Spermatogenesis and the structure of the sperm
Spermatogenesis happens in the seminiferous tubules in four phases. In the multiplication phase the primary germ cells (2N) divide by mitosis many times and make spermatogonia (2N). In the growth phase the spermatogonia store food and become primary spermatocytes (2N). In the maturation phase each primary spermatocyte divides by meiosis I into two secondary spermatocytes (N), and these divide by meiosis II into four haploid spermatids (N). The number of chromosomes is halved in this phase. In the metamorphosis phase the spermatids change into sperm. A sperm has four parts. The head holds the nucleus with 23 chromosomes, and in front of it the acrosome with the enzyme hyaluronidase, which dissolves part of the coat of the egg. The neck holds two centrioles, which take part in the first division of the fertilized egg. The mid-piece holds mitochondria that supply energy. The tail has an axis ending in a tail piece and it moves the sperm.
| Part of the sperm | Contains | Job |
|---|---|---|
| Head | nucleus with 23 chromosomes; acrosome with hyaluronidase | carries the genetic material; dissolves part of the egg’s coat |
| Neck | two centrioles | help the first division of the fertilized egg |
| Mid-piece | mitochondria | supply energy for movement |
| Tail | an axis ending in a tail piece | moves the sperm |
Make a phase chart, with a grown-up. Draw four boxes in a row, one per phase, and in each box write the name of the phase, the cell that results and its chromosome number (use 46 for body cells). Colour the box where the number is halved. Underneath draw a very simple sperm with its four parts named. Do not try to find out more than the lesson gives.