imaage
Figure 3-16. Isolated metaphase spindles by using different methods of light microscopy (After Salmon and Segall, 1980). a — differential-interference-contrast microscopy; b — phase-contarst microscopy; c — polarized-light microscopy.
In lower eukaryotic organisms — unicellular green algae,
dinoflagellates, euglenes, paramecia, fungi, etc., which possess nuclear
envelope but do not form typical division spindle, cell division is realized by
modes and mechanisms different from the typical mitotic division. As far as
evolution has kept intermediate stages between complete absence and
well-formed mitotic spindle, the obtained results of the investigations in this
field are very diverse. Kubai and Ris (1969) have registered nuclear
division in dinoflagellate alga Girodinium cohnii, where it is seen that its
division is realized by means of the formed synaptonemal complex, through
which the chromosomes just “tear” and are distributed in the two new
daughter nuclei (Fig. 3–17 A). The authors have also successfully
constructed models of tridimensional reconstruction of the nucleus in the
process of division (Fig. 3–17 B). The mode of division described by them
illustrates a nuclear reproduction in case of closed mitosis with internal
division spindle (see Dodge and Vickerman, 1980) or a mechanism with a
scanty information about it. Reasonably they have called it “a new type of
nuclear reproduction”.
As it was already mentioned, normally the cells have one nucleus and
divide in two. This can be accepted as “ideal” model of cell reproduction.
But living nature has not stopped up to here, i.e. to divide the cells only in
two. In their vast majority, especially when they exist as unicellular
organisms, the division is in more than two. Division in two is widely spread
in specialized and differentiated cells, which build the tissues and organs of
multicellular organisms. Depending on that, how many daughter cells will
be obtained, so many nuclei will be formed in the mother cell.
There exist also multicellular cells with disturbed cell cycle — the
symplasts in cross-furrowed muscle fibres, osteoclasts of bone tissue,