Fertilization

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 Blastocyst stage embryo Oocyte

Growth and maturation of oocyte Growth of oocytes –Primordial follicles Commitment for development and differentiation PrimordialPrimarySecondaryAntral Oocyte growth Cumulus-oocyte complex Oocyte Cumulus cells

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) PrimordialPrimarySecondaryAntral Oocyte growth Cumulus-oocyte complex Oocyte Cumulus cells

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 Zona pellucida 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 ovulation Adapted from Hafez, 1993

Oocyte in the oviduct Lifespan of oocyte – 20 to 24 hours after being released from the ovulatory follicle Infundibulum Ampulla Isthmus 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 Ampulla Isthmus Fimbriae Ampullary- isthmic Junction Uterotubal Junction