Histochemistry of granular (poison) secretion in the skin of the frog, Rana pipiens

There are two types of granular glands in the skin of leopard frogs ( Rana pipiens ). Cellular glands produce fine intracellular granules rich in carbohydrate, carboxylic acids, tyrosine and sulfur‐containing amino acids. Luminal glands consist of peripherally located nuclei with scant cytoplasm. Fine granular material in these glands resembles cellular gland secretion, except that the carbohydrate is less susceptible to periodate oxidation and that a diphenol (DOPA?) is present instead of tyrosine. The prominent lumen contains large granules composed of phospholipoprotein, and are also rich in primary catecholamine and a sulfur‐containing amino acid. There was no evidence of 5‐hydroxy‐tryptamine or steroids.