mammalian gametes & fertilisation

• sperm has a flagellum which enables it to swim to the egg
• sperm contains many mitochondria that provide energy for movement of the flagellum
• sperm cells have an acrosome that contains digestive enzymes to break down the protective glycoprotein layer (a jelly-like coating known as zona pellucida) surrounding the egg cell - sperm cells must penetrate this layer in order to fertilise the egg
• egg cells have follicle cells that form a protective coating
• egg cells have a jelly-like glycoprotein layer, known as the zona pellucida, that forms an impenetrable barrier after fertilisation by a sperm cell has occurred, to prevent other sperm nuclei from entering the egg
• during fertilisation, the head of a sperm cell releases enzymes that digest a path through the protective outer layer of the egg cell (zona pellucida). allowing the sperm to pass through the egg cell membrane

  this process is known as the acrosome reaction
• once this occurs, the egg cell immediately releases the contents of vesicles known as cortical granules into the space between the egg cell membrane and the zona pellucida

  the chemicals contained within the cortical granules cause the zona pellucida to rapidly thicken and harden preventing any more sperm cells from entering, ensuring only one sperm cell can fertilise the egg cell

  this process is called the cortical reaction
• the nucleus of the sperm cell then enters the egg and fuses with the nucleus of the egg cell

  when the male and female gamete nuclei fuse, they become a zygote (fertilised egg cell)
• the zygote contains 46 chromosomes (23 pairs) half from mother and half from father
• zygote divides by mitosis to form two new cells which then continue to divide like this until an embryo is formed after a few days

  cell division continues and eventually many of the new cells produced become specialised to perform particular functions and form all the body tissues of the offspring
• the sperm cell: male gamete which contains a long tail, an acrosome and lots of mitochondria and is specialised to fertilise an egg cell

  
• egg cell: female gamete which contains zona pellucida, follicle cells and yolk droplets