The ultrastructure of germ cells in foetal and neonatal male rats

Abstract

Male germ cells of foetal and neonatal rats (14 · 5 days p.c. to 7 days p.p.) were examined with the electron microscope. Female germ cells of young specimens (14 · 5 to 16 · 5 days p.c.) were re-examined for comparison. Shortly after sex differentiation, male and female germ cells have many features in common. The endoplasmic reticulum in oögonia, however, is the more highly developed, while the outline of gonocytes becomes markedly more angular as development proceeds. Between 15-5 and 18-5 days p.c., mitochondria in the male aggregate at one pole of the cell; those of the female remain randomly distributed. The significance of mitochondrial ‘polarization’ is obscure, but it is possible that the process is in some way related to cessation of mitotic activity. From 18 · 5 days p.c. onwards, when mitochondria become re-distributed evenly, groups of specific organelles (A- and B-bodies) appear in the cytoplasm. Three types of B-bodies have been identified. Of these, one type resembles lysosomes. The incidence of B-bodies increases during late intra-uterine life and reaches its peak between birth and 2 days p.p. Thereafter, they become rarer, as do germ cells situated in the centre of the seminiferous cord. At this time, a high proportion of gonocytes grow cytoplasmic extensions establishing contact with the basement membrane; their nucleus and nucleolus change their shape, while their mitochondria contain more regularly arranged parallel cristae than do those of ‘unattached’ gonocytes. Cells showing these signs of ‘streaming’ or migration are believed to correspond to the ‘transitional’ cells identified under the light microscope, and to be the immediate precursors of the first generation of spermatogonia type-A. After 2 days p.p., B-bodies appear to persist almost only in those gonocytes that have failed to establish contact with the basement membrane. The possible significance of B-bodies in spontaneous degeneration and high radiosensitivity of gonocytes is discussed. A few isolated germ cells show degenerative changes during the first 3 days after birth. These frequently show swollen endoplasmic reticulum, abnormal mitochondria and an abundance of B-bodies. Widespread degeneration does not set in until about 5 days, and largely affects ‘unattached’ gonocytes at inter-phase; a few degenerating cells undergoing mitosis have been observed. Degeneration takes the form of lysis: the cell membrane ruptures, the cytoplasm streams out over a wide area, and the nucleus becomes distorted. The process by which lysing germ cells are eliminated from the testis has not been fully established. The ultrastructure of spermatogonia type-A is essentially similar to that of their precursors. Most of the small germ cells corresponding to spermatogonia type-A seen under the light microscope lie with their long axis parallel to the basement membrane.