This histochemical study comprises a comprehensive overview of acid phosphatase distribution along the entire length of the digestive tract from esophagus through large intestine of growing and metamorphosing larvae and mature frogs of a single anuran species, R a m pipiens. The main site of enzyme localization throughout the length of the tube is the supranuclear area of the mucosal epithelium. However, activity of the enzyme varies markedly, not only in different locations (e.g., weak in the esophagus, strong in the small intestine), but also in different stages of larval development. During limb bud-early metamorphic stages (I-XX), activity is localized only in epithelial cells (supranuclear zone and scattered basal foci) and in glands of the stomach. Activity in these sites increases when remodeling of the digestive tract begins in the latter stages of this period. By mid-metamorphosis (XXI-XXII), shortening of the tract has become maximal, and acid phosphatase-positive epithelial cells have degenerated and sloughed into the lumen. In contrast, enzyme activity appears (albeit transitorily) in a new location, namely, in connective tissue thickening along the length of the tract. A similar histochemical picture can be induced precociously by thyroxine treatment. By the end of metamorphosis (XXIII-XXV), activity again appears in mucosal epithelium, but in newly differentiated adulttype cells as well as in gastric and initially-forming esophageal glands (the only two types of multicellular glands in the digestive tract). In adult frogs, activity remains in these sites and in scattered points (probably phagocytes) in the connective tissue; however, activity is generally weaker than in early-stage larvae.

Histochemical patterns of alkaline phosphatase activity were demonstrated in specific regions of the digestive tube in a series of growing and metamorphosing larvae and frogs of Rana pipiens. In growing larvae activity in the mucosal epithelium was intense and sharply localized in the striated border of the duodenum posterior to the bile duct and of the mid-small intestine. No activity was found in the epithelial lining of the esophagus, stomach (or its glands), duodenum anterior to the bile duct, end of small intestine, or large intestine. Connective tissue and blood vessels throughout the length of the digestive tract displayed moderate activity. As remodeling of the gut began in metamorphosing larvae, the high phosphatase activity in the striated border decreased sharply, but activity in the connective tissue of the entire digestive tract increased substantially. As metamorphosis neared completion and a new mucosal epithelium developed, activity became strong in the striated border throughout the small intestine, including the region anterior to the bile duct and the end-small intestine. In adult well-fed frogs activity remained in this location, although in unfed frogs kept in the cold it diminished in the striated border and became prominent in subepithelial blood vessels. Enzyme activity was not detected in connective tissue (excepting random blood vessels) in any region of the frog digestive tract. These histochemical observations represent an overview of the distribution of alkaline phosphatase activity along the length of the digestive tract during larval and adult stages of a single anuran species.Metamorphic changes in the amphibian digestive tract, such as dramatic shortening and histological reorganization, have often been described. Degeneration and regeneration of the mucosal epithelium and increase in thickness of the connective tissue and muscle layers occur during normal metamorphosis (see references in Griffiths, '61; also, Liu and Lee, '30; Sussbier, '36; Joly, '37) and metamorphosis induced precociously by thyroid treatment (Swingle,'18; Lim, '20; Champy,'22; Janes, '34; Kaywin, '36; McGovern and Charriper, '57). In addition, fine structural changes have been reported for the normally metamorphosing stomach (Forte et al., '69; Forte and Forte, '701, small intestine (Bonneville, '63; Hourdry, '69a,b, '71a, '731, and miscellaneous regions of the alimentary canal (Fox et al., '721, as well as of the small intestine of thyroxine-stimulated tadpoles (Bonneville, '62; Hourdry, '70, '71b, '73).Despite these many histological investigations, as well as intensive study of alkaline phosphatase in the avian and mammalian intestine (see, e.g., review of Moog, '621, very little attention has been given to changes in activity or localization of this enzyme in the digestive tract of metamorphosing amphibians. A marked decrease in alkaline phosphatase activity occurred in intestinal homogenates during normal and induced metamorphosis of Rhacophorus schlegerii and Rana esculenta (Yanagasawa, '54; ...