Fertilization & Pregnancy Campbell pages 989-993

Quick Recap http://pennhealth.com/health_info/pregnancy/000078.htm Egg production Sperm production Conception

Fertilization Sexual intercourse; millions of sperm are ejaculated into a female’s vagina Sugar in ejaculate  fuel for the “journey” Some sperm find their way through the cervical opening  uterus  fallopian tubes Typical location of fertilization is fallopian tube Gametes fuse together

Cortical Reaction When the two gametes fuse, cortical granules (on the oocyte) release enzymes These enzymes cause a chemical change in the zona pellucida (ovum layer) Makes it impermeable to any more sperm Happens in a few seconds This ensures only one sperm cell fertilizes the oocyte Why is this important? Fertilized ovum = zygote

Human gestation Pregnancy is preceded by conception Averages 266 days (38 weeks) from conception or 40 weeks from the start of the last menstrual cycle Trimesters Body size and extent of development determine length of gestation Elephants: 600 days (yikes!) Rodents: 21 days

First Trimester Organogenesis Development of organs Heart beats ~4 weeks End of 8th week: major structures in rudimentary form Most sensitive to drugs and radiation Birth defects

First Trimester (cont.) Embryo secretes the hormone human chorionic gonadotropin (HCG) Maintains secretion of progesterone and estrogen by the corpus luteum (without it, menstruation and spontaneous abortion of the embryo would occur) Levels of HCG are so high some is excreted in the urine (pregnancy tests) High levels of progesterone initiate: Increased mucus in the cervix (protective plug) Growth of the placenta Enlargement of the uterus Cessation of ovulation and menstrual cycle Breasts enlarge

Development of the Placenta 4th week Organ that contains embryonic and maternal blood vessels Umbilical cord At no time does blood from the fetus and mother exchange; there IS exchange of materials, but NOT blood

Exchange within Placenta Materials passed from fetus to mother Materials passed from mother to fetus Carbon dioxide Oxygen Urea Nutrients (glucose, amino acids) Water Hormones (HCG) Hormones Vitamins, minerals Alcohol, drugs, nicotine, some viruses

Second Trimester The fetus grows rapidly to ~30 cm and is very active Hormone levels stabilize as HCG declines Corpus luteum deteriorates The placenta secretes its own progesterone (which maintains the pregnancy) Switch is a pivotal time Women start to show

Third Trimester Rapid growth Fetal activity decreases ~3.5 kg, 50 cm Fetal activity decreases Mothers abdominal organs become compressed and displaced The need to urinated frequently, back pain, indigestion