Meckel's cartilage in Xenopus laevis during metamorphosis: a light and electron microscope study.

Meckel's cartilage, in Xenopus laevis prior to metamorphosis, is a tissue exhibiting very large lacunae, separated by thin rims of matrix, presenting a net-like appearance, similar to that of cartilage in invertebrates. The cells on the periphery of the tissue are rather more flattened, and more closely packed. On the lateral aspects of the cartilage distinct columns of apparently dividing cells are evident. During metamorphic climax, the amount of matrix separating the lacunae increases, with an associated decrease in lacunar size, and some of the deeper cells develop cilia, which are not seen either before or after climax. By the end of metamorphic climax there is a considerable increase in the amount of matrix present in the tissue, while many cells at all depths in the cartilage show the presence of lysosome-like structures, possibly associated with the changing shape of the cartilage. Intramembranous ossification is proceeding around Meckel's cartilage, but there is no evidence of endochondral ossification up to the end of metamorphosis.