Evolution of the lining bodies in the embryonic chick gonad.

Gonads from White Leghorn chick embryo of various ages ranging from 4 to 10 days of incubation, were fixed in half-strength Karnovsky's fixative, osmicated and embedded in Araldite. Thin sections were stained with urany] acetate and lead citrate. Our observations were especially concentrated on the germinal epithelium. Two cytological characteristics not previously described in embryonic chick gonads were observed: (1) A network of fine microfilaments was present in the somatic cells of the germinal epithelium. These microfilaments were especially concentrated in the apical pole of these cells and appeared to be more numerous in 5and 6-day gonads. Since similar microfilaments have been described in other embryonic epithelia in which growth of glands or cords is taking place, their presence in the embryonic gonad may be related to the formation of sex cords. (2) Lining bodies, organelles which are characteristic of follicular cells in the adult ovary, were found in the embryonic gonads of both sexes. They first appeared in some of the 5-day embryos and were present thereafter in all 6-day left gonads, in all left ovaries and in those left testes in which regression of the germinal epithelium had not been completed. Lining bodies were never observed in right germinal epithelia. It is classically accepted that follicular cells of the adult originate from the somatic cells of the germinal epithelium. The presence of lining bodies in the epithelial cells of young embryos appears to indicate that, despite the fact that follicles are not formed until after hatching, follicular cells initiate their differentiation much earlier. The fact that this differentiation is also present in gonads of male embryos can be interpreted in the light of current theories on sex differentiation which assume the bisexuality of embryonic gonads. Short cytoplasmic processes, representatives of a merocrine secretory process, have been described by other authors in the germinal epithelium of young embryos. We confirmed these observations and found, in addition, that these processes were smaller, both in size and number, in the right gonads. This fact, together with the previously mentioned absence of lining bodies in right gonads, appears to be the expression, at the ultrastructural level, of the well known asymmetry of the avian embryonic gonads.