[Genome multiplication mechanisms in the development of albumen gland polyploid cell of Succinea lauta (Gastropoda:Pulmonata). VIII. Pseudomitosis in the terminally differentiated cells].

Using a mature albumen gland of Succinea lauta, a study was made of the chromatin structure and RNA synthesis activity in polyploid nuclei (16c-32c-64c) of terminally differentiated cells, by means of electron microscopy and 3H-uridine radioautography of squash preparations. No 3H-thymidine incorporation in these cells was observed, which suggested the absence of DNA synthesis and endomitotic cycles at the examined functional stage of gland development. Some different stages of secretory cells could be distinguished: a resting period of the terminally differentiated cells, with the least 3H-uridine incorporation; a phase of intense heterosynthetic transcription, with the highest 3H-uridine incorporation; intervals in daily cycles; nuclear degeneration in ageing cells. At all these states the nuclei demonstrated some morphological features of endomitosis. This may be possible, because in the course of terminal cell differentiation the main chromatin mass is compacting like chromatids or chromocentres with thin euchromatinous filaments, which can be covered with perichromatinous granules. Since the true endomitosis is deliberately excluded at this stage, the above described morphological state of polyploid nuclei is regarded as postendomitotic and is designated as pseudoendomitosis. The view of the existence of "two fundamentally different types of endomitosis", one of which is "endomitosis in general", and the other "shows up as stationary" (Therman et al., 1983), is accepted as groundless.