RELATIONSHIP BETWEEN EPITHELIAL AND CONNECTIVE TISSUES IN THE STOMACHOF THE FROG RANA-TEMPORARIA DURING METAMORPHOSIS - AN ULTRASTRUCTURAL-STUDY

In the course of metamorphosis of the stomach of Rana temporaria tadpo les there is a marked increase in the amount of active mesenchymal fib roblasts and extracellular matrix underlying the regenerating gastric epithelium. At the onset of metamorphosis, a thick PAS-positive baseme nt membrane is developed around the epithelial component of the mucosa , formed by the apical, degenerating larval epithelium and the basal, regenerative epithelial cords, Under the electron microscope, a folded basement membrane is usually revealed under the apical degenerating e pithelium while a compact basement membrane (up to 1-2 mu m thick), fo rming both patches and more extensive areas, is frequently seen around the regenerative glandular cords. Cytoplasmic processes, extending fr om both the epithelial and mesenchymal fibroblastic cells, cross the b asement membrane and make physical contact between the two cellular ty pes. At mid-metamorphosis, areas of thick PAS-positive basement membra ne are still observed around the differentiating glandular outlines, b efore disappearing completely at late metamorphosis. The probable invo lvement of intertissue interactions between epithelium and connective elements in the morphogenesis, proliferation and differentiation of se condary, definitive frog stomach is discussed. Early contacts between epithelium and phagocytes, probably related to the invasion of epithel ium by the phagocytic cells, have also been observed.