Reproduction notes

Reproduction and Gametes

Reproduction may be asexual or sexual. Sexual reproduction involves the formation of male and female gametes by meiosis and their fusion during fertilisation. The male gamete is usually small, numerous and motile, while the female gamete is usually large, few in number and non-motile.

In humans, the sperm is a flagellated, motile gamete and the ovum is a non-motile gamete. The fusion of two haploid gametes produces a diploid zygote, which develops into a new individual.

• Spermatogenesis is the process by which spermatozoa are produced.
• Oogenesis is the process by which ova or egg cells are formed.
• A haploid gamete contains one set of chromosomes.
• A zygote is formed when two haploid gametes fuse during fertilisation.
• Oogamy is sexual reproduction in which a small, flagellated, motile sperm fertilises a large, non-motile egg.
• Male gametes are generally many, minute and motile; female gametes are generally few, large and non-motile.

Male Reproductive System

The male reproductive system consists of a pair of testes, ducts, accessory glands and external genitalia. The testes are present in the scrotum, outside the abdominal cavity. The lower temperature of the scrotum supports normal sperm formation.

Each testis contains many seminiferous tubules. Spermatozoa are produced in these tubules. Between the seminiferous tubules are interstitial cells, also called Leydig cells, which secrete testosterone.

Sperm pass from the seminiferous tubules into the vasa efferentia, then into the epididymis. They are temporarily stored and mature in the epididymis. From there, the vas deferens carries them towards the urethra.

• Testes produce spermatozoa and secrete the male sex hormone testosterone.
• Seminiferous tubules are the site of sperm production.
• Interstitial cells between seminiferous tubules secrete testosterone.
• Vasa efferentia transport sperm from the testes to the epididymis.
• The epididymis is the site of sperm maturation and temporary storage.
• The vas deferens carries sperm from the epididymis towards the urethra.
• The urethra is the tube that passes from the urinary bladder through the penis.
• The external male genitalia include the penis, which acts as the male copulatory organ, and the scrotum.
• The cervix is not a part of the male reproductive system.

Functions of Male Reproductive Structures and Hormones

Accessory glands add fluids to the sperm. These fluids provide a medium for sperm movement and contribute to semen formation. Sertoli cells are present in the seminiferous tubules and support developing sperm cells.

Testosterone is secreted by interstitial cells under hormonal control. It supports the development and maintenance of male reproductive organs and secondary male characteristics.

• Semen consists of spermatozoa mixed with secretions from accessory glands.
• Sertoli cells nourish and support developing sperm cells.
• Fluid associated with Sertoli cells helps provide a suitable medium for developing and transporting sperm.
• Testosterone is needed for the development of secondary male characteristics.
• Examples of secondary male characteristics include a deeper voice, growth of facial and body hair, increased muscle development and broadening of shoulders.
• Testosterone also supports sperm formation and the functioning of male reproductive organs.

Female Reproductive System

The female reproductive system consists of ovaries, oviducts, uterus, cervix, vagina and external genital structures. The ovaries produce ova and secrete the female sex hormones estrogen and progesterone.

Each oviduct, also called a fallopian tube or uterine tube, receives the ovulated oocyte. Its funnel-shaped end has finger-like projections called fimbriae. These projections help sweep the oocyte into the oviduct.

Fertilisation generally occurs in the oviduct, commonly in its wider part called the ampulla. The uterus receives and supports the developing embryo. Its lower narrow region is the cervix, which opens into the vagina.

• Ovaries produce ova and secrete estrogen and progesterone.
• The oviduct is also called the fallopian tube or uterine tube.
• Fimbriae at the end of the oviduct help receive the ovulated oocyte.
• The oviduct receives the egg released from the ovary during ovulation.
• The oviduct is the usual site of fertilisation in humans.
• The uterus is the organ in which the embryo implants and develops.
• The cervix is the lower, narrow portion of the uterus.
• The vagina receives the penis during copulation and forms the birth canal.

Menstrual Cycle and Ovulation

The menstrual cycle is a regular series of changes in the ovaries and uterus. In a typical cycle, an ovarian follicle develops and releases an oocyte. The cycle is controlled by hormones from the pituitary gland and ovaries.

Menstruation is the discharge of blood, mucus and cellular debris from the uterus. It occurs when fertilisation and implantation do not take place. The proliferative phase follows menstruation and is mainly controlled by estrogen, which stimulates repair and growth of the uterine lining.

Ovulation is the release of an oocyte from a mature ovarian follicle. The remaining follicle changes into the corpus luteum, which secretes progesterone. Progesterone maintains the uterine lining for possible implantation.

• Menstruation is the discharge of blood and cell debris from the uterus.
• The proliferative phase of the menstrual cycle is mainly controlled by estrogen.
• Ovulation is the release of an oocyte from an ovary.
• The ovaries may release an egg from alternate ovaries in successive cycles, although strict alternation is not guaranteed in every person.
• The fertile period is commonly given as days 9 to 15 of the menstrual cycle.
• The corpus luteum is formed from the Graafian follicle after ovulation.
• The corpus luteum produces progesterone.
• If fertilisation does not occur, the corpus luteum degenerates and progesterone levels fall.
• A fall in progesterone contributes to breakdown of the uterine lining and menstruation.

Fertilisation, Development and Birth

During fertilisation, a sperm penetrates the egg and the nuclei of the two gametes fuse. The resulting zygote is diploid. It undergoes repeated mitotic divisions and forms an embryo.

The embryo implants in the uterine wall. The placenta develops at the maternal and embryonic interface and allows exchange of materials between mother and embryo. Pregnancy continues as the embryo develops into a foetus.

Development may include direct development or indirect development. In indirect development, the young stage differs greatly from the adult and changes through a process called metamorphosis.

• Fertilisation is the fusion of male and female gametes.
• The zygote is the first diploid cell of a new individual.
• A flagellated sperm fertilising a non-motile egg is an example of oogamy.
• Metamorphosis is the series of changes by which a larva changes into an adult after embryonic development.
• An organism or condition with two heads and one trunk is called Siamese twins or conjoined twins.
• The placenta performs functions related to exchange of nutrients, gases and wastes between mother and foetus.
• Implantation is the attachment of the early embryo to the lining of the uterus.

Reproduction in Plants and Seed Formation

The flower is the reproductive structure of flowering plants. It bears stamens, which produce pollen grains, and carpels, which contain ovules. Pollination is followed by pollen tube growth and fertilisation.

The pollen tube enters the ovule through the micropyle. At the micropylar end of the embryo sac are the egg cell and two synergids. The filiform apparatus in the synergids helps guide the pollen tube into a synergid.

Meiosis produces haploid spores and gametes in plants. Because one meiotic division produces four products, the number of meiotic divisions can be calculated from the number of required products, when each division is counted as producing four products.

• The flower is the reproductive part of an angiosperm and is essential for continuation of the species.
• Stamens are the male reproductive organs of a flower.
• Carpels contain the ovary, style and stigma; ovules are present inside the ovary.
• The pollen tube carries male gametes towards the ovule.
• The filiform apparatus helps in the entry of the pollen tube into a synergid.
• One meiotic division produces four haploid products.
• To produce 200 seeds, 50 meiotic divisions are needed if each division directly produces four required products; however, in seed-production calculations involving both male and female meiotic requirements, the exact total depends on the stated pathway.
• For pea seed calculations stated as 200 seeds requiring 250 meiotic divisions, the corresponding 400 seeds require 500 meiotic divisions.

Sexually Transmitted Diseases

Sexually transmitted diseases, or STDs, are infections commonly spread through sexual contact. They may be caused by bacteria, viruses or other infectious agents. Some infections can also pass through contaminated blood, shared needles or from an infected mother to her child.

STDs may affect the reproductive organs and can cause infertility, pregnancy complications and serious disease. Some infections show few or no symptoms, so prevention and medical testing are important.

• Gonorrhoea is a bacterial disease that may cause painful urination and discharge from the reproductive tract.
• Syphilis is a bacterial disease that can progress through several stages and damage body organs if untreated.
• Chlamydia is a bacterial infection that may cause reproductive tract inflammation and infertility if untreated.
• AIDS is caused by human immunodeficiency virus, or HIV, which attacks the immune system.
• Hepatitis B can be transmitted sexually and may damage the liver.
• STDs can be reduced by mutual faithfulness, avoidance of unsafe sexual contact, screened blood and not sharing needles.
• Antibiotics may treat bacterial STDs, but they do not cure viral infections such as HIV.
• Early diagnosis and treatment help reduce complications and transmission.