Cell division in the pennate diatomDiatoma vulgare

Summary

Mitosis and cytokinesis in the pennateDiatoma is described. Prior to division, a doubled “Persistent Polar Complex” (PPC), the focus of numerous cytoplasmic microtubules, migrates from near the nucleus to one side of the cell near the girdle bands, followed by the nucleus. The central dense core of the doubled PPC breaks down as a central spindle grows between the two PPCs, which now each become characteristically associated with a small vacuole and other features of unknown significance. The tubules of the central spindle terminate in a layer, the “Spindle Insertion” (SI), close to the PPCs; other, “polar” tubules radiate from each SI, mostly toward the nucleus, which becomes increasingly deformed by them until the nuclear envelope is ruptured. The elongating spindle enters one side of the nucleus laterally; as shown previously, the central spindle consists of two interdigitated half spindles, but the polar tubules which diverge laterally from the SI, by metaphase form a complex, cone-shaped array emanating from each pole. Some polar tubules penetrate the chromatin and may represent true kinetochore tubules; others, which persist conspicuously throughout mitosis, extend tangentially past the chromatin, out into the cytoplasm, intersecting with those from the other pole. The core structure of the PPCs alters from being plate-shaped at prophase, to being rod-like by metaphase. The chromosomes form a donut-shaped mass of chromatin penetrated by the central spindle throughout meta-and anaphase. Chromosomal separation is accomplished in two stages: the chromatin splits and moves up to the SI, and then the central spindle elongates, concurrent with its overlap region decreasing markedly in extent. Thus, this latter part of anaphase movement could be generated by microtubule sliding past microtubule. Then each PPC distinctly separates from its SI, and moves away from the daughter nucleus during cytokinesis; it again becomes the focus of numerous tubules and often ends up in one corner of the daughter cell. Meanwhile, the central spindle and the SIs, now surmounted by the reforming telophase nuclei, slowly disintegrate during cleavage.

Cytokinesis proceeds in two stages. The cleavage furrow is very broad around the cell periphery; this broadened profile is maintained thereafter, and may later serve to mould the edge of the newly secreted valve. From the broad furrow grows a much narrower cleavage furrow, whose ingrowing edge is lined with dense (contractile -?) filaments; some larger cell organelles are drawn in to line the surface of this furrow. Secretion of the new valve is briefly described.