oogenesis

• oogenesis is the production of ova in the ovaries
• Before birth, millions of oogonia develop in ovaries. these cells are diploid and divide by mitosis to produce cells that eventually develop into ova
• By birth, each ovary contains several hundred thousand oogonia. which have undergone growth phase to become primary oocytes. these primary oocytes begins prophase of first meiotic division, but the process stops here. primary oocytes are surrounded by a single layer of cells which forms a primary follicle
• At puberty, process of follicle growth and maturation begins. as follicle matures, primary oocytes within completes stage of first meiotic division, producing cells that are unequal in size.

  • the large one called the secondary oocytes receives half the chromosome but nearly all the cytoplasm
  • the smaller cell called the first polar body receives other half of chromosomes but very little cytoplasm 
• Secondary oocyte immediately starts second meiotic division but stops at metaphase
• Ovulation occurs, the follicle ruptures, expelling secondary oocyte and polar body
• when expelled, the secondary oocyte enters uterine tube and if it is penetrated by spermatozoon, meiosis is quickly completed, this also produces cells of unequal size with the large one developing into an ovum and the smaller cell being the second polar body. the first polar body may also undergo second meiotic division, producing two additional polar bodies 
• All polar bodies disintegrate