Most mammalsMost 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)

 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.  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 mucusmucus 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

 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

 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

 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

 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  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

 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

 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.

 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.

 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  In species without an electrical block, polyspermy is prevented by secretion of materials (a mechanical barrier to polyspermy)

 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)

 The cortical reaction ( zona reaction): when a sperm unites with the egg plasma membrane  The zona pellucida which prevents other sperm from binding to and entering the egg  The cortical reaction is exocytosis of the egg cortical granules  Cortical granules are secretory vesicles

 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

 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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