470
Cellular Interactions
2.2
Cell Architecture
2.2.1
Action of Signaling Agents on the
Cytoskeleton
The fertilization-competent egg has a
variety of molecular scaffolds at distinct
locations within the cell. Cortical granules
that exocytose after fertilization, create a
block to polyspermy and are enriched
along the cell periphery of the spherical
egg cell. Actin Flaments also are enriched
at the cell periphery, while unique arrays
of intermediate Flaments in the form of
‘‘cytoskeletal sheets’’ are in the cell interior
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The only microtubules in the cell at this
time in the cell cycle are the spindle
microtubules with their associated DNA in
the form of chromosomes. This entire unit
(i.e. the spindle and the chromosomes)
at meiotic metaphase II is polarized
immediately
subjacent
to
the
plasma
membrane. The egg surface is covered
with microvilli except for the region
immediately above the meiotic spindle.
Even the mitochondria have a polarized
distribution with more mitochondria in
the hemisphere containing the meiotic
spindle.
In mammalian eggs, the microtubule
network is acted on by MAP kinase to con-
vert the interphase array of microtubules
into the M-phase array of microtubules.
The M-phase array of microtubules is the
meiotic spindle and this appears to be serv-
ing as a molecular scaffold that is involved
in the regulation of the complex signaling
pathways in the egg.
The actin Flament network that is greatly
enriched at the cell periphery is a location
where many structural changes occur.
Correlative data suggest that signaling
agents may act on the actin network,
such as the localization of active PKC
in the cell periphery, shortly after egg
activation. However, there is a paucity of
direct evidence for this interaction in the
mammalian egg.
The intermediate Flament network in
mammalian eggs is composed of a highly
cross-linked network of Flaments in the
oocyte, fertilization-competent egg, and
early embryo. The intermediate Flaments
are composed of cytokeratins 5, 6, 16,
and Z, which are produced by the mater-
nal genome. These cytoskeletal elements
referred to as ‘‘sheets’’ serve as a ma-
ternal form of intermediate Flaments
that are involved partly with cell adhe-
sion events during embryonic compaction
and extensively with cell adhesion as the
blastocyst implants into the uterine wall.
This specialized network of intermedi-
ate Flaments has been identiFed in the
fertilization-competent eggs of a variety
of mammals including mouse, rat, ham-
ster, humans, cows, and pigs. PKC acts
on these ‘‘sheets’’ as part of the mech-
anism of downregulating the kinase. As
described in more detail in an earlier
section, the fertilization-induced rise in
calcium activates PKC causing it to translo-
cate to the plasma membrane. At the cell
periphery, the PKC signal is downreg-
ulated by cleaving the catalytic subunit
of PKC from its calcium and membrane
binding domain. The catalytic subunit is
then free to diffuse into the cell interior
(in a form referred to as PKM) where
it can phosphorylate substrates in both
a membrane- and calcium-independent
fashion and then PKM becomes enriched
on cytoskeletal sheets. Cytokeratin 16, be-
comes phosphorylated, along with other
components in the sheets, at the same time
that this intermediate Flament network
undergoes extensive remodeling. A recent
investigation by Coonrod and coworkers
has demonstrated that a peptidylarginine
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