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