The Questions of Developmental Biology

Snapshot Summary: Metamorphosis, Regeneration, and Aging
1. Amphibian metamorphosis includes both morphological and biochemical changes. Some
structures are remodeled, some are replaced, and some new structures are formed.
2. Many changes during amphibian metamorphosis are regionally specific. The tail epidermis
dies, the head epidermis does not. An eye will persist even if transplanted into a degenerating tail.
3. The hormones responsible for amphibian metamorphosis are the thyroid hormones thyroxine
(T 4 ) and triiodothyronine (T 3 ). The coordination of metamorphic changes appears to be due to
early changes that occur at low concentrations of the thyroid hormones. This is called the
threshold concept. The molecular basis for the autoinduction of thyroid hormones may be the
ability of thyroid hormones to induce production of more thyroid hormone receptor protein.
Thyroid hormones act predominantly at the transcriptional level.
4. Heterochrony involves changing the relative rate of development in different parts of the
animal. In animals with direct development, the tadpole stage has been lost. Some frogs, for
instance, form limbs while in the egg.
5. In neoteny, the juvenile (larval) form is slowed down, while the gonads and germ cells mature
at their normal rate. In progenesis, the gonads and germ cells mature rapidly, while the rest of the
body matures normally. In both instances, the animal can mate while in its larval form.
6. In ametabolous insects, there is direct development. In hemimetabolous insects, there is a
nymph stage wherein the immature organism is usually a smaller version of the adult. In
holometabolous insects, there is a dramatic metamorphosis from larva to pupa to sexually mature
adult.
7. In the period between larval molts, the larva is called an instar. After the last instar stage, the
larva undergoes a metamorphic molt to become a pupa. The pupa will undergo an instar molt to
become an adult.
8. During the pupal stage, the imaginal discs and histoblasts grow and differentiate to produce the
structures of the adult body.
9. The anterior-posterior, dorsal-ventral, and proximal-distal axes are sequentially specified and
involve interactions between different compartments in the imaginal discs.
10. Molting is caused by the hormone hydroxyecdysone. In the presence of high titres of juvenile
hormone, the molt is an instar molt. In low concentrations of juvenile hormone, the molt produces
a pupa; and if no juvenile hormone is present, the molt is an imaginal molt.
11. The ecdysone receptor gene can produce nRNA that can form at least three different proteins.
The types of ecdysone receptors in a cell may influence the response of that cell to
hydroxyecdysone. The ecdysone receptors bind to DNA to activate or repress transcription.
12. There are three major types of regeneration. In epimorphosis (such as regenerating limbs),
tissues dedifferentiate into a blastema, divide, and re-differentiate into the new structure. In
morphallaxis (characteristic of hydra), there is a repatterning of existing tissue with little or no
growth. In compensatory regeneration (such as in the liver), cells divide but retain their
differentiated state.