Chapter 3 The sequential stages in the differentiation and maturation of primordial germ cells into gametes in male and female animals are referred to as gametogenesis. Primordial germ cells of the epiblast migrate via the dorsal mesentery to the developing gonads. During migration these cells undergo mitosis, producing large numbers of germ cells which populate the gonads. Germ cells undergo similar sequential development in male and female animals. Primordial germ cells undergo a series of mitotic divisions, producing stem cells which, in association with mesodermal cells, form seminiferous cords in the developing testis. In this location, they remain quiescent until the onset of puberty when sexual maturation begins. At puberty, these dormant germ cells become activated and, through a series of mitotic divisions, produce clones of cells referred to as type A spermatogonia (Fig 3.1). Subsequently, some type A cells divide, giving rise to type B spermatogonia, from which primary spermatocytes arise. Figure 3.1 Stages in the development of spermatozoa from a primordial germ cell. Primordial germ cells, which remain dormant until puberty, differentiate into spermatogonia and, following meiosis, spermatozoa are formed from spermatids. The diploid primary spermatocytes undergo the first stage of meiotic division resulting in the formation of haploid secondary spermatocytes. When these haploid secondary spermatocytes undergo the second stage of meiotic division, they form haploid spermatids (Fig 3.1). The process whereby a spermatid undergoes metamorphosis into a spermatozoon is termed spermiogenesis (Fig 3.2). Initially the spermatid has the organelles of a typical mammalian cell including a spherical nucleus, a Golgi complex, mitochondria, paired centrioles and endoplasmic reticulum. Granules, which are synthesised in the Golgi complex, fuse forming a single large acrosomal vesicle. When this vesicle covers the anterior aspect of the condensed nucleus, it is referred to as the acrosome. The centrioles, which migrate to the pole of the nucleus opposite the acrosome, form the axial filament from which the tail of the spermatozoon develops. Mitochondria aggregate in the proximal region of the filament forming the middle piece of the spermatozoon. Portions of cytoplasm shed from individual spermatids are collectively referred to as residual bodies. A unique feature of spermatogenesis is that the cytoplasmic divisions of the dividing spermatogonia are incomplete as the spermatocytes remain attached by cytoplasmic bridges. The time required for the production of spermatozoa from type A spermatogonia may range from 40 to 60 days, depending on the species. Figure 3.2 The morphological changes whereby a mammalian spermatid is converted into a spermatozoon.
Gametogenesis
Spermatogenesis
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