Brent Cornell

    
The male and female reproductive gametes (sperm and egg) have specialised structures which reflect their functions

• The male gamete (sperm) is small and motile and only contributes the male’s haploid nucleus to the zygote
• The female gamete (egg) is large and non-motile and contributes all the organelles and cytoplasm to the zygote

Sperm

• A typical human spermatozoa can be divided into three sections – head, mid-piece and tail 
• The head region contains three structures – a haploid nucleus, an acrosome cap and paired centrioles
  • The haploid nucleus contains the paternal DNA (this will combine with maternal DNA if fertilisation is successful)
  • The acrosome cap contains hydrolytic enzymes which help the sperm to penetrate the jelly coat of the egg
  • The centrioles are needed by a zygote in order to divide (egg cells expel their centrioles within their polar bodies)
• The mid-piece contains high numbers of mitochondria which provide the energy (ATP) needed for the tail to move
• The tail (flagellum) is composed of a microtubule structure called the axoneme, which bends to facilitate movement

Egg

• A typical egg cell is surrounded by two distinct layers – the zone pellucida (jelly coat) and corona radiata
  •  The zona pellucida is a glycoprotein matrix which acts as a barrier to sperm entry
  • The corona radiata is an external layer of follicular cells which provide support and nourishment to the egg cell
• Within the egg cell are numerous cortical granules, which release their contents upon fertilisation to prevent polyspermy
• Although diagrams of egg cells commonly include a haploid nucleus, no nucleus will form within the egg until after fertilisation has occurred (the egg cell is arrested in metaphase II until it becomes fertilised by a sperm)

Diagram of Human Gametes  (click to show / hide labels)

Video of an Unfertilised Human Egg

 Adapted from a video published by the Frisco Institute for Reproductive Medicine