Germ-cell line and sexual differentiation in birds

Abstract

The present paper deals with recent investigations on the germ line and the sexual organogenesis in birds. The analysis has been limited to the following problems: origin of the germ line, physiology of the germinal epithelia, determinism of the migration of gonocytes, and differentiation of the germ cells during sexual organogenesis. It has been clearly established that in birds the germ line is precociously determined and that the anterior germinal crescent is a secondary formation. The hypothesis according to which the primary localization of the germ cells in birds (and more generally in Amniotes) would be posterior, is discussed. It appears to be the most plausible. The germinal epithelia are secretory and excretory organs. The excretory function is of the merocrine type. Kinetic studies of the protein turnover in the germinal epithelia demonstrate the existence of at least two categories of cytoplasmic proteins: structural proteins, and exportable proteins; the latter are excreted via protrusions of the epithelial cells. The hypothesis according to which the excretory function is associated with the attractive power exerted by the sexual primordia upon the migrating primordial germ cells is examined. The mechanism controlling the entry into the genital ridge of the gonocytes circulating in the embryonic blood stream is of a chemotactic nature. Finally, during early sexual organogenesis, as well as during later stages of sexual differentiation, the germ cells undergo important ultrastructural, histochemical and physiological (migratory properties) changes.