Fertilization and pregnancy Fertilization Blocks to polyspermy n

Fertilization and pregnancy:

Fertilization:

Blocks to polyspermy n If more than one sperm were to fertilize the egg, then the genetic complement would be 3 n. n In order to prevent multiple sperm penetrations, two responses have evolved in the egg. n First, as soon as the first sperm head penetrates the egg, it triggers a massive influx of Na+. n This influx depolarizes the egg, making it positive inside. This repels the positively charged sperm, inhibiting penetration of more sperm.

n Second, the depolarization triggers an influx of Ca 2+. This Ca 2+ facilitates the exocytosis of a number of secretory vesicles, known as cortical vesicles. n The contents of these vesicles surrounds the egg, swells with water and gels, pushing other sperm away from the egg and blocking their entry.

Implantation:

Chorionic villi

Placental hormones: n During early pregnancy, HCG is secreted by the syncitial trophoblasts. n Later, the placenta secretes estradiol, progesterone, relaxin and somatomammotropin.

Function of placental hormones: n HCG is similar to LH and maintains the corpus luteum in a functional state for 3 -4 months. n n n This keeps progesterone levels high and they maintain the functional endometrium. Relaxin increases flexibility in the pelvic joints, as well as suppressing release of oxytocin. Placental progesterone keeps the uterine wall intact. Somatomammotropin acts like prolactin and triggers the mammary glands to develop. Estrogen increases the sensitivity of the myometrium to mechanical irritation, as well as oxytocin stimulation.

Labour: n n n Towards the end of pregnancy, relaxin secretion falls off, thus, the uterus becomes more sensitive to oxytocin. Initially, the fetus secretes oxytocin into the maternal circulation. The oxytocin stimulates contractions, which push the head down against the cervix.

n This pressure on the cervix stimulates the release of oxytocin from the maternal pituitary gland. n The maternal oxytocin causes more contractions of the uterus, forcing the head of the fetus against the cervix even harder. n This is a positive feedback system.

Labour and delivery: n As the head of the fetus is pressed down against the cervix, it thins and then starts to dilate. n n This stage is known as the Dilation Stage and can last several hours, or days (usually around 8 hours). Once the cervix has dilated, the fetus starts moving through the birth canal. Contractions are maximal and come about 2 -3 minutes. This is known as the Expulsion Stage. n If the vaginal wall has not stretched enough, tearing may occur. n

Labour and delivery cont. : n There is also a chance that the fetus will get stuck in the birth canal (usually caused by insufficient molding of the head. n n In these cases, a cesarean section is performed. Finally, after expulsion of the fetus, the placenta detaches from the uterine wall and is delivered through the birth canal. n This is known as the Placental Stage.

Nursing: n Two hormones are involved, PRL and oxytocin. n PRL stimulates milk production, while oxytocin is required for the expression of milk from the breast.

Fertility issues: