Fertilization
Growth and maturation of oocyte
• Growth of the oocyte– Essential for successful
fertilization and embryonic development
• Before the ovulation
Immature oocyte surroundedby cumulus cells
Blastocyst stage embryo
Oocyte
Growth and maturation of oocyte
• Growth of oocytes
– Primordial follicles • Commitment for
development and differentiationPrimordial Primary Secondary Antral
Oocyte growth
Cumulus-oocyte complex
Oocyte
Cumuluscells
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)
Primordial Primary Secondary Antral
Oocyte growth
Cumulus-oocyte complex
Oocyte
Cumuluscells
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
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
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
Polar body
Zonapellucida
Oocyte
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
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
Oocyte in the oviduct
• 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
Fimbriae
Infundibulum
Ampulla
Isthmus
Ampullary-Isthmic Junction
Uterotubal Junction
Ovary
Site of ovulationAdapted from Hafez, 1993
Oocyte in the oviduct
• Lifespan of oocyte – 20 to 24 hours after being released from the
ovulatory follicle
Infundibulum
AmpullaIsthmus
Fimbriae
Ampullary-isthmic Junction
Uterotubal Junction
Oocyte in the oviduct
• 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
Infundibulum
AmpullaIsthmus
Fimbriae
Ampullary-isthmic Junction
Uterotubal Junction