Fertilization and Fertilization and pregnancy:pregnancy:

Fertilization:Fertilization:

Blocks to polyspermyBlocks to polyspermy If more than one sperm were to fertilize the If more than one sperm were to fertilize the

egg, then the genetic complement would be 3n.egg, then the genetic complement would be 3n.

In order to prevent multiple sperm In order to prevent multiple sperm penetrations, two responses have evolved in penetrations, two responses have evolved in the egg.the egg.

First, as soon as the first sperm head First, as soon as the first sperm head penetrates the egg, it triggers a massive influx penetrates the egg, it triggers a massive influx of Naof Na++..

This influx depolarizes the egg, making it positive This influx depolarizes the egg, making it positive inside. This repels the positively charged sperm, inside. This repels the positively charged sperm, inhibiting penetration of more sperm.inhibiting penetration of more sperm.

Second, the depolarization triggers Second, the depolarization triggers an influx of Ca an influx of Ca 2+2+ . This Ca . This Ca 2+2+ facilitates the exocytosis of a facilitates the exocytosis of a number of secretory vesicles, known number of secretory vesicles, known as cortical vesicles.as cortical vesicles.

The contents of these vesicles The contents of these vesicles surrounds the egg, swells with water surrounds the egg, swells with water and gels, pushing other sperm away and gels, pushing other sperm away from the egg and blocking their entry.from the egg and blocking their entry.

Implantation:Implantation:

Chorionic villiChorionic villi

Placental hormones:Placental hormones:

During early During early pregnancy, HCG is pregnancy, HCG is secreted by the secreted by the syncitial syncitial trophoblasts.trophoblasts.

Later, the placenta Later, the placenta secretes estradiol, secretes estradiol, progesterone, relaxin progesterone, relaxin and and somatomammotropin.somatomammotropin.

Function of placental Function of placental hormones:hormones:

HCG is similar to LH and maintains the corpus HCG is similar to LH and maintains the corpus luteum in a functional state for 3-4 months.luteum in a functional state for 3-4 months. This keeps progesterone levels high and they This keeps progesterone levels high and they

maintain the functional endometrium.maintain the functional endometrium. Relaxin increases flexibility in the pelvic joints, Relaxin increases flexibility in the pelvic joints,

as well as suppressing release of oxytocin.as well as suppressing release of oxytocin. Placental progesterone keeps the uterine wall Placental progesterone keeps the uterine wall

intact.intact. Somatomammotropin acts like prolactin and Somatomammotropin acts like prolactin and

triggers the mammary glands to develop.triggers the mammary glands to develop. Estrogen increases the sensitivity of the Estrogen increases the sensitivity of the

myometrium to mechanical irritation, as well myometrium to mechanical irritation, as well as oxytocin stimulation.as oxytocin stimulation.

Labour:Labour: Towards the end of Towards the end of pregnancy, relaxin pregnancy, relaxin secretion falls off, secretion falls off, thus, the uterus thus, the uterus becomes more becomes more sensitive to sensitive to oxytocin. oxytocin.

Initially, the fetus Initially, the fetus secretes oxytocin secretes oxytocin into the maternal into the maternal circulation.circulation.

The oxytocin The oxytocin stimulates stimulates contractions, which contractions, which push the head down push the head down against the cervix.against the cervix.

This pressure on This pressure on the cervix the cervix stimulates the stimulates the release of oxytocin release of oxytocin from the maternal from the maternal pituitary gland.pituitary gland.

The maternal The maternal oxytocin causes oxytocin causes more contractions more contractions of the uterus, of the uterus, forcing the head forcing the head of the fetus of the fetus against the cervix against the cervix even harder. even harder.

This is a positive This is a positive feedback system.feedback system.

Labour and delivery:Labour and delivery:

As the head of the fetus is pressed down As the head of the fetus is pressed down against the cervix, it thins and then starts to against the cervix, it thins and then starts to dilate.dilate. This stage is known as the Dilation Stage and This stage is known as the Dilation Stage and

can last several hours, or days (usually around 8 can last several hours, or days (usually around 8 hours).hours).

Once the cervix has dilated, the fetus starts Once the cervix has dilated, the fetus starts moving through the birth canal. moving through the birth canal. Contractions are maximal and come about Contractions are maximal and come about 2-3 minutes.2-3 minutes. This is known as the Expulsion Stage.This is known as the Expulsion Stage. If the vaginal wall has not stretched enough, If the vaginal wall has not stretched enough,

tearing may occur.tearing may occur.

Labour and delivery cont. Labour and delivery cont. ::

There is also a chance that the fetus will There is also a chance that the fetus will get stuck in the birth canal (usually caused get stuck in the birth canal (usually caused by insufficient molding of the head.by insufficient molding of the head. In these cases, a cesarean section is performed.In these cases, a cesarean section is performed.

Finally, after expulsion of the fetus, the Finally, after expulsion of the fetus, the placenta detaches from the uterine wall placenta detaches from the uterine wall and is delivered through the birth canal.and is delivered through the birth canal. This is known as the Placental Stage.This is known as the Placental Stage.

Nursing:Nursing:

Two hormones are Two hormones are involved, PRL and involved, PRL and oxytocin. oxytocin.

PRL stimulates PRL stimulates milk production, milk production, while oxytocin is while oxytocin is required for the required for the expression of milk expression of milk from the breast.from the breast.

Fertility issues:Fertility issues: