Conway, Simon James; (1993) Molecular analysis of the gastrulating mouse embryo and its germ cell lineage. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Gastrulation occurs between 6.5 and 8.0 days post coitum in the mouse and is associated with a dramatic increase in cell proliferation, being initiated when the embryo consists of some 500-600 cells and resulting in a neural plate stage embryo containing 50,000 cells or more. There is evidence to suggest that some of these areas may be determined with respect to their future development quite early in the process of gastrulation. During this period of development it is clear that the growth rate of the whole embryo, and of its constituent parts, is under intrinsic control and capable of controlled variation aimed presumably at generating an organogenesis stage embryo of a notional target size in which the parts are of appropriate relative size. Investigations in the mouse have shown that growth of the embryo parts and their co-ordination can be profoundly disturbed in early embryonic stages by insult with the cytotoxic agent, Mitomycin C. MMC is an alkylating agent that inhibits DNA synthesis. It causes cell death and reduces pre-organogenesis stage embryos to 10% of normal size as measured by cell number. During the progression of embryogenesis the normal co-ordination of development of different parts of the embryo is abnormal. Considerable and localized growth control is obviously operating in these stages, thus the proto-oncogenes with known growth regulatory functions were looked at as likely candidates controlling the dramatic growth patterns observed. During the gastrulation movements the primordial germ cells are located at the base of the allantois and appear to 'escape' the movements and rearrangements of gastrulation by migrating into the extraembryonic membranes. The primordial germ cells then appear to undergo their own differentiation, proliferate and migrate into the genital ridges. The germ cells multiply as they migrate until some 5000 cells ultimately invade the gonads. The proto-oncogene c-kit expression was analysed, as it has been recently identified with the dominant white spotting (W) locus of the mouse. The W mutations have pleiotropic effects on mouse development, one of which is to cause an impairment of proliferation and/or migration of primordial germ cells. It appears that the c-kit product is necessary for the maintenance of the primordial germ cells during their migratory/mitotically active phase. This finding holds true for overall expression of c-kit in structures of the developing embryo. As the testes develop and become functional, the germ cells undergo further differentiation. There is evidence from Y chromosome deletion mapping that there is a multiple copy gene on the long arm of the mouse Y which is needed for the normal development of the sperm head. A Y-specific genomic DNA sequence (Y353/B) has previously been described which is present in multiple copies on the long arm of the mouse Y and is specifically transcribed in the testis. Using quantitative Northern analysis it was shown that the RNA transcripts are from the multiple copies. In situ hybridization was used to demonstrate that the transcripts are In the germ line and not In the soma, and that the expression is predominantly in round spermatids. This was the first evidence for a Y chromosome gene being transcribed in the testis (and probably anywhere). Y353/B is thus a candidate for being the multiple copy 'sperm morphology gene' on the mouse Y chromosome.

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