Summary
Highlights
Human prenatal development begins with fertilization in the fallopian tube. The fertilized egg, known as a conceptus, undergoes rapid mitotic cell divisions (cleavage) as it travels to the uterus. After approximately 3 days and 16 cells, it becomes a morula, whose cells are totipotent and can differentiate into any cell type.
The morula continues to divide in the uterus, forming a blastocyst with about 100 cells. The blastocyst consists of an inner cell mass (embryoblast) destined to become the embryo and an outer shell (trophoblast) that nourishes it and eventually forms the chorion. The trophoblast aids in 'hatching' and implantation onto the endometrium around the end of the first week. The syncytiotrophoblast, formed from the trophoblast, secretes HCG to maintain pregnancy by instructing the corpus luteum to produce progesterone.
By the middle of week 2, the embryoblast develops into a two-layered embryonic disc: hypoblast and epiblast, establishing the dorsal/ventral axis. By the end of week 2, these layers form the yolk sac and amnion, respectively. The yolk sac provides nutrients and facilitates gas exchange and is the first site of embryonic blood cell production. The amnion later forms a protective fluid-filled sac around the embryo.
In week 3, gastrulation transforms the two-layered disc into three germ layers: ectoderm, mesoderm, and endoderm. This process begins with the primitive streak forming on the epiblast, which sets the cranial/caudal axis. Epiblast cells migrate through the primitive streak to displace the hypoblast, forming the endoderm, then fill the space between the endoderm and epiblast to create the mesoderm. The remaining epiblast cells become the ectoderm. Each germ layer differentiates into specific organs and tissues, with primitive forms present by the end of the embryonic period.
The yolk sac gives rise to the allantois, which forms part of the umbilical cord and primitive urinary bladder. The amnion grows around the embryo by week 4, filled with amniotic fluid that protects the embryo from trauma, regulates temperature, and allows movement. The yolk sac, amnion, allantois, and chorion are the four embryonic membranes crucial for supporting the developing embryo.