Oogenesis

Overview
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 mammals, 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-coordinating centrosomes.

It consists of several processes: oocytogenesis, ootidogenesis and the final maturity to form an ovum. Folliculogenesis is a separate process during ootidogenesis.

Creation of oogonia
The creation of oogonia traditionally doesn't belong to oogenesis, but to the common path of gametogenesis together with spermatogenesis.

Oocytogenesis
Oogenesis starts with oogonial transformation into primary oocytes, called oocytogenesis. 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 weeks of gestational age, when there are approximately seven million of them.

Recently, however, two publications have challenged the ovarian biology dogma that a finite number of oocytes are set around the time of birth. 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.

However, although this process begins at prenatal age, it stops at prophase I. In 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 dictyate 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, and it's halted in the metaphase of meiosis II. However, this only lasts until fertilization, if such occurs. When meiosis II is completed, an ootid and another polar body is created.

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.

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 menstrual cycle.