REPRODUCTION IN HUMANS

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REPRODUCTION IN HUMANS

• Formation of gametes
• A Spermatogenesis
• B Oogenesis

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SPERMATOGENESIS

• Production of sperm
• Begins between ages of 11 to 15 and continues
  until death
• 100 to 200 million sperm are made a day

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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

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SPERMATOGENESIS

• Primary spermatocytes then divide by meiosis.
• After first meiotic division two secondary
  spermatocytes are formed.

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SPERMATOGENESIS

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

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SPERMATOGENESIS

• The spermatids mature over the next few weeks
  into spermatozoon.
• The whole process takes about 64 days.

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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.

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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

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Removing an ovarian cyst!
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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.

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OOGENESIS

• They remain half way through this division for
  many years.
• Most of the primary oocytes disappear by
  puberty about 400 000 remain.

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OOGENESIS

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

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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

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OOGENESIS

• The meiotic division commenced as an embryo now
  completes.
• This results in a large secondary oocyte and a
  tiny polar body

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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.