The Questions of Developmental Biology

The more T 3 receptors a tissue has, the more competent it should be to respond to small
amounts of T 3 . Thus, metamorphic climax, that time when the visible changes of metamorphosis
occur rapidly, may be brought about by the enhanced production and induction of more T 3
receptors. The TR does not work alone, however, but forms a dimer with the retinoid receptor,
RXR. This dimer binds thyroid hormones and can enter the nucleus to effect transcription (Wong
and Shi 1995; Wolffe and Shi 1999).
The hormone prolactin has been found to inhibit the up-regulation of TR and TRβ mRNAs.
Moreover, if the up-regulation of the TR is experimentally blocked by prolactin, the tail is not
resorbed, and the adult-specific keratin gene is not activated (Tata et al. 1991; Baker and Tata
1992). Injections of prolactin stimulate larval growth and inhibit metamorphosis (Bern et al.
1967; Etkin and Gona 1967), but there is dispute as to whether this finding reflects the natural
role of prolactin (Takahashi et al. 1990; Buckbinder and Brown 1993). We still do not know the
mechanisms by which levels of thyroid hormone are regulated in the tadpole, nor do we know
how the reception of thyroid hormone elicits different responses (proliferation, differentiation,
cell death) in different tissues.
Raising Tadpoles
Most temperate zone frogs do not invest time or energy in providing for their tadpoles.
However, among tropical frogs, there are numerous species in which adult frogs take painstaking
care of their tadpoles. The poison arrow frog, Dendrobates, for example, is found in the rain
forests of Central America. Most of the time, these highly toxic frogs live in the leaf litter of the
forest floor. After laying eggs in a damp leaf, a parent (sometimes male, sometimes female)
stands guard over the eggs. If the ground gets too dry, the frog will urinate on the eggs to keep
them moist. When the eggs mature into tadpoles, the guarding frog allows them to wriggle onto
its back (see Figure 18.6A). The frog then climbs into the canopy until it finds a bromeliad with a
small pool of water in its leaf base. Here it deposits one of its tadpoles, then goes back for
another, and so on, until the brood has been placed in numerous small pools. Then each day the
female returns to these pools and deposits a small number of unfertilized eggs into them,
replenishing the dwindling food supply for the tadpoles until they finish metamorphosis (Mitchell