Egg activation process


















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Prior to the fertilization process commencing both the gametes oocyte egg and spermatozoa sperm require completion of a number of biological processes. Endocrinology - Diagram of the comparative anatomy of the male and female reproductive tracts. Clinical guidelines have typically identified the "fertile window" between days 10 and 17 within the typical 28 day menstrual cycle. Spermatozoa and oocyte fusion in the membrane adhesion area requires the presence of 3 membrane proteins spermatozoa Izumo1; oocyte receptor Juno and Cd9.

The sperm-specific protein Izumo is named for a Japanese shrine dedicated to marriage and is essential for sperm-egg plasma membrane binding and fusion. It interacts with the spermatozoa folate receptor 4 Folr4. A recent study in mice has shown that after fertilization the maternal proteins present in the original oocyte are quickly degraded by the zygote stage. MII oocytes have , different peptides while zygotes contain only 85, peptides. The abnormalities listed below relate to genetic abnormalities resulting in infertility or occurring during fertilisation.

There are of course many additional genetic abnormalities inherited and introduced by the recombined maternal and paternal genomes, other than trisomy 21 these will not be covered here. Note the sex different genes responsible shown for the examples of male and female infertility. Asthenozoospermia - asthenospermia term for reduced spermatozoa motility.

Azoospermia - term for no spermatozoa located in the ejaculate. Globozoospermia - term for spermatozoa with a round head and no acrosome. Primary ovarian insufficiency - depletion or dysfunction of ovarian follicles with cessation of menses before age 40 years.

Oocyte maturation arrest - arrest of human oocytes may occur at different stages of meiosis. Other than mammals, many different species plants, insects, reptiles can develop from unfertilized eggs. An embryo so formed without sperm contribution. Blocking of parthenogenesis in mammals appears to be related to genomic imprinting.

Abnormal parthenogenic processes can occur in mammals, and more recently a parthenogenic mouse has been made in the laboratory. Trisomy 21 Down's or Down syndrome is caused by nondisjunction of chromosome 21 in a parent who is chromosomally normal and is one of the most common chromosomal aneuploidy abnormalities in liveborn children.

The frequency of trisomy 21 in the population is approximately 1 in to 1, live births, in Australia between there were 2, Trisomy 21 infants. There are other less frequently occurring trisomies Trisomy 18 , Trisomy 13 and Trisomy X. Male infertility-related molecules involved in sperm-oocyte fusion. Location and expression of Juno in mice oocytes during maturation. The zinc spark is an inorganic signature of human egg activation. Sci Rep , 6 , Post-ovulatory ageing of the human oocyte and embryo failure.

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Sci Projects. Movies Movies. One Minute. Once released, the calcium ions can diffuse directly, or they can facilitate the release of more calcium ions by binding to calcium-sensitive receptors located in the cortical endoplasmic reticulum McPherson et al. The binding of calcium ions to these receptors releases more calcium, and this released calcium binds to more receptors, and so on. The resulting wave of calcium release is propagated throughout the cell, starting at the point of sperm entry; and the cortical granules, which fuse with the cell membrane in the presence of high calcium concentrations, respond in a wave of exocytosis that follows the calcium ions.

Mohri and colleagues have shown that IP 3 -released calcium is both necessary and sufficient for initiating the wave of calcium release. IP 3 is similarly found to release calcium ions in vertebrate eggs. As in sea urchins, waves of IP 3 are thought to mediate calcium release from sites within the endoplasmic reticulum Lechleiter and Clapham ; Miyazaki et al.

Blocking the IP 3 receptor in hamster eggs prevents the release of calcium at fertilization. Xu and colleagues found that blocking the IP 3 -mediated calcium release blocks every aspect of sperm-induced egg activation, including cortical granule exocytosis, mRNA recruitment, and cell cycle resumption. The question then becomes, what initiates the production of IP 3?

In other words, what activates the phospholipase C enzymes? This question has not been easy to address, since 1 there are numerous types of PLC, 2 they can be activated through different pathways, and 3 different species can use different mechanisms to activate them.

This PLC is activated by protein kinases. This movie shows the evidence for this inhibition. The next question, then, is, what protein tyrosine kinase is activated? This question has not yet been satisfactorily answered. According to one model, the sperm receptor protein crosses the egg plasma membrane and has a protein tyrosine kinase activity in its cytoplasmic domain Figure 7. This structure would make it a classic receptor tyrosine kinase see Chapter 5.

However, the sequence of the putative bindin receptor reveals neither transmembrane nor kinase domains Just and Lennarz According to a second model, the bindin receptor is linked to a protein tyrosine kinase and can activate the kinase, perhaps as a consequence of receptor crosslinking by the sperm Figure 7.

A third possibility is that the activation of the IP 3 pathway is caused not by the binding of sperm and egg, but by the fusion of the sperm and egg plasma membranes.

McCulloh and Chambers have electrophysiological evidence that sea urchin egg activation does not occur until after sperm and egg cytoplasms are joined. They suggest that the egg-activating components are located on the sperm plasma membrane or in the cytoplasm. It is even possible that when the fusion of gamete membranes occurs, the sperm receptor tyrosine kinases activated by the egg jelly to initiate the acrosomal reaction activate the IP 3 cascade for calcium release in the egg see Gilbert In this scenario, shown in Figure 7.

Still another possibility is that the agent active in releasing the sequestered calcium comes from the sperm cytosol Figure Here, a single intact human sperm is injected directly into the cytoplasm of the egg. This results in egg activation, the formation of a male pronucleus, and normal embryonic development Van Steirtinghem Kimura and colleagues have shown that the isolated head of a mouse sperm is capable of activating the mouse oocyte, and that the active portion of the sperm head appears to be the proteins surrounding the haploid nucleus.

It is not known what role these perinuclear components may play in the normal physiology of egg activation. The roles of inositol phosphates in initiating calcium release from the endoplasmic reticulum and the initiation of development. Possible mechanisms of egg activation. A The bindin receptor in the egg plasma membrane has tyrosine kinase activity. The tyrosine kinase activates PLC. B The bindin receptor activates a cytoplasmic tyrosine kinase. C An activated tyrosine kinase more In certain salamanders, this developmental function of fertilization has been totally divorced from the genetic function.

The silver salamander Ambystoma platineum is a hybrid subspecies consisting solely of females. Sperm penetration into the egg A series of steps allows the sperm to penetrate the shell and finally bind to the outer egg membrane oolemma , which is just within the outer shell.

Then the sperm head can release its contents into the center of the egg. The main thing that the sperm brings in are the 23 chromosomes. Prev Previous. Next Next.

Share on facebook. Share on twitter. Share on linkedin. Related Articles. Varicoceles in Men and Infertility When should surgery be done? Numerous sperm — at high magnification on a counting chamber. Advanced Fertility Center of Chicago. Fertility BLOG.



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