OOGENESIS

• The process of formation of a mature female gamete is called oogenesis.
• Some of the germinal epithelial cells divide by mitosis to produce a large number of gamete mother cells or oogonia
• Oogoniamultiply by mitosis and form primary oocytes.

Growth phase-

• Each primary oocyte then gets surrounded by a layer of granulosa cells and called primary follicle.
• The primary oocytes enlarge and mature by obtaining food from follicle cells.
• The primary follicles get surrounded by more layers ofgranulosa cells and a new theca and called secondary follicles.
• The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum
• The theca layer isorganised into an inner theca interna and an outer theca externa.
• The primary oocyte within the tertiary follicle grows in size and completes its first meiotic division which is an unequal division and forms a large secondary oocyte and tiny first polar body.
• The tertiary follicle changes into the mature follicle or Graafian follicle.
• The secondary oocyte forms a new membrane called zonapellucida
• The Graafian follicle then ruptures to release the secondary oocyte from the ovary by the process called ovulation
• If a sperm can enter the secondary oocyte through zonapellucida layer, the secondary oocyte completes meiosis II and thus results in the formation of second polar body and an ovum.

fig. Oogenesis

 

Fig. structure of human ovum