Oogenesis

Oogenesis (/ˌoʊ.əˈdʒɛnɪsɪs/) or ovogenesis is the differentiation of the ovum (egg cell) into a cell competent to further develop when fertilized. It is developed from the primary oocyte by maturation. Oogenesis is initiated during embryonic development.

In humans and other 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.

Oogonium —(Oocytogenesis)—> Primary Oocyte —(Meiosis I)—> First Polar body (Discarded afterward) + Secondary oocyte —(Meiosis II)—> Second Polar Body (Discarded afterward) + Ovum

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 traditionally does not belong to oogenesis proper, but, instead, to the common process of gametogenesis, which, in the human female, begins with the processes of folliculogenesis, oocytogenesis, and ootidogenesis. Oogonia enter meiosis during embryonic development, becoming oocytes. Meiosis begins with DNA replication and meiotic crossing over. It then stops in early prophase.

Mammalian oocytes are maintained in meiotic prophase arrest for a very long time—months in mice, years in humans. Initially, the arrest is due to lack of sufficient cell cycle proteins to allow meiotic progression. However, as the oocyte grows, these proteins are synthesized, and meiotic arrest becomes dependent on cyclic AMP. The cyclic AMP is generated by the oocyte by adenylyl cyclase in the oocyte membrane. The adenylyl cyclase is kept active by a constitutively active G protein-coupled receptor known as GPR3 and a G protein, Gs, also present in the oocyte membrane.

Maintenance of meiotic arrest also depends on the presence of a multilayered complex of cells, known as a follicle, that surrounds the oocyte. Removal of the oocyte from the follicle causes meiosis to progress in the oocyte. The cells that comprise the follicle, known as granulosa cells, are connected to each other by proteins known as gap junctions, that allow small molecules to pass between the cells. The granulosa cells produce a small molecule, cyclic GMP, that diffuses into the oocyte through the gap junctions. In the oocyte, cyclic GMP prevents the breakdown of cyclic AMP by the phosphodiesterase PDE3, and thus maintains meiotic arrest. The cyclic GMP is produced by the guanylyl cyclase NPR2.