The Questions of Developmental Biology

18. Metamorphosis, regeneration, and aging
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.
Metamorphosis: The Hormonal Reactivation of Development
In most species of animals, embryonic development leads to a larval stage with
characteristics very different from those of the adult organism. Very often, larval forms are
specialized for some function, such as growth or dispersal. The pluteus larva of the sea urchin, for
instance, can travel on ocean currents, whereas the adult urchin leads a sedentary existence.
The caterpillar larvae of butterflies and moths are specialized for feeding, whereas their adult
forms are specialized for flight and reproduction, often lacking the mouthparts necessary for
eating. The division of functions between larva and adult is often remarkably distinct (Wald
1981). Cecropia moths, for example, hatch from eggs and develop as wingless juveniles
(caterpillars) for several months. All this development enables them to spend a day or so as fully
developed winged insects, mating quickly before they die. The adults never eat, and in fact have
no mouthparts during this short reproductive phase of the life cycle. As might be expected, the
juvenile and adult forms often live in different environments.
During metamorphosis, developmental processes are reactivated by specific hormones,
and the entire organism changes to prepare itself for its new mode of existence. These changes
are not solely ones of form. In amphibian tadpoles, metamorphosis causes the developmental
maturation of liver enzymes, hemoglobin, and eye pigments, as well as the remodeling of the
nervous, digestive, and reproductive systems. Thus, metamorphosis is often a time of dramatic
developmental change affecting the entire organism.
Amphibian Metamorphosis
Morphological changes associated with metamorphosis
In amphibians, metamorphosis is generally associated with the changes that prepare an
aquatic organism for a primarily terrestrial existence. In urodeles (salamanders), these changes
include the resorption of the tail fin, the destruction of the external gills, and a change in skin
structure. In anurans (frogs and toads), the metamorphic changes are more dramatic, and almost
every organ is subject to modification (see Figure 2.4; Table 18.1).