reproduction in humans formation of gametes a spermatogenesis b oogenesis

REPRODUCTION IN HUMANS

Formation of gametes

A Spermatogenesis

B Oogenesis

SPERMATOGENESIS

Production of spermBegins between ages

of 11 to 15 and continues until death

100 to 200 million sperm are made a day

SPERMATOGENESIS

Each seminiferous tubule is surrounded by a layer of epithelium on which sit spermatogonia

These divide by mitosis to form dipolid cells

Some of these cells move towards the middle of the tubule becoming primary spermatocytes

SPERMATOGENESIS

Primary spermatocytes then divide by meiosis.

After first meiotic division two secondary spermatocytes are formed.

SPERMATOGENESIS

Two or three days later the secondary spermatocytes undergo a second meiotic division to produce haploid spermatids.

SPERMATOGENESIS

The spermatids mature over the next few weeks into spermatozoon.

The whole process takes about 64 days.

SPERMATOGENESIS

At all stages the developing sperm are supported and nourished by much larger non-dividing cells called Sertoli cells or nurse cells.

The fully developed sperm are carried by a fluid released by the Sertoli cells.

OOGENESIS

Each ovary weighs about 15g and contains many eggs at different stages of development.

The process begins when a girl is still an embryo

Removing an ovarian cyst!

OOGENESIS

5 or 6 weeks after zygote forms some cells in embryo ovary undergo mitosis to produce diploid oogonia

By 24 weeks the embryo contains millions of oogonia

From then until 6 weeks after birth the oogonia undergo a first meiotic division to form primary oocytes.

OOGENESIS

They remain half way through this division for many years.

Most of the primary oocytes disappear – by puberty about 400 000 remain.

OOGENESIS

As development recommences some of the surrounding ovary cells form the primordial follicle which later develops into the primary follicle

OOGENESIS

At puberty hormones stimulate the primary follicle to become a secondary follicle

One of these will develop each 28 days into an ovarian follicle containing the developing primary oocyte

OOGENESIS

The meiotic division commenced as an embryo now completes.

This results in a large secondary oocyte and a tiny polar body

OOGENESIS

It is at this stage that the oocyte is released at the moment of ovulation.

The meiotic division does not actually complete until after the sperm has entered it.