The Nucleus and Cell Division

The Interphase Nucleus:

The euglenoid nucleus is easily seen with the light microscope.   It is generally spherical in shape and is positioned near the center or toward the posterior of the cell.  It  is surrounded by a nuclear envelope and has a prominent nucleolus, sometimes with multiple nucleoli.  In euglenoids the nucleolus is often referred to as the endosome.  Unlike the case for many other eukaryotes, the chromosomes are condensed at interphase and are easily seen with the light and electron microscopes.

Mitosis:

Light Microscopy

In most eukaryotes, the beginning of mitosis is signaled by the disappearance of the nucleolus and the breakdown of the nuclear envelope. In euglenoids, the nucleolus and the nuclear envelope remain intact throughout division.  In the light microscope, euglenoid mitosis is signaled by the elongation of the nucleolus.   This elongation of the  nucleolus and concomitant elongation of the nucleus may continue until the nucleus is stretched across the cell.   The nuclear envelope then begins to constrict around the center of the elongate  nucleus and gives the telophase nucleus a dumbbell shape.   In some euglenoids, continued constriction of the nuclear envelope pinches off  the nucleus and generates the two daughter nuclei.  In other euglenoids, the central portion of the dumbbell only is severed as the developing cleavage furrow moves toward the posterior end of the cell.

Mitotic Sequence from Peranema trichophorum

Many euglenoids undergo active metaboly, squirming about, during cytokinesis.   The cleavage furrow begins to develop at the anterior of the cell ("a" in the figure below) and progresses posteriorly (b) until the cells are connected only by a thin tube of cytoplasm (c).  This will break as the cells actively writhe about and pull apart (d).  

Cytokinesis in Peranema trichophorum

Electron Microscopy

The electron microscope gives a more detailed picture of mitosis.   The following general description characterizes mitosis in most euglenoids.

   At the onset of mitosis, the nucleolus begins to elongate and stretches across the equatorial plane of the nucleus (Fig.1).  The nuclear envelope and the nucleolus remain intact throughout the mitotic process.  As mitosis proceeds, microtubules appear within the nucleus, often near the nuclear envelope.  The chromosomes develop multi-layered kinetochores to which the microtubules attach (Fig.2). By mid division, a number of sub-spindles have developed in the nucleus (Fig.3). Each sub-spindle appears to function separately and attaches to, and segregates a few of the chromosomes (Fig. 4). Although chromosomes may group near the equatorial plane of the nucleus, a true metaphase plate is never formed.   Figure 5 shows the aggregation of the chromosomes (Ch) near the center of the nucleus and some of the subspindles (arrows). Also note that portions of the elongated nucleolus (Nu) are visible at the poles. Elongation of the nucleus and nucleolus continues at anaphase (Fig. 6). As the nucleus elongates, the daughter chromatids separate.   Movement of the chromosomes to the spindle poles due to shortening of the chromosomal microtubules does not occur until nuclear elongation has begun.  In essence, Anaphase B precedes Anaphase A in euglenoids.  By telophase, the nucleus extends across the cell and is often dumbbell-shaped (Fig. 7).  Most of the remaining spindle microtubules are now compressed into a midbody (Mb) connecting the two reforming daughter nuclei (Fig. 8). The final stages in the separation of the daughter nuclei vary among taxa.  The nuclear membranes of the daughter nuclei may constrict around the midbody leaving it behind in the cytoplasm where it gradually breaks down.  In some taxa, the midbody breaks near the center and about one half remains associated with each daughter nucleus where it is ultimately resorbed.  Alternatively, the central spindle can remain in place between the nuclei and be severed by the descending cleavage furrow.    Mitosis culminates with the reformation of the daughter nuclei, one of which is distributed to each daughter cell (Fig. 9).