Fertilization

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Fertilization
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Growth and maturation of oocyte

• Growth of the oocyte
• Essential for successful fertilization and
  embryonic development
• Before the ovulation

Immature oocyte surrounded by cumulus cells
Oocyte
Blastocyst stage embryo
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Growth and maturation of oocyte

• Growth of oocytes
• Primordial follicles
• Commitment for development and differentiation

Oocyte
Cumulus cells
Cumulus-oocyte complex
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Growth and maturation of oocyte

• Specialized granulosa cells that surround the
  oocyte
• Cumulus cells
• Assist growth of the oocyte
• Produce growth factors and hormones that are
  essential for normal oocyte growth and
  development
• Combined structure of the oocyte and cumulus
  cells
• Cumulus-oocyte complex (COC)

Oocyte
Cumulus cells
Cumulus-oocyte complex
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Growth and maturation of oocyte

• The oocyte increases its size and volume during
  its growth phase
• No mitosis
• The maximal number of oocytes present on the
  ovaries
• Set before birth
• numbers decrease throughout fetal and adult life
• Undergo meiosis
• ultimately results in reduction of number of
  chromosomes by half (46 to 23)
• Haploid cells

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Growth and maturation of oocyte

• Process of meiosis
• Begins before primordial follicles are formed
• Arrested during oocyte growth
• Growth of the oocyte
• Almost completed by the time the follicle reaches
  the antral stage
• Meiosis
• Arrested until LH surge
• Granulosa cells produce factors that inhibit
  premature resumption of meiosis

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Growth and maturation of oocyte

• Oocyte after LH surge
• Formation of a polar body
• a small, round structure in the space between the
  cytoplasm (inside of the cell) and the clear
  outer membrane of the oocyte (zona pellucida)
• Contains a portion of maternal chromosomes
• Expelled as a part of meiosis in order to reduce
  number of chromosomes present in the oocyte

Oocyte
Zona pellucida
Polar body
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Growth and maturation of oocyte

• Resumption of meiosis
• Changes in the function of cumulus cells
• Loss of contact with each other and with the
  granulosa cells,
• Essential for the oocyte to be released from the
  preovulatory follicle
• Production of sticky mucus
• Necessary for the COC to be picked up by fimbriae
  after being released from the preovulatory
  follicle

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Release of the oocyte

• The COC
• Expelled to the outer surface of the ovary after
  the ovulatory follicle ruptures
• Covered with sticky mucus
• Remains on the site of ovulation unless being
  picked up by the fimbriae of the oviduct

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Oocyte in the oviduct
Ovary
Infundibulum

• The pickup of COC
• The structural design of the fimbriae
• Close proximity of the ovary to the oviduct
• Coordinated contraction of ligaments surrounding
  the ovaries and uterus

Ampulla
Ampullary- Isthmic Junction
Fimbriae
Isthmus
Uterotubal Junction
Adapted from Hafez, 1993
Site of ovulation
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Oocyte in the oviduct
Uterotubal Junction
Ampullary- isthmic Junction
Fimbriae
Isthmus
Ampulla
Infundibulum

• Lifespan of oocyte
• 20 to 24 hours after being released from the
  ovulatory follicle

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Oocyte in the oviduct
Uterotubal Junction
Ampullary- isthmic Junction
Fimbriae
Isthmus
Ampulla
Infundibulum

• Time to reach uterus from the infundibulum of the
  oviduct
• Approximately 90 hours in cows
• Approximately 48 to 72 hours in women
• Oocyte must be fertilized at appropriate time to
  ensure survival of the embryo when it enters the
  uterus