| Summary: | Cytokinesis separates replicated chromosomes and cytoplasm into two
daughter cells. In animal cells, this is achieved by the formation of a cleavage
furrow that bisects the mitotic (or meiotic) spindle. It is known that the mitotic
apparatus defines the cell cleavage plane. However, it is not clear how the mitotic
apparatus initiates the cleavage furrow. Each part of the mitotic apparatus; namely
asters, central spindle (microtubule arrays and the spindle midzone), and
chromosomes, has been found capable of inducing a cleavage furrow in certain
cell types. Yet it is uncertain which part is the essential source of the signal and
whether all parts act in concert.
This thesis systematically examines in grasshopper spermatocytes 1) which
spindle constituent is the essential source of furrow signal; 2) the impact of
microtubules on distribution of actin filaments and positioning of cell cleavage
relative to spindle reorganization; 3) the independent role of the spindle midzone
relative to microtubules in furrow initiation and ingression. These examinations
combine micromanipulation with digital-enhanced polarization microscopy and
epifluorescence microscopy, in which mitotic spindles in living cells are
mechanically dissected and rearranged as desired as well as microfixed to evaluate
and propose models for cell cleavage.
This thesis has come to the conclusion that none of structural constituents
of the spindle apparatus is essential for cell cleavage induction except
microtubules. First, furrow induction occurs regardless of a particular spindle
constituent, so long as sufficient microtubules are present to form bipolar arrays.
Second, microtubules continuously dictate distribution of actin filaments and
positioning of cell cleavage. Asymmetric alterations of spindle microtubules
dynamically affect the location of the spindle midzone, distribution of actin
filaments, and ultimately position of the cleavage furrow in cells containing a
bipolar spindle, monopolar spindle, or half-spindle. Third, actin filaments are
distributed to the furrow region by microtubule-mediated transport, but organized
by the midzone, which is essential for furrow ingression, but not initiation. These
results suggest that during post-anaphase spindle assembly, actin filaments are
excluded by bipolar microtubule arrays to the equatorial cell cortex where they
bundle into a contractile ring with cytokinetic factors. === Graduation date: 2004
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