Sexual reproduction in humans Revision Notes

The testes are oval-shaped organs located within the scrotum. Their primary function is to produce sperm cells, the male gametes necessary for fertilisation. Additionally, the testes secrete testosterone, the hormone responsible for developing male secondary sexual characteristics such as facial hair and a deeper voice.

Sperm production occurs in the seminiferous tubules inside the testes through a process called spermatogenesis. This process requires a temperature slightly lower than the body's core temperature, which is why the testes are positioned outside the abdominal cavity.

• Produce sperm cells through spermatogenesis.
• Secrete testosterone to regulate male characteristics.

The scrotum is a pouch of skin that holds and protects the testes. Its main role is to maintain the testes at an optimal temperature, about 2–3°C below normal body temperature, which is essential for efficient sperm production.

Muscles in the scrotum contract or relax to move the testes closer to or further from the body, adjusting temperature. For example, in cold conditions, the muscles contract to reduce heat loss by bringing the testes closer to the body.

• Houses the testes outside the body.
• Regulates temperature to optimise sperm production.

Once sperm are produced in the testes, they travel through the sperm ducts, also called vas deferens. These muscular tubes transport sperm from the testes to the urethra during ejaculation.

The sperm ducts also serve as storage sites for mature sperm before ejaculation. Their muscular walls contract rhythmically to propel sperm forward.

• Carry sperm from testes to urethra.
• Store mature sperm prior to ejaculation.

The prostate gland surrounds the urethra just below the bladder. It produces a slightly alkaline fluid that forms part of the semen, the medium that carries sperm.

This fluid nourishes sperm and neutralises the acidic environment of the female reproductive tract, increasing sperm survival and mobility.

• Secretes nourishing fluid to mix with sperm.
• Neutralises acidity to protect sperm.

The urethra is a tube running through the penis that serves two functions: it carries urine from the bladder to the outside of the body, and it transports semen during ejaculation.

A valve mechanism prevents urine and semen from passing through the urethra simultaneously, ensuring effective sperm delivery during intercourse.

• Transports urine out of the body.
• Carries semen during ejaculation.

The penis is the external organ that delivers sperm into the female reproductive tract during sexual intercourse.

During sexual arousal, erectile tissue in the penis fills with blood, causing an erection that facilitates penetration and sperm transfer.

• Delivers sperm to female reproductive system.
• Becomes erect to enable intercourse.

The ovaries are a pair of small, oval-shaped organs located on either side of the uterus in the lower abdomen. Their primary function is to produce female gametes called eggs or ova. Each ovary contains many follicles, each housing an immature egg that matures during the menstrual cycle.

Besides producing eggs, the ovaries secrete important hormones such as oestrogen and progesterone. These hormones regulate the menstrual cycle and prepare the body for pregnancy. For example, oestrogen stimulates the thickening of the uterine lining, while progesterone maintains it after ovulation.

• Produce and release mature eggs approximately once every menstrual cycle.
• Secrete hormones that regulate the menstrual cycle and reproductive system.

The oviducts, also known as fallopian tubes, are narrow tubes that connect each ovary to the uterus. After an egg is released from an ovary during ovulation, it enters the oviduct.

The oviducts have ciliated epithelial cells lining their walls, which gently move the egg towards the uterus. Fertilisation typically occurs within the oviduct if sperm are present. The fertilised egg then continues its journey to the uterus for implantation.

If fertilisation does not occur, the egg disintegrates and is expelled during menstruation.

• Transport eggs from the ovary to the uterus using cilia and muscular contractions.
• Provide the site where sperm and egg meet for fertilisation.

The uterus is a hollow, muscular organ shaped like an inverted pear. It has a thick lining called the endometrium, which changes throughout the menstrual cycle.

If fertilisation occurs, the fertilised egg implants into the endometrium, where it receives nutrients and oxygen to develop into an embryo and later a fetus. The muscular walls of the uterus support the growing fetus and contract during childbirth to help deliver the baby.

If fertilisation does not occur, the endometrium sheds during menstruation.

• Supports implantation and development of the fertilised egg.
• Contracts during childbirth to expel the baby.

The cervix is the narrow, lower part of the uterus that opens into the vagina. It acts as a gateway controlling the passage between the uterus and vagina.

During most of the menstrual cycle, the cervix produces mucus that can either block or facilitate sperm entry depending on its consistency. During childbirth, the cervix dilates (widens) to allow the baby to pass through the birth canal.

The cervix also helps keep the uterus sterile by preventing bacteria from entering.

• Controls passage of sperm into the uterus and menstrual blood out.
• Dilates during childbirth to allow baby’s passage.

The vagina is a muscular, elastic canal extending from the cervix to the outside of the body. It serves multiple functions in reproduction.

It receives the penis during sexual intercourse, allowing sperm to be deposited near the cervix. It also acts as the birth canal through which the baby is delivered during childbirth.

Additionally, the vagina provides an exit for menstrual blood and tissue during menstruation.

• Receives sperm during intercourse.
• Acts as the birth canal during delivery.
• Allows menstrual flow to leave the body.

Fertilisation is a fundamental biological process in sexual reproduction where two specialised cells, called gametes, combine to form a new organism. In humans, these gametes are the male sperm and the female egg cell (ovum). Each gamete carries half the number of chromosomes typical for the species, making them haploid cells. The fusion of these gametes restores the full chromosome number, producing a diploid cell called a zygote.

• Fertilisation occurs after sexual intercourse when sperm reach the egg.
• It is essential for producing offspring with genetic material from both parents.

The male gamete, sperm, is a small, motile cell designed to travel through the female reproductive tract to reach the egg. The female gamete, the egg cell, is larger and contains nutrients to support the early stages of development. Both gametes contain nuclei that hold genetic material in the form of chromosomes.

Each nucleus contains 23 chromosomes in humans, half the normal diploid number of 46. This haploid condition ensures that when the nuclei fuse, the resulting zygote has the correct chromosome number.

• Sperm cells are motile and adapted to reach the egg.
• Egg cells are larger and provide nutrients for the embryo.
• Both gametes have haploid nuclei with 23 chromosomes each.

Fertilisation begins when a sperm cell successfully penetrates the outer layers of the egg cell. Once a single sperm enters, the egg's membrane changes to prevent other sperm from entering, ensuring only one sperm fertilises the egg.

The nuclei of the sperm and egg then move towards each other and merge in a process called syngamy. This fusion combines the genetic material from both parents into a single nucleus, forming the zygote.

• Sperm penetrates the egg's outer layers.
• Egg membrane changes to block additional sperm.
• Nuclei of sperm and egg fuse to form a zygote nucleus.

The zygote is the first cell of a new individual, containing a complete set of chromosomes (46 in humans). It undergoes multiple rounds of cell division and differentiation to develop into an embryo.

This fusion of genetic material introduces genetic variation, as the offspring inherits a unique combination of genes from both parents. This variation is important for the survival and evolution of species.

• Zygote has a diploid nucleus with 46 chromosomes.
• It divides to form an embryo.
• Genetic variation arises from combining parental genes.

Consider a human sperm cell carrying 23 chromosomes meeting an egg cell also with 23 chromosomes. When fertilisation occurs, their nuclei merge, resulting in a zygote with 46 chromosomes. This zygote will develop into a baby with traits inherited from both parents.

This process ensures that each generation maintains the species' chromosome number and genetic diversity.

• 23 chromosomes from sperm + 23 from egg = 46 in zygote.
• Zygote develops into a genetically unique individual.

The menstrual cycle is a repeating series of changes in the female reproductive system that lasts about 28 days on average. It involves two main organs: the ovaries, where eggs develop and are released, and the uterus, where the lining changes to support a possible pregnancy.

The cycle prepares the body each month for fertilisation and pregnancy. If fertilisation does not occur, the cycle ends with menstruation, the shedding of the uterine lining, and then starts again.

• Average length: 28 days (can vary between individuals).
• Involves changes in ovaries and uterus.
• Prepares for pregnancy each month.

The cycle can be divided into four main phases: the follicular phase, ovulation, the luteal phase, and menstruation. Each phase involves specific changes in the ovaries and the uterine lining.

• Follicular phase: follicles in the ovary mature.
• Ovulation: release of a mature egg.
• Luteal phase: preparation of the uterine lining.
• Menstruation: shedding of the uterine lining if no fertilisation occurs.

During the follicular phase, several follicles (fluid-filled sacs containing immature eggs) begin to develop in the ovaries. Usually, only one follicle fully matures to release an egg.

Ovulation occurs around day 14, when the mature follicle releases the egg into the oviduct (fallopian tube).

After ovulation, the ruptured follicle transforms into a structure called the corpus luteum, which plays a role in maintaining the uterine lining during the luteal phase.

• Follicles mature during follicular phase.
• One follicle releases an egg at ovulation.
• Corpus luteum forms after ovulation.

At the start of the cycle, the uterine lining (endometrium) is thin following menstruation.

During the follicular phase, the lining thickens and becomes rich in blood vessels to prepare for a possible embryo implantation.

If fertilisation does not occur, the corpus luteum degenerates, causing the uterine lining to break down and be shed during menstruation.

• Uterine lining thickens after menstruation.
• Becomes rich in blood vessels to support embryo.
• Sheds if no fertilisation occurs (menstruation).

The follicular phase lasts about 14 days, during which follicles mature and the uterine lining thickens.

Ovulation happens around day 14, releasing the egg.

The luteal phase follows ovulation, lasting about 14 days, during which the uterine lining is maintained for possible implantation of a fertilised egg.

• Follicular phase: days 1–14.
• Ovulation: around day 14.
• Luteal phase: days 15–28.
• Menstruation marks the start of a new cycle.