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Achondroplastic Dwarfism

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Achondroplastic24 (ah-con-dro-PLAS-tic) dwarfism is a condition in which the long bones of the limbs stop growing in childhood, while the growth of other bones is unaffected. As a result, a person has a short stature but a normal-sized head and trunk (fig. 7.12). As its name implies, achondroplastic dwarfism results from a failure of cartilage growth—specifically, failure of the chondrocytes in zones 2 and 3 of the metaphysis to multiply and enlarge. This is different from pituitary dwarfism, in which a deficiency of growth hormone stunts the growth of all of the bones and a person has short stature but normal proportions throughout the skeletal system.

Achondroplastic dwarfism results from a spontaneous mutation that can arise any time DNA is replicated. Two people of normal height with no family history of dwarfism can therefore have a child with achondroplastic dwarfism. The mutant allele is dominant, so the children of a heterozygous achondroplastic dwarf have at least a 50%

Before You Go On

Answer the following questions to test your understanding of the preceding section:

1. Describe the stages of intramembranous ossification. Name a bone that is formed in this way.
2. Describe the five zones of a metaphysis and the major distinctions between them.

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