Anatomy and Physiology Resource Site

10.7 - Embryology

Within one hour after fertilization the sperm and ovum nuclei fuse to create the new diploid zygote.

Here are the basic developmental stages following the zygote before the formation of a true embryo:

Cleavage - begins approximately 24 hours after fertilization; the zygote divides rapidly as it travels down the oviduct
Morula - solid ball of 16 cells; has now reached the uterus
Blastocyst - hollow ball of 100 cells, secretes human chorionic gonadotropin (hCG), a hormone similar to LH that keeps the corpus luteum from degenerating

Trophoblast - outer layer of the blastocyts
Inner cell mass - small dense ball of cells at the edge of the blastocyts
Implantation of the embryo in the uterine lining

Gastrula stage

The primary germ layers begin to form in the inner cell mass

Cell differentiation

Cell differentiation is the process by which cells become specialized for a specific purpose in the body.

All cells start out pluripotent and capable of differentiating to become any cell type in the body. All zygote cells up until the blastocyst stage are pluripotent and undifferentiated.

During the blastocyst stage, cells become multipotent as they begin to differentiate. The primary germ layers form, which are composed of three types of multipotent cells:

Ectoderm - will form the nervous system, epidermis, hair and nails
Endoderm - will form the mucous membranes and associated glands
Mesoderm - will form the dermis, cardiovascular, muscular, skeletal and excretory systems

Development of the fetus

Before the third week of development the placenta forms to allow the placental villi to exchange materials between the fetus and the maternal blood. The placental villi are bundles of capillaries that float in a pool of maternal arterial blood to facilitate the exchange.

The umbilical cord contains the umbilical artery and vein and carries blood between the fetal circulation and the placenta.

By week 12 the placenta takes over estrogen and progesterone production to maintain the uterine lining for the duration of a pregnancy.

The chorion is the thickened trophoblast layer from the blastocyst and fuses with the amnion to create the amniotic sac, filled with amniotic fluid. The amniotic fluid cushions and protects the fetus.