Oogenesis
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'''Oogenesis''' or rarely '''oögenesis''' is the creation of an [[ovum]] (egg cell). It is the female process of [[gametogenesis]]. It involves the various stages of [[immature ova]].
==Oogenesis in mammals==
In [[mammal]]s, oogenesis occurs in the [[ovarian follicle]] of the [[ovary]].
It is interesting to note that such an important process in animal life cycles is done completely without the aid of [[Spindle apparatus|spindle]]-coordinating [[centrosomes]].
It consists of several processes: oocytogenesis, [[ootidogenesis]] and the final maturity to form an ovum. [[Folliculogenesis]] is a separate process during ootidogenesis.
{| class="wikitable"
| '''Cell type''' || '''[[ploidy]]''' || '''Process''' || '''Process completion'''
|-
| [[Oogonium]] || diploid || '''Oocytogenesis''' ([[mitosis]]) || third [[trimester]] (forming oocytes)
|-
| primary [[Oocyte]] || diploid || '''Ootidogenesis''' ([[meiosis]] 1) ([[Folliculogenesis]]) || [[Dictyate]] in prophase I until ovulation
|-
| secondary [[Oocyte]] || haploid || '''Ootidogenesis''' ([[meiosis]] 2) || Halted in metaphase II until fertilization
|-
| [[Ootid]] || haploid || ? || Minutes after fertilization
|-
| [[Ovum]] || haploid ||
|}
Oogonium --(Oocytogenesis)--> Primary Oocyte --(Meiosis I)-->First Polar Body (Discarded afterward) + Secondary oocyte --(Meiosis II)--> Secondary Polar Body(Discarded afterward) + Ovum
===Creation of oogonia===
The creation of oogonia traditionally doesn't belong to oogenesis, but to the [[Gametogenesis#Common path|common path]] of gametogenesis together with spermatogenesis.
===Oocytogenesis===
Oogenesis starts with [[oogonium|oogonial]] transformation into primary [[oocyte]]s, called ''oocytogenesis''<ref>[http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=dbio.section.4695 NCBI - The saga of the germ line]</ref>. Oocytogenesis is completed either before or shortly after birth.
====Number of primary oocytes====
It is commonly said that when oocytogenesis is completed, no additional primary oocytes are created, in contrast to the male spermatogenesis, where gametocytes are continuously created. In other words, oocytes reaches their maximum at ~20<ref>[http://humrep.oxfordjournals.org/cgi/content/full/18/9/1762 Early ovarian ageing: a hypothesis
What is early ovarian ageing?
pmid = 16051153}} </ref><ref>{{cite journal | author = Johnson J, Canning J, Kaneko T, Pru J, Tilly J | title = Germline stem cells and follicular renewal in the postnatal mammalian ovary | journal = Nature | volume = 428 | issue = 6979 | pages = 145–50 | year = 2004 | pmid = 15014492 | doi = 10.1038/nature02316}}</ref> Renewal of ovarian follicles from germline stem cells (originating from bone marrow and peripheral blood) was reported in the postnatal mouse ovary. Due to the revolutionary nature of these claims, further experiments are required to examine the dynamics of small follicle formation.
===Ootidogenesis===
The succeeding '''ootidogenesis''' is the step in which the [[primary oocyte]] turns into an ootid. It is achieved by [[meiosis]]. The primary oocyte is even defined from its role to undergo meiosis<ref>[http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-P/primary_oocyte.html Biochem]</ref>.
However, although this process begins at prenatal age, it stops at prophase I. In late fetal life, all oocytes, still primary oocytes, have taken this halt in development, called [[dictyate]]. First after menarche they continue to develop, although only a few does so every menstrual cycle.
====Meiosis I====
[[Meiosis I]] of ootidogenesis starts at embryonic age, but halts in diplotene of prophase I until puberty. For those primary oocytes continuing to develop in each menstrual cycle, however, synapsis occurs and [[tetrad]]s form, enabling and [[Chromosomal crossover|crossing over]]. As a result of meiosis I, the primary oocyte becomes the [[secondary oocyte]] and the first [[polar body]].
====Meiosis II====
Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this, too is halted in metaphase II. However, this only lasts until [[fertilization]], if such occurs. When meiosis II is completed, an ootid and another polar body is created.
====Folliculogenesis====
{{Main|Folliculogenesis}}
Synchronously as ootidogenesis, the [[ovarian follicle]] surrounding it develops from a primordial follicle to a preovulatory one.
===Maturation into ovum===
Both polar bodies at the end of Meiosis II disintegrate leaving only the ootid which undergoes maturation and eventually matures into an ovum.
The function of forming polar bodies is to discard the extra haploid set of chromosome(n)
== Oogenesis in non-mammals ==
Many [[protist]]s produce egg cells in structures termed [[archegonia]]. Some [[algae]] and the [[oomycete]]s 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 [[plant]]s, oogenesis occurs inside the female [[gametophyte]] via [[mitosis]]. In many plants such as [[bryophyte]]s, [[fern]]s, and [[gymnosperm]]s, egg cells are formed in [[archegonia]]. In [[flowering plant]]s, the female gametophyte has been reduced to an eight-celled [[embryo sac]] within the [[ovule]] inside the [[Ovary (plants)|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 menstrual cycle.
== See also ==
*[[Oogonium]]
*[[Archegonium]]
*[[Oocyte]]
*[[Spermatogenesis]]
==References==
{{Reflist}}
==External links==
*http://distance.stcc.edu/AandP/AP/AP2pages/reprod/oogenesi.htm
*[http://www.ufp.pt/~pedros/qfisio/reproduction.htm Reproductive Physiology]
{{Female reproductive system}}
[[Category:Developmental biology]]
[[Category:genetics]]
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