Ultrastructural changes in the intestinal connective tissue of Xenopus laevis during metamorphosis.

Ultrastructural changes in the intestinal connective tissue of Xenopus laevis during metamorphosis have been studied. Throughout the larval period to stage 60, the connective tissue consists of a few immature fibroblasts surrounded by a sparse extracellular matrix: few collagen fibrils are visible except close to the thin basal lamina. At the beginning of the transition from larval to adult epithelial form around stage 60, extensive changes are observed in connective tissue. The cells become more numerous and different types appear as the collagen fibrils increase in number and density. Through gaps in the thickened and extensively folded basal lamina, frequent contacts between epithelial and connective tissue cells are established. Thereafter, with the progression of fold formation, the connective tissue cells become oriented according to their position relative to the fold structure. The basal lamina beneath the adult epithelium becomes thin after stage 62, while that beneath the larval epithelium remains thick. Upon the completion of metamorphosis, the connective tissue consists mainly of typical fibroblasts with definite orientation and numerous collagen fibrils. These observations indicate that developmental changes in the connective tissue, especially in the region close to the epithelium, are closely related spatiotemporarily to the transition from the larval to the adult epithelial form. This suggests that tissue interactions between the connective tissue and the epithelium play important roles in controlling the epithelial degeneration, proliferation, and differentiation during metamorphic climax.