NSCI 1404 Study Guide - Midterm Guide: Somatic Cell, Gastrulation, Cell Adhesion
Course CodeNSCI 1404
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1. Gametogenesis; fertilization (see previous chapter)
Gametogenesis is a biological process by which diploid or haploid precursor cells undergo cell division
and differentiation to form mature haploid gametes.
Fertilization (also known as conception, fecundation, syngamy and impregnation) is the fusion of gametes
to initiate the development of a new individual organism. In animals, the process involves the fusion of an
ovum with a sperm, which first creates a zygote and then leads to the development of an embryo.
2. Events that take place at fertilization, esp. sperm-egg interactions, acrosome reaction, fast and slow
blocks to polyspermy. Possible essay question
•Sperm Egg Recognition (mostly for external fertilization)- the ocean has so many species emitting
sperm and eggs into water, so if wrong species fertilizes egg, no organism will develop
•Acrosome Reaction-The acrosome is the tip of the sperm head. The acrosomal reaction is a change
in the sperm that is common to many animals. Its function is best understood in the sea urchin!
During fertilization, a sperm must first fuse with the plasma membrane and then penetrate the
female egg in order to fertilize it. Fusing to the egg usually causes a little problem, whereas
penetrating through the egg's hard shell can present more of a problem to the sperm. Therefore
sperm cells go through a process known as the acrosome reaction which is the reaction that occurs
in the acrosome of the sperm as it approaches the egg. !
The acrosome is a cap-like structure over the anterior half of the sperm's head. As the sperm
approaches the zona pellucida of the egg, which is necessary for initiating the acrosome reaction,
the membrane surrounding the acrosome fuses with the plasma membrane of the oocyte, exposing
the contents of the acrosome. The contents include surface antigens and numerous enzymes which
are responsible for breaking through the egg's tough coating and allowing fertilization to occur.
1. Receptor proteins in the sperm plasma membrane contact the sea urchin jelly coat
(vitelline layer). This contact between receptor proteins and the jelly coat (vitelline layer)
causes the acrosomal membrane to dissolve, releasing acrosomal enzymes
2. In the egg, Na+ channels open in the plasma membrane (BELOW the jelly coat/vitelline
layer.) Normally, Na+ concentration is higher outside the cell than inside. So Na+ ions
flow down their gradient into the egg and the plasma membrane depolarizes (positive
charges neutralize the more negative charge inside the egg cytoplasm.) This
depolarization causes the FAST BLOCK TO POLYSPERMY.
3. The depolarization (neutralization of charge difference) causes voltage-sensitive Ca2+
channels to open in the egg endoplasmic reticulum (ER).
4. Digestive enzymes from the acrosomal vesicle digest the jelly coat and vitelline
membrane. Ca2+ also activates a Na+:H+ ion exchanger, which pumps H+ out of the cell,
increasing intracellular pH. This pH change causes the polymerization of actin subunits
(pink) into microfilament cables that thrust acrosomal processes toward the egg plasma
membrane. Bindin released from the acrosomal vesicle coats the acrosomal process.
5. The increase in intracellular calcium causes water to enter the cell, increasing hydrostatic
pressure. This aids in the extension of the acrosomal process. At last the acrosome fuses
with the egg's plasma membrane (BENEATH the vitelline layer). The sperm head now
has access to the cytoplasm.
6. The Ca2+ moves in a wave across the cell. This Ca++ results in the fusion of cortical
vesicles with the egg plasma membrane, releasing their contents into the space
surrounding the egg, called the perivitelline space. This raises the vitelline membrane,
and inactivates bindin receptors on the vitelline membrane. Thus, any additional sperm
are released from the vitelline membrane and no more bind. This is known as the SLOW
BLOCK TO POLYSPERMY
7. The sperm head now enters the cytoplasm, where it forms a male pronucleus. The
pronucleus fuses with the egg nucleus, regenerating 2N chromosomes. Mitosis (first
cleavage) then occurs. Fertilization is complete.
•Fast Block-The fast block to polyspermy involves the opening of Na+ channels in the egg plasma
membrane. Na+ flows into the egg cell, depolarizing the membrane. This depolarization prevents
additional sperm from fusing to the egg plasma membrane.
•Slow Block-consists of a physical barrier to further sperm penetration into the egg. Cortical granule
exocytosis results in the formation of the fertilization envelope
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