MODEL RELEASED. Boys with the genetic condition ectodermal dysplasia. Two young brothers aged 4 years (left) and 2 1/2 years, both of whom have X-linked hypohidrotic ectodermal dysplasia (HED). Ectodermal dysplasia (ED) describes a group of developmental conditions affecting the ectoderm, the layer of cells that form the skin and nervous system. Typical symptoms include very fine hair, thin skin, missing or mis-shapen teeth and high susceptibility to eczema and asthma. In HED, there is the added complication that the sufferer has a reduced capacity to sweat. As sweating is the main way the body loses heat, particular care must be taken to prevent hyperthermia, especially in the summer. X-linked HED results from a mutaion of the EDA gene on the X chromosome. Males only need one copy of this to have the condition, but are unable to pass it on to their children, the mother is the carrier.