Transcript Structure of ovum, sperm (acrosome reaction and polyspermi)

Most mammals ovulate an "egg“ matured into a oocyte II
The secondary oocyte that is fertilized
The secondary oocyte is produced along with the first polar
body as → first meiotic division
These cells are encased in a glycoprotein shell called the
zona pellucida
The secondary oocyte that arrives in the oviduct is in
metaphase of the second meiotic division.
Mature sperm, have:
* nucleus (head, oval shape)
* during spermiogenesis, sperm cell develops a tail
and all of its mitochondria → midpiece
* acrosome → gigantic lysosome → have two faces –
the inner acrosomal membrane faces the nucleus,
and the outer acrosomal membrane is in close
contact with the plasma membrane
Mammalian eggs are ovulated from ovarian as
cumulus-oocyte complexes
In fertilization, the ovulated → transported into
oviduct through an opening called the ostium
In mammals the ovarian end of the oviduct → called
the fimbria (cover the ovary)
The fimbria → ciliated epithelial cells → ostium into
the oviduct
Rodents and dogs, the ovary → the eggs after
ovulation → into the oviduct
Oocyte enters the oviduct → by ciliary into the
ampulla (where fertilization takes place)
The oviduct provides the environment not only for
fertilization, but for early embryonic development
(three days)
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Semen is ejaculated and deposited initially into one of
two sites:
◦ the vagina (e.g. humans, cattle, rabbits) or
◦ the uterus (e.g. horses, pigs, rodents).
◦ In species such as dogs, semen is deposited largely into the
vagina, but also forced into the uterus.
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A few hundred to a few thousand sperm → oviducts (1%
sperm in the ejaculate)
The vagina: a hostile environment for sperm and survival
depends on more regions of the female tract
From vagina to oviduct, sperm barriers → eliminates
sperm
Connects the vagina to the uterus
The cervical epithelium is richly → mucus-secreting cells,
and, as a consequence, the lumen is filled with mucus
Consistency and viscosity of cervical mucus is under
endocrine control
Estrogen levels are high & progesterone levels low
(ovulation) → cervical mucus becomes watery →
facilitates sperm through the cervical canal
Progesterone concentrations are high (luteal phase) →
cervical mucus → viscous and disorganized →
precludes entry of sperm into the uterus
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does not an active barrier
Several species → sperm from uterus → oviducts in a
few minutes
Dead sperm: transported through the uterine lumen.
The sperm transport in the uterus → uterine contractions
the uterus is also a site hostile to sperm
In many animals, sperm within the uterus are rapidly
phagocytosed
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The region joining the tip of the uterine to the
oviduct
The uterotubal junction → varies among
species, morphology and structure → barrier to
sperm
Rodents & pigs → the sperm are deposited
directly in uterus
Ejaculated sperm are lost → between the cervix
and oviduct
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Sperm produces hyaluronidase to penetrate
the follicular cell layer (corona radiata)
ZP3 receptor, the head of the sperm
releases its contents (acrosin) and through
the zp and perivitelline space - the
acrosomal reaction
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Receptor has been activated, a series of
reactions to prevent polyspermy (the
entrance of more than one sperm)
◦ First, the cell surface will be depolarized
◦ Then, cortical granules (lysosomes) released
into the perivitelline space
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Thus, only one sperm can enter the ovum
since only one receptor can be activated
Fusion of the plasma membranes of the oocyte
and sperm occurs
The sperm nucleus is released into the
cytoplasm of the oocyte
The rest of the sperm degenerates
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Entrance of sperm into the oocyte causes
oocyte II to complete meiotic II
Male pronucleus forms and swells;
pronuclei membranes become porous
Membranes breakdown, chromosomes
condense and 46 chromatids line up on
metaphase plate in preparation for mitosis
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During fertilization, sperm must first fuse and
then penetrate the female egg in order to
fertilize it.
Therefore sperm cells go through a process
known as 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.
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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 sperm
The contents include surface: antigens and
numerous enzymes which are responsible
for breaking through the egg's tough
coating and allowing fertilization to occur.
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polyspermy is an egg that has been fertilized
by more than one sperm
The cell resulting from polyspermy, contains
three or more copies of each chromosome -one from the egg and one each from multiple
sperm
The male-male competition among sperm spills
over to create sexual conflict
Electrical polyspermy block: a positively charged
molecule in the sperm surface membrane is
repelled by the positive charge at the egg surface,
but not in mammals
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In species without an electrical block, polyspermy
is prevented by secretion of materials (a
mechanical barrier to polyspermy)
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In mammals, fertilization occurs internally, fewer
sperm reach the fertilization site in the oviduct
female genital tract being adapted to minimize
the number of sperm reaching the egg
polyspermy preventing mechanisms: the cortical
reaction modifies the extracellular coat of the
egg (the zona pellucida)
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The cortical reaction ( zona reaction): when a
sperm unites with the egg plasma membrane
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The zona pellucida which prevents other sperm
from binding to and entering the egg
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The cortical reaction is exocytosis of the egg
cortical granules
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Cortical granules are secretory vesicles
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in fertilizing: contacts sperm-egg membrane, it
causes calcium to released in the egg
This triggers fusion of the cortical granule
membranes with the egg membrane
Fusion sperm contact: wave of calcium release
sweeps around the egg, a wave of cortical
granule fusion results
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the granule contents a protein coat on the outside of the
plasma membrane (the vitelline layer) so that it is
released from the membrane
The released cortical granule proteins exert a colloid
osmotic pressure causing water to enter the space
between the plasma membrane and the vitelline layer,
and the vitelline layer expands away from the egg
surface
The cortical reaction prevents polyspermic fertilization
cortical granule component:
◦ polysaccharide-rich hyalin,
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