Gastrulation and Germ Layer Derivatives

Immediately after implantation, the inner cell mass of the blastocyst differentiates into two layers: an outer epiblast, which becomes the ectoderm, and an inner hypoblast, which becomes the endoderm. Cells that remain in the space between these two layers go on to form the mesoderm. This whole movement of blastomeres, which converts the blastocyst into a three-layered gastrula bearing the three primary germ layers, is called gastrulation, and every subsequent tissue, organ and organ system in the body is built from one of these three layers during the process of organogenesis. Specifically, the ectoderm gives rise to the central nervous system (brain and spinal cord), the peripheral nervous system, the epidermis and its derivatives, and the mammary glands. The mesoderm gives rise to connective tissue, cartilage and bone, muscle, and the organs of the urinogenital system, including the kidney, ureter and gonads. The endo …