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Anatomy and physiology
Published in Suzanne Everett, Handbook of Contraception and Sexual Health, 2020
Follicles are developing oocyte and follicular tissue. The primordial follicle comprises cuboidal cells, which divide to form granulosa cells. Outside the follicle the interstitial cells change and become the theca folliculi; the inner layer of this produces oestrogens. The follicle increases in size and becomes the secondary follicle. The secondary follicle matures, and the granulosa cells split to form the corona radiata around the oocyte and the outer layer forms the membrane granulosa. This becomes the Graafian follicle which moves the surface of the ovary. A mature ovum develops within the follicle, the primordial germ cells differentiate into oogonia and by 12 weeks of intrauterine life they have undergone mitotic division to produce primary oocytes.
The Gonads
Published in George W. Casarett, Radiation Histopathology: Volume II, 2019
The cortical connective tissue of the ovaries contains the ovarian follicles which are of various sizes and in various stages of development and contain the ovocytes (Figure 6). Unlike the male gonad with its continuous production of spermatogonia throughout life, the production of oogonia is probably complete by the time of birth or shortly thereafter. Most of the ovarian follicles are small, primitive follicles located mainly in the peripheral cortex, and consisting of a primary oocyte surrounded by a layer of flattened granulosa cells. These give rise throughout life to all of the larger developing follicles. The primary oocyte is analogous to the primary spermatocyte. After birth, a human female starts out with a limited number of primary oocytes and ovarian follicles (perhaps one-half million) and the number is progressively reduced with the passage of time, mainly through involution or atresia of follicles (degeneration and disappearance), which begins in utero and is completed after the menopause. The process of ovulation once a month during the period of sexual activity accounts for the loss of only several hundred ova and follicles. Since the process of involution of follicles may start at any stage of follicular development, the normal ovary contains degenerating follicles in various sizes or stages of development.
Gametogenesis
Published in Frank J. Dye, Human Life Before Birth, 2019
As in the male, primordial germ cells arise on the yolk sac, then migrate into the developing ovaries on about the 38th day of development. Once in the ovaries, the germ cells are called oogonia. The ovaries contain the greatest number of germ cells before birth, when they number in the millions. At birth, the number is in the hundreds of thousands; at puberty (menarche) in the tens of thousands; and at menopause, the number is essentially zero. The number of germ cells in a woman's ovaries is in constant decline. A relatively small number of germ cells is lost through ovulation, and a much larger number is lost through normal cell death (Figure 6.5). In addition, the ovaries “do not have stem cell populations” to renew the supply of germ cells; however, see a dogma challenged at the end of this chapter.
Oocyte Survival and Development during Follicle Formation and Folliculogenesis in Mice Lacking Aromatase
Published in Endocrine Research, 2022
Jessica M. Toothaker, Kristen Roosa, Alexandra Voss, Suzanne M. Getman, Melissa E. Pepling
The process of oocyte development and follicle formation begins in the fetus with the migration of primordial germ cells to the developing ovary.2 The germ cells then undergo several rounds of mitosis, and during this time they are referred to as oogonia. Groups of oogonia, connected by intracellular bridges, are formed as the result of incomplete cytokinesis after each round of mitosis.3 These groups of oogonia are referred to as germ cell cysts and become oocytes when they enter meiosis.4,5 In mice, cysts first fragment into smaller cysts which then reassociate so that clusters contain some oocytes connected by intercellular bridges and other oocytes associated by aggregation.6 During this time, the oocytes progress through the first stages of meiotic prophase I and become arrested at an extended diplotene stage called dictyate.7,8 Beginning at 17.5 days post coitum (dpc) the cells separate and individual oocytes become surrounded with pregranulosa cells.9 This process is accompanied by apoptosis of several oocytes from each cyst.10 There is evidence that the oocytes that are lost serve to support or “nurse” the surviving oocytes.11 Those that remain become enclosed by pregranulosa cells to make up the ovarian reserve consisting of diplotene-arrested oocytes housed within primordial follicles.12 Despite the significance of this process for female fertility, the precise mechanisms that regulate cyst breakdown and follicle formation in mammals remain poorly understood.
Premature ovarian insufficiency – the need for a genomic map
Published in Climacteric, 2021
During ovarian development, PGCs emerge in the wall of the yolk sac on day 18 as a small cluster of cells. Simultaneously, the genital ridges develop from the intermediate mesoderm. Between days 28 and 36, the PGCs migrate to the genital ridge. Subsequently, the oogonia undergo mitosis and increase up to 600,000 in number by 8 weeks. These oogonia then either continue multiplying through mitosis up until 28 weeks or continue down a path of meiosis for the generation of non-dividing PGCs, or undergo oogonial atresia. As a result, up to 7 million PGCs are present by the 20th week; however, after this time, atresia dominates, resulting in 1–2 million germ cells at the time of birth [62]. The majority of the primordial follicles will undergo atresia, occurring from birth through to the menopause. Therefore, inadequate in utero production of the PGC pool can result in POI [8,63].
Sexual dimorphism in ultradian and 24h rhythms in plasma levels of growth hormone in Indian walking catfish, Clarias batrachus
Published in Chronobiology International, 2021
Raj Naresh Gopal, Dhanananajay Kumar, Vinay Kumar Singh, Atanu Kumar Pati, Bechan Lal
C. batrachus is a consumer preferred and an economically important cultured catfish of India. It is a seasonal breeder. In India, they commence their reproductive activities, such as gametogenesis during late February/March (the early-recrudescence phase). During this phase, the mitotic proliferation of spermatogonia and oogonia commences in testis and ovary, respectively, with an increase in the duration of photoperiod and a rise in ambient water temperature. In April (mid-recrudescence phase), these spermatogonia and oogonia further differentiate into spermatocytes and oocytes, respectively. During the mid-recrudescence phase, vitellogenic oocytes start accumulating yolk proteins. In May/June (the late-recrudescence phase), with further increase in temperature and photoperiod, testes attain peak spermatogenic activity (the meiotic division of spermatocytes). In contrast, in the ovary, the ovarian follicles further enlarge with massive up-take of yolk proteins. During July/August, i.e., the spawning phase, the advanced germ cells, such as secondary spermatocytes/spermatids and fully grown vitellogenic oocytes (eggs), respectively, undergo spermiogenesis and final oocytes maturation (germinal vesicle breakdown; GVBD). Finally, oocytes are ovulated from the follicles and released into the water in synchrony with the release of sperms from the spermiating male during late July/August (the spawning phase) for fertilization and further embryonic development.