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NCBI Bookshelf. The interaction of sperm and egg generally proceeds according to five basic steps Figure 7. Summary of events leading to the fusion of egg and sperm plasma membranes in the sea urchin A and the mouse B. A Sea urchin fertilization is external. The chemoattraction Sex big kenye com the sperm to the egg by soluble molecules secreted by the egg.

The binding of the sperm to the extracellular envelope vitelline layer or zona pellucida of the egg. Sometimes steps 2 and 3 are reversed as in mammalian fertilization and the sperm binds to the egg before releasing the contents of the acrosome. After these five steps are accomplished, the haploid sperm and egg nuclei can meet, and the reactions that initiate development can begin. In many species, the meeting of sperm and egg is not a simple matter.

Many marine organisms release their gametes into the environment. That environment may be as small as a tide pool or as large as an ocean. Moreover, it is shared with other species that may shed their sex cells at the same time.

These organisms are faced with two problems: How can sperm and eggs meet in such a dilute concentration, and how can sperm be prevented from trying to fertilize eggs of another species? Two major mechanisms have evolved to solve these problems: species-specific attraction of sperm and species-specific sperm activation. Species-specific sperm attraction has been documented in numerous species, including cnidarians, molluscs, echinoderms, and urochordates Miller ; Yoshida et al.

In many species, sperm are attracted toward eggs of their species by chemotaxisthat is, by following a gradient of a chemical secreted by the egg. InMiller demonstrated that the eggs of the cnidarian Orthopyxis caliculata not only secrete a chemotactic factor but also regulate the timing of its release.

Developing oocytes at various stages in their maturation were fixed on microscope slides, and sperm were released at a certain distance from the eggs. Miller found that when sperm were The formation of egg and sperm to oocytes that had not yet completed their second meiotic division, there was no attraction of sperm to eggs.

However, after the second meiotic division was finished and the eggs were ready to be fertilized, the sperm migrated toward them. Thus, these oocytes control not only the type of sperm they attract, but also the time at which they attract them.

The mechanisms of chemotaxis differamong species see Metz ; Ward and Kopf One chemotactic molecule, a amino acid peptide called resact, has been isolated from the egg jelly of the sea urchin Arbacia punctulata Ward et Teen brunette pussy all fours. Resact diffuses readily in seawater and has a profound effect at very low concentrations when added to a suspension of Arbacia sperm Figure 7.

Within seconds after a minute amount of resact is injected into the drop, sperm migrate into the region of the injection and congregate there. As resact continues to diffuse from the area of injection, more sperm are recruited into the growing cluster.

Resact is specific for A. Sperm chemotaxis in Arbacia. The position The formation of egg and sperm the micropipette is indicated in A. A A 1-second photographic exposure showing sperm swimming in tight more Resact also acts as a sperm-activating peptide. Sperm-activating peptides cause dramatic and immediate increases in mitochondrial respiration and sperm motility Tombes and Shapiro ; Hardy et al. This activates the mitochondrial ATP-generating apparatus as well as the dynein ATPase that stimulates flagellar movement in the sperm Shimomura et al.

A second interaction between sperm and egg is the acrosomal reaction. In most marine invertebrates, the acrosomal reaction has two components: the fusion of the acrosomal vesicle with the sperm plasma membrane an exocytosis that results in the release of the contents of the acrosomal vesicle and the extension of the acrosomal process Colwin and Colwin The acrosomal reaction in sea urchins is initiated by contact of the sperm with the egg jelly. The sequence of these events is outlined in Figure 7.

Acrosomal reaction in sea urchin sperm. A—C The portion of the acrosomal membrane lying directly beneath the sperm plasma membrane fuses with the plasma membrane to release the contents of the acrosomal vesicle.

D The actin molecules assemble more In sea urchins, the acrosomal reaction is thought to be initiated by a fucose-containing polysaccharide in the egg jelly that binds to the sperm and allows calcium to enter into the sperm head Schackmann and Shapiro ; Alves et al. Girls kissing on lips exocytosis of the acrosomal vesicle is caused by The formation of egg and sperm calcium-mediated fusion of the acrosomal membrane with the adjacent sperm plasma membrane Figures 7.

Acrosomal reaction in hamster sperm. A Transmission electron micrograph of hamster sperm undergoing the acrosomal reaction. The acrosomal membrane can be seen to form vesicles.

B Interpre- tive diagram of electron micrographs showing the fusion of more The second part of the acrosomal reaction involves the extension of the acrosomal process see Figure 7. This protrusion arises through the polymerization of globular actin molecules into actin filaments Tilney et al. Once the sea urchin sperm has penetrated the egg jelly, the acrosomal process of the sperm contacts the surface of the egg Figure 7.

A major species-specific recognition step occurs at Busty dressed mature milf point. The acrosomal protein mediating this recognition is called bindin. InVacquier and co-workers isolated this nonsoluble 30,Da protein from the acrosome of Strongylocentrotus purpuratus and found it to be capable of binding to dejellied eggs of the same species Figure 7.

Further, its interaction with eggs is relatively species-specific Glabe and Vacquier ; Glabe and Lennarz : bindin isolated from the acrosomes of S. Using immunological techniques, Moy and Vacquier demonstrated that bindin is located specifically on the acrosomal process—exactly where it should be for sperm-egg recognition Figure 7.

Species-specific binding of acrosomal process to egg cell surface in sea urchins. A Actual contact of a sea urchin sperm acrosomal process with an egg microvillus. B In vitro model of species-specific binding. The agglutination of dejellied eggs by more Localization of bindin on the acrosomal process. A Immunochemical technique used to localize bindin. Rabbit antibody was made to the bindin protein, and this antibody was incubated with sperm that had undergone the acrosomal reaction.

If bindin was more Biochemical studies have shown that The formation of egg and sperm bindins of closely related sea urchin species are indeed different. Such receptors were also suggested by the experiments of Vacquier and Paynewho saturated sea urchin eggs with sperm. As seen in Figure 7. Even at saturating numbers of sperm approximatelythere appears to be room on the ovum for more sperm heads, implying a limiting number of sperm-binding sites. The bindin receptor on the egg has recently been isolated Giusti et al.

This kDa protein may have several regions that Amrita rao bondage facebook nude with bindin. At least one of these sites recognizes only the bindin of the same species. The other site or sites appear to recognize a general bindin structure and can recognize the bindin of many species. The bindin receptors are thought to be aggregated into complexes on the egg cell surface, and hundreds of these complexes may be needed to tether the sperm to the egg Figure 7.

Thus, species-specific recognition of sea urchin gametes occurs at the levels of sperm attraction, sperm activation, and sperm adhesion to the egg surface. Bindin receptors on the The formation of egg and sperm. A Scanning electron micrograph of sea urchin sperm bound to the vitelline envelope of an egg. Although this egg is saturated with sperm, there appears to be room on the surface for more sperm, implying the existence of a more In the early s, fertilization research was framed by a dispute between F.

Lillie and Jacques Loeb, who disagreed over whether the sperm recognized the egg through soluble factors or through cell-cell interactions. The zona pellucida in mammals plays a role analogous to that of the vitelline envelope in invertebrates.

This glycoprotein matrix, which is synthesized and secreted by the Full pc hd nude erotic oocyte, plays two major roles during fertilization: it binds the sperm, and it initiates the acrosomal reaction after the sperm is bound Saling et al. The binding of sperm to the zona is relatively, but not absolutely, species-specific.

Species-specific gamete recognition is not a major problem when fertilization occurs internally. The binding of mouse sperm to the mouse zona pellucida can be inhibited by first incubating the sperm with zona The formation of egg and sperm. Bleil and Wassarmanisolated an kDa glycoprotein, ZP3from the mouse zona that was the active competitor for binding in this inhibition assay.

The other two zona glycoproteins they found, ZP1 and ZP2, failed to compete for sperm binding Figure 7. Moreover, they found that radiolabeled ZP3 bound to the heads of mouse sperm with intact acrosomes. Thus, ZP3 is the specific glycoprotein in the mouse zona pellucida to which the sperm bind. ZP3 also initiates the acrosomal reaction after sperm have The formation of egg and sperm to it. The mouse sperm can thereby concentrate its proteolytic enzymes directly at the point of attachment at the zona pellucida.

Mouse ZP3 as the zona protein that binds sperm. A Diagram of the fibrillar structure of the mouse zona pellucida. The major strands of the zona are composed The formation of egg and sperm repeating dimers of proteins ZP2 and ZP3.

These strands are occasionally crosslinked together more The molecular mechanism by which the zona pellucida and the The formation of egg and sperm sperm recognize each other is presently The formation of egg and sperm studied.

The current hypothesis of mammalian gamete binding postulates a set of proteins on the sperm capable of recognizing specific carbohydrate regions of ZP3 Figure 7. Removal of these threonine- or serine-linked carbohydrate groups from ZP3 abolishes its ability to bind sperm. Several proteins have been identified on the sperm cell surface that specifically bind to the ZP3 carbohydrates. Moreover, the deletion of these proteins from the sperm can inhibit or eliminate sperm-zona binding see Kopf Sperm ZP3-binding proteins at the zona pellucida.

A ZP3-binding proteins on the mouse sperm are located in the plasma membrane, overlying the acrosome.


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