Section II • Early Embryology
During the first 4 to 5 days ofthe first week, the zygote undergoes rapid mitotic division (cleavage) in the oviduct to form a blastula, consisting of increasingly smaller blastomeres. This becomes the morula (32-cell stage).
A blastocyst forms as fluid develops in the morula. The blastocyst consists ofan inner cell mass known as the embryoblast, and the outer cell mass known as the trophoblast, which becomes the placenta.
At the end of the first week, the trophoblast differentiates into the cytotropho blast and syncytiotrophoblast and then implantation begins (see below).
Clinical Correlate
Ectopic Pregnancy
Tubal
•The most common form of ectopic pregnancy; usually occurs when the blastocyst implants within the ampulla ofthe uterine tube because of delayed transport
•Risk factors: endometriosis, pelvic inflammatory disease (PID), tubular pelvic surgery, or exposure to diethylstilbestrol (DES)
•Clinical signs: abnormal or brisk uterine bleeding, sudden onset of abdominal pain that may be confused with appendicitis, missed menstrual period (e.g., LMP 60 days ago), positive human chorionic gonadotropin (hCG) test, culdocentesis showing intraperitoneal blood, positive sonogram
Abdominal
•Most commonly occurs in the rectouterine pouch (pouch of Douglas)
Implantation
The zona pellucida must degenerate for implantation to occur.
The blastocyst usually implants within the posterior wall ofthe uterus.
The embryonic pole ofblastocyst implants first.
The blastocyst implants within the functional layer of the endometrium during the progestational phase ofthe menstrual cycle.
Chapter Summary
•Fertilization occurs in the ampulla ofthe uterine tube with the fusion of the male and female pronuclei to form a zygote. During the first 4-5 days of
the first week, the zygote undergoes rapid mitotic division (cleavage) in the oviduct to form a morula before entering the cavity ofthe uterus.
•A blastocyst forms as fluid develops in the morula, resulting in a blastocyst that consists of an inner cell mass known as the embryoblast (becomes the embryo) and the outer cell mass known as the trophoblast (becomes the placenta). At the end ofthe first week, the trophoblast differentiates into the cytotrophoblast and syncytiotrophoblast and then implantation begins.