The Questions of Developmental Biology

Fibronectin is likely to be an important substrate for PGC migration (ffrench-Constant et
al. 1991), and germ cells that lack the integrin receptor for such extracellular matrix proteins
cannot migrate to the gonads (Anderson et al. 1999). Directionality may be provided by a
gradient of soluble protein. In vitro evidence suggests that the genital ridges of 10.5-day mouse
embryos secrete a diffusible TGF-β1-like protein that is capable of attracting mouse PGCs (Godin
et al. 1990; Godin and Wylie 1991). Whether the genital ridge is able to provide such cues in vivo
still must be tested.
Although no germ plasm has been found in mammals, the retention of totipotency has
been correlated with the expression of a nuclear transcription factor, Oct4. This factor is
expressed in all of the early-cleavage blastomere nuclei, but its expression becomes restricted to
the inner cell mass.