Ultrastructural study of Haematococcus lacustris (Girod.) Rostafinski (Volvocales). II. Mitosis and cytokinesis.

In this article are presented the results of the ultrastructural study of the cell division in Haematococcus lacustris. The more significative data are as follows: the nuclear envelope is maintained intact practically throughout mitosis, as it only breaks down at the end of telophase when the daughter nuclei are separated. The disappearance of the nucleolus, which in the interphase nuclei display the two typical structural components (granulous and fibrillar), is sometimes delayed, since in late metaphase-early anaphase nucleolus clearly organised were observed. The spindle microtubules (NITS) seem to have an exclusively endonuclear origin, as neither through polar fenestrae nor nuclear pores they appear to be related with any extranuclear structure. In metaphase and anaphase we can see that the above endonuclear spindle is formed by “continuous” and chromosomic MTS. The former are persistent in the interzonal region up to telophase and the latter are deep in the chromatin, sometimes in apparently differentiated chromosomic regions (centromeres). In late anaphase-early telophase, the nucleus is seen more elongated, sometimes curved and repeatedly with dilated ends (dumbell shaped). The nuclear envelope of the telophasic nucleus breaks down in the middle-region and surrounds the two chromosomic groups (daughter nuclei). Between them, a growing cleavage furrow (invagination of the plasmalemma) ends by leading to the division of the protoplast (cytokinesis). This cleavage furrow is always associated to a microtubular system (phycoplast) similar to that which has been described in other algae.