REPRODUCTION IN HUMANS Formation of gametes A Spermatogenesis B Oogenesis SPERMATOGENESIS Production of sperm Begins 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.