Reproduction in Mammals

1. The male produces sperms (also called spermatozoa).
2. The female produces ova (also called eggs).

Spermatozoa and ova differ in both structure and size, with sperm being smaller and motile, while ova are larger and immobile.

Gonads are specialized reproductive organs responsible for the production of gametes. The process of gamete production is called gametogenesis.

1. Sperms are produced in the male testes (singular: testis).
2. Ova are produced in the female ovaries.

Fertilization refers to the process by which male and female gametes fuse. Fertilization can be:

1. Internal Fertilization: Occurs inside the body of the animal.
2. External Fertilization: Takes place outside the body of the animal (this is more common in aquatic species).

Humans and other higher mammals have specialized organs for reproduction, which form a system known as the reproductive system. The male and female reproductive systems are distinct, with specialized structures designed to support the functions of reproduction.

The human male reproductive system is composed of several organs: the testes, epididymis, vas deferens, prostate gland, Cowper's gland, seminal vesicle, urethra and penis.

a. Front view b. Side view

Testes

The testes (or testicles) are the main male reproductive organs. They are enclosed in a sac-like pouch called the scrotal sac or scrotum. They are oval in shape. They are found between the thighs, behind and below the penis. Testes are organs where sperms and male sex hormones are produced.

Scrotum (scrotal sac)

It is a poach like sac of skin that holds testis together. It hangs behind the penis. It is well supplied with nerves and blood vessels.

It protects testes from mechanical injury. Being outside it makes the temperature of the scrotum to be lower by about 2–3°C compared to body temperature. This temperature is suitable for sperm production and storage.

Seminiferous tubules

These are small coiled tubules inside the testes. This is a place where sperms are produced. They contain interstitial cells which produce testosterone hormone.

Vas efferentia

These are much smaller tubes that collect sperm from inside the testis and transfer them to the epididymis.

Epididymis

Is a long coiled tube located at the back of each testicle. The epididymis stores the sperms produced in seminiferous tubule.

Vas deferens

This is long muscular tube that transports sperms from testes to urethra. It is also called the sperm duct. Vas deferens has a narrow lumen and muscular walls which contract during ejaculation, propeling sperms towards the urethra.

Penis

Penis is a copulatory organ which is used to introduce sperms into the vagina during sexual intercourse or copulation. The tissues of the penis are highly supplied with blood vessels. The penis has spongy tissues called erectile tissues in which the blood accumulates. When the spongy tissue is filled with blood, the penis becomes firm and erect. This enables the penis to penetrate into the vagina. The head of the penis is known as the glans which is very sensitive to stimulation. The glans is protected by the foreskin which is called prepuce. The foreskin is usually removed during circumcision to reduce the risk of STIs and STDs infection as well as for hygienic purposes. During ejaculation, urethra muscles contract. Contraction of urethra muscles propels semen out of the erect penis. Semen is a fluid made up of sperm and fluid from various glands in the male reproductive system.

The female reproductive system consists of several organs: the ovaries, fallopian tubes, uterus, cervix and vagina.

a. Front view b. Side view

Ovaries

The ovaries are the main female reproductive organs. They are oval-shaped cream-coloured organs found in the lower part of the abdominal cavity. There is one ovary on either side of the uterus. Ovaries are attached to the uterus by a membrane. They produce the female gamete called the egg or ovum and female sex hormones which are oestrogen and progesterone. Oestrogen is important in repair of the uterine lining after menstruation. Progesterone is involved in maintaining the thickness of the uterine wall ready for implantation. Each ovary has two layers, an outer cortex and an inner medulla. The medulla has blood capillaries, nerve fibres and smooth muscles. The cortex has follicles which give rise to ova.

Each follicle consists of an immature ovum surrounded by epithelial cells.

Fallopian tube (oviduct)

This is the tube through which the ovum moves from the ovary to the uterus.

The oviduct is funnel-shaped at the end adjacent to the ovary to enable the released ovum to be directed towards the uterus.

Normally fertilisation takes place in the fallopian tube. The walls of the oviduct have cilia that assist in the movement of the ovum towards the uterus.

Uterus

This is a thick-walled muscular organ in which the embryo implant grows, develops, and obtains its nourishment during pregnancy. The outer layer of the uterus is called myometrium and the inner layer is called endometrium. After fertilisation the embryo is implanted in the uterus wall and develops until birth.

The uterus has elastic tissues that allow its expansion during development of foetus and can attain its original size after delivery. It is around 7.5 cm in length and 5 cm in width but it can enlarge four to five times of this size in order to accommodate the fully grown foetus.

Cervix

The cervix is a ring of muscles that closes the lower end of the uterus. Cervix separates the uterus from the vagina.

It allows sperm to pass through during copulation. It also produces mucus, which helps the sperm to swim through the uterus. The cervix is narrow to ensure that the embryo is not expelled from the uterus before the right time. During birth, the cervix expands to allow the baby to pass through into the vagina. The cervix also allows the menstrual blood to flow out of the uterus.

Vagina

This is a muscular tubular structure that links the uterus to the outside of the body.

Vagina is also called copulatory canal or birth canal. During sexual intercourse, the sperms are deposited into the vagina. Then they pass through the cervix into the uterus and up into the oviduct where fertilisation takes place. The vagina has elastic walls that stretch during sexual intercourse and during birth. The opening of the vagina is surrounded by flaps of tissue called labia which protect the vaginal opening. There are two sets namely labia majora and labia minora. The main functions of the vagina are to allow the baby to pass during birth, to pass out menstrual blood and receive male organ during sexual intercourse.

Vulva

This is the external opening of the female genitalia that surrounds the opening to the vagina. Its walls contain vestibule glands, which release mucus when the female is sexually aroused, the mucus reduces friction during sexual intercourse.