Oogenesis - wikipedia Oogenesis Oogenesis Dorlands / Elsevier 12339107 Anatomical terminology ( edit on Wikidata ) Oogenesis, ovogenesis, or oögenesis / ˌoʊ. əˈdʒɛnɪsɪs / is the differentiation of the ovum ( egg cell ) into a cell competent to further development when fertilized. It is developed from the primary oocyte by maturation. Contents 1 Oogenesis in mammals 1.1 The creation of oogonia 2 Human oogenesis 2.1 Oocytogenesis 2.1. 1 Number of primary oocytes 2.2 Ootidogenesis 2.2. 1 Meiosis I 2.2. 2 Meiosis II 2.2. 3 Folliculogenesis 2.3 Maturation into ovum 2.4 In vitro maturation 3 Oogenesis in non-mammals 4 See also 5 References 6 External links Oogenesis in mammals Diagram showing the reduction in number of the chromosomes in the process of maturation of the ovum. ( In mammals, the first polar body normally disintegrates before dividing, so only two polar bodies are produced. ) In mammals, the first part of oogenesis starts in the germinal epithelium, which gives rise to the development of ovarian follicles, the functional unit of the ovary. Oogenesis consists of several sub-processes : oocytogenesis, ootidogenesis, and finally maturation to form an ovum ( oogenesis proper ). Folliculogenesis is a separate sub-process that accompanies and supports all three oogenetic sub-processes. Cell type ploidy / chromosomes chromatids Process Time of completion Oogonium diploid / 46 ( 2N ) 2C Oocytogenesis ( mitosis ) Third trimester primary oocyte diploid / 46 ( 2N ) 4C Ootidogenesis ( meiosis I ) ( Folliculogenesis ) Dictyate in prophase I for up to 50 years secondary oocyte haploid / 23 ( 1N ) 2C Ootidogenesis ( meiosis II ) Halted in metaphase II until fertilization Ootid haploid / 23 ( 1N ) 1C Ootidogenesis ( meiosis II ) Minutes after fertilization Ovum haploid / 23 ( 1N ) 1C Oogonium -- ( Oocytogenesis ) -- > Primary Oocyte -- ( Meiosis I ) -- > First Polar Body ( Discarded afterward ) + Secondary oocyte -- ( Meiosis II ) -- > Second Polar Body ( Discarded afterward ) + Ovum It should be noted that oocyte meiosis, important to all animal life cycles yet unlike all other instances of animal cell division, occurs completely without the aid of spindle - coordinating centrosomes. The creation of oogonia The creation of oogonia traditionally does n't belong to oogenesis proper, but, instead, to the common process of gametogenesis, which, in the female human, begins with the processes of folliculogenesis, oocytogenesis, and ootidogenesis. Human oogenesis Oocytogenesis Oogenesis starts with the process of developing oogonia, which occurs via the transformation of primordial follicles into primary oocytes, a process called oocytogenesis. Oocytogenesis is complete either before or shortly after birth. Number of primary oocytes It is commonly believed that, when oocytogenesis is complete, no additional primary oocytes are created, in contrast to the male process of spermatogenesis, where gametocytes are continuously created. In other words, primary oocytes reach their maximum development at ~ 20 weeks of gestational age, when approximately seven million primary oocytes have been created ; however, at birth, this number has already been reduced to approximately 1 - 2 million. Recently, however, two publications have challenged the belief that a finite number of oocytes are set around the time of birth. The renewal of ovarian follicles from germline stem cells ( originating from bone marrow and peripheral blood ) has been reported in the postnatal mouse ovary. In contrast, DNA clock measurements do not indicate ongoing oogenesis during human females ' lifetimes. Thus, further experiments are required to determine the true dynamics of small follicle formation. Ootidogenesis The succeeding phase of ootidogenesis occurs when the primary oocyte develops into an ootid. This is achieved by the process of meiosis. In fact, a primary oocyte is, by its biological definition, a cell whose primary function is to divide by the process of meiosis. However, although this process begins at prenatal age, it stops at prophase I. In late fetal life, all oocytes, still primary oocytes, have halted at this stage of development, called the dictyate. After menarche, these cells then continue to develop, although only a few do so every menstrual cycle. Meiosis I Meiosis I of ootidogenesis begins during embryonic development, but halts in the diplotene stage of prophase I until puberty. The mouse oocyte in the dictyate ( prolonged diplotene ) stage actively repairs DNA damage, whereas DNA repair is not detectable in the pre-dictyate ( leptotene, zygotene and pachytene ) stages of meiosis. For those primary oocytes that continue to develop in each menstrual cycle, however, synapsis occurs and tetrads form, enabling chromosomal crossover to occur. As a result of meiosis I, the primary oocyte has now developed into the secondary oocyte and the first polar body. Meiosis II Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this process is also halted at the metaphase II stage until fertilization, if such should ever occur. When meiosis II has completed, an ootid and another polar body have now been created. Folliculogenesis Main article : Folliculogenesis Synchronously with ootidogenesis, the ovarian follicle surrounding the ootid has developed from a primordial follicle to a preovulatory one. Maturation into ovum Both polar bodies disintegrate at the end of Meiosis II, leaving only the ootid, which then eventually undergoes maturation into a mature ovum. The function of forming polar bodies is to discard the extra haploid sets of chromosomes that have resulted as a consequence of meiosis... In vitro maturation Main article : In vitro maturation In vitro maturation ( IVM ) is the technique of letting ovarian follicles mature in vitro. It can potentially be performed before an IVF. In such cases, ovarian hyperstimulation is n't essential. Rather, oocytes can mature outside the body prior to IVF. Hence, no ( or at least a lower dose of ) gonadotropins have to be injected in the body. However, there still is n't enough evidence to prove the effectiveness and security of the technique. Oogenesis in non-mammals Main article : Evolution of sexual reproduction Diagram of oogenesis in a digenean ( Platyhelminthes ) Some algae and the oomycetes produce eggs in oogonia. In the brown alga Fucus, all four egg cells survive oogenesis, which is an exception to the rule that generally only one product of female meiosis survives to maturity. In plants, oogenesis occurs inside the female gametophyte via mitosis. In many plants such as bryophytes, ferns, and gymnosperms, egg cells are formed in archegonia. In flowering plants, the female gametophyte has been reduced to an eight - celled embryo sac within the ovule inside the ovary of the flower. Oogenesis occurs within the embryo sac and leads to the formation of a single egg cell per ovule. In ascaris, the oocyte does not even begin meiosis until the sperm touches it, in contrast to mammals, where meiosis is completed in the estrus cycle. See also Anisogamy Archegonium Evolution of sexual reproduction Female infertility Female reproductive system Meiosis Oncofertility Oogonium Oocyte Origin and function of meiosis Sexual reproduction Spermatogenesis References Jump up ^ Merriam - Webster Online Dictionary Definition : Oogenesis Jump up ^ Gilbert, Scott F. ( 2000 - 01 - 01 ). `` Oogenesis ''. Jump up ^ Szollosi D, Calarco P, Donahue RP ( 1972 ). `` Absence of centrioles in the first and second meiotic spindles of mouse oocytes ''. J Cell Sci. 11 ( 2 ) : 521 -- 541. PMID 5076360. Jump up ^ Manandhar G, Schatten H, Sutovsky P ( January 2005 ). `` Centrosome reduction during gametogenesis and its significance ''. Biol. Reprod. 72 : 2 -- 13. PMID 15385423. doi : 10.1095 / biolreprod. 104.031245. Jump up ^ NCBI - The saga of the germ line Jump up ^ Lobo RA ( September 2003 ). `` Early ovarian ageing : a hypothesis. What is early ovarian ageing ? ''. Hum. Reprod. 18 ( 9 ) : 1762 -- 4. PMID 12923124. doi : 10.1093 / humrep / deg377. Jump up ^ Johnson J, Bagley J, Skaznik - Wikiel M, et al. ( July 2005 ). `` Oocyte generation in adult mammalian ovaries by putative germ cells in bone marrow and peripheral blood ''. Cell. 122 ( 2 ) : 303 -- 15. PMID 16051153. doi : 10.1016 / j. cell. 2005.06. 031. Jump up ^ Johnson J, Canning J, Kaneko T, Pru J, Tilly J ( 2004 ). `` Germline stem cells and follicular renewal in the postnatal mammalian ovary ''. Nature. 428 ( 6979 ) : 145 -- 50. PMID 15014492. doi : 10.1038 / nature02316. Jump up ^ Forster P, Hohoff C, Dunkelmann B, Schürenkamp M, Pfeiffer H, Neuhuber F, Brinkmann B ( 2015 ). `` Elevated germline mutation rate in teenage fathers ''. Proc R Soc B. 282 : 20142898. PMC 4345458. PMID 25694621. doi : 10.1098 / rspb. 2014.2898. Jump up ^ Biochem Jump up ^ Guli CL, Smyth DR ( 1988 ). `` UV - induced DNA repair is not detectable in pre-dictyate oocytes of the mouse ''. Mutat Res. 208 ( 2 ) : 115 -- 119. PMID 3380109. doi : 10.1016 / s0165 - 7992 ( 98 ) 90010 - 0. ^ Jump up to : Vejledning om kunstig befrugtning 2006 ( Danish ) Bibliography Manandhar G, Schatten H and Sutovsky P ( 2005 ). Centrosome reduction during gametogenesis and its significance. Biol Reprod, 72 ( 1 ) 2 - 13. External links Reproductive Physiology Female reproductive system Internal Adnexa Ovaries Follicles corpus hemorrhagicum luteum albicans Theca of follicle externa interna Follicular antrum Follicular fluid Corona radiata Zona pellucida Membrana granulosa Perivitelline space Other Germinal epithelium Tunica albuginea cortex Cumulus oophorus Stroma Medulla Fallopian tubes Isthmus Ampulla Infundibulum Fimbria Ostium Ligaments Ovarian ligament Suspensory ligament Wolffian vestiges Gartner 's duct Epoophoron Vesicular appendages of epoophoron Paroophoron Uterus Regions Body Uterine cavity Fundus Cervix External orifice Cervical canal Internal orifice Supravaginal portion Vaginal portion Uterine horns Layers Endometrium epithelium Myometrium Perimetrium Parametrium Ligaments Round ligament Broad ligament Cardinal ligament Uterosacral ligament Pubocervical ligament General Uterine glands Vagina Fossa of vestibule of vagina Vaginal fornix Hymen External Vulva Labia Mons pubis Labia majora Anterior commissure Posterior commissure Pudendal cleft Labia minora Frenulum of labia minora Frenulum of clitoris Vulval vestibule Interlabial sulci Bulb of vestibule Vaginal orifice vestibular glands / ducts Bartholin 's glands / Bartholin 's ducts Skene 's glands / Skene 's ducts Clitoris Crus of clitoris Corpus cavernosum Clitoral glans Hood Urethra Urethral crest Other G - spot Urethral sponge Perineal sponge Human physiology and endocrinology of sexual reproduction Tanner scale Menstrual cycle Menarche Menstruation Follicular phase Ovulation Luteal phase Gametogenesis Spermatogenesis ( spermatogonium spermatocyte spermatid sperm ) Oogenesis ( oogonium oocyte ootid ovum ) Germ cell ( gonocyte gamete ) Human sexual behavior Sexual arousal Sexual intercourse Masturbation Erection Orgasm Ejaculation Insemination Fertilisation / Fertility Implantation Pregnancy Postpartum period Mechanics of sex Life span Prenatal development / Sexual dimorphism / Sexual differentiation ( Feminization Virilization ) Puberty ( Gonadarche Pubarche Menarche Spermarche Adrenarche ) Maternal age / Paternal age Climacteric ( Menopause Late - onset hypogonadism ) Egg Ovum Oviposition Oviparity Ovoviviparity Vivipary Reproductive endocrinology and infertility Hypothalamic -- pituitary -- gonadal axis Hypothalamic -- pituitary -- prolactin axis Andrology Hormone Breast Thelarche Breast development Lactation Breastfeeding Sex Biological terms Sexual dimorphism Male Female Sexual differentiation Feminization Virilization Sex - determination system XY ZW XO Temperature - dependent Haplodiploidy Heterogametic sex Homogametic sex Sex chromosome X chromosome Y chromosome Testis determining factor Hermaphrodite Sequential hermaphroditism Intersex Sexual reproduction Evolution of sexual reproduction Anisogamy Isogamy Germ cell Reproductive system Sex organ Meiosis Gametogenesis Spermatogenesis Oogenesis Gamete spermatozoon ovum Fertilization External Internal Sexual selection Plant reproduction Fungal reproduction Sexual reproduction in animals Sexual intercourse Human reproduction Sexuality Plant sexuality Animal sexuality Human sexuality Mechanics Differentiation Activity Retrieved from `` https://en.wikipedia.org/w/index.php?title=Oogenesis&oldid=805732019 '' Categories : Developmental biology Genetics Human female endocrine system Meiosis Hidden categories : Medicine infobox template using Dorlands parameter Talk Contents About Wikipedia Aragonés বাংলা Беларуская Беларуская ( тарашкевіца ) ‎ Bosanski Català Deutsch Eesti Español Esperanto Euskara فارسی Français Galego Հայերեն Ido Bahasa Indonesia Italiano עברית Қазақша Latviešu Lietuvių Nederlands Polski Português Русский Slovenščina Српски / srpski Suomi Svenska ไทย Українська 中文 Edit links This page was last edited on 17 October 2017, at 08 : 19. 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Question: when do females produce all of their oocytes which will develop into eggs
Answer: at ~20[6] weeks of gestational age