Chapter Summary

The hallmark of sexual reproduction is the formation of male and female gametes in the testis and ovary respectively. Spermatogenesis and oogenesis both entail meiotic divisions. Fusion of the sperm and the egg results in the formation of a diploid zygote, and its development is initiated by a temporary rise in intracellular Ca2+. Early development is also associated with rapid translation of maternal mRNA molecules.

[fig. 2.2]
Primordial germ cells undergo meiosis in the ovary and give rise to haploid oocytes. The primordial germ cell in the extended prophase of meiosis I is called the primary oocyte and its nucleus is called the germinal vesicle. Meiosis I gives rise to two cells: a bigger cell called the secondary oocyte and a much smaller cell called the first polar body, each containing a pair of homologous chromosomes. The secondary oocyte may remain arrested in meiosis II, and signals such as those triggered by fertilization release such an arrest, thereby leading to the completion of meiosis II and giving rise to a haploid egg. The egg is a huge storehouse of maternal materials that are necessary for early embryonic development. Such materials in the form of mRNA and protein molecules are present in specific regions of the egg, and their localization has important consequences on embryonic development, especially precise axial determination.

Cells called spermatogonia undergo mitosis continuously near the periphery of the seminiferous tubules. A few of the descendants that undergo meiosis are called primary spermatocytes. Meiosis I gives rise to two secondary spermatocytes which undergo meiosis II to produce four spermatids. Spermatids undergo spermiogenesis wherein most of their cytoplasm and organelles are lost. Their nucleus becomes more compact, and the cell is then called a mature spermatozoan/sperm.

[fig. 2.10]
Syngamy, the fusion of two haploid nuclei which gives rise to a diploid nucleus, is the hallmark of fertilization. The cell so formed by the fusion of the egg and sperm is called the zygote. The process of fertilization requires the activation of both the sperm and the egg. Each of the gametes undergoes changes in membrane properties and metabolism, ultimately leading to the fusion of the male and female gametes.

Fertilization is associated with a wave of calcium release that passes through the egg. The fertilization envelope is the major barrier to the occurrence of polyspermy and is formed due the release of material from the cortical granules into the perivitelline space. Polyspermy is also prevented by the change in membrane potential following the entry of the first sperm. Fertilization in many species is followed by a burst of protein synthesis from maternal mRNA molecules.

A map by which we are able to predict the fate of a particular region of an egg or an early embryo is called a fate map. Such maps may be constructed by using vital dyes or lineage tracers. On the basis the information about the mitotic descendants of a particular cell, one is able to construct a lineage diagram. Early cell divisions in the egg are associated with little or no growth and are called cleavage. This phase is associated with a rapid increase in cell surface area.

Further Reading

Gametogenesis and Fertilization Resources

Other resources from SDB
This Web site owned and maintained by the Society for Developmental Biology links various laboratories studying different aspects of early embryonic development including gametogenesis and fertilization.

Movies of the Calcium Ion Wave in Activated Fish Eggs
This movie shows the wave of intracellular calcium release which is visualised using a Ca binding dye. On binding Ca, released from intracellular vesicles, fluorescence is indicative of a localized increase in Ca concentration.


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