The Metamorphosis Antidote

Previews by catalyzing the removal of iodine from the inner ring. When the function of D3 was inactivated by iopanoic acid (IOPA), a specific inhibitor of the enzyme, the asym-metric growth pattern in response to thyroid hormone What if you woke up one morning and your eyes were was lost, and both dorsal and ventral CMZ grew equally. on the top of your head? You would be confused to see In complementary experiments, they introduced a D3 the sky in front of you and would worry that if you took transgene under the control of a ubiquitous or neuron-a step to the rear you would smash into the ground. specific promotor, which resulted in the inhibition of the This could never happen unless you lived in a Kafka-growth burst in the ventral retina during metamorphosis. esque story and had been transformed into a giant These results clearly show that the expression of D3 clawed frog. Xenopus really do experience such a in the dorsal CMZ inactivates thyroid hormone locally, change in the position of their eyes during their Meta-which prevents the cells from responding to the high morphosis, but they have an antidote to this disorienting level of circulating thyroid hormone during the metamor-morphogenetic movement. phic climax. In Xenopus, the laterally positioned eyes of the filter-The steroid hormones are known to regulate various feeding tadpole move dorsally and frontally during meta-aspects of nervous system development, and there are morphosis, which results in the predatory frog's binocu-several cellular mechanisms to regulate the region-spe-lar overlapping visual field (Grobstein and Comer, 1977; cific responses. The first mechanism, commonly found Grant and Keating, 1986). Unlike the retina in mammals in other endocrine systems, is the expression of the or birds, the frog retina continues to grow throughout receptor only in target tissues. The second is the expres-life by adding newly born cells to the ciliary marginal sion of different receptor subtypes in each target organ. zone (CMZ) at the periphery (Glü cksmann, 1940; Perron A striking example of this strategy is found in insect et al., 1998). It has long been known that retinal growth metamorphosis. In Drosophila, it has been established in the ventral CMZ is much greater than in the dorsal that the differential responses to ecdysone, a steroid CMZ during and after metamorphosis (Hollyfield, 1971; hormone that induces metamorphosis in insects, are Straznicky and Tay, 1977; Beach and Jacobson, 1979). regulated …