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In-Depth Information
only present after the first minute of anaphase.
11
This suggests that a transient signal travels
from the spindle to the cell cortex during metaphase or the very beginning of anaphase.
Components of this signal have now been identified, mainly through the genetics of
D. melanogaster. They include chromosomal passenger proteins such as the inner centromere
protein, INCENP, and serine-threonine kinases of the Aurora family. These proteins bind
chromosomes during prometaphase and metaphase but fall away at anaphase,
12,13
probably
being released by the anaphase-promoting complex that was mentioned in Chapter 22. Once
free of chromosomes, these proteins bind the microtubules at the equator of the spindle and
assist in their stabilization as the chromosomes part. Mutations that block the ability of
INCENP or Aurora proteins to associate with microtubules block the onset of cytokinesis.
14,15
It has been suggested that the high concentration of these proteins at the equator signals to
the overlying cortex to initiate ingress of a cleavage furrow; some, such as INCENP,
may interact directly with the furrow.
16
This theory does not, however, provide a ready
explanation of how chromosome-free cells can still orientate cytokinesis properly.
Whether the astral model, the equatorial model, or some combination of both wins the day,
orientation of cell division is controlled by the orientation of the mitotic spindle. A compli-
cation about cytokinesis that has been appreciated only very recently is that there are differ-
ences between cytokinesis in simple cultured cells and in cells in their proper tissue context.
In monolayer cultures of epithelial cells, for example, the spindle is in the middle of the cell
and the cleavage furrow ingresses from the entire circumference around the spindle mid-
point. When cell division in intact embryonic mouse gut epithelia is observed carefully,
17
a much less symmetrical behaviour is observed. To begin with, while dividing cells show
the rounding up characteristic of mitosis in culture, the spindle is located well towards the
apical end of the cell; contact with the basement membrane is maintained only through
a thin stalk (
Figure 23.6
a). The mitotic spindles are located within 30 degrees of the plane
FIGURE 23.6
Asymmetrical location of themitotic spindle, and ingress of the cleavage furrow, in epithelial cells in
vivo. The diagrams summarize a study made in mouse gut.
17
(a) Shows the dividing cell rounding up so that mitosis
happens towards the apical side of the epithelium, contact with the basement membrane being maintained by a thin
stalk. (b) Shows the cytokinesis furrow invading from the stalk rather than from the whole circumference of the cell.
