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, 306 (1), 20-33

Amphibian Metamorphosis


Amphibian Metamorphosis

Donald D Brown et al. Dev Biol.


Fig. 1
Fig. 1. Schematic representation of some key events during X. laevis tadpole development
The time post fertilization is on the x axis. Above the line are “tadpole” blots of animals incubated for 24 h with Na 125I, fixed with formaldehyde, and washed to lower background (see Brown, 1997for the method). The tadpoles were dried on filter paper and filmed. X. laevis has 2 thyroid glands located on either side of the midline (solid arrow). The first incorporation of iodine into the thyroid gland occurs 10 days after fertilization (NF46). The solid line represents the concentrations of both TH and TRβ during development. TRβ is a direct response gene of TH.
Fig. 2
Fig. 2. The negative feedback loop between the thyroid gland and the anterior pituitary regulates the circulating levels of T4
Throughout tadpole growth TSH is secreted by the anterior pituitary and stimulates the thyroid gland to synthesize increasing amounts of T4. However, TSH production is not shut down until D2 is up-regulated at metamorphic climax in the anterior pituitary (Huang et al., 2001) and converts local T4 to the active hormone T3. This figure shows where pharmacological inhibitors interfere with the formation or inactivation of thyroid hormone.
Fig. 3
Fig. 3. Limb phenotypes at climax (NF62)
(A) Control. A dominant negative form of the thyroid receptor is expressed in (B) neural tissue; (C) collagen and skeletal structures; (D) muscle. This figure was taken from Brown et al. (2005). Scale bar: 2 mm.
Fig. 4
Fig. 4. Skin remodeling
The tadpole epidermis (upper) has 3 cell layers covering a collagen lamella and contains no glands. The cells in all three layers replicate. At climax (middle) a dermis forms, and two different kinds of skin glands appear for the first time. The skin becomes a germinative epithelium with replication only in the basal cell layer.
Fig. 5
Fig. 5. Intestine remodeling
Left, before climax (NF58); right, froglet (NF66). Below each picture is an H&E cross section of the anterior intestine (red line on upper pictures). In the 8 days of climax the intestine shrinks 75% in length and undergoes changes from a simple tube with one involution to the crypts and villi of a typical vertebrate intestine. Throughout metamorphosis there is extensive DNA replication of all of the endothelial cells. Even at the end of metamorphosis (lower right) all of the endothelial cells are replicating (Schreiber et al., 2005). Several months later replication becomes specific to the epithelial crypts. The stomach differentiates during climax. We are grateful to Alex Schreiber for these pictures.

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