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Human Reproductive System

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Biology

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:

  • 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

Oogenesis is the process that produces egg cells.

It occurs in the outer layers of the ovaries. Fig. 2 provides an overview 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 (1st 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 (2nd 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:

  • 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

Spermatogenesis is the process that produces sperm. It occurs in the testes, the male gonads.

  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.

Human fertilisation is the fusion of the nuclei of a sperm cell and an egg cell, forming a zygote, or fertilised egg.

  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.

Human Reproductive System - Key takeaways

    • The ovaries are the female gonads where egg cells are produced.

    • The testes are the male gonads where sperm cells are produced.

    • Oogenesis is the process of egg production.

    • Spermatogenesis is the process of sperm production.

    • Fertilisation is the fusion of the sperm and egg nuclei to create a zygote

Human Reproductive System

Sexual reproduction creates genetic diversity within a species. Genetic diversity is essential for the continuation of life, as it protects species from disease and changes in environments, as well as allowing for animals to adapt and evolve over time. 

Compared to asexual reproduction, sexual production is more time consuming – it takes time (and energy) to find a mate. Also, sexual reproduction is not always successful as some mates can be infertile. Another disadvantage is that fewer offspring are produced on average, when compared to asexual reproduction, which can produce an offspring whenever required. 

During ejaculation, sperm from the male is transferred into the female. The sperm cells then make their way to reach the egg cell in order to fertilise it. 

Sexual reproduction leads to genetic variation through mutations, crossing-over and independent assortment. 

Humans naturally reproduce via sexual reproduction, meaning that when a male ejaculates into a female, sperm cells travel up the vaginal canal, through the cervix, into the uterus. An egg descends from the one of the ovaries and meets the sperm cell. They then fuse, fertilising the egg and creating a zygote, which then grows within the woman until birth. The alternative is In-Vitro Fertilisation, or IVF, where the egg cells are removed from the woman. The egg is then incubated with the male's sperm cells to allow fertilisation to take place. If this does not occur, the sperm cell can be injected into the egg in a process known as intracytoplasmic sperm injection. The eggs are then incubated to confirm fertilisation, before being implanted into the mother. 

Final Human Reproductive System Quiz

Question

What cells are contained in the ovaries?

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Answer

Oocytes,  which mature to become egg cells or ova. 

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Question

The fimbriae connect what to what? 

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Answer

Ovary to fallopian tubes

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Question

What is the uterus?

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Answer

A muscular, pear-shaped organ, becomes known as the womb during pregnancy. 

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Question

What is the cervix?

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Answer

The lowering end of the uterus, containing the opening to the vaginal canal. 

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Question

What process creates gametes?

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Answer

Meiosis

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Question

What cell is created in the testicles? 

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Answer

Sperm

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Question

The vas deferens carry what to where? 

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Answer

Sperm from the epididymis to the ejaculatory ducts

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Question

What process forms egg cells? 

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Answer

Spermatogenesis

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Question

What process forms sperm cells

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Answer

Spermatogenesis

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Question

What is the outer layer of the egg cell that sperm cells bind to called?

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Answer

Zona Pellucida

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Question

The acrosome reaction does what?

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Answer

Creates a tunnel into the zona pellucida to allow sperm cells to reach the membrane, by using enzymes to digest the outer layers of the egg.  

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Question

What reaction occurs after fertilisation of the egg cell?

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Answer

The cortical reaction

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Question

What occurs during the cortical reaction?

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Answer

Calcium ions released by nuclear fusion stimulates the release of cortical granules into the zona pellucida. These harden the zona pellucida and remove binding proteins, preventing further sperm entry. 

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Question

A fertilised egg is known as?

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Answer

A zygote

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Question

The binding of the blastocyst to the uterus wall is known as?

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Answer

Implantation

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