CH. 28: Pregnancy and Human Development
I. From Egg to Embryo
Fertilization occurs when a sperm fuses with an egg to form a
Millions of sperm
ejaculated into the female reproductive tract
due to leakage from the
vaginal canal, destruction by the acidic environment of the vagina, inability to pass the cervical
mucus, or destruction by defense cells of the uterus.
In order to fertilize an egg, sperm must be
, a process involving weakening of the sperm
cell membrane in order to allow release of acrosomal hydrolytic enzymes.
When sperm cells bind to the zona pellucida surrounding the egg, they undergo an
, where acrosomal enzymes are released to the oocyte.
a. Hundreds of sperm cells must release their acrosomal enzymes before fertilization can occur.
b. Once a sperm cell binds to membrane receptors on the oocyte membrane, its nucleus is pulled
into the cytoplasm of the oocyte.
(fertilization by more than one sperm cell):
to polyspermy occurs when the membrane of the oocyte depolarizes and prevents
similar binding by other sperm cells.
to polyspermy results in destruction of sperm receptors, and the formation of a
swollen membrane that removes other sperm cells from the surface of the oocyte.
Once a sperm enters an oocyte, it loses its tail and midpiece, and migrates to the center of the
oocyte while the oocyte completes meiosis II.
a. After meiosis II is completed, male and female pronuclei fuse and produce a zygote, which almost
immediately enters into mitosis.
Begins with fertilization and continues with the movement of the preembryo to the uterus, where it
implants in the uterine wall.
= The mitotic divisions after fertilization which occur without much growth between
divisions, resulting in progressively smaller cells.
a. Cleavage forms two identical cells, blastomeres, then four identical cells, etc. This process,
completed by 72 hours, produces a solid ball of cells called a
b. By the 4
day, the morula has formed a hollow ball of cells called the
about to enter the uterus.
occurs after 6–7 days; the
adheres to the endometrium, and produces
enzymes that irritate the endometrium.
a. Uterine capillaries become permeable and leaky, and the trophoblast proliferates, forming the
b. Trophoblast cells secrete