctuation growth is suppressed by interactions with the radiation prior to the epoch of decoupling of matter and radiation, whereas weakly interacting particles are able to cluster freely as long as the dominant form of density is ordinary matter rather than radiation. Since the density in the very early universe was dominated by radiation, fluctuation growth in the presence of dark matter is enhanced by about a factor of 10, equivalent to the expansion factor between the epochs of ordinary matter dominance when fluctuation growth first commences and the last scattering of the radiation. The associated fluctuations in the cosmic microwave background required in order to form structures by a given epoch are reduced by a corresponding factor. The detection of cosmic microwave background temperature fluctuations at a level of about one part in 105, initially by the COBE satellite and subsequently by more than 20 experiments, means that the precursor fluctuations of the largest structures, such as galaxy clusters, have been identified, in a statistical sense, in the sky. Dark matter plays an essential role in reconciling the level of the observed fluctuations with the limited growth period available since the universe was first matter-dominated, approximately 10,000 years after the big bang. This matter consists of massive weakly interacting particles whose existence is predicted by the theory of supersymmetry .

The implications of particle dark matter for structure formation are considerable. If the particles are massive, they are slowly moving at the onset of fluctuation growth, when the universe is first matter-dominated. Such particles are called cold dark matter. As structure develops, cold dark matter clusters, first on the smallest scales, then on progressively larger scales. This leads to a bottom-up scenario of hierarchical clustering. Unique predictions are made for both the microwave background fluctuations and the density fluctuations that are measured in large-scale structure studies.
Reconciliation of large-scale structure and galaxy formation with cold dark matter has proven remarkably successful on the largest scales. Most data point to a universe in which the density of cold dark matter is about 30% of the critical value. The cosmic microwave background temperature fluctuations demonstrate that the universe is at critical temperature to account for the locations of the observed angular peaks in the fluctuation distribution observed on the microwave sky. In a universe that has a near-Euclidean geometry, the positions of these peaks are displaced because of the bending of the light rays that have traversed the universe since the epoch when the microwave background photons were last scattered by the matter. A Euclidean geometry requires that the universe must be at critical density, if most of the energy density is in the form of the vacuum energy that is associated with the cosmological constant term introduced by Albert Einstein. One prediction of such a cosmological model is that the expansion of the universe is currently accelerating as a consequence of the nature of the additional energy. Data from use of distant supernovae to measure the deceleration of the universe suggest that the universe is indeed accelerating. Large-scale structure still provides asevere constraint on the nature of the dark matter.

In a universe with a critical density of dark matter, excessively strong clustering of galaxies occurs. Dark energy, which is uniform and smooth, does not participate in gravitational clustering. Three independent observational results contribute to make a strong case for a standard model of the modern universe. These are the cosmic microwave background temperature fluctuation peaks, the acceleration of the universe as inferred from the distances to remote supernovae, and the large-scale structure of the galaxy distribution. The standard model of the universe consists of 30% dark matter, 65% dark energy, and 5% baryons. Variations in the standard model introduce a component of hot dark matter. This matter consists ofneutrinos that are assumed to have a small mass, sufficient to account for about one-quarter of the critical density. However, the resulting mixture of hot and cold dark matter gives poor agreement with the astrophysical data on fluctuations at all scales. According to a much less accepted alternative viewpoint, the dark matter is entirely baryonic. It consists of very low mass stars or of burnt-out stars such as white dwarfs. In this case, the matter density of the universe is only about one-tenth of the critical density, with the rest of the critical density being made up of dark energy. One then finds that adiabatic fluctuations in a baryon-dominated universe, supplemented by a subdominant admixture of hot dark matter, can result in temperature fluctuations that agree with the observational constraints.

A further possible advantage of this interpretation is that by reducing the Hubble constant to about two-thirds of the currently preferred value, it is also possible to dispense with dark energy if one ignores the evidence for acceleration of the universe inferred from distant supernovae. The universe would then contain a critical density of baryons, along with some hot dark matter. Another scenario appeals to warm dark matter, for which a massive neutrino is the expected candidate. However, this option requires an admixture of isothermal fluctuations in order to allow early structure formation. The isothermal fluctuations are consistent with the cosmic microwave background data, provided they are subdominant. In this case, a complex baryon genesis scenario is required, in which matter is created with spatial inhomogeneities in the number of baryons relative to the number of photons. By far the simplest model is one in which the observed structure is seeded by primordial adia-batic density fluctuations generated during inflation.
Reference : McGraw - Hill Encyclopedia of Science and Technology

History of Glider

An unpowered flying device that attempts to copy the flight of soaring birds as accurately as possible.

Early development.
Otto Lilienthal made hundreds of flights with several designs of hang gliders before his fatal crash in 1896. His gliders were of rigid construction, generally without movable control surfaces, and were controlled by shifting the pilot's weight. These gliders had no landing gear other than the pilot's legs.

The next major advance in glider design was made by Wilbur and Orville Wright, who were inspired by Lilienthal. The Wrights' 1902 glider in its final version made hundreds of perfectly controlled glides and set a distance record of 622 ft (189 m) and a duration record of 26 s on the dunes of Kitty Hawk, North Carolina. This glider did not depend on weight shift for control, but had aerodynamic controls consisting of movable elevator, rudder, and wing tips. This system, in principle, has been used to the present time, even on very large and fast gliders and powered aircraft. The success with this glider led the Wrights to construct a slightly larger aircraft, with engine and propellers, that enabled them to take off and fly from level ground for the first time on December 17,1903. The first successful powered vehicle, now called an airplane, was not a practical or useful aircraft, but with engineering development did reach that goal in a few years.

Methods of flight.

In October 1911, Orville Wright made a gliding flight of nearly 10 min duration, and demonstrated that gliders could stay up for long periods in the rising air caused by the wind blowing against a sand dune or hill. This condition of flight, called slope soaring, was the basic method of soaring flight until about 1930. Thermal soaring, the next step, was accomplished by flying in areas of rising convection currents, which are almost always present to some degree in the atmosphere. The development of thermal soaring and its practice is now principally due to the use of the variometer, calibrated as a sensitive rate-of-climb instrument, which enables the pilot to find the thermal and make the best use of it. By the use of thermal flight, the modern glider can fly almost anywhere in the world for extended time and distances over 500 mi (800 km) in one flight. Other methods of soaring make use of clouds and standing-wave phenomena in the atmosphere. These techniques have enabled gliders to achieve altitudes of 46,000 ft (14,000 m) under the proper conditions. High-performance gliders (sailplanes;)may be launched by towing behind powered aircraft to a release height of about 2000 ft (700 m), by winch-launching to about 800 ft (250 m), or by car towing, which is used to a lesser extent. Some gliders have been fitted with a small motor and propeller, which enables them to take off and climb to an altitude where rising air permits them to soar unpow-ered. Francis M. Rogallo

Sailplanes.
Sailplane construction traditionally has been of wood and plywood, although the use of aluminum alloy has become common. The greater strength and stiffness of modern aluminum alloy permits higher aspect ratios and improved performance. The use of fiber glass as primary structure has also come into prominence, since it is possible to produce the external shapes in accurate molds with greater precision, resulting in improved performance.
Sailplanes are equipped with dive brakes on the wings for emergency descent and for landing in small areas. Properly designed brakes hold the aircraft to its "never-exceed" dive speed in a vertical dive. The laminar-flow airfoil achieves its lowest drag in a certain range of lift coefficient and corresponding speed range. By the use of properly designed flaps, this range can be shifted to higher or lower speeds at the pilot's control, which provides improved performance at a wider range of speeds.
As sailplane performance increases, each new drag item becomes important, and there has been a tendency to use reclining, and almost full reclining, positions for the pilots. This permits fuselages with overall heights as low as 30 in. (75 cm) but introduces control system and visibility problems.

Flexible-wing gliders.

In the late 1950s the National Aeronautics and Space Administration (NASA) investigated various methods of returning crewed spacecraft to Earth. Two kinds of glider were investigated: aircraft with rigid delta or lifting body shapes and very high landing speeds, which later evolved into the space shuttle; and flexible-wing craft which could be packed like parachutes and deployed for slow, controlled landings in almost any open field, a concept that had been proposed 10 years earlier. After NASA demonstrated flexible-wing capability, many segments of the Department of Defense and industry also became interested in flexible-wing gliders for a variety of applications. Crewed and radio-controlled flights were made with flexible-wing gliders with or without power or towed by cars or aircraft. Some gliders were completely flexible, and some were stiffened with springy battens, aluminum tubes, or fabric tubes either pressurized or ram-inflated. Although extensive military and space applications are still undeveloped, flexible wings have had an effect on sport flying devices such as kites, hang gliders, and deployable gliders or gliding canopies used by sky divers. The completely flexible, deploy-able gliders have maximum glide ratios of only 3 to 4, which are very low compared to those of sailplanes, but a substantial change from the zero glide of the traditional parachute or the glide ratio of less than unity obtained from modified parachutes. Flexible, deployable gliders are used by sky divers who have demonstrated great ability to maneuver, penetrate the wind, and land on a chosen spot.

Modern foot-launched hang gliders with aluminum tube frames are the result of many improvements by private individuals and small manufacturers. Hang gliders have flown over 100 mi (160 km) in straight-line distance, have reached about 20,000 ft (6000 m) altitude, and have remained aloft more than 15 h. But it is not so much their performance that makes hang gliders popular, as their low cost, their convenience of folding into a small package for transport or storage, and the fact that no license is required for glider or pilot. Hundreds of thousands of people have learned to fly hang gliders.

Propellers and motors of about 10 hp (7.5 kW) have been attached to some hang gliders, enabling them to take off from level ground and climb in still air if necessary, while still able to soar in updrafts with the engine off. Some hang gliders have wheels that can be used optionally. In the summer of 1979 five powered hang gliders took off from California and, after many stops for rest and refueling, landed on the east coast.
Reference : McGraw - Hill Encyclopedia of Science and Technology

Stem Cells - A new chance at life

Cells that have the ability to self-replicate and to give rise to mature cells. The concept of stem cells was originally based on renewing tissues. Many adult tissues, such as the skin, blood, and intestines, consist of mostly mature and short-lived cells that must be continuously replaced. Stem cells were postulated as the source of the self-renewal. In the early 1960s, Canadian scientists Ernest A.

McCulloch and James E. Till provided the first experimental proof of the existence of stem cells in the blood system. They revealed that a type of cell in bone marrow possesses the capacity to replicate itself and to differentiate to various lineages of mature blood cells. Self-renewal, together with the capacity for differentiation, defined the properties of stem cells. This definition is generally used in stem cell biology today.

Stem cells can be found at different stages of fetal development and are present in a wide range of adult tissues. Many of the terms used to distinguish stem cells are based on their origins and the cell types of their progeny. There are three basic types of stem cells. Totipotent stem cells, meaning that their potential is total, have the capacity to give rise to every cell type of the body and to form an entire organism. Pluripotent stem cells, such as embryonic stem cells, are capable of generating virtually all cell types of the body but are unable to form a functioning organism. Multipotent stem cells can give rise only to a limited number of cell types. For example, adult stem cells, also called organ- or tissue-specific stem cells, are multipotent stem cells found in specialized organs and tissues after birth.

Their primary function is to replenish cells lost from normal turnover or disease in the specific organs and tissues in which they are found.

Totipotent and embryonic stem cells.

Totipotent stem cells occur at the earliest stage of embryonic development. The union of sperm and egg creates a single totipotent cell. This cell divides into identical cells in the first hours after fertilization. All these cells have the potential to develop into a fetus when they are placed into the uterus. [To date, no such totipotent stem cell lines (primary cell cultures) have been developed.] The first differentiation of totipotent cells forms a sphere of cells called the blastocyst, which has an outer layer of cells and an inner cell mass. The outer layer of cells will form the placenta and other supporting tissues during fetal development, whereas cells of the inner cell mass go on to form all three primary germ layers: ectoderm, mesoderm, and endoderm. The three germ layers are the embryonic source of all types of cells and tissues of the body.

Embryonic stem cells are de-rivedfrom the inner cell mass of the blastocyst. They retain the capacity to give rise to cells of all three germ layers. However, embryonic stem cells cannot form a complete organism because they are unable to generate the entire spectrum of cells and structures required for fetal development. Thus, embryonic stem cells are pluripotent, not totipotent, stem cells.

Embryonic germ cells.

Embryonic germ cells differ from embryonic stem cells in the tissue sources from which they are derived, but appear to be similar to embryonic stem cells in their pluripotency. Human embryonic germ cell lines are established from the cultures of the primordial germ cells obtained from the gonadal ridge of late-stage embryos, a specific part that normally develops into the testes or the ovaries. Embryonic germ cells in culture, like cultured embryonic stem cells, form embryoid bodies, which are dense, multilayered cell aggregates consisting of partially differentiated cells. The embryoid body-derived cells have high growth potential. The cell lines generated from cultures of the embryoid body cells can give rise to cells of all three embryonic germ layers, indicating that embryonic germ cells may represent another source of pluripotent stem cells.

Growing mouse embryonic stem cells.

Much of the knowledge about embryonic development and stem cells has been accumulated from basic research on mouse embryonic stem cells. The techniques forsep-arating and culturing mouse embryonic stem cells from the inner cell mass of the blastocyst were first developed in the early 1980s. To maintain their growth potential and pluripotency, mouse embryonic stem cells can be grown on a feeder layer, usually consisting of mouse embryonic fibroblast cells. The feeder cells support embryonic stem cells by secreting a cytokine growth factor, the leukemia inhibitory factor, into the growth medium. Alternatively, purified leukemia inhibitory factor can be added to the growth medium without the use of a mouse embryonic feeder layer. (The leukemia inhibitory factor serves as an essential growth factor to maintain embryonic stem cells in culture.) A line of embryonic stem cells can be generated from a single cell under culture conditions that keep embryonic stem cells in a proliferative and undifferentiated state. Embryonic stem cell lines can produce indefinite numbers of identical stem cells. When mouse embryonic stem cells are integrated into an embryo at the blas-tocyst stage, the introduced embryonic stem cells can contribute to cells in all tissues of the resulting mouse. In the absence of feeder cells and the leukemia inhibitory factor in cultures, embryonic stem cells undergo differentiation spontaneously Many studies are focused on directing differentiation of embryonic stem cells in culture. The goal is to generate specific cell types. Formation of cell aggregates with three-dimensional structure during embryonic stem cell differentiation in culture may allow some of the cell-cell interaction to mimic that of in vivo development. The culture conditions can be designed to support and select specific cell types. With these experimental strategies, preliminary success has been achieved to generate some cell types, such as primitive types of vascular structures, blood cells, nerve cells, and pancreatic insulin-producing cells.

Growing human embryonic stem cells.

Since 1998, research teams have succeeded in growing human embryonic stem cells in culture. Human embryonic stem cell lines have been established from the inner cell mass of human blastocysts that were produced through in vitro fertilization procedures. The techniques for growing human embryonic stem cells are similar to those used for growth of mouse embryonic stem cells. However, human embryonic stem cells must be grown on a mouse embryonic fibro-blast feeder layer or in media conditioned by mouse embryonic fibroblasts (see illustration).

There are anumber of human embryonic stem cell lines being generated and maintained in laboratories in the United States and other nations, including Australia, Sweden, India, South Korea, and Israel. The National Institutes of Health has created a Human Embryonic Stem Cell Registry, which lists stem cell lines that have been developed and can be used for research. Human embryonic stem cell lines can be maintained in culture to generate indefinite numbers of identical stem cells for research. As with mouse embryonic stem cells, culture conditions have been designed to direct differentiation into specific cell types (for example, neural and hematopoietic cells).

Adult stem cells.

Adult stem cells, also referred to as somatic stem cells, occur in a wide variety of mature tissues in adults as well as in children. Like all stem cells, adult stem cells can self-replicate. Their ability to self-renew can last throughout the lifetime of individual organisms. Unlike embryonic stem cells, though, it is usually difficult to expand adult stem cells in culture. Adult stem cells reside in specific organs and tissues but account for a very small number of the cells in tissues. They are responsible for maintaining a stable state of the specialized tissues. To replace lost cells, stem cells typically generate intermediate cells called precursor or progenitor cells, which are no longer capable of self-renewal. However, they continue undergoing cell division, coupled with maturation, to yield fully specialized cells. Such stem cells have been identified in many types of adult tissues, including bone mar-row,blood, skin, gastrointestinal tract, dental pulp, retina of the eye, skeletal muscle, liver, pancreas, and brain. Adult stem cells are usually designated according to their source and their potential. Adult stem cells are multipotent because their potential is normally limited to one or more lineages of specialized cells. However, a special multipotent stem cell that can be found in bone marrow, called the mesenchymal stem cell, can produce all cell types of bone, cartilage, fat, blood, and connective tissues.

Blood stem cells.

Blood stem cells, or hematopoietic stem cells, are the most studied type of adult stem cells. The concept of hematopoietic stem cells is not new, as it has been long realized that mature blood cells are constantly lost and destroyed. Billions of new blood cells are produced each day to make up the loss. This process of blood cell generation, called hematopoiesis, occurs largely in the bone marrow. The presence of hematopoietic stem cells in the bone marrow was first demonstrated by E. A. McCulloch and J. E. Till in a mouse model in the early 1960s. The first experimental work on stem cells was an unexpected outcome from their study for measuring the effects of radiation. They found that the blood system of a mouse that has been subjected to heavy radiation can be restored by infusion of bone marrow. The stem cells responsible for reconstituting the blood system generate visible cell colonies on the spleen of the recipient mouse. Each of the spleen colonies consists of one or more types of blood cells, and all the cells in a colony are derived from a single cell. Self-renewal capacity of the colony-forming cells is demonstrated by their ability to form secondary spleen colonies. Such blood stem cells, known as colony forming unit-spleen cells, qualify as pluripotent hematopoi-etic stem cells because they can replicate and give rise to multiple types of mature blood cells. A definitive proof of blood stem cells is their ability to reconstitute the blood system. Bone marrow transplantation demonstrates the restorative powers of blood stem cells in humans.

Isolating blood stem cells.

Like other adult stem cells, blood stem cells are rare and difficult to isolate. Only about1in100,000cellsin the bone marrow is a stem cell. Scientists have used cell-sorting methods to enrich and purify blood stem cells. Stem cells differ from mature cells in their surface markers, which are specific protein molecules on the cell membrane that can be tagged with monoclonal antibodies. By using a set of surface markers, some expressed mainly on stem cells and others on mature blood cells, nearly pure populations of stem cells can be separated from bone marrow. The stem cells purified by this approach can engraft (begin to grow and function) and reconstitute the blood system in the recipient. In animal studies, as few as 30 purified stem cells can rescue a mouse that has been subjected to heavy radiation. Besides the bone marrow, a small number of blood stem cells can be found in circulating blood. In addition, stem cells in the bone marrow can be mobilized into the bloodstream by injecting the donor with certain growth factors or cytokines. This approach can result in a large number of stem cells circulating in peripheral blood, from which they can be collected and used for transplant therapy.
Umbilical cord blood and cord blood banks.

An alternative source of blood stem cells is human umbilical cord blood, a small amount of blood remaining in the placenta and blood vessels of the umbilical cord. It is traditionally treated as a waste material after delivery of the newborn. However, since the recognition of the presence of blood stem cells in umbilical cord blood in the late 1980s, its collection and banking has grown quickly. Similar to bone marrow, umbilical cord blood can be used as a source material of stem cells for transplant therapy. In 1989, the first successful cord blood transplant was reported for treating a 6-year-old boy suffering from Fanconi's anemia (an inherited disease that primarily affects the bone marrow, resulting in decreased production of blood cells) in Paris. Since then, over 6000 cord blood stem cell transplants have been performed worldwide, mainly in patients with blood conditions and in some cancer therapies. However, because of the limited number of stem cells in umbilical cord blood, most ofthe procedures are performed on young children of relatively low body weight. A current focus of study is to promote the growth of umbilical cord blood stem cells in culture in order to generate sufficient numbers of stem cells for adult recipients.

Many blood banks have been established to collect and cryopreserve cord blood cells. Commercial banks offer services of storing cord blood of healthy newborns for potential future use by themselves or their siblings. Although it is considered a biological insurance, the chance ofa child using his or her own cord blood is estimated at 1 per 20,000 collections. Of the estimated 6000 cord blood transplants, only 14 were performed using autologous sources.
Public banks encourage donation of cord blood for unrelated transplants. The Stem Cell Research and Therapeutic Act of 2005 (H.R. 2520) established a national umbilical cord blood program, providing federal funding to collect and store cord blood for blood cell transplants. The program functions to provide a national inventory of 150,000 cord blood units for public use and to establish a registry network integrated with the national marrow donor registry administered by the National Marrow Donor Program (NMDP).

Mesenchymal stem cells.

Mesenchymal stem cells (MSCs) are a type of multipotent adult stem cells, and they are defined by the capacity to give rise to a variety of connective tissue lineages, including bone, cartilage, tendon, muscle, and fat cells. Classic studies found a type of cells in bone marrow stroma capa-bleofgenerating fibroblast-like cell colonies. These clonogenic cells were termed colony forming unit-fibroblasts (CFU-F). CFU-F share some characteristics of MSCs. MSCs appear as fibroblast-like spindle-shaped cells. CFU-F assay is still used to evaluate MSCs in cell cultures. MSCs can be distinguished and isolated from other cells based on phenotypic characteristics. Typically, MSCs express specific surface
antigens SH2, SH4, and STRO-1 and lack blood cell markers CD45 and CD34. MSCs can replicate as multipotent cells. The mesenchymal cell lineage potential can be demonstrated in vitro with appropriate culture conditions. Differentiation can be induced to osteocytes by dexamethasone and ascorbate, to chrondrocytes by transforming growth factor-^3, or to adipocytes by dexamethasone and insulin.
MSCs are primarily obtained from bone marrow stromal cells. They are also found in small numbers in umbilical cord blood. In addition, adipose-derived stem cells (ASCs) have been shown to be similar to MSCs. Fat tissue is of mesenchymal origin and contains stromal components. ASCs can be isolated from fat tissue by the method of liposuction. Human ASCs have been shown to exhibit the capacity to give rise to fat, bone, cartilage, muscle, and possibly neurons. Thus, ASCs may provide a potential source of multipotent adult stem cells.
Neural stem cells. Neural stem cells, the multipotent stem cells that generate nerve cells, are a new focus in stem cell research. Active cellular turnover does not occur in the adult nervous system as it does in renewing tissues such as blood or skin. Because of this observation, it had been dogma that the adult brain and spinal cord were unable to regenerate new nerve cells. However, since the early 1990s, neural stem cells have been isolated from the adult brain as well as from fetal brain tissues. Stem cells in the adult brain are found in the areas called the subventricular zone and the ventricle zone. Brain ventricles are small cavities filled with cerebrospinal fluid. Another location of brain stem cells occurs in the hippocampus, a special structure of the cerebral cortex related to memory function. Stem cells isolated from these areas are able to divide and to give rise to nerve cells (neurons) and neuron-supporting cell types in culture.

Plasticity. Stem cell plasticity refers to the phenomenon of adult stem cells from one tissue generating the specialized cells of another tissue. The longstanding concept of adult organ-specific stem cells is that they are restricted to producing the cell types of their specific tissues. However, a series of recent studies have challenged the concept of tissue restriction of adult stem cells. Much of the experimental evidence is derived from transplant studies with blood stem cells. Bone marrow stem cells have been shown to contribute to liver, skeletal muscle, and cardiac cells in human recipients. In mouse models, purified blood stem cells have been demonstrated to generate cells in nonblood tissues, including the liver, gut, and skin. Although the stem cells appear able to cross their tissue-specific boundaries, crossing occurs generally at a low frequency and mostly only under conditions of host organ damage. The finding of stem cell plasticity is unorthodox and unexpected (since adult stem cells are considered to be organ/tissue-specific), but it carries significant implications for potential cell therapy. For example, if differentiation can be redirected, stem cells of abundant source and easy access, such as blood stem cells in bone marrow or umbilical cord blood, could be used to substitute stem cells in tissues that are difficult to isolate, such as heart and nervous system tissue. However, the concept of plasticity has been the subject of controversy.

The observed frequency of lineage conversion is generally low. An alternative explanation to plasticity is the phenomenon of fusion of host and donor cells. Recent findings suggest that blood cells contribute to other tissues by fusing with preexisting cells rather than by converting to other cell lineages.

Potential clinical applications.

Stem cells hold great potential for developing cell therapies to treat a wide range of human diseases. Already in clinical use is blood stem cell transplant therapy, well known as bone marrow transplant therapy for the treatment of patients with certain types of blood diseases and cancers. The discovery of stem cells in various adult tissues, stem cell plasticity, and human embryonic stem cells brings new excitement and opportunities. Stem cells offer the possibility of cell replacement therapy for many human diseases, such as Parkinson's and Alzheimer's diseases, spinal cord injury, diabetes, heart disease, and arthritis, that result from loss or damage of cells in a specialized tissue of the body. Stem cell therapy might revolutionize the treatment and outcome of these diseases. Stem cell science is still in the very early stage. Much more research is required to understand the biological mechanisms that govern cell differentiation and to identify factors that direct cell specialization. Future cell therapy will depend largely on advances in the understanding of stem cell biology and the ability to harness the process of stem cell growth and differentiation. Somatic cell nuclear transfer (SCNT) stem cells.
SCNT involves a micromanipulation procedure in which the nucleus of an egg is removed and replaced by a nucleus taken from somatic cells, typically skin cells. Successful nuclear transfer requires reprogramming of the donor nucleus. The cells so created may divide in cultures to generate embryonic stem cells that can initiate embryogenesis. This is the technique being used in cloning animals, such as the first cloned mammal, Dolly the sheep. However, cloning by nuclear transfer is observed with extremely low efficiency, probably due to faulty and incomplete reprogramming of the donor nucleus. The mechanisms governing the transition from a differentiated genome to a totipotent state remain largely unknown.
A major interest in SCNT is the prospect of creating patient-specific embryonic stem cells. These cells would be genetically identical to the nuclear donor except for maternal mitochondrial deoxyribonucleic acid (mtDNA) of the oocyte. Therefore, the problem of graft rejection would be avoided if the cells could be used for transplant therapy for the donor patients. The concept of using SCNT to generate customized stem cells for cell therapy is also referred to as therapeutic cloning. However, there are hurdles and limitations to using embryonic stem cells in clinical applications. A major challenge is to achieve the directed differentiation and controlled growth before stem cells can be used for transplant therapy. Another issue on SCNT in human stem cells is the sourcing of human eggs. The procedure requires a large number of eggs from women, and poses an ethical and technical challenge.

The success in producing embryonic stem cell lines by the SCNT technique has been demonstrated in mice. In an article published in Science in 2005, a team led by Hwang Woo Suk of South Korea claimed the establishment of patient-specific stem cell lines by using the SCNT technique. However, the paper was later retracted as the results were fabricated and the claim a fraud. The field is still left uncertain if somatic nuclear replacement is feasible in humans.
Ethical and regulatory issues. The use of human embryonic stem cells raises ethical, social, and legal issues. The major concern centers on the source of stem cells. Human embryonic stem cell lines are made from the inner cell mass of a blastocyst stage embryo. Most embryos used to produce stem cells are left over from in vitro fertilization (TVF) treatment. The embryos are destroyed by the procedure of extracting stem cells. The early embryo has the biological potential to develop into a person. However, society has not reached consensus on when human life begins. The attention on stem cell research and cloning calls for regulation and legislation from governments. In the United States, current policy allows federal funds to be used for research only on existing human embryonic stem lines. The human embryonic stem cell lines that meet the eligibility criteria are listed in the Human Embryonic Stem Cell Registry by the National Institutes of Health (NIH).

One concern about SCNT is that it may lead to the reproductive cloning of humans. In theory, the embryo created via SCNT could be used to clone a human if it were implanted into a womans uterus. In the United States, the legislators in the House of Representatives and the Senate have introduced bills proposing a ban of all forms of cloning, including research cloning, or inhibiting reproductive cloning while preserving therapeutic cloning research. However, these bills have not been passed, and no federal law has been established on human embryonic stem cell research. The Canadian Parliament has passed Bill C-6 that prohibits creation of a human clone, sale of sperm or ova, and commercial surrogacy. The bill permits the use of stem cells obtained from discarded products of in vitro fertilization, that is, excess and unused embryos. In the United Kingdom, a law permits the use of embryos in research and therapeutic cloning research but bans reproductive cloning, and implanting a cloned embryo in a human uterus is liable to criminal prosecution. Cloning research must be licensed from the Human Fertilization and Embryology Authority that governs embryonic and stem cell research in the United Kingdom.
Reference : McGraw - Hill Encyclopedia of Science and Technology