ectodermal cells because of a genetic defect that happens early in development. The most common type of ectodermal dysplasia is called hypohidrotic ectodermal dysplasia or HED. Patients with HED are unable to sweat because the ectoderm never formed sweat cells. Some of these patients also have disfigurement of the face and missing teeth, wrinkling of the skin near the eyes, thin hair, and a misshapen nose. This is an X-linked recessive disease affected males almost exclusively. Females, however, can be carriers of the disease.
Mesodermal Cells The mesoderm is the middle embryonic layer, which is packed between the endoderm layer and the ectoderm layer. It forms non-epithelial blood cells and muscle cells. It forms the mesentery of the human/mammal and the part of the gonads that is not made directly from gametes. Like other aspects of differentiation, the formation of the various organs relies on intercellular signaling that polarizes cells into different areas of the embryo. Besides making these structures, the mesoderm is the inducing factor for the development of the neural plate (which is ectodermal-derived tissue). The separation of the embryo into the three different layers and the process of gastrulation occurs in the third embryonic week. During this process, three types of mesoderm are produced: paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm. Each has its own function. Paraxial mesoderm forms the bones, cartilage, and subcutaneous skin tissue. Cells called somites are responsible making these tissues and are turned on by nearby embryonic structures, such as the epidermis, neural tube, and neural cord. The intermediate mesoderm gives rise to the urogenital organs of the body, including the gonads (that aren’t gametes), kidneys, adrenal glands, and the ducts that connect these structures. The lateral plate mesodermal cells make the heart, the blood vessels, and the blood cells. Parts of the limbs deep to the skin are created from mesoderm. All muscle types come from mesoderm, including skeletal muscle, cardiac muscle, and smooth muscle. Connective tissue, bone, cartilage, subcutaneous fat, blood cell endothelium, white blood cells, kidneys, adrenal cortex, and red blood cells all come from mesoderm. The notochord production is under the control of the mesoderm, forming the neural tube and the linear axis of the embryo, extending from the head of the embryo to the tail of the embryo. The part of the mesoderm that forms the paraxial mesoderm forms the covering over the notochord. The process of making the notochord happens between the fifteenth and seventeenth day of embryonic development. The paraxial mesoderm is intimately connected to spinal cord development. It is these cells that grow up and down the embryo, covering the neural structures and ultimately forming the backbone or axial skeleton. There are different somites in the mesoderm that differentiate into their own compartment that contains specific muscles, cartilage, and tendons—each of which is serviced by a different dermatome that comes from the ectoderm. As mentioned, multiple different somites from the mesodermal germ layer separate into the various bones of the spinal column. There is notochord protein that builds up in the mesoderm, allowing the somites to be established. Protein SHH is activated by the cells of the notochord
