Sexual Reproduction: Meiosis, Germ Cells, and ... - U-Cursos

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1296 Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, and FertilizationMITOSISMEIOSIS IMEIOSIS IIspermatogoniumspermatogoniaprimaryspermatocytescytoplasmic bridgessecondaryspermatocytesspermatidsFigure 21–31 Cytoplasmic bridges indeveloping sperm cells and theirprecursors. The progeny of a singlematuring spermatogonium remainconnected to one another by cytoplasmicbridges throughout their differentiationinto mature sperm. For the sake ofsimplicity, only two connected maturingspermatogonia are shown enteringmeiosis, eventually to form eightconnected haploid spermatids. In fact,the number of connected cells that gothrough meiosis and differentiatesynchronously is very much larger thanshown here. Note that in the process ofdifferentiating, most of the spermatidcytoplasm is discarded as residual bodies,which are phagocytosed by Sertoli cells.differentiatingspermatidsresidual bodies+mature spermatozoaDaz genes is similarly essential for spermatogenesis in the fly: Daz-deficientmale flies are infertile because they cannot make sperm, but, remarkably, theycan be cured by a human Daz transgene. RNA-binding proteins are especiallyimportant in spermatogenesis, because many of the genes expressed in thesperm lineage are regulated at the level of RNA translation.SummaryA sperm is usually a small, compact cell, highly specialized for the task of fertilizing anegg. Whereas in women a large pool of oocytes is produced before birth, in men spermatogoniaonly begin to enter meiosis to produce spermatocytes (and sperm) after sexualmaturation, and they continue to do so from then on. Each diploid primary spermatocytegives rise to four mature haploid sperm. The process of sperm differentiationoccurs after meiosis is complete, requiring five weeks in humans. Because the maturingspermatogonia and spermatocytes fail to complete cytokinesis, however, theprogeny of a single maturing spermatogonium develop as a large syncytium. Thus, theproducts encoded by both parental chromosomes direct sperm differentiation, eventhough each sperm nucleus is haploid.
FERTILIZATION 1297FERTILIZATIONOnce released, egg and sperm alike are destined to die within minutes or hoursunless they find each other and fuse in the process of fertilization. Through fertilization,the egg and sperm are saved: the egg is activated to begin its developmentalprogram, and the haploid nuclei of the two gametes come together toform the diploid genome of a new organism. Fertilization was originally studiedmost intensively in marine invertebrates such as sea urchins and starfish, wherefertilization occurs in seawater after the release of huge numbers of both spermand eggs. Such external fertilization is easier to study than the internal fertilizationof mammals, which normally occurs in the female reproductive tract aftermating. In the late 1950s, however, it became possible to fertilize mammalianeggs in vitro, opening the way to an analysis of the cellular and molecular eventsin mammalian fertilization.In this section, we focus on mammalian fertilization. We begin by consideringthe maturation of sperm that occurs during their passage through the femalegenital tract. We then discuss the binding of sperm to the egg coat (zona pellucida),which induces the acrosome reaction, required for the sperm to burrowthrough the zona and fuse with the egg. We next consider the binding of thesperm to the egg plasma membrane and its subsequent fusion with this membrane.After discussing how the fusion of a sperm activates the egg and how thehaploid nuclei of the two gametes come together in the zygote to complete fertilization,we briefly consider the burgeoning field of assisted reproductive technology,which has revolutionized the treatment of human infertility and openedup new ways of manipulating the reproductive process.Ejaculated Sperm Become Capacitated in the Female GenitalTractOf the 300,000,000 or so human sperm ejaculated during coitus, only about 200reach the site of fertilization in the oviduct. Once a sperm finds an egg, it mustfirst migrate through the layers of granulosa cells that surround the egg and thenbind to and cross the zona pellucida. Finally, it must bind to and fuse with theegg plasma membrane.Ejaculated mammalian sperm are initially not competent to accomplishany of these tasks. They must first be modified by conditions in the femalereproductive tract. Because it is required for sperm to acquire the capacity tofertilize an egg, the process is called capacitation. Capacitation takes about 5–6hours in humans and is completed only when the sperm arrive in the oviduct.The sperm undergo extensive biochemical and functional changes, includingchanges in glycoproteins, lipids, and ion channels in the sperm plasma membraneand a large change in the resting potential of this membrane (the membranepotential moves to a more negative value so that the membrane becomeshyperpolarized). Capacitation is also associated with an increase in cytosolicpH, tyrosine phosphorylation of various sperm proteins, and the unmasking ofcell-surface receptors that help bind the sperm to the zona pellucida. Capacitationalters two crucial aspects of sperm behavior: it greatly increases themotility of the flagellum, and it makes the sperm capable of undergoing theacrosome reaction.Capacitation can occur in vitro in the appropriate culture medium and isusually a required part of in vitro fertilization. Three critical components areneeded in the medium, all of which are normally in high concentration in thefemale genital tract—albumin, Ca 2+ , and HCO 3 – . The albumin protein helpsextract cholesterol from the plasma membrane, increasing the ability of thismembrane to fuse with the acrosome membrane during the acrosome reaction.The Ca 2+ and HCO 3 – enter the sperm and directly activate a solubleadenylyl cyclase enzyme in the cytosol to produce cyclic AMP (discussed inChapter 15), which helps to initiate many of the changes associated withcapacitation.
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