[
Mol Reprod Dev,
2013]
Formation of the germline in an embryo marks a fresh round of reproductive potential, yet the developmental stage and location within the embryo where the primordial germ cells (PGCs) form differs wildly among species. In most animals, the germline is formed either by an inherited mechanism, in which maternal provisions within the oocyte drive localized germ-cell fate once acquired in the embryo, or an inductive mechanism that involves signaling between cells that directs germ-cell fate. The inherited mechanism has been widely studied in model organisms such as Drosophila melanogaster, Caenorhabditis elegans, Xenopus laevis, and Danio rerio. Given the rapid generation time and the effective adaptation for laboratory research of these organisms, it is not coincidental that research on these organisms has led the field in elucidating mechanisms for germline specification. The inductive mechanism, however, is less well understood and is studied primarily in the mouse (Mus musculus). In this review, we compare and contrast these two fundamental mechanisms for germline determination, beginning with the key molecular determinants that play a role in the formation of germ cells across all animal taxa. We next explore the current understanding of the inductive mechanism of germ-cell determination in mice, and evaluate the hypotheses for selective pressures on these contrasting mechanisms. We then discuss the hypothesis that the transition between these determination mechanisms, which has happened many times in phylogeny, is more of a continuum than a binary change. Finally, we propose an analogy between germline determination and sex determination in vertebrates-two of the milestones of reproduction and development-in which animals use contrasting strategies to activate similar pathways.
[
Ann N Y Acad Sci,
1959]
A detailed summary of methods for axenic cultivation of Caenorhabditis briggsae is given. Results of axenic culture on chemically defined basal media (GM and GS) and on these media supplemented with undefined preparations of horse liver and chick embryos are reported in detail, with a review of the formulation of the GM and GS designs and of the chronology of changes made therein. The best growth so far realized with C. briggsae in axenic culture is suboptimal as compared with growth in the presence of bacteria, and maturation takes longer (4 to 5 days instead of about 3 days at 20C). Suitable media of the GM design give good axenic growth with relatively low levels of complex supplements-Liver Protein Fraction C (LPF-C) and chick embryo extract (CEE), both of which presumably include a protein-linked requirement, Factor Rb. With GM-16 plus CEE or certain GSs plus CEE, requirements have been variously demonstrated for 6 B-vitamins: folic acid, niacinamide, pantothenic acid, pyridoxine, riboflavin, and thiamine; one of these-folic acid-had already been shown to be required. Only niacinamide is also demonstrated as a requirement in the presence of low levels of LPF-C. In the presence of CEE we have tested the essentiality of the other 5 vitamins only by omitting them singly from vitamin mixes added at increased (5 to 50 times GS) levels to media of GM or GS type. Preliminary evidence is given that the ten "rat-essential" amino acids are required. Improvement of nutritional balance with respect to amino acid levels and to relative levels of amino acids in relation to vitamins or salts is discussed as an explanation of differential growth on different media. Possibly the variations of DM-GS so far tested contain unnecessarily high amino acid levels. The definition of nutritional requirements for C. briggsae still presents many