Embryology

Follicle Maturation and Ovulation Oocytes ~2 million at birth ~40,000 at puberty ~400 ovulated over lifetime Leutinizing Hormone surge (from pituitary gland) causes changes in tissues and within follicle: • Swelling within follicle due to increased hyaluronan • Matrix metalloproteinases degrade surrounding tissue causing rupture of follicle Egg and surrounding cells (corona radiata) ejected into peritoneum Corona radiata provides bulk to facilitate capture of egg. Corona radiata Zona pellucida (ZP-1, -2, and -3) Cortical granules The egg (and corona radiata) at ovulation Transport through the oviduct At around the midpoint of the menstrual cycle (~day 14), a single egg is ovulated and swept into the oviduct. Fertilization usually occurs in the ampulla of the oviduct within 24 hrs. of ovulation. Series of cleavage and differentiation events results in the formation of a blastocyst by the 4th embryonic day. Inner cell mass generates embryonic tissues Outer trophectoderm generates placental tissues Implantation into the uterine wall occurs ~6th embryonic day (day 20 of the menstrual cycle) Embryologists Fertilization age: moment of fertilization is dO Division of pregnancy corresponding to development: 0-3 weeks –early development 3-8 weeks –embryonic period (organogenesis) 8 wks-term –fetal period Total gestation time = 38 weeks Clinicians Menstrual age: last menses is dO Division of pregnancy into trimester