Chapter 16 Section 16.3-1 Fertilization and Changes in the Female Reproductive System
Fertilization • Fertilization occurs in the Fallopian tube or in the uterus. • It is the union of a single sperm cell with a secondary oocyte. • Once they have come together, the secondary oocyte is now a fertilized ovum or a zygote. • The zygote has 46 chromosomes, 23 from the sperm cell and 23 from secondary oocyte.
Once fertilized the zygote will travel through the Fallopian tube to the uterus. (3-5 days) • During this travel time the zygote undergoes many cellular divisions in a process called cleavage. • Cleavage is the equal division of the cells without any increase in size or change in the number of chromosomes. • As a result of this cleavage the cells of the zygote become smaller.
By the time it reaches the uterus, in about six days, the zygote had developed into a fluid filled structure called a blastocyst. • The blastocyst consists of an outer sphere of cells, from which the extraembryonic structures develop, and an inner mass, from which the embryo develops. • Extraembryonic structures include the chorion and the amnion.
Chorion – contributes to the placenta. • Amnion – fluid filled structure surrounding fetus and amniotic fluid. • Once in the uterus, the blastocyst attaches to the wall of the endometrium, in a process called implantation.
Cleavage of a zygote. Blastocyst forming.
Changes in the Female Reproductive System • The zygote first transforms into a blastocyst and finally 7 days after fertilization into an embryo. • It remains an embryo until the eighth week, then becomes a fetus. • Gestation period is 9 months. • During these 9 months the female body will change to ensure the protection of the fetus.
Menstruation stops • endometrium could not be shed without dislodging the embryo from the uterus. • Corpus luteum remains in tact • estrogen and progesterone must remain high or uterine contractions will occur and dislodge fetus • chorion produces human chorionic gonadotropic hormone (hCG) for the first 3 months • hCG maintains corpus luteum • Pregnancy structures develop
Pregnancy Structures amnion yolk sac amniotic cavity allantois umbilical cord embryo chorion chorionic villi Extraembryonic coelom endometrium
Structures and Functions • Amnion • fluid filled sac surrounding the fetus • Amniotic cavity • the fluid filled cavity surrounding the embryo/fetus • insulates it • protects it from infection, dehydration, impact and changes in temperature
Chorion • layer surrounding the extraembryonic coelom that contributes to the placenta • produces human chorionic gonadotropic hormone (hCG) • hCG maintains corpus luteum and therefore maintains estrogen and progesterone levels • Extraembryonic coelom • body cavity between the amnion and the chorion
Endometrium • inner membrane on the uterus • helps form placenta • Chorionic villi • projections of the chorion • increase surface area • ensure that a large number of fetal blood vessels are exposed to maternal blood vessels
Umbilical cord • connects the fetus to the placenta • Placenta • site of exchange of nutrients and gases between the mother and fetus • produces estrogen and progesterone after 4th month of pregnancy
Allantois • provides umbilical blood vessels to the placenta. • Yolk sac • a sac that forms during embryo development • is the site of early blood vessel formation • later contributes to the digestive tract
Homework: • Read section 16.3