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Sexual Reproduction in Humans - The First Stages
Like in plants it is the male gamete that needs to be transferred to the female gamete. The female gamete is fertilised and develops inside the mother's body so the reproductive systems of both males and females are highly adapted for this.
Production of sperm is called spermatogenesis.
It takes place in the gonads of the male - the testes. Over 100 million can be made in one day!
Each testis is composed of numerous tiny tubes called seminiferous tubules. It is in the walls of these tubules that sperm production actually takes place.
Development begins in the outer side of the wall in a layer of cells called the germinal epithelium. As the immature sperm cells become more mature they move to the inner side and break way into the lumen of the tubule to be carried away to the epididymis for storage. The process of this production is shown in the next two diagrams.
The Sertoli cells are present to provide nourishment to the developing gametes and to aid with the development of the specialised shape of the sperm cells.
At the end of this production line, mature sperm look like this:
In between the tubules, inside the testes, are interstitial cells called Leydig cells. These secrete the hormone testosterone.
There are also blood vessels in close proximity, delivering nutrients and carrying away some testosterone to other target cells for the development and maintenance of secondary sexual characteristics, e.g. facial and pubic hair, deepening of the voice. The testosterone also stimulates the cells inside the testis involved in spermatogenesis.
The control centres are the pituitary gland and the hypothalamus in the brain.
The hypothalamus secretes GnRH (gonadotrophin releasing hormone). This is released into the blood and stimulates the anterior lobe of the pituitary gland.
The anterior lobe of the pituitary gland secretes ICSH (interstitial cell stimulating hormone).
ICSH: this stimulates the leydig cells that produce testosterone.
FSH: this stimulates the seminiferous tubules, including the Sertoli cells. They produce sperm in response.
Note: Testosterone also acts on the seminiferous tubules and stimulates sperm production.
The testosterone feeds back to the hypothalamus and pituitary gland to switch off GnRH and ICSH release.
The Sertoli cells produce a hormone called inhibin that feeds back to the pituitary gland to switch off FSH release.
Since the action of the interstitial cells and Sertoli cells are inhibited, less testosterone and inhibin are released as a result. The inhibition of the hypothalamus and pituitary is lifted and the process can start again. Due to the levels of the hormones and their effects, the process is not noticeably cyclical - there aren't noticeable peaks and troughs in the levels of the hormones.
The production of eggs is called oogenesis. It takes place in the ovaries and begins before birth.
The outer layer of the ovary (the germinal epithelium) produces primary oocytes. It also produces follicle cells that congregate around the oocytes, forming a structure called the primary follicle.
By the time a baby girl is born, the primary oocytes in the primary follicles have started the first meiotic division but the process halts at the first stage (prophase I).
After puberty one of these develops each month. It completes meiosis I to form a secondary oocyte and first polar body (the latter of which will eventually disintegrate). The follicle cells around it proliferate to form a wall many cells thick called the theca. Fluid collects inside the structure to form a fluid-filled cavity.
The whole structure is called a Graafian follicle.
At a time controlled by hormones the secondary oocyte is released from the Graafian follicle and it leaves the ovary - a process called ovulation. The secondary oocyte with some surrounding follicle cells leaves the ovary and enters one of the oviducts. What is left behind on the surface of the ovary turns into a structure called the corpus luteum.
As with males, the process of egg production is controlled by 2 centres in the brain - the hypothalamus and the anterior lobe of the pituitary gland. Unlike males, however, the process is cyclical with rises and falls in hormone levels. The cycle, called the menstrual cycle - takes about 28 days, with ovulation occurring in the middle at about day 14.
- The hypothalamus releases GnRH: this stimulates the anterior lobe of the pituitary gland.
- The pituitary gland releases FSH: this stimulates the development of follicles within the ovary.
- The follicle secretes oestrogen: this stimulates the repair of the uterus wall. It also inhibits the further release of FSH so no new follicles develop in case pregnancy results. It also inhibits the release of LH from the pituitary gland until shortly before ovulation. Then the level of oestrogen rises which actually stimulates the release of LH and FSH.
- A surge of LH: this causes ovulation and the development of the corpus luteum. LH feeds back to inhibit oestrogen release so, as a result, the levels of LH and FSH begin to fall.
- The corpus luteum secretes progesterone: this stimulates the thickening and the vascularisation of the uterus wall in preparation for pregnancy...
- If no pregnancy occurs: the corpus luteum degenerates, progesterone is no longer released and the lining of the uterus breaks down. The discharge of the wall is called menstruation. FSH release is no longer inhibited so the cycle can begin again and a new follicle can develop.
- If pregnancy occurs: the corpus luteum persists due to a hormone (CG - chorionic gonadotrophin) being released.