Oogenesis: The Formation of the Egg - Essay Example

? “Gametogenesis" Introduction Gametogenesis is the formation of male and female gametes or sex cells. Spermatogenesis and oogenesis are the formation of male and female gametes, respectively, by the process of mitosis and meiosis. Gametes unite in the process of fertilization. The most important consequence of meiosis is that it produces an enormous amount of genetic variation through the production of different combinations of chromosomes. the completion of meiosis does not necessarily mean that gametes have formed. Meiosis produces four haploid cells. These cells must now mature into functional gametes, eggs and sperm. The meiotic process that produces haploid cells and the subsequent maturation of these cells into functioning gametes is called gametogenesis. (Rahman et al, 2008) Oogenesis: The formation of the Egg In the early stage of embryonic development, groups of cells become committed to differentiate into various cell types such as liver cells, nerve cells and muscle cells. One group of cells becomes committed to form the germ cell line, cells that eventually develop into eggs if the embryo is female or sperm if the embryo is male. These are the only cells that undergo meiosis. These committed cells, primordial germ cells increase in number through mitotic cell division. Later, they divide meiotically to produce mature sperm or eggs. The process of forming mature eggs is called oogenesis (Aberts et al, 2002; Snustad and Simmons, 2002). The primordial germ cells that migrate to the developing ovary during early embryogenesis become oogonia. They multiply rapidly, undergoing several rounds of mitotic cell division and eventually differentiate into primary oocytes. The primary oocytes begin meiotic cell division and complete the diplotene stage of prophase I and then division stops. They remain in this suspended prophase. During this period, the oocyte undergoes many changes that prepare it for the completion of meiosis and for fertilization. It acquires a special coat that protects the developing egg from mechanical damage and in many cases acts as a barrier to sperm from other species. Just beneath the membrane cortical granules develop that alter the egg coat so that only a single sperm fertilizes an egg. In addition, the primary oocyte accumulates large quantities of nutrients and other molecules that nourish the early embryo and coordinate as well as direct its early development. With the attainment of sexual maturity, the next stage of oocyte development occurs, triggered by hormones. The oocyte completes meiosis I and two haploid nuclei are formed, each containing one member of each chromosome pair in a replicated state. But cytokines are very asymmetrical. One cell, called secondary oocyte gets virtually all the cytoplasm and is ancestral to the mature egg. The other cell, called a polar body, gets very little cytoplasm. Both of these cells , the secondary oocyte and the polar body undergo meiosis II to produce four haploid nuclei. Again cytokinesis is asymmetrical: Meiosis II in the secondary oocyte produces one large cell, the ovum, or egg, which has virtually all the cytoplasm and a small polar body with very little cytoplasm. Thus, of the four meiotic products, only one forms the mature egg. the polar bodies, which are small with little cytoplasm to support their metabolism, eventually degenerate (Aberts et al, 2002; Snustad and Simmons, 2002). Oogenesis occur in the ovaries of the female. Each primary oocyte is surrounded by a spherical cluster of cells in a cavity or sac called the primary or Graafian follicle. In response to hormone signals, the primary oocyte completes meiosis I to become a secondary oocyte. the follicle then ruptures, releasing the secondary oocyte into the oviduct, where it begin the second meiotic division. in humans, the oviduct is called the Fallopian tube. the release of the secondary oocyte from the Graafian follicle is called ovulation (Aberts et al, 2002; Snustad and Simmons, 2002). As the secondary oocyte travels down the oviduct, it enters the second meiotic division. if it is fertilized, meiosis II is completed. If it is not fertilized meiosis II is not completed and the egg degenerates. Upon fertilization, both male and female nuclei occupy the egg cytoplasm. The haploid nuclei fuse, restoring the diploid chromosomes number. The zygote, or fertilized egg, passes down the oviduct into the uterus and attaches there (Aberts et al, 2002; Snustad and Simmons, 2002). The Role of the gonad as an endocrine gland Anatomy of female reproductive system involves the external genitalia called vulva encompassing labia majora, labia minora and clitoris. The uterus is connected to the vulva by vagina. The cervix is the neck of the uterus. The wall of the uterus are thick and are called myometrium, inner lining of the uterus is endometrium. Endometrium proliferates for zygote while shed in menstrual cycle. Uterus is connected to ovaries through fallopian tube, one on each side of the uterus. Fertilization occurs in the fallopian tube. Ovary is the female gonad (Web. Reproductive System). Production of egg is carried out in ovaries and primary oocytes begin to develop, contained in the follicle and surrounded by the supporting cells. At the mid of the menstrual cycle, ovarian follicles rupture and egg is released into fallopian tube (Web. Female Reproductive System). Hormones are produced in the ovaries especially in the developing follicles comprising progesterone and estrogen. During the entire menstrual cycle, the level of hormones vary. For instance, on the first day of menstruation, when the cycle onsets, the FSH (Follicle Stimulating Hormone) elevates and this stimulate the follicle development in the ovaries, as a result, follicles generate estrogen. Of the several follicles only one matures and there is an increase in the estrogen level throughout the maturation process. Gradually progesterone level starts elevating, the estrogen in turn generates a negative feedback a kind of check mechanism on GnRH and gonadotropin secretion. On the 14th day of the menstrual cycle, there is a tremendous rise in the estrogen level and a positive feedback mechanism onsets as a result there is an elevation in LH (Luteinising Hormone) which initiates the process of ovulation. Consequently, egg is released as soon as the follicle ruptures. Following the process of ovulation, the follicle collapses. This results in the transformation of the cells inside it generating a new form called the corpus luteum. The luteal cells generate less estrogen and more progesterone as a result, progesterone level in the blood elevates more as compared to the estrogen concentration. The corpus luteum survives for 14 days followed by a diminish in the estrogen and progesterone levels. This decreasing concentration of estrogen eliminates the negative feedback on Gonadotropin Releasing Hormone and Follicle Stimulating Hormone. The cycle again onsets from this stage (Web. Female Reproductive System). References Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter P., 2002. Molecular Biology of the Cell. 4th ed. Publisher Garland Science. Female Reproductive System. [online] Available at [Accessed 28 March 2012]. Reproductive System. [online] Available at [Accessed 28 March 2012]. Rahman, A.N.M.A., Abdullah, R.B. and Wan-Khadijah, W. E. 2008. Gametogenesis, Fertilization and Early Embryogenesis in Mammals with Special Reference to Goat: A Review. Journal of Biological Sciences, 8: 1115-1128. Snustad, D.P., Simmons, M.J. 2002. Principles of Genetics. 3rd ed. Publisher Wiley. Read More