The Human Reprodcutive System

Female reproductive system

There are several organs and tissues that make up the female reproductive system, these include that ovaries, fallopian tubes, uterus, vagina, vulva, mammary glands and breasts. They are also involved in producing and transporting gametes and in the production of sex hormones. The female reproductive system is also responsible for fertilizing the ova by the sperm cells and supporting the development of the offspring during the process of pregnancy and infancy.

The ovaries are located in the abdominal cavity, they are held in place by ligaments. The surface of the ovaries is covered with germinal epithelial tissue which consists of cells called Oogonia. These cells produce ovum. Oogonia cells undergo mitosis to produce primary oocytes, diploid cells, and produce follicle cells which are responsible for protecting and nourishing the egg before it is released into the oviducts during ovulation.

The funnel of the fallopian tube is located near to the ovary. This funnel is lined with fimbrae, finger like structures which, when activated by hormones, catch a released egg and move it down into the fallopian tube. The fimbrae is lined with cilia.

Secondary oocytes (the oocyte that is created by the first meiosis division) are pushed along the fallopian tube by the cilia and the muscles of the fallopian tubes. These fallopian tubes lead to the uterus and join the uterus at a point called the uterine horn.

Myometrium is the name of the smooth muscle that makes up the uterus. The uterus is lined with the endometrium which is a tissue with high blood supply. The Blastocyst will embed into this tissue.

The bottom end of the uterus has an opening which is called the cervix. The cervix leads into the vagina which allows the cervix to be connected to the outer environment through the vulva.

The vulva is made up of several folds of skin which are called the labia. There are two inner folds called the labia minora and two outer folds called the labia majora.

The clitoris is also a part of the female reproductive system, it is a small body of erectile tissue which is enclosed within the labia. It is a highly sensitive tissue which swells with blood during sexual stimulation.

Male reproductive system

The male reproductive system is responsible for the production, maintenance and transportation of sperm and protective fluids. The sperm and protective fluids are to be discharged within the female reproductive tract during sexual intercourse.

There are three important glands within the male reproductive system which have ducts joining the urethra. These glands are called the seminal vesicles, prostate gland and the Cowper's gland. These glands are responsible for nourishing the sperm and raising it's pH, they do this by secreting fluids. The purpose of making the sperm alkaline is to neutralise the acidic conditions in the urethra and the vagina. If the sperm had a more acidic pH the chances of conception could be impaired.

Each testis is broken up into several different compartments called lobules which contain the seminiferous tubules. These seminiferous tubules are lined with a layer of cells which are called the germinal epithelial. Inside the seminiferous tubules are cells called spermatogonia which produce sperm. These spermatogonia cells undergo mitosis in order to produce primary spermatocytes (diploid cells).

The seminiferous tubules connect together in order to form a network of tubules, this network is called the vas efferentia. These then connect and form the epididymis which is a long tube and lies outside of the testis.

The epididymis connects to the vas deferens which leaves the scrotal sac and then joins the urethra. Sperm is stored in these sections of the testis until ejaculation occurs. During the process of ejaculation, sperm and fluids are released into the urethra are released from the end of the penis. This mixture of sperm and fluids is called semen.

Reproductive processes

Gamete production

Gamete production is known as gametogenesis, this happens in the gonads. Sex cells are developed in the testes and ovaries during gametogenesis through the process of meiosis. During gametogenesis meiosis occurs to produce gametes which are haploid. This is done in order to produce a diploid offspring during fertilisation.

Spermatogenesis

Spermatogenesis is the process of forming sperm cells in the testes. This process takes place in the seminiferous tubules and begins when a human male reaches puberty. Spermatogonia cells undergo mitosis to produce primary spermatocytes. These primary spermatocytes then divide via meiosis in order to form secondary spermatocytes which develop into spermatids.

Spermatids contain the correct number of chromosomes in order to be classified as gametes but they lack the structure of a sperm cell. They do not have certain organelles like the flagellum which can help them swim to the ovum.

In order for the spermatids to develop into mature sperm cells they need to be nourished and protected. There are Sertoli cells present in the wall of the seminiferous tubules which secrete a fluid that nourishes and protects the spermatids.

Testosterone stimulates the Sertoli cells, testosterone is released by a cell near the seminiferous tubules called the Leydig cells.

Hormonal control of Spermatogenesis

The hypothalamus releases a hormone called the Gonadotropin-releasing hormone (GnRH) which stimulates the anterior pituitary gland to release Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH).

The FSH travels to the testis to the spermatogenic cells in the seminiferous tubules which will stimulate spermatogenesis.

The LH is going to travel to the interstitial cells which will then release testosterone. In certain target tissues testosterone will be converted into dihydrotestosterone (DHT). DHT and testosterone will bind to certain target receptors called androgen receptors in specific target tissues and be responsible for secondary sexual characteristics such as facial hair in males.

Testosterone and FSH also travel to the Sertoli cells which will release androgen binding protein (ABP). The effect of ABP is the exertion of tropic effects on spermatogenesis.

When spermatogenesis will reach a certain level the Sertoli cells will release inhibin which inhibits the FSH release. Testosterone will also inhibit at the level of the anterior pituitary gland and the hypothalamus.

Oogenesis

Oogenesis is the process of forming ova in the ovaries. It begins before birth while the female is a foetus. Oognoia divide in order to form primary oocytes.

Hormonal control of Oogenesis

Cells which are located within the germinal epithelium divide in order to form follicle cells. These follicle cells then surround primary oocytes to form primary follicles. Meiosis then begins in the primary oocytes but stops at prophase 1.

During puberty FSH is produced by the pituitary gland and stimulates primary follicle cells to develop. A few follicles will begin to develop each month but usually only one of them would mature to form a Graafian follicle. FSH also stimulates the formation of oestrogens.

Inside of the Graafian follicle the primary oocyte completes the first meiotic division in order to create primary oocytes and a polar body. The follicle cells which surround the secondary oocyte grow and several fluid filled spaces form.

The Graafian follicle matures and travels to the surface of the ovary. LH instigates the rupture of the mature Graafian follicle which allows secondary oocytes to be released, this process is called ovulation.

The menstrual cycle

The menstrual cycle is a period of time which starts on the day that a woman’s period begins until the day prior to the next period. The menstrual cycle begins during puberty from the age of 10 and ends when the woman reaches the age of 55. This is because of menopause, menopause is caused by the decline of oestrogen levels which prevents the regulation of the menstrual cycle.

The menstrual cycle is divided into multiple phases: The menstrual phase, the follicular phase, the ovulation phase and the luteal phase.

Menstrual phase

This phase begins on the first day of menstruation and lasts until the fifth day of the menstrual cycle. During this phase of the cycle the uterus sheds its inner lining of soft tissue and blood vessels which exits the vagina in the form of menstrual fluid. It is common for females to experience abdominal cramps which are caused by contractions of the uterine and abdominal muscles. These muscles contract in order to get rid of menstrual fluid.

Follicular phase

This phase begins at the same time as the menstrual phase, but it lasts until the thirteenth day of the menstrual cycle. During this phase the pituitary gland secretes a hormone called FSH which stimulates the eggs in the ovaries to grow. One of those egg cells will mature inside of a follicle, it takes an egg 13 days to reach maturity. While the egg cell is maturing a hormone called FH stimulates the uterus to form an endometrium (lining of blood vessels and soft tissue).

Ovulation phase

This phase begins on the fourteenth day of the cycle. During this phase the pituitary gland releases LH which causes the ovary to release the mature egg cell. The released egg is swept into the fallopian tube by the cilia of the fimbriae.

Luteal phase

This phase begins on the fifteenth day and lasts until the end of the menstrual cycle. During this phase the egg cell which was released during the ovulation phase is kept in the fallopian tube for 24 hours. If the egg cell isn’t penetrated by a sperm cell within that time, the egg cell will be destroyed. At the end of this phase the LH gets used up and the next stage of the cycle continues.