The Human Reproductive System

The Process of Fertilization

The process of fertilization is the progression which gives living things their reproductive characteristic. In humans, fertilization occurs in the reproductive system of the female after copulation. It involves the fusion of the male sperm and the female ovum, which results in the formation of a diploid cell called a zygote (Gray, Williams & Bannister, 1995). This process occurs in the fallopian tube. The process begins soon after sperms have been deposited in the vagina during ejaculation. These sperms first undergo capacitation, a process which enables one of them to penetrate the membrane of the released ovum efficiently. The tough membrane of the ovum known as the zona pellucida surrounds the oocytes. The penetration of the sperm cell is also aided by the bursting of the sperms acrosome, releasing enzymes that make it even easier for the sperm to penetrate the ovum. This fusion of the sperms and the ovum in the fallopian tube is referred to as human fertilization. Only a single sperm fertilizes the egg, therefore, immediately after fertilization has taken place, the secondary oocytes fuse with the cell membrane of the ovum in a cross link making the membrane hard and impermeable to any more sperm cells (Jones & Lopez, 2006). This process is known as the cortical reaction and is essential towards ensuring that only a single sperm fertilizes the ovum. According to Jodar, Selvaraju, Sendler, Diamond and Krawetz (2013), after the cortical reaction, the sperm tail degenerates allowing the sperm head and the oocytes to undergo the fusion process. 

Oogenesis and Spermatogenesis

The sperm cells and the ovum responsible for the process of fertilization are both produced in the human reproductive system. The process through which an ovum, also known as the egg cell, is produced, is called oogenesis while the male equivalent, and the process through which sperm cells are produced is known as spermatogenesis. The two processes are essential in the human reproductive system, seeing as they give rise to the cells that fuse during fertilization. Spermatogenesis occurs in the seminiferous tubules in the testes and takes approximately 74 days (Amann, 2008). Here, the sperm cells are produced by sperm stem cells through the process of mitosis and meiosis. This process begins during puberty and never stops until death, provided that there are optimal conditions favorable for the process to occur. Oogenesis, on the other hand, occurs in the ovary of the female reproductive system. The process, unlike in males, begins at birth during the formation of the primary oocytes during a process called oocytogenesis. After this process, no more primary oocytes are produced by the ovary (Johnson, Canning, Kaneko, Pru and Tilly, 2004). Through the process of meiosis, these oocytes undergo cell division at puberty and eventually mature into the egg cells. However, only a few cells are produced during each menstrual cycle. Both spermatogenesis and oogenesis, therefore, leads to the creation of the human reproductive cells. However, spermatogenesis begins at puberty and goes on until death and involves both mitosis and meiosis while oogenesis begins at birth and only involves the process of mitosis. 

Estrogen and Testosterone

Estrogen and testosterone are hormones that play vital roles in the female and male reproductive system respectively. These hormones are often present in both the males and females but in different amounts. Estrogens are thus present in large amounts in females as compared to males, and the same applies for the testosterone. In the female reproductive system, estrogens are produced by the ovaries. The main function of estrogen is the regulation of the menstrual cycle. These hormones are also essential for the development of secondary sex characteristics in the female body. This includes the growth of breasts, pubic hair, maintaining the vaginal lining and also providing the vagina with optimal lubrication. Testosterone, on the other hand, is produced in the testes and is essential for the development of both the penis and testes right after birth. Like the estrogens, they also influence the development of secondary sex characteristics in the male. They are also believed to regulate sex drive in males, unlike the estrogen which does not regulate the female’s urge for sex. Both estrogen and testosterone are human reproductive hormones by definition because they are both produced by the endocrine glands in the human reproductive system and have specific effects on the development and activities of other organs. 

References

Amann, R. (2008). The Cycle of the Seminiferous Epithelium in Humans. Journal of Andrology , 29(5), pp.469-487. Available at: http://dx.doi.org/10.2164/jandrol.107.004655 [Accessed 24 Oct. 2016]

Gray, H., Williams, P. and Bannister, L. (1995). Gray's anatomy . New York: Churchill Livingstone.

Jodar, M., Selvaraju, S., Sendler, E., Diamond, M. and Krawetz, S. (2013). The presence, role and clinical use of spermatozoal RNAs. Human Reproduction Update , 19(6), pp.604-624. Available at: http://dx.doi.org/10.1093/humupd/dmt031 [Accessed 24 Oct. 2016].

Johnson, J., Canning, J., Kaneko, T., Pru, J. and Tilly, J. (2004). Germline stem cells and follicular renewal in the postnatal mammalian ovary. Nature , 430(7003), pp.1062-1062. Available at: http://dx.doi.org/10.1038/nature02868 [Accessed 24 Oct. 2016].

Jones, R. and Lopez, K. (2006). Human reproductive biology . Amsterdam: Elsevier Academic Press.