Differentiation of primordial germ cells in the embryonic development of Thermobia domestica, Pack. (Thysanura): an ultrastructural study.

The primordial germ cells(PGCs) of Thermobia domestica undergo some morphological changes during the embryonic development. Most conspicuous are the changes in the ultrastructure of the nucleus, whose envelope shows a high degree of activity. Two types of vesicles bled off from the nucleus; the ones with the light interior are called the accessory nuclei, the others, with electron-opaque contents, have been termed the dense bodies. The nucleolus, initially clustered at the nucleus centre, undergoes dispersion and assembles again towards the end of embryonic development. At the same time, the sex differentiation of PGCs takes place. It is preceded by an increase in the activity of Golgi complexes and in the volume of lysosomes and lamellar bodies, the latter giving rise to lipid droplets. At the early stages of postembryonic development, preoogonia and prespermatogonia can readily be distinguished. Preoogonia have a wavy-surfaced nucleus and their cytoplasm contains dense bodies. In prespermatogonia, the nucleus is spherical with smooth envelope and there are no dense bodies in the cytoplasm. Throughout the period studied there occur nucleolus-like bodies and nuage material considered to be the germ-cell determinants in this species.