The Questions of Developmental Biology

Organ-specific response to thyroid hormones is dramatically demonstrated by
transplanting a tail tip to the trunk region or by placing an eye cup in the tail (Schwind 1933;
Geigy 1941). The tail tip placed in the trunk is not protected from degeneration, but the eye
retains its integrity despite the fact that it lies within the degenerating tail (Figure 18.5). Thus, the
degeneration of the tail represents an organ-specific programmed cell death. Only specific tissues
die when a signal is given. Such programmed cell deaths are important in molding the body. The
degeneration of the human tail during week 4 of development resembles the regression of the
tadpole tail (Fallon and Simandl 1978).
Coordination of developmental changes.
One of the major problems of metamorphosis is the coordination of developmental
events. For instance, the tail should not degenerate until some other means of locomotion the
limbs has developed, and the gills should not regress until the animal can utilize its newly
developed lung muscles. The means of coordinating metamorphic events appears to be a
difference among tissues and organs in their responsiveness to different amounts of hormone
(Saxén et al. 1957; Kollros 1961). This model is called the threshold concept. As the
concentration of thyroid hormones gradually builds up, different events occur at different
concentrations of the hormones. If tadpoles are deprived of their thyroids and are placed in a
dilute solution of thyroid hormones, the only morphological effects are the shortening of the
intestines and accelerated hindlimb growth. However, at higher concentrations of thyroid
hormones, tail regression is seen before the hindlimbs are formed. These experiments suggest that
as thyroid hormone levels gradually rise, the hindlimbs develop first and then the tail regresses.
Similarly, when T 3 is given to tadpoles, it induces the earliest-forming bones at the lowest
dosages and the last bones at higher dosages, mimicking the natural situation (Hanken and Hall
1988). Thus, the timing of metamorphosis appears to be regulated by the sensitivity of different
tissues to thyroid hormones.