STRUCTURAL STUDY OF THE FROG RANA-TEMPORARIA LARVAL STOMACH

The gastric wall of Rana temporaria tadpoles consists of a well-develo ped mucosa and thin muscular and serosa layers. Three cellular types - mucous, ciliated and endocrine cells - make up the lining epithelium. Different types of endocrine cells exist. Argyrophylic endocrine cell s can be recognized in semithin sections of plastic-embedded material while non-agyrophylic endocrine cells can only be identified under the electron microscope. Glands are composed mainly of well-differentiate d oxyntic cells and, occasionally, scarce endocrine cells. Oxyntic cel ls show abundant mitochondria and smooth endoplasmic reticulum, but do not contain zymogen granules as do those present in adults. Secretory canaliculi with microvilli are also well-developed. The lamina propri a contains numerous vascular sinuses and nerve bundles which innervate the endothelium and some endocrine cells. The neuroendocrine regulati on of frog gastric functions seems therefore to have developed in youn g tadpoles. Nerve fibers also innervate the muscular propria, which is composed of a single layer of smooth muscle cells. Underlying the mus cle, connective fibers and a flattened layer of mesothelial cells make up the serosa. In summary, the structure of the frog larval stomach s hows a well-differentiated histological pattern, especially referring to surface epithelium and glands. Some of the histological traits will also be present in adult frogs while others are characteristic of the tadpole's stage.