ROLE OF THE MESONEPHROS IN THE DEVELOPMENT OF THE SHEEP FETAL OVARY

Sexually indifferent gonads, ovaries and mesonephroi of sheep embryos and fetuses from day 24 of development through term (day 145) were studied by high resolution light microscopy and electron microscopy. From day 24 at least day 58, the sexually indifferent gonads and the ovaries are massively invaded and colonized by cells mobilized from a peculiar nephron situated in the proximal third of the mesonephros and referred to, because of its large size, as 'giant'. The mobilization process occurs simultaneously with the involution of the giant nephron, and involves the mesangial and epithelial cells of the glomerulus and, in late stages, also the epithelial cells of the walls of the excretory tubules. At first sporadically and, then, in the form of a prominent, compact cellular mass, these cells leave their sites of origin and, advancing along cranio-caudal and dorso-ventral directions, invade the gonads. Here the mesonephric cells establish a close and permanent associative relationship with the germinal cells, also seeding the gonads at these times. During sequential stages of ovarian development, the relationship between mesonephric and germinal cells assumes characteristics of increasing organization; at first, it involves isolated cells, then cellular clusters and, ultimately, the ovigerous cords, elongated formations of mesonephric and germinal cells lined by a basal lamina from which follicles eventually develop.