Embryos
Embryos of almost all vertebrates contain certain cells which are singled out during the early stages of development as gamete progenitors .These specialized cells known as the primordial germ cells enter the developing gonads ,that forms the testes in males and ovaries in females.Mitotic division of primordial germ cell is followed by meiotic differentiation to form mature gametes-either sperm or eggs.Fusion of these mature gametes is the intial process of embryogenesis whreby another begins by the subsequent production of new primordial cells. (Anjubault ,2006)
The unfertilized egg in most mammals especially the vertebtates is asymmetrical and contain varying cytoplasmic regions that has various mRNA and protein sets.Egg fertilization is followed by repeated division to produce early embronic cells which become the primordial germ cells after inheriting certain molecules situated at a particular area of the egg cytoplasm .However, the mammalian egg is more of symmetrical and initial divisions of the fertilized egg produce totipotent cells-that is, they produce any type of body cells.Signals from neighbouring cells induce some of the embryonic cells in mammals to form primordial germ cells. (Boczkowski, 2008)
The primordial cells differentiate and are taken back into the embryo proper together with the invaginating hindgut. These cells then shift to the developing gonads through the gut. The extracellular proteins from the adjacent somatic cells facilitate the proliferation, survival and migration of the primordial cells.Arrival of the primordial germ cell in the developing gonad , which according to this stage is the genital ridge,causes them to continually proliferate for afew days.They enter a pathway of development that makes them a sperm or an egg depending on whether the genital ridge has started to become a testis or an ovary respectively. The genital ridge’s somatic cells have sex chromosomes that determines which type of a gonad the ridge develops to .(Wilhelm,2013)
Mammalian sex chromosomes include the X and the Y chromosomes.The female somatic cells have two X chromosomes while those of the male have one Y and one X chromosome.The Y chromosome serve as the sex determinant factor whereby indivuals having a Y chromosomes become male and those without the Y chromosome become the femalesUnder the influence of the Y chromosome , a individuals sex is determined through the induction of the somatic cells on the genital ridge to become testis instead of an ovary.
The sry gene on the Y chromosome determine this testis formation function.Sry also encodes the sry-related protein Sox9 essential for the development of the sertoli cells.The genital ridge develops into an ovary in absence of either the Sox9or the sry where the somatic cells develops into follicle cells rather than the sertoli cells. The secretion of oestrogen hormone instead of testerone hormone occurs at puberty and the primordial cells become the eggs and not the sperm hence the development of the animal into a female.
References
Anjubault, E., & Exbrayat, J. M. (2006). Development of gonads. In Reproductive Biology and Phylogeny of Gymnophiona: Caecilians (pp. 305-316). CRC Press
Boczkowski, K. (2008). Male and female differentiation of the human gonad. Clinical Genetics, 4(3), 213- 219. doi: 10.1111/j.1399-0004.1973.tb01145.xC
Wilhelm, D., Yang, J. X., & Thomas, P. (2013). Mammalian sex determination and gonad development. In Current topics in developmental biology (Vol. 106, pp. 89-121). Academic Press.