Gametogenesis
Frank J. Dye in Human Life Before Birth, 2019
Let us follow the fate of a single diploid oogonium as it undergoes oogenesis (Figure 6.6). The oogonium begins to grow and then is called a primary oocyte. As such, it will be arrested in prophase I until at least menarche. Just before ovulation from the adult ovary, the primary oocyte completes meiosis I, producing two haploid cells: one large secondary oocyte and one small first polar body. The secondary oocyte proceeds through meiosis II until it reaches metaphase II, in which it remains until it is fertilized or dies. If fertilization occurs, the secondary oocyte completes meiosis II. As in meiosis I, cytokinesis is very unequal: it produces one large, haploid ootid (also known as the ovum or egg) and one small, haploid second polar body.
Ovotoxic Environmental Chemicals: Indirect Endocrine Disruptors
Rajesh K. Naz in Endocrine Disruptors, 2004
During fetal development, primordial germ cells (oogonia) that are formed invade the indifferent gonad and undergo rapid hyperplasia. Oogonia become oocytes, once they stop dividing and become arrested at the diplotene stage (prophase) of the first meiotic division. The oocyte does not commence meiosis again unless triggered to ovulate, should that occur. As a result, the lifetime supply of oocytes is set at the time of birth. Around the time of birth, individual oocytes within the ovary become surrounded by a single layer of flattened somatic cells (pre-granulosa cells) and a basement membrane to form primordial follicles.[4] Association of the granulosa cells with the oocyte is critical at all subsequent times for maintenance of viability, and follicle growth and development.[13]
Regulation of Reproduction by Dopamine
Nira Ben-Jonathan in Dopamine, 2020
Oogenesis can be divided into five stages. The first three occur prenatally, stage 4 occurs during ovulation, and stage 5 is at the time of fertilization. In stage 1, each primordial germ cell undergoes regular mitosis to produce two diploid oogonia. In stage 2, each oogonium undergoes mitosis to produce two diploid primary oocytes. In stage 3, each primary oocyte, enclosed in a primordial follicle consisting of a single layer of squamous follicular cells, starts to undergo the first meiotic division. This involves alignment of the homologous chromosomes, pairing and formation of chiasmata, crossing over, and exchange of genetic material. The oocyte then becomes arrested at prophase of the first meiotic division and remains in a genetically dormant state for many years to come.
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.
Related Knowledge Centers
- Fetus
- Ovary
- Ploidy
- Electron Microscope
- Oocyte
- Cell Nucleus
- Germ Cell
- Gametangium
- Mitosis
- Somatic