ovarian development
The most important genes controlling the initial phase of gonadal development, identical in females and males, are Wilms’ tumor suppressor 1 (WT1) and steroidogenic factor 1 (SF1).
Four genes are likely to be involved in the subsequent stages of ovarian development (WNT4, DAX1, FOXL2 and RSPO1), but none is yet proven to be the ovarian determining factor.
Ovarian organogenesis
Primordial germ cells (PGCs) derive from the epiblast, the outer ectodermal layer of the embryo.
Primordial germ cells (PGCs) subsequently move to the yolk sac wall and then migrate along the dorsal mesentery of the hind gut to the gonadal ridge.
During migration, PGCs undergo cell division and, once in the genital ridge (by the end of the 5th week), lose their motility, begin to aggregate and continue to proliferate by mitosis.
In the female embryos, PGCs differentiate to oogonia and continue to divide by mitosis.
Shortly before and during the arrival of PGCs, the epithelium of the genital ridge proliferates and the epithelial cells penetrate the underlying mesenchyme forming the primitive sex cords, which surround the oogonia.
During the 7th week the proliferating epithelium gives rise to a second generation of cords, the cortical cords.
At the 10th week some oogonia will arrest their division and differentiate to oocytes.
In the 4th month the cortical cords split into clusters surrounding one or more of the oocytes and the earliest primary follicles appear.
The oocytes increase rapidly in number and by the 5th month of gestation the total number of oocytes in the ovary reaches its maximum.
However, most oocytes undergo apoptosis, their number diminishes and many follicles become atretic.
At birth, the total number of oocytes is estimated to range from 600,000 to 800,000; subsequently the majority of follicles will become atretic and at the beginning of puberty approximately 400,000 follicles will remain and less than 500 will proceed to ovulation.
Open references
Ovarian organogenesis in mammals: mice cannot tell us everything. Jiménez R. Sex Dev. 2009;3(6):291-301. PMID: #20130386# [Free]
Antagonism of the testis- and ovary-determining pathways during ovotestis development in mice. Wilhelm D, Washburn LL, Truong V, Fellous M, Eicher EM, Koopman P. Mech Dev. 2009 May-Jun;126(5-6):324-36. PMID: #19269320# [Free]
References
Brennan J, Capel B. One tissue, two fates: molecular genetic events that underlie testis versus ovary development. Nat Rev Genet. 2004 Jul;5(7):509-21. #PMID: 15211353#
Kobayashi A, Behringer RR. Developmental genetics of the female reproductive tract in mammals. Nat Rev Genet. 2003 Dec;4(12):969-80. PMID: #14631357#