Underdevelopment and overdevelopment of the skeleton, respectively. Skeletal growth is a complex process and can be distributed in many ways. For example, overstimulation by excessive growth-hormone production during childhood can produce gigantism. This is usually due to a pituitary tumor. Insufficient stimulation of skeletal growth, resulting from hormonal, metabolic, or nutritional disturbances, leads to reduced height with normal body proportions. The shortness of stature depends on the degree of the disturbance; the designation proportionate dwarfism is used in severe cases. Proportionate dwarfism may or may not be genetic. See also Pituitary gland.
Commonly, the term dwarfism is applied to short individuals with abnormal body proportions resulting from disturbances of bone growth itself. The bones of either limbs or trunk can be most affected. The disorders, called chondrodysplasias, number well over 100 and show a wide variation in clinical features: some appear at birth, others become evident in later childhood. The severity ranges from conditions in which infants die at birth to conditions compatible with normal life. In most instances, however, the abnormal skeleton creates medical problems.
Almost all of the chondrodysplasias are inherited as mendelian or single-gene traits. However, because of reduced fertility and a high incidence of new mutations for certain traits, most individuals with these disorders are born to parents of average stature. See also Human genetics; Skeletal system disorders.
