Development never ceases. Throughout life, we continuously generate new blood cells, lymphocytes, keratinocytes, and digestive tract epithelium from stem cells. In addition to these continuous daily changes, there are instances in which development during adult life is obvious—sometimes even startling. One of these instances is metamorphosis, the transition from a larval stage to an adult stage. In many instances of metamorphosis, a large proportion of the animal's structure changes, and the larva and the adult are unrecognizable as being the same individual (see Figure 2.4). Another startling type of development in the adult is regeneration, the creation of a new organ after the original one has been removed. Some adult salamanders, for instance, can regrow limbs after these appendages have been amputated. The third category of developmental change in the adult is a more controversial area. It encompasses those alterations of form associated with aging. Some scientists believe that the processes of degeneration are not properly part of the study of developmental biology. Other investigators point to the genetically determined, species-specific patterns of aging and claim that gerontology, the science of aging, studies an important part of the life cycle and is therefore rightly included in developmental biology. Whatever their relationship to normative development, metamorphosis, regeneration, and aging are poised to be critical topics for the biology of the twenty-first century.

Contents

• Metamorphosis: The Hormonal Reactivation of Development
• Regeneration
• Aging: The Biology of Senescence
• References