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When the tail of the Xenopus laevis tadpole resorbs at the end of metamorphosis, various cell types, including muscle, fibroblasts, skin, and spinal cord, are lost at about the same time. However, feeding frogs with tails can be produced by inhibiting thyroid hormone production at the climax of metamorphosis with the goitrogen methimazole. These tails lose their fast muscle preferentially, showing that the different cell types of the tail have different fates and confirming that more than one cell death program is involved in tail resorption. Both normal and methimazole tails contain "cords," novel structures that consist of two dorsal and two ventral parallel rows of slow muscle bundles joined by collagen fibers that run the length of the tail. The cords persist until the very end of tail resorption, being the last structure to dissolve. When thyroid hormone induces expression of proteolytic enzymes in the notochord sheath, the notochord, a structural rod that runs the length of the tail, begins to buckle, demonstrating that the tail is under tension. When sections of the tail that contain cords are surgically separated from the notochord, they contract in vitro, suggesting that the cords contribute to the tension that augments tail resorption.
FIG. 1. Tail structure. (A) This cross section through the tail of a large premetamorphic tadpole (NF57) was immunostained (F59 antibody)
to reveal muscle. The large muscle masses (m) flank the notochord (n). (B) A similar section, immunostained with S58 antibody, shows slow
muscle at the periphery, as well as a ventral cord (c). (C) A section of a tail at NF64, immunostained with F59, shows the persistence of two
dorsal muscle cords. (D) A hematoxylin-stained section of a NF63 tail reveals two distinct muscle cords (c), flanking the dorsal bloodvein
(v). Scale bar in (A), 1 mm for A, B, C; 0.25 mm for D.
FIG. 2. Tail muscle cords. (A) A NF64 tail, late in metamorphosis, was isolated, bisected, and immunostained as a whole mount with S46
antibody. The periodic staining appears as four rows, representing the two dorsal and the two ventral muscle cords. Scale bar, 1 mm. (B)
The periodic muscle cords are visible in a fixed, unstained NF63 tail. (C) When the skin is peeled from the tail, the muscle cords usually
isolate with the skin (NF62). (D) Immunostained cord isolates from a NF62 tail (top) and a NF63 tail (bottom) show that both the muscle
(dark) and the intervening collagen (light) are reduced in size, as the tail is resorbed. Scale bar, 1 mm.
FIG. 3. Electron micrograph of muscle cords. The long axis of the cord is parallel to the scale bars. (A) The muscles are typical striated
muscle. Scale bar, 2 mm. (B) The gaps between the muscles are filled with collagen fibers. Scale bar, 1 mm.
FIG. 4. Origin of the cords. (A) A just-feeding tadpole (NF46), immunostained for slow muscle (S58), shows staining of the most dorsal
fibers of each myotome. Scale bar, 1 mm. (B) Higher magnification of dorsal immunostaining. Scale bar, 0.25 mm. (C) The ventralmost fibers
are also slow muscle. Scale bar, 0.25 mm. (D) Dorsal view of a large NF60 tadpole, immunostained for slow muscle, shows the attenuation
of the m. longissimus dorsi of the trunk (right) into the dorsal cords of the tail (left). Scale bar, 1 mm.
FIG. 5. Tailed frog produced by methimazole treatment. (A) This tailed frog had been feeding for about a month. Its snoutâvent length was
24 mm. (B) A higher magnification of the bend in the tail, in the same orientation as in (A), shows the dorsal and ventral blood vessels
running a straighter course than the central part of the tail, which contains the notochord.
FIG. 6. Tail structure in a tailed frog, produced by methimazole. (A) A hematoxylin-stained cross section shows a large notochord (n),
peripheral muscle, dorsal and ventral cords (c), and the absence of the large chevron muscles. (B) A cross section, immunostained with F59
for fast and slow muscle, shows the paucity of muscle. Compare this section to Fig. 1A. (C) A cross section, immunostained with S46, shows
that most if not all of the muscle, seen in (B), is the slow type. Scale bar in (A), 0.5 mm for A, B, C.
FIG. 7. Compression of the notochord. (A) In this cleared animal at NF64, the notochord in the tail is bent and folded. (B) This tail was
cut from the animal (NF63) and carefully bisected, so as not to injure the notochord. The muscle-containing pieces contracted back, and
the notochord had a wrinkled appearance. Scale bar in (A), 2 mm for A, 1 mm for B.
FIG. 8. Dissection of the tail from a tailed frog. The tail was sliced
longitudinally to isolate, from top to bottom, the dorsal fin, the
dorsal muscle cord, the notochord, the ventral muscle cord, and the
ventral fin. The strips of tissue containing the dorsal and ventral
muscle cords are contracted relative to the fins and the notochord.
Scale bar, 2 mm.
