Sexual reproduction in mammals

In a nutshell

Reproduction is a fundamental process for the survival of a species. Sexual reproduction in mammals involves the fusion of the nuclei from spermatozoa and ova. This summary will explain the process of sexual reproduction; from gametogenesis to embryo development.

Reproduction

Definition

Reproduction is a fundamental process for the survival of a species. It is the process of producing new individuals from an existing members (or member in asexual reproduction!) of the same species. Sexual reproduction forms genetically varied offspring that contain chromosomes from both parents. The gametes carry the genetic information from the parents. They contain half the normal chromosome number and will fuse together during fertilisation to create a zygote that contains the normal chromosome number.

Human reproductive system

Male

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Sexual reproduction in mammals — 1. Sperm duct, 2. Bladder, 3. Erectile tissue, 4. Penis, 5. Urethra, 6. Testes, 7. Glans, 8. Foreskin, 9. Ureter, 10. Seminal vesicle, 11. Prostate gland, 12. Scrotum, 13. Coiled tubes (epididymis)

Female

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Sexual reproduction in mammals — 1. Oviduct, 2. Ovary, 3. Uterus, 4. Ureter, 5. Cervix, 6. Rectum, 7. Vagina, 8. Bladder, 9. Urethra, 10. Clitoris, 11. Labia, 12. Vulva

Determining sex

The genetic material determines the sex of a mammal. The SRY gene is a gene on the Y chromosome and its presence means the zygote is a male. If zygote does not have a Y chromosome, the SRY gene will not be present and therefore, the zygote will be female.

Gametogenesis

Gamete production is known as gametogenesis and it is a process that occurs in the gonads.

Sex	Gonad	Gamete
Male	Testes	Spermatozoa (singular is spermatozoon or sperm cell).
Female	Ovaries	Ova (singular is ovum or egg).

Gametogenesis in males and females produces different gametes but the process is very similar.

Stage	Description	Spermatogenesis produces	Oogenesis produces
Proliferation	Cells multiply by mitosis in the germinal epithelium. These cells could all become gametes.	Diploid ( $2n$ ) spermatogonia.	Diploid ( $2n$ ) oogonia.
Growth	The cells grow.	Diploid ( $2n$ ) primary spermatocytes.	Diploid ( $2n$ ) primary oocytes.
Maturation	Meiosis occurs and forms haploid gametes.	The primary spermatocyte undergoes meiosis I to produce two haploid (n) secondary spermatocytes. The secondary spematocytes undergo meiosis II which produces four gametes.	The primary oocyte undergoes meiosis I to produce one haploid (n) secondary oocyte and a polar body. The polar body is a smaller haploid cell which is lost. The secondary oocyte will eventually undergo meiosis II to produce a larger ovum and a smaller polar body which is also lost.

Oogenesis

Oogenesis is the process through which the female gametes are produced. The process is described below.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Sexual reproduction in mammals

1.	The oogonia are formed by mitosis in the germinal epithelium.
2.	The oogonia grow and form oocytes.
3.	The primary follicle is formed which is the oocyte and a layer of follicle cells.
4.	During the menstrual cycle, a few primary follicles start to develop. They complete meiosis I and form a secondary oocyte. The first polar body is formed when the cytoplasm divides unequally. This is lost as it does not develop.
5.	The follicle develops into a Graafian follicle which is a fluid-filled sac and it contains the secondary oocyte. *Note:* The secondary oocyte is in prophase of meiosis II and does not develop further until fertilisation.
6.	Ovulation occurs and the follicle bursts which releases the egg cell. In an adult female, this occurs roughly every $28\ days$ .
7.	The remains of the follicle develop into an endocrine gland called the corpus luteum.
8.	If fertilisation does not occur, the corpus luteum is broken down.

Secondary oocyte

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Sexual reproduction in mammals — 1. Haploid nucleus, 2. Cortical granules, 3. Centrioles, 4. Polar body, 5. Layer of follicle cells, 6. Cytoplasm, 7. Cell surface membrane, 8. Zona pellucida.

Spermatogenesis

Spermatogenesis is the process through which the male gametes are produced. The process is shown below.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Sexual reproduction in mammals — 1. Sperm duct, 2. Blood capillaries, connective tissue and intersitial cells, 3. Epididymis, 4. Site of sperm production (seminiferous tubules), 5. Basement membrane, 6. Germinal epithelial cells, 7. Sertoli cell, 8. Spermatid, 9. Spermatazoa, 10. Secondary spermatocytes

A sperm cell can be divided into three parts.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Sexual reproduction in mammals — 1. Tail, 2. Middle section with mitochondria, 3. Cell surface membrane, 4. Haploid nucleus, 5. Centriole, 6. Acrosome

Head	The head contains a haploid nucleus and an acrosome. The acrosome contains hydrolytic enzymes to digest the zone pellucida. Whilst in the epididymis, the sperm cell head is coated with glycoproteins.
Middle	The middle section contains lots of mitochondria that produce ATP to allow the sperm cell to swim.
Tail	The tail contains microtubules in a similar arrangement to the cilium.

Fertilisation

After ovulation, the follicle bursts, releasing the secondary oocyte. Cilia in the lining of the oviduct moves the secondary oocyte. Millions of sperm are released into the vagina and of those, a few hundred will reach the upper part of the oviduct where fertilisation takes place.

In preparation for fertilisation, the sperm cell undergoes some changes in a process called capacitation. The two steps of capacitation are described below.

1.	The glycoproteins on the head of the sperm cell are removed.
2.	The acrosome reaction occurs which is triggered by chemicals released by the ovum. The acrosome swells and fuses with the sperm cell surface membrane. The hydrolytic enzymes in the acrosome are released. The membrane around the acrosome can fuse with the membrane of the secondary oocyte.

Cortical reaction

The cortical reaction is triggered by the contact of the sperm cell and the oocyte cell membrane. Microvilli on the oocyte engulf the head of the sperm cell and calcium ions are released. The increase in calcium concentration causes the oocyte to release cortical granules. These cause the hardening of the zona pellucida. The reason this occurs is to reduce the chance of polyspermy (the fusion of multiple sperm and one oocyte).

The secondary oocyte will then undergo meiosis II to produce a haploid ovum and a haploid polar body. The haploid ovum can then fuse with the sperm cell.

The full process of fertilisation is described below.

Biology; Cells, viruses and reproduction of living things; KS5 Year 12; Sexual reproduction in mammals

1.	The sperm cell passes through the follicle cells and reaches the jelly coat (zona pellucida) of the secondary oocyte.
2.	Capacitation then occurs to prepare the sperm cell for fertilisation.
3.	The hydrolytic enzymes that are released will digest the zona pellucida and create a path for the sperm cell to follow. The sperm cell will reach the oocyte cell membrane.
4.	The cell membranes of the oocyte and sperm cell will fuse together.
5.	The cortical reaction occurs and causes the zona pellucida to harden. This prevents the entry of any more sperm cells.
6.	The oocyte nucleus undergoes meiosis II.
7.	The resultant haploid nuclei fuse together to form a diploid nucleus. The diploid nucleus undergoes mitosis.
8.	The cytoplasm divides and forms two cells of the embryo.

Early embryo development

After fertilisation occurs, the beating of the cilia lining the oviduct moves the zygote to the uterus. This process takes between three and four days and in this time, the zygote is undergoing mitosis and cell division. The cell division is called cleavage division and it produces blastomeres.

Note: The zygote is not growing at this point, the blastomeres are just becoming smaller.

Between four and seven days after fertilisation, cell division continues and the cell grows into a hollow ball called a blastocyst. The outermost cells are called trophoblasts and the inner cells are the blastomeres. Inside the cell is also the blastocoel which is a fluid-filled cavity.

During this time, the cells also differentiate. The inner cells will become the embryo (and eventually the foetus), the outer trophoblasts will form a membrane, called the amnion, around the embryo that contains amniotic fluid.

When the blastocyst reaches the $8$ -cell stage, the maternal mRNA that was used to create the secondary oocyte, is destroyed.

The blastocyst embeds itself in the endometrium and uses the nutrients directly from the endometrium. As the blastocyst grows it will require more nutrients. In order to facilitate the increasing nutrient needs, the placenta develops. The placenta is made from both maternal and embryonic tissue.

The blastocyst will remain in the uterus for almost $300$ days. This is called gestation.

Note: For the first $60$ days, the zygote is called an embryo and for the remaining time it is referred to as a foetus.