cells for continuously growing teeth form from tissue layers called the stellate reticulum and the suprabasal layer of the surface ectoderm. Clinical significance Ectodermal dysplasia Ectodermal dysplasia is a rare but severe condition where the tissue groups (specifically teeth, skin, hair, nails and sweat glands) derived from the ectoderm undergo abnormal development. This is a diffuse term, as there are over 170 subtypes of ectodermal dysplasia. It has been accepted that the disease is caused by a mutation or a combination of mutations in certain genes. Research of the disease is ongoing, as only a fraction of the mutations involved with an ectodermal dysplasia subtype have been identified. Hypohidrotic ectodermal dysplasia (HED) is the most common subtype of the disease. Clinical cases of patients with this condition display a range of symptoms. The most relevant abnormality of HED is hypohidrosis, the inability to produce sufficient amounts of sweat, which is attributed to missing or dysfunctional sweat glands. This aspect represents a major handicap particularly in the summer, limits the patient's ability to participate in sports as well as his working capacity, and can be especially dangerous in warm climates where affected individuals are at risk of life-threatening hyperthermia. Facial malformations are also related to HED, such as pointed or absent teeth, wrinkled skin around the eyes, a misshaped nose along with scarce and thin hair. Skin problems like eczema are also observed in a number of cases. Most patients carry variants of the X-chromosomal EDA gene. This disease typically affects males more severely because they have only one X chromosome, while in females the second, usually unaffected X chromosome may be sufficient to prevent most symptoms. See also Ectoderm specification Coelom Embryology Endoderm Gastrulation Mesoderm Neural plate List of human cell types derived from the germ layers References Germ layers Animal developmental biology Embryology Gastrulation