The Reproductive System
The reproductive system occurs in both male and female.
Like in plants it is the male gamete that needs to be transferred to
the female gamete. The female gamete is fertilised and develops inside
the mother’s body so the reproductive systems of both males and
females are highly adapted for this.
Production of sperm is called spermatogenesis.
It occurs at puberty and for the rest of there life.
It takes place in the gonads of the male - the testes. Over 100
million can be made in one day!
Each testis is composed of numerous tiny tubes called seminiferous
tubules. It is in the walls of these tubules that sperm production
actually takes place.
Development begins in the outer side of the wall in a layer of cells
called the germinal epithelium. As the immature sperm cells become
more mature they move to the inner side and break way into the lumen
of the tubule to be carried away to the epididymis for storage. The
process of this production is shown in the next two diagrams.
In between the tubules, inside the testes, are interstitial cells
called Leydig cells. These secrete the hormone testosterone.
There are also blood vessels in close proximity, delivering nutrients
and carrying away some testosterone to other target cells for the
development and maintenance of secondary sexual characteristics, e.g.
facial and pubic hair, deepening of the voice. The testosterone also
stimulates the cells inside the testis involved in spermatogenesis.
Hormonal control of spermatogenesis
The control centres are the pituitary gland and the hypothalamus in
The hypothalamus secretes GnRH (gonadotrophin releasing hormone). This
is released into the blood and stimulates the anterior lobe of the
The anterior lobe of the pituitary gland secretes ICSH (interstitial
cell stimulating hormone).
ICSH: this stimulates the leydig cells that produce testosterone.
FSH: this stimulates the seminiferous tubules, including the Sertoli
cells. They produce sperm in response.
Note: Testosterone also acts on the seminiferous tubules and
stimulates sperm production.
The testosterone feeds back to the hypothalamus and pituitary gland to
switch off GnRH and ICSH release.
The Sertoli cells produce a hormone called inhibin that feeds back to
the pituitary gland to switch off FSH release.
Since the action of the interstitial cells and Sertoli cells are
inhibited, less testosterone and inhibin are released as a result. The
inhibition of the hypothalamus and pituitary is lifted and the process
can start again. Due to the levels of the hormones and their effects,
the process is not noticeably cyclical – there aren’t noticeable peaks
and troughs in the levels of the hormones.
The production of eggs is called...