Two-step oligoclonal development of male germ cells

Abstract

During mouse development, primordial germ cells (PGCs) that give rise to the entire germ line are first identified within the proximal epiblast. However, long-term tracing of the fate of the cells has not been done wherein all cells in and around the germ-cell lineage are identified. Also, quantitative estimates of the number of founder PGCs using different models have come up with various numbers. Here, we use tetrachimeric mice to show that the progenitor numbers for the entire germ line in adult testis, and for the initiating embryonic PGCs, are both 4 cells. Although they proliferate to form polyclonal germ-cell populations in fetal and neonatal testes, germ cells that actually contribute to adult spermatogenesis originate from a small number of secondary founder cells that originate in the fetal period. The rest of the ''deciduous'' germ cells are lost, most likely by apoptosis, before the reproductive period. The second ''actual'' founder germ cells generally form small numbers of large monoclonal areas in testes by the reproductive period. Our results also demonstrate that there is no contribution of somatic cells to the male germ cell pool during development or in adulthood. These results suggest a model of 2-step oligoclonal development of male germ cells in mice, the second step distinguishing the heritable germ line from cells selected not to participate in forming the next generation.stem cells ͉ testis ͉ premordial germ cells ͉ apoptosis chimeras T he germ line is made up of a highly protected and strictly regulated group of cells that transmit genetic information accurately to the next generation. It was initially proposed (1), and later shown that they usually originate as a very small founding population that is segregated from somatic cells early in development, at least in organisms where the overall body plan is established early (2, 3). However, retroviral marking of mouse early embryos at the 4-to 16-cell stage suggests that at least 3 cells contribute to the germ line and are set aside before somatic tissue allocation (4), indicating that the origin of germ cells is not generally monoclonal. In species such as Drosophila and Caenorhabditis elegans, germ plasm, a unique cluster of mRNAs and organelles, is a good marker to follow the origin and development of early germ cells (3). However, there is no visible germ plasm in mammals, making it difficult to follow early populations of germ cells (5). It has been shown that inner cell mass (ICM) or epiblast cells earlier than embryonic day (E)5.5 can contribute to any lineage of somatic and germ cells (6-8).Tissue nonspecific alkaline phosphatase (TNAP) has long been used as a marker for candidate primordial germ cells (PGCs) (9, 10). Also, many markers for PGCs have been identified, including SSEA-1 (11), Oct3/4 (12), Stella/PGC-7 (13, 14), Fragilis (14), and Blimp-1 (15). By using these markers, earlier candidate PGCs that cannot be identified with TNAP could be detected. It has been shown that they are stella positive and fragilis highly posit...

Stem cells are units of natural selection for tissue formation, for germline development, and in cancer development

“…In the adult testis of these mice, the number of independent colors in germline clones was two or three, which by Poisson distribution is consistent with only four germline stem cells separating from the epiblast somatic cells (13). The adult testis contained dozens of adjacent seminiferous tubules of the same color, demonstrating that single germline stem cells are capable of inhabiting many adjacent seminiferous tubules.…”

“…We propose that genetic changes occur in most of the cells, and most will not give rise to viable germline-derived sperm. Others have noted that the time period in which we see the maximal cell death coincides with the movement within the germline of transposable DNA elements, many of which are the survivors of horizontally transmitted retroviruses that have invaded the germline (13,14). During this movement, copies of particular retrotransposons are made and insert throughout the genome, creating, for the most part, interruptions of genes and, perhaps, enhancers retained in the transposable element.…”

Identification of side population cells in mouse primordial germ cells and prenatal testis

“…Scaldaferri et al set (McLean et al, 2003). It has been recently suggested that adult SSCs are already selected among PGCs before 13.5 dpc (Ueno et al, 2009), but the molecular mechanisms underlying SSC fate commitment remain obscure.…”

“…Scaldaferri et al set (McLean et al, 2003). It has been recently suggested that adult SSCs are already selected among PGCs before 13.5 dpc (Ueno et al, 2009), but the molecular mechanisms underlying SSC fate commitment remain obscure.Previous studies in the mouse have shown that SSCs can be identified by their ability to efflux dyes such as Hoechst 33342 (Falciatori et al, 2004;Lassalle et al, 2004). This ability, defined as the "side-population" (SP) phenotype, is common to several stem/progenitor cell types (Challen and Little, 2006) and relies on the activity of ATP-binding cassette (ABC) transporter superfamily, including multidrug resistance 1 (Mdr1a/1b, mouse; MDR1, human) and breast cancer resistance protein 1 (Bcrp1, mouse; ABCG2 human) (Bunting et al, 2000;Zhou et al, 2001).…”

How One Thing Led to Another

“…It seemed likely that tissue stem cell biology could account for the programmed life span and could reveal the genetic units of intraorganismal natural selection of stem cell lineages (189)(190)(191). These studies were the basis for our experiments to test for a diversity of competing HSCs in aging, a diversity of germline stem cells in testis and ovary development in mice (192), and the concept of clonal competitions in cancers (161,(193)(194)(195)(196)(197)(198)(199).…”