Sexual Reproduction In Mammals


Gametogenesis describes the formation of the gametes that make sexual reproduction possible.

Gametogenesis

Specification Reference

"Understand the processes of oogenesis and spermatogenesis."

Spermatogenesis

  1. A primordial germ cell divides twice by mitosis to form 4 primary spermatocytes
  2. These cells then divide twice through meiosis to each produce 4 sperm cells.
The Sperm
  • Acrosome enzymes in the head digest the zona-pellucida (acrosome reaction)
  • Nucleus contains haploid chromosomes
  • Mitochondria stores lots of energy for movement
  • Tail to generate the movement

Oogenesis

  1. A primordial germ cell again divides twice by mitosis to form 4 primary oocyte's
  2. Only one of these will continue to grow as all materials are invested into the egg
  3. The first meiotic division results in a secondary oocyte and a polar body
  4. This secondary oocyte then divides through meiosis after fertilisation to form an ovum (n) and a second polar body
The Ovum
  • The egg is large with lots of food reserves
  • Additional polar bodies can provide support
  • Zona-pellucida acts as a protective layer
  • Cortical granules harden the membrane preventing further sperm entry

Fertilisation

Specification Reference

"Understand the events of fertilisation from the first contact between the gametes to the fusion of nuclei."
This occurs when a male and female gametes fuse together to form a zygote. This process triggers a number of mechanisms which prevent polyspermy. The process of fertilisation is outlined below:

Capacitation
The surface of the sperm cell undergoes changes that are essential to enabling the acrosome reaction and sperm entry.

Acrosome Reaction
Enzymes from the head of the sperm are released, digesting a pathway through the zona-pellucida.

Fusion of the Sperm Head
The sperm and the egg fuse resulting in a depolarisation. This acts as a fast block to further sperm entry.

Cortical Reaction
The egg releases cortical granules which harden the egg membrane. This acts as a slow more permanent block to further sperm entry.

Totipotent cells can divide into any type of cell. Pluripotent cells can divide into any type of cell apart from placental.

Development of the Embryo

Specification Reference

"Understand the early development of the embryo to blastocyst stage."
Immediately after fertilisation the embryo divides rapidly. The cell divisions increase the number of cells but not size.
  • After 3 days the morula is formed
  • After 6-8 days the morula develops into the blastocyst and makes contact with the uterine lining providing fetal-maternal interaction
  • The hormone HGC is produced which nourishes the embryo