oogenesis

Oogenesis or oögenesis (pronounced /ˌoʊ.əˈdʒɛnɨsɪs/^[1])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 mammals, the first part of oogenesis starts in the germinal epithelium which give rise to ovarian follicles(the functional unit of the ovary.)

Note that such an important process in animal life cycles is done completely without the aid of oo 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.

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 common path of gametogenesis together with spermatogenesis.

Process of Human Oogenesis

At the start of the menstrual cycle some 12-20 Primary follicles begin to develop under the influence of elevated FSH to form secondary follicles. The Primary follicles have formed from primordial follicles which developed in the ovary at around 10–30 weeks after conception. By around day 9 of the cycle only one healthy secondary follicle is remaining, with the rest having undergone atresia. The remaining follicle is called the dominant follicle and is responsible for producing large amounts of oestradiol during the late follicular phase. Oestradiol production depends on co-operation between the theca and granulosa cells. On day 14 of the cycle an LH surge occurs which is triggered by positive feedback of oestradiol. This causes the secondary follicle to turn into a tertiary follicle which ovulates some 24–36 hours later. An important event in the tertiary follicle is that the primary oocyte completes the first meiotic division with formation of a polar body and a secondary oocyte. The empty follicle then forms a corpus luteum.^{[citation needed]}

Oocytogenesis

Oogenesis starts with oogonial transformation into primary oocytes, called oocytogenesis^[2]. 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 reach their maximum at ~20^[3] weeks of gestational age, when there are seven million of them; however at birth this 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.^[4][5] 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^[6].

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 do 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 tetrads form, enabling and 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 are created.

Folliculogenesis

Synchronously with ootidogenesis, the ovarian follicle surrounding it develops 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 undergoes maturation and eventually matures into an ovum.

The function of forming polar bodies is to discard the extra haploid set of chromosomes(n)

Oogenesis in non-mammals

Many protists produce egg cells in structures termed archegonia. 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

• Oogonium
• Archegonium
• Oocyte
• Spermatogenesis

References

External links

• Reproductive Physiology