Testicular differentiation, as evidenced by the appearance of a distinct tunica albuginea and formation of plate-like aggregates of germ cells and mesenchymal cells, occurs at 16 days in the rabbit fetus. The germ cells at this stage of development are isolated large, round to oval cells that are easily distinguished from the smaller elongated mesenchymal elements surrounding them. The germ cell nuclei have a smooth spherical contour and a prominent eccentric nucleolus. Cytoplasmic organelles are sparse, consisting of scattered polyribosomes and spherical or ovoid mitochondria distributed singly in the cell. At 18 days, the cell groups form into distinct cords of germ cells and supporting cells, the earliest stage of seminiferous tubule formation. Between 22 and 25 days, proliferating germ cells show increasing condensation of nuclear chromatin and increased nucleolar complexity. The cytoplasm contains groups of mitochondria, dense polyribosomal complexes, lipid aggregates, scattered smooth vesicles and prominent Golgi complexes. Intercellular bridges appear between germ cells on day 22 and increase in number on succeeding days. During the last week of fetal development, germ cells become aligned in rows at the periphery of the primitive tubules. Their arrangement at this time resembles that of spermatogonia in the adult testis. However, the fetal "prespermatogonia" lack

Spermatogenesis begins in the rabbit at seven to eight weeks of age. During the preceding postnatal period, seminiferous tubules contain round to oval prespermatogonia arranged in pairs at the tubular periphery and elongated Sertoli cells lying perpendicular to the basal lamina. Prespermatogonia are distinguished from fetal gonocytes by their abundant cytoplasm and the presence of intercellular bridges. Preceding the onset of spermatogenesis, prespermatogonia undergo diminution in cell size and cytoplasmic condensation. At the same time there is marked nucleolar enlargement, with elaborate proliferation of the nucleolonema. By the end of the seventh week, characteristic type A and B spermatogonia are present. Spermatocytes are seen at the end of the eighth week. Spermatids appear in the twelfth week, coincident with tubular lumen formation.

No abstract