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Oogenesis and Ovarian Cycle


  • —Gametes are derived from PGCs.
  • Formed in the epiblast during 2nd week and move to the wall of yolk sac and reach gonads by end of 5th week

—In preparation for fertilization germ cells undergo: —

  1. Gametogenesis
  2. —Cytodifferentiation


—Is the sequence of events by which germ cells oogonia are transformed into mature oocytes.


  • —Maturation of oocytes begins before birth. —
  • Accelerates at puberty.
  • —Ends at menopause.

Morphological changes during maturation of gametes

Maturation of oocytes begin before birth.

A. Primordial germ cell

B. Oogonium

C. Primary oocyte

D. Secondary oocyte

◦By 5th month number of germ cells reaches 7 million in the ovaries.

◦By 7th month majority of oogonia and primary oocyte degenerate.

◦All surviving primary oocytes are surrounded by flat epithelial cells to form primordial follicle

Maturation of oocytes continue at puberty

—At birth: Ovaries have about 2 million primordial follicles and all Primary oocytes are arrested in the diplotene stage till puberty due to OMI secreted by follicular cells. —

By puberty: Number drops to about 40,000. Rising FSH triggers start of ovarian cycle —

Ovarian cycle: Fewer than 500 ovulate in the reproductive life of a female

Recruited Primordial follicle

A. Primordial follicle

B. Growing follicle

C. Primary follicle

Secondary/Antral Follicle

  • —Longest stage
  • —Few —
  • Rapid enlargement
  • —Stratum granulosum 6-12 cell layers —
  • Liquor folliculi (hyaluronic acid)
  • —Formation of antrum —
  • Oocyte eccentric
  • —Well defined theca interna & externa

Cumulus oophorus

—Is a column of granulosa cells that attaches the oocyte to the follicle wall. At ovulation, this column of cells is broken or separates to release the oocyte from its follicle attachment. —

Corona radiata is composed of cumulus cells that send microvilli through zona pellucida that communicate with microvilli of oocyte

Tertiary or Graafian Follicle (Pre ovulatory)

  • —Spans entire width of cortex & produces a bulge on the surface of ovary
  • —St. granulosum appears to be thinner
  • —One large antral cavity
  • —Cumulus oophorus & corona radiata (loose connection) —
  • First meiotic division being completed: Primary oocyte divides into a Secondary oocyte and a polar body

—Females undergo regular monthly reproductive cycles commencing at puberty.

  1. Ovarian cycle
  2. Menstural cycle

Corpus atreticum

—Oocytes & its  surrounding follicular cells degenerate and replaced by scar tissue.

Ovulation (Release of secondary oocyte)

  • —Maximum size of follicle —
  • Sudden surge of LH
  • —Increase in volume & pressure of follicular fluid —
  • Overlying ovarian cortex become thin & translucent
  • —Stigma formation —
  • Increased collegenase & PG activity
  • —Increased muscular contractions

Corpus Luteum

  • —After ovulation, Follicle collapses
  • —Granulosa and theca interna cells hypertrophy, leutenized & vasularized
  • —Secretes Progesterone  and some estrogens.

Corpus albicans (Absence of HCG)

1.Corpus luteum degenerates and becomes corpus albicans

2.GnRH increases under low estrogen and progesterone levels


•Sweeping movements of fimbriae

• Rhythmic contractions

•Motion of cilia

•Fluid currents

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