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.
Anatomy and physiology
Suzanne Everett in 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.
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]
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: an International Menopause Society White Paper
Published in Climacteric, 2020
N. Panay, R. A. Anderson, R. E. Nappi, A. J. Vincent, S. Vujovic, L. Webber, W. Wolfman
One of the key differences between the male and female reproductive systems is that, in the male, gametogenesis continues throughout adult life with little diminution, whereas female reproduction is characterized by its finite duration and indeed the significant reduction in gamete quality in the later reproductive years. The biological basis for this is the presence of spermatogonial stem cells within the testis, which undergo unequal division to form daughter spermatogonia but with retention of the key stem cell characteristics in one product of that division. By contrast, in the female it is understood that all oocytes are formed during fetal life, following a wave of meiosis in the late first and early second trimesters of pregnancy. All oogonia within the ovary thus enter meiosis with subsequent arrest at prophase of meiosis I, with formation at that same time of the pool of primordial follicles, which constitutes the ovarian reserve.
Pythium insidiosum keratitis: Review of literature of 5 years’ clinical experience at a tertiary eye care center
Published in Seminars in Ophthalmology, 2023
Pratima Vishwakarma, Bhupesh Bagga
There are two phases (asexual and sexual) in the life cycle of a typical Pythium sp., which are excited by different environmental circumstances.8 The asexual cycle is characterized by the generation of sporangia which may either propagate directly to form a germ tube (direct germination) or by cytoplasmic cleavage it may get differentiated into uninucleate, biflagellate zoospores (indirect germination).7 Upon interaction with a mammalian host, their flagella are lost, they encyst followed by secretion of an adhesive-like material that helps to keep them attached to the host. These encysted zoospores can develop a germ tube that points toward the wound and penetrates the host tissues, leading to pythiosis.9 The sexual cycle generates thick-walled oospores that can survive under harsh environmental conditions. These are produced by a process called oosporogenesis in which a female oogonium and a male antheridium are produced that later fuses leading to fertilization and thereby to the development of an oospore. These resting spores can germinate under suitable conditions to produce single or multiple germ tubes which can then form sporangia thereby recapitulating the asexual cycle of the pathogen.8(Figure 1)
Related Knowledge Centers
- Fetus
- Ovary
- Ploidy
- Electron Microscope
- Oocyte
- Cell Nucleus
- Germ Cell
- Gametangium
- Mitosis
- Somatic