Twenty-eight foetal and post-natal human ovaries (6 weeks post conception to 11 years post partum) have been examined with the electron microscope. Stages in the normal differentiation of germ cells (oogonia; oocytes at the pre-leptotene, leptotene, zygotene, pachytene, and diplotene stages of meiotic prophase) were identified by comparison with (a) corresponding cells in histological and squash preparations, and (b) similar cells in the rat ovary. Atretic cells (oogonia in mitosis; oocytes at pachytene and diplotene) were also examined.
The nuclei of oogonia contain an evenly dispersed fibrillar matrix which becomes organized into irregular strands in oocytes at pre-leptotene. At leptotene these strands become the sheaths surrounding unpaired, electron-dense axial threads or ‘cores’, which become associated in pairs during the transitory zygotene stage, and tend to be polarized within the nucleus. Single and paired threads are replaced by ‘tripartite ribbons’ (synaptinemal complexes) at pachytene; longitudinal subdivision of the lateral components of the ribbon is observed in some nuclei and may represent a later stage. At diplotene, essentially unpaired cores are observed as at leptotene, although they are thicker and more intimately related to the surrounding fibrillar sheath. The latter is also more highly organized than at earlier stages. This chromosomal structure is retained in oocytes in promordial follicles in post-natal ovaries. The dictyate stage observed in rat ovaries is not found in man.
The organization of the cytoplasm does not change markedly as oogenesis advances, although organelles become more numerous, and the internal structure more complex, as the cells enlarge towards the diplotene stage.
Oogonia degenerating during mitosis differ from normal cells in that the chromosomes become fused to form an irregular mass. Abnormal membrane-bound areas are also observed in the cytoplasm. Atretic oocytes at pachytene coalesce to form ‘pools’ of cytoplasm containing several nuclei in various stages of pyknosis. Atretic cells at diplotene may contain grossly swollen cytoplasmic organelles and clumped, homogeneous chromosomes. The ‘atretic divisions’ appear to be phagocytosed by somatic cells, but the means of elimination of degenerating meiocytes was not determined.