Gametogenesis could be defined as the process in which a haploid cell is formed from a diploid cell through meiotic cell division and cell differentiation. In higher plants and animals, two distinct types of gametes are observed via these processes.
In animals, a tissue is dedicated to forming gametes and is called a germline and individual cells in a germline are called germ cells. In animals, these germ cells undergo meiosis to produce gamete cells which directly develop into gametes.
In plants, diploid cells undergo meiosis to produce haploid spores which develop into haploid cells and gives rise to the haploid generation called gametophyte.
In this topic, we will learn about gametogenesis in humans which takes place in two processes. These are
We will discuss each process in brief.
The human male starts producing sperms when they reach puberty which is when they are around 10-16 years old and the process continues till death. Approximately 200million sperms are produced each day which increases the likelihood of a sperm reaching the egg.
Sperm production takes place in the testes of the human male, in the seminiferous tubules to be more specific. The seminiferous tubules are separated from the systematic circulation by the blood-testis barrier.
Blood-testes barrier: This barrier is formed by Sertoli cells and prevents hormones and constituents of the human circulatory system from affecting the developing sperm. It also prevents the immune system from recognising the sperm as a foreign object. (The sperm is genetically different from the male it can be recognised as an antigen) The Sertoli cells also play a role in the development of the spermatozoa.
Spermatogonia: It is the initial pool of diploid cells that divide via mitosis to produce identical cells. One cell will go under meiosis to form a sperm cell and the other cell will be used to replenish the spermatogonia. The cells which replenish the spermatogonia are called A1 spermatogonia.
The type B spermatogonia duplicate via mitosis many times to form identical diploid cells which are linked by cytoplasm bridges and they are called spermatocytes. The primary spermatocytes undergo meiosis I to produce two secondary spermatocytes. Then the secondary spermatocytes undergo meiosis II to produce four haploid cells called spermatids.
Then the cytoplasmic bridge breaks down and the spermatids are released into the lumen of the seminiferous tubule in a process called spermiation. Then they mature into spermatozoa in a process called spermiogenesis and travel along the seminiferous tubules till they arrive at the epididymis.
The sperm travels to the rete testes after they travel from the seminiferous tubule. The rete testes act to concentrate the sperm as it removes excess fluid. Before the sperm moves to the epididymis, it undergoes the final stages of maturation.
The entire process of spermatogenesis takes around 70 days and new groups of spermatogonia develop every 16 days. It is a continuous process as multiple spermatogonic processes take place simultaneously.
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Oogenesis in human females begins before birth in the foetus. Primordial germ cells in the yolk sack of the embryo move to colonise the cortex of the primordial gonad and duplicate by mitosis and reach around 7 million in numbers by mid-gestation which is approximately 20 weeks. After reaching the peak, cell death takes place and around 2 million cells begin meiosis I before birth. These cells are known as primary oocytes. Which means, a human female is born with 2 million primary oocytes arrested in meiosis I. These oocytes are arranged in the gonads in clusters and they are surrounded by epithelial cells which form primordial follicles.
Once puberty begins at the age of 15-20 for human females, a number of primary oocytes begin to mature each month and they undergo three stages to attain full maturation.
Pre-antral stage: The primary oocytes grow although they are arrested in meiosis I. The follicular cells grow and proliferate to form a stratified cuboidal epithelium. The calls are called granulosa cells and secrete glycoproteins to form the zona pellucida around the primary oocyte cells. The surrounding connective tissue cells differentiate to become the theca folliculin which responses to LH and can secrete androgens under the hormone’s influence.
Antral stage: Fluid-filled spaces form between granulosa cells. Eventually, these spaces combine to form the antrum which is a central fluid-filled space. The follicles are now known as secondary follicles. During the menstrual cycle each month, one of these secondary follicles becomes dominant and develops further under the influence of oestrogen, LH and FSH.
Pre-ovulatory stage: This stage is induced by a surge in LH and meiosis I is completed. Two haploid cells of different sizes are formed in the follicle. One daughter cell receives less cytoplasm and develops into a polar body. The other haploid cell is called the secondary oocyte. Both cells undergo meiosis II and the polar body will be replicated to produce two polar bodies whilst the growth of the secondary oocyte is arrested in the metaphase of meiosis II approximately 3 hours before ovulation.
In this stage, the follicle is fully grown and mature and it is called a Graafian follicle. A surge in LH increases collagenase activity which weakens the follicular wall. Furthermore, muscular contractions of the ovarian wall result in the release of ovum from the ovary and travel to the fallopian tube.
The secondary oocyte will only complete meiosis II on fertilisation producing a third polar body and fertilised egg. If fertilisation does not occur, the oocyte degenerates 24 hours after ovulation.
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1. What is capacitation?
A sperm cell undergoes capacitation once it enters the female reproductive tract. It is a process in which cholesterol and glycoproteins are removed from the sperm cell head which allows the cell to bind to the zona pellucida of the egg cell or ovum. It is the penultimate step in the maturation of mammalian spermatozoa and is important to render the sperm cell to be competent to fertilise an oocyte or egg. Capacitation is a biochemical event and prior to this the sperm cell is mature and shows normal motility.
2. What role do hormones play in gametogenesis?
Generally there are three hormones in which have a direct role in gametogenesis. These are
Gonadotropin-releasing hormone or GnRH: GnRH is a small peptide that travels via blood circulation and affects the cells in the anterior pituitary. It regulates the secretion of the Follicle Stimulating Hormone and the Luteinizing Hormone from this part of the hypothalamus.
FSH or Follicle Stimulating hormone: This hormone acts on gamete-producing cells to regulate or control gametogenesis.
LH or Luteinizing hormone: This hormone acts on the hormone-producing cells or the endocrine system by stimulating the release of steroid sex hormones.