The initial event that begins the study of embryology is fertilisation. The joining of a sperm and egg cell. Following fertilisation the oocyte (egg) forms two blastomeres, two cells of equal size. These cells undergo mitosis copying to produce a mass of cells, 16 in total. The cellular mass is called a morula.
The Morula reorganises its structure forming a cavity. This cavity is called a blastocyst and is made of two layers of cells.
The trophoblast which is the outermost layer of cells, this makes contact with the endometrium (part of the uterus) to assist with implanting the blastocyst into the uterus wall and gives rise to the placenta.
The embryo blast forms the inner part of the blastocyst which goes on to form the embryo itself. 14 days after fertilisation the blastocyst undergoes accelerated cellular differentiation. The inner embroblast specialises into different cell lines, the bilaminar disk which is made up of two types of cell, the epiblast and hypoblast. the trophoblast divides to form the syncytiotrophoblast and cytotrophoblast layers.
The blastocyst must fuse with the uterus wall. This occurs by syncytiotrophoblast and maternal sinusoids (blood vessels) fusing, this allows formation of the placenta.
Gastrulation occurs during week 3 and allows the embryo to develop the cranial-caudal axis giving rise to the three germ cell lines that will go on to form the systemic tissues.
During gastrulation a primitive node and rod form within the bilaminar disk. This node shape caused by the lining up of cells will form the cranial end (the primitive node) the axis of the body will form along the primitive rod. The cells of the epiblast being migration and differentiation around the primitive node which gives rise to three types of cell:
Lining of pulmonary and digestive tracts, bladder and reproductive system, the liver and pancreas
Notochord, muscular system, skeletal system, smooth muscles of the GI, heart and lungs
Epidermis of skin, cornea and lens of the eye, brain and spinal cord.
Now the germ cell lines are formed, the rest of the internal structures can begin formation.