Pathways to Pluripotency: How Germ Cells Make Stem Cells - Male Germline Stem Cells Developmental and Regenerative Potential - page 16

regulating the formation of testicular tumors and, therefore, of the formation of
pluripotent EC cells (reviewed in Matin and Nadeau 2005 ). Therefore, these studies
provide valuable information about the mechanisms by which pluripotency is regu-
lated. What is still somewhat controversial is whether the cell type of origin in
humans is the same as that in mice and, therefore, whether data from human genetic
studies is relevant to our understanding of the control of pluripotency. Nevertheless,
studies using inbred strains of mice have identified several genes involved in the
formation of teratomas or teratocarcinomas. Most mouse strains have a very low
incidence of testicular germ cell tumors. Importantly, Stevens identified a strain of
mice that showed an increased incidence of testicular teratocarcinomas, the 129Sv/J
strain, which had a TGCT incidence of about 5% (Stevens and Hummel 1957 ).
Further he demonstrated that the cell type of origin of these tumors were PGCs
(Stevens 1967a ). In the normal course of PGC development in males, PGCs enter
the developing gonad and, at about the same time that the somatic cells show the first
signs of differentiation, PGCs begin to enter mitotic arrest and form gonocytes.
These cells will remain arrested in mitosis until after birth, at which time they will
resume mitosis and give rise to spermatogonia. In mouse strains that are susceptible
to TGCT, small groups of PGCs in the developing gonads continue to proliferate
after the time at which they would normally have entered mitotic arrest. These small
nests of proliferating PGCs give rise to EC cells. After birth these EC cells differenti-
ate into differentiated cells representative of those found in the embryo and adult
forming a benign tumor called a teratoma. These studies therefore identified PGCs
as the stem or progenitor cell of these tumors. Having identified a mouse strain with
susceptibility to developing TGCT it was possible to introduce gene mutations onto
that strain background and therefore analyze the effect on the incidence of TGCT
and, consequently, to identify genes that influence that process both negatively and
positively. These genes include the C-Kit receptor tyrosine kinase, the Kit-ligand
(KL), and Agouti (Fig. 1.4 ). In addition, Stevens subsequently identified a locus
termed Te r (for Teratocarcinoma ) that arose spontaneously in the 129Sv/J strain that
is a powerful modifier of TGCT (Stevens 1973 ). Modern techniques of genome
analysis have allowed identification of the role of many of these genes as well as the
identification of other genes and chromosomal regions conferring susceptibility to
TGCT. One of the loci identified as being a modifier of TGCT was the Steel ( Sl )
locus on mouse chromosome 10. Subsequent cloning of this locus demonstrated that
the Sl locus encodes a transmembrane growth factor that can be cleaved to give rise
to a soluble growth factor. Both transmembrane and soluble factors act as ligands for
the C-Kit receptor tyrosine kinase and are termed Kit-ligand. Both KL and its recep-
tor have been shown to play a key role in PGC development and have been proposed
to play a critical role in regulating PGC survival. Many mutations at the Sl locus and
the W locus encoding the C-Kit receptor cause dramatic reductions in PGC numbers
and can cause reduced fertility or complete sterility. Introduction of different Sl
alleles onto the 129/SvJ background has allowed further dissection of the role of KL
in TGCT. Multiple mutants have been described at the Sl locus and include intragenic
mutations, complete deletions, and mutations in the regulatory elements that leave
the coding regions intact. Some Sl alleles, such as Sl and Sl j , which delete the entire