Human Reproductive System

Humans reproduce sexually through intercourse between biological parents, but what are the structures, processes and physiological reactions that allow this to be possible? Let’s find out more about the human reproductive system.

Female reproductive system

While the following structures of the female reproductive system serve different purposes, together, they contribute to the creation and maintenance of an egg:

Fig. 1 - Structure of the female reproductive system

• Ovaries – The ovaries are the female gonads – the primary organ in the female reproductive system. It is mainly responsible for producing egg cells, or ova, as well as hormones such as oestrogen and progesterone. These hormones are responsible for controlling various reproductive processes in females and thickening and maintaining the uterus lining.

• Fimbriae – The fimbriae are tissues attached to the ovaries that transfer the oocytes (immature eggs) from the ovaries to the fallopian tubes, or oviduct.

• Fallopian tubes (oviduct) – The fallopian tubes are a pair of tubes that stretch from the ovaries to the uterus. They are responsible for transporting the oocytes to the uterus. Fertilisation normally occurs here.

• Uterus – The uterus is a muscular, pear-shaped organ where the fertilised egg will be implanted and developed.

• Cervix – The cervix is the lower part of the uterus, a canal between the uterus and the vagina through which the foetus (developing baby) will exit during childbirth.

• Vagina – The vagina is a passage leading to the uterus which accommodates the penis during sexual intercourse.

Oogenesis

It occurs in the outer layers of the ovaries. Fig. 2 provides an overview of oogenesis.

Fig. 2 - The process of oogenesis

1. Germ cells, which are the body’s reproductive cells, undergo cell division several times by mitosis, forming two oogonia. Oogonia are immature reproductive cells containing two complete sets of chromosomes, one from each parent, i.e., they are diploid.

   Remember that a diploid cell has two sets of chromosomes with two chromatids each. While chromatids are twins (they contain the same genetic information), paired chromosomes will code for the same traits but might have different alleles for those traits. If you need a refresher, check our article on Mitosis.

2. One oogonium continues to grow into a primary oocyte, an immature egg.
3. The cell (the primary oocyte) then undergoes division by meiosis (1^st meiotic division), forming a secondary oocyte (if you need a refresher, check out our article Meiosis). This oocyte is a haploid cell containing only half of the original cell’s chromosome. This is now the female set of chromosomes for the zygote. Additionally, a small cell buds off the oocyte called a polar body.

   Polar bodies are also haploid cells but are generally unable to be fertilised. It usually dies by apoptosis.

4. The secondary oocyte undergoes meiosis (2^nd meiotic division), forming an ovum (egg cell) and another polar body. The polar bodies continue to divide and degenerate as the ovum develops. The maturation of the oocyte to form an ovum is arrested, or ‘paused’, in prophase until ovulation (after puberty). Then, it is allowed to continue until metaphase II, at which stage meiotic division is arrested again until the sperm fertilise the egg.

Male reproductive system

While the following structures of the male reproductive system serve different purposes, together, they contribute to the creation and storage of sperm:

Fig. 3 - Structure of the male reproductive system

• Penis – The penis delivers sperm to the vagina during sexual intercourse.

• Testes – The testes are the male gonads – the primary sex organ in the male reproductive system. They produce sperm and hormones, such as testosterone. Testosterone is crucial for the production of sperm in males.

• Scrotum – The scrotum is a pouch-like skin sac containing the testes.

• Epididymis – The epididymis is responsible for storing sperm created in the testes.

• Vas deferens – The vas deferens are long tubes, or ducts, that connect the epididymis and the ejaculatory ducts. The ejaculatory ducts receive sperm from the vas deferens and deliver it into the urethra, where it exits the male body.

Spermatogenesis

Fig. 4 - The process of spermatogenesis

1. Germ cells undergo cell division several times through mitosis to form spermatogonia.
2. The spermatogonia grow and divide by mitosis to form primary spermatocytes.
3. Primary spermatocytes undergo meiosis to form secondary spermatocytes.
4. The secondary spermatocytes undergo meiosis for the second time, forming spermatids, which are haploid sex cells – I.e., the cells contain one complete set of chromosomes.
5. The spermatids differentiate (mature) to become spermatozoa (sperm cells).

Process and Stages in human reproduction

Sexual reproduction involves various stages and human fertilization is one of those steps.

Fig. 5 - Acrosome reaction

1. Process of Reproduction
2. Sperm cells surround the egg cell. One sperm cell binds to special sperm-binding proteins on the outer layer of the egg. The egg’s outer layer is called the zona pellucida (jelly coat). Once a sperm cell binds to the zona pellucida, it releases digestive enzymes from its acrosome (see Fig. 5).The acrosome is an organelle that covers the head of the sperm cell and contains digestive enzymes.
3. The acrosome reaction follows. This reaction occurs when the digestive enzymes break down and digest a tunnel through the zona pellucida so that the sperm can reach the egg’s membrane. See Fig. 5 below for a diagram of the acrosome reaction.
4. The membranes of both sperm and egg fuse, and the sperm nuclei enter the cytoplasm of the egg.
5. The cortical reaction follows. The release of calcium ions (triggered by the fusion of sperm and egg nuclei) stimulates some vesicles to release cortical granules into the zona pellucida. These cortical granules harden the zona pellucida and remove sperm binding proteins, preventing multiple sperm from penetrating and fertilising the egg, called polyspermy. It ensures that the zygote formed is a diploid, receiving half of its chromosome from the sperm cell and half from the egg.

Implantation

The newly formed zygote undergoes cleavage; this is a process of division by mitosis without the interphase step of the cell cycle. It divides several times to form a ball of cells called a blastocyst. This blastocyst implants itself into the lining of the uterus, where it can absorb nutrients from the mother and start the development of the foetus.