466
Cellular Interactions
these structural and functional changes do
not occur correctly, then the organism fails
to propagate its genes. Additional features
that make mammalian eggs a good system
for investigating signaling events include
the following: (1) Inherent in the produc-
tion of (as well as fertilization of) eggs are
natural cell cycle arrest points. The natural
cell cycle arrest points provide for syn-
chronous release from the cell cycle arrest.
This has permitted the analysis of many of
the signaling mechanisms involved in the
cell cycle. When eggs are employed, these
pathways involve the meiotic cell cycle;
only after fertilization does the Frst mi-
totic cell cycle initiate. Investigations have
revealed features unique to the meiotic cell
cycle, such as the function of c-mos in the
cell cycle arrest. These investigations have
also identiFed many cell cycle signaling
events common to both meiosis and mi-
tosis. (2) In mammalian eggs, the DNA is
condensed in the form of chromosomes
and cannot participate in transcription.
Consequently, at this time in development,
the regulation of changes in cell function
(i.e. from egg to zygote) is under the con-
trol of cytoplasmic signaling events and
occurs largely at the protein level.
The illustration (±ig. 1) indicates the
pathways that have been reported to be
engaged as a result of mammalian egg
fertilization and cell cycle resumption. Cal-
cium acts at the top of a hierarchy of an
extensive signaling cascade and triggers
more than one signaling pathway. ±ur-
thermore, in mammalian eggs, the initial
calcium rise is followed by oscillations in
the level of free calcium above the basal
level for several hours. A recent study by
Ducibella and coworkers show that cal-
cium oscillations drive the progression of
several signaling events that convert the
egg to a zygote. Once the fertilization-
induced rise in intracellular-free calcium
occurs, other calcium-dependent pathways
are activated as indicated in ±ig. 1.
Calcium/calmodulin-dependent protein
kinase II (CaM KII) is activated in mam-
malian eggs, and the activation of CaM
Fertilization
Elevated
CaM KII
activity
[Ca
2
+
]
i
MPF inactivation
(Cyclin B1 degradation)
Cortical granule exocytosis
2
nd
PB closure of contractile ring
Potentiation of MAPK activity
Metaphase II to anaphase II/telophase II
Elevated
PKC
activity
Continued
MAPK
activity
2
nd
PB initiation
Cortical granule exocytosis
Interphase cytoplasm
PKM formed
Prohibits pronuclear
formation
Fig. 1
The illustration indicates pathways that are involved as a result of the rise in
intracellular-free calcium that accompanies mammalian fertilization. 2
nd
PB
=
second
polar body.
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