Fetal alcohol syndrome (FAS) is a disease that occurs when the individual’s mother drank alcohol during the pregnancy in which they were conceived. There are many symptoms including brain damage, intellectual disabilities, and heart disease. These symptoms come from the passage of ethanol to the fetus from the placenta. The ethanol impairs nervous system development and negative impacts on other organ and developmental systems.

During the 1950’s, thalidomide was an over-the-counter drug advertised and used to treat morning sickness in pregnant women. Thalidomide was later found to cause limb and skeletal deformities in resulting fetuses. Subsequently, the sale and use of the drug has ended.

The blastocoel is a fluid-fill cavity region of the blastocyst. The blastocoel is formed through cell polarization, when trophoblasts and embryoblasts differentiate. The trophoblasts are the outer layer of cells that compact to become watertight. Embryoblasts polarize to one end of the cell and form gap junctions for cell-cell communication and developmental coordination. The trophoblasts then secrete fluid into the cavity, which forms the blastocoel.

The blastocoel is a fluid-filled cavity that is a part of the blastocyst. A compacted trophoblast cell layer surrounds the cavity, making it watertight. Trophoblast cells then secret fluid into the cavity.

The zona pellucida is a glycoprotein layer secreted by follicle cells that surrounds the released oocyte and developing embryo. Once the blastocoel grows in size during blastula development, the blastocyst “hatches” out of the zone pellucida, which then disintegrates.

Implantation is the process in which a blastocyst adheres to the female uterine wall. This is preceded by the exit of the blastocyst form the zona pellucida as well as the degradation of the endometrial extracellular matrix by proteolytic enzymes secreted by the blastocyst and initial loose connections with the endometrium. The blastocyst then adheres to the uterine wall through ligand-receptor systems and cell invasion.

The leading cause of implantation failure is decreased uterine receptivity. Other causes include problems with the developing embryo. Certain medications can be administered to increase chances of successful implantation.

The epiblast is an inner cell layer in the blastocoel that sits above the hypoblast. The epiblast ingresses to form the mesoderm and invaginates to form the endoderm.

The primitive streak is an arrangement of cells that forms during gastrula development. The location of the primitive streak establishes the left/right axes, cranial/caudal axes, and bilateral symmetry of the developing embryo.

In gastrulation, the process of ingression forms the mesoderm. In this process, the epiblast ingresses at the primary streak into the area between the epiblast and hypoblast cell layers.