The ultrastructure of oogonia and oocytes in the foetal and neonatal rat

Abstract

Ovaries from eighty foetal and neonatal rats (aged 16$\cdot $0 days post coitum to 4 days post partum) were examined under the electron microscope. All the normal developmental stages (oogonia and oocytes in the leptotene, zygotene, pachytene, diplotene and dictyate phases of meiotic prophase) were identified. Patterns of degeneration ('atretic divisions', 'Z' cells and atresia at the diplotene phase), whose histological appearance and incidence have been recorded by Beaumont & Mandl (1962), were also recognized. The nuclei of oocytes at the leptotene phase contain single electron dense threads which become aligned in parallel pairs at the following phase (zygotene). A third finer fibril half-way between them appears at pachytene (tripartite ribbon). The longitudinal segments of threads, visible in ultra-thin sections, become shorter, presumably due to coiling. Nuclei at the diplotene phase contain single threads essentially similar to those seen at leptotene. As the oocyte enters the dictyate or resting phase, electron dense threads disappear from the nucleus. These observations suggest that the threads represent chromosomal 'cores'. Nucleolus-like components persist throughout meiotic prophase, and at the diplotene phase regain the more complex form typical of oogonia. The cytoplasmic organelles become more numerous and complex as the oocyte approaches the dictyate phase. 'Atretic divisions' in oogonia are characterized by the persistence of long segments of nuclear membrane and the appearance of vesicles enveloped by a double membrane resembling the nuclear envelope. The dense masses of 'chromatin' (mitotic chromosomes) are more rounded than in normal cells at metaphase, and tend to coalesce. Spindle fibres have not been observed. Cytoplasmic organelles tend to increase in number and complexity. 'Z' cells (cells degenerating largely at the pachytene phase) show heavy 'chromatin' condensation around the tripartite ribbons. The major cytoplasmic changes consist in swelling of the endoplasmic reticulum, vacuolation of mitochondria and increase in incidence of multilamellar bodies. Atretic oocytes at the diplotene phase differ markedly from 'Z' cells in that 'chromatin' condensation around electron dense threads (single) is markedly less prominent. Cytoplasmic changes are similar to those of 'Z' cells, but also include a rise in the incidence of multivesicular and other complex bodies. All three types of degenerating cells are removed from the ovary by the phagocytic activity of neighbouring somatic cells.