Spermatogonial germ layers. Germinal differentiation in the testis starts

Spermatogonial stem cells are derived from growth-arrested
gonocytes of the newborn testis. Primordial germ cells are pluripotent and are
capable of forming all three primordial germ layers.  Germinal differentiation in the testis starts
from a small population of cells (2–3 × 104 per adult mouse testis)
designated Asingle (As) spermatogonia (de Rooij and
Grootegoed, 1998). Lining the basal membrane of the seminiferous tubule, they
show a characteristic morphology with a nucleus devoid of heterochromatin. A
series of mitotic divisions sequentially generate the Aaligneds
cells, which remain connected by cytoplasmic bridges. The latter in turn
generate the A1 to A4, Intermediate and B spermatogonia,
which eventually divide into preleptotene spermatocytes that will subsequently
enter the ?rst meiotic division. SSCs can undergo two types of cell divisions
namely symmetrical and asymmetrical. Daughter stem cells are always results of
symmetrical divisions while asymmetrical divisions give rise to both stem cells
and differentiating cells. Functionally, SSCs characterized by their ability to
re-establish permanent spermatogenesis upon transplantation into recipient
testes devoid of germ cells.

Unlike other stem cells, SSCs are distinct in that they
can transmit genetic information to the next generation. Unlike Embryonic
stem  cells, however, SSCs retain some
features of the original PGCs, including genome-wide demethylation, erasure of
genomic imprints and reactivation of X-chromosomes, the degree of which likely
reflects the developmental stages of the PGCs from which they are derived (Yu et al., 2008; Maser and Depinho, 2002).
The use of SSCs has several advantages over conventional methods based on
eggs/oocytes. First, SSCs are established from postnatal testes, whereas
Embryonic stem cells are derived from embryos. Second, SSCs have very stable
genetic and epigenetic properties, probably reflecting the features of SSCs as
committed stem cells. This is in contrast to Embryonic stem cells. Because they
are uncommitted to a specific lineage, Embryonic stem cells can easily
differentiate in to other lineages, but they may lose their germ-line
potential. Therefore, it can be considered that SSCs will become a target for
mutagenesis in many animal species. Unlike mice, most animals do not ovulate
large numbers of oocytes and they require a long period of time to reach sexual
maturity. These factors limit the genetic manipulation of such animal species.
Therefore, SSCs technology has an advantage in many animal species that produce
small numbers of offspring i.e. Livestock species.

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