LIGHT AND ELECTRON-MICROSCOPIC OBSERVATIONS ON CELL-DIVISION IN 2 LARGE PENNATE DIATOMS, HANTZSCHIA AND NITZSCHIA .1. MITOSIS INVIVO

Abstract

Mitosis and cytokinesis were followed in live cells using Nomarski and birefringence optics. Prophase is protracted. The small square or rectangular spindle is brillantly birefringent and situated close to one side of the cell. It grows very slowly until suddenly it begins to elongate rapidly, doubling or trebling its length in a few minutes. This prometaphase stage is immediately accompanied by very active, oscillatory movements of the previously quiescent chromosomes, along invisible tracks directed at either pole. Independently, each pair of chromatids soon attaches to the other pole as well; immediately, its oscillations cease, and it becomes stretched across the central spindle. Some chromosomes attach almost as soon as the spindle enters the nucleus, others much later. The overlap in the central spindle becomes discernable during mid prometaphase; it stays roughly the same length while the total length of the spindle increases to a temporarily stable maximum at metaphase, which is quiescent except for late chromosome attachments and which lasts around 10 min. Then suddenly and synchronously, the chromatids split and immediately move polewards as if tension has been released in them. About a minute later, the spindle recommences elongation, but now the overlap diminishes in step with elongation. At this stage, Nitzschia and Hantzschia differ markedly in behavior. In Hantzschia, like other diatoms, the half spindles become coarsely striated near the poles, and the elongated central spindle stays intact after reaching its maximum length, until broken by the cleavage furrow, whereupon the broken halves slowly disassemble. In contrast, the half spindles in Nitzschia never display such striations, and after maximum elongation, the central spindle rapidly breaks down entirely, before cleavage is complete. The cleavage furrow grows inward very slowly during metaphase. Its ingrowth is stimulated during late anaphase, and it moves inwards at about 20 .mu.m/min. Most of the cleavage is accomplished in about 4 mins. The chloroplasts are pulled inwards and finally pinched in 2 by the furrow. These events are discussed with emphasis on the dynamics and mechanics of spindle assembly, elongation and disassembly.