[Genome multiplication mechanisms in the development of albumen gland polyploid cells of Succinea lauta (Gastropoda:Pulmonata). V. Polyploidizing mitosis and endomitosis]

Abstract

Mechanisms of polyploidization of albumen gland cells of S. lauta have been studied using squash preparations in combination with autoradiography of DNA synthesis. During the growth period of the gland normal and abnormal (polyploidizing) mitoses were noticed in addition to endomitoses. Diploid foot cells and poorly differentiated secretory cells are reproduced by the normal mitosis (2c : 2c). These poorly differentiated secretory cells go then through polyploidization by different means. Tetraploid nuclei are substantially formed by abnormal mitoses (4c), which take 40-60% of all mitoses. Metaphase blocks (C-mitosis type) dominate among them; anaphase blocks and their sequences (bladed nuclei) occur rarely. The abnormal polyploid series 3c-6c ... and hypoploid (2c, 3c) mitoses originate from asymmetric mitoses (1c : 3c). Singular nuclei are defined as endomitotic even in the first cycles (4c, 6c). The second and following cycles of the main polyploid nuclei series ... 8c-16c-32c and of alternative series ... 12c-24c pass through endomitosis of classical (insect) type (Geitler, 1939). Unlabelled endomitoses were observed in an hour, while labeled ones were seen in 8-24 hours after 3H-thymidine injection. So, endomitosis can be defined as a real phase of the cell cycle. Integrity of the nuclear envelope as the formal feature of endomitosis was proven at the ultrastructural level (Anisimov, 199). It is emphasized that the normal mitosis is followed by an abnormal polyploidizing mitosis and then by endomitosis. These processes could be considered as compatible and successive polyploidization mechanisms. This conclusion is in agreement with dynamics and speed of DNA and RNA synthesis during cell polyploidization and differentiation (Anisimov, 1988, 1994b, 1995b).