CUTANEOUS GRANULAR GLANDS AND AMPHIBIAN VENOMS

Through gradual acquisition of a set of adaptations, both morphofuncti onal and behavioural, the first amphibians were able to spend longer p eriods in a terrestrial environment. The multicellular exocrine glands of the skin represent one such adaptation. Two fundamental types of c utaneous glands are to be observed in the amphibians: mucous and granu lar (serous or venom) glands. The mucous glands produce a mucus which plays a part in a variety of functions: cutaneous respiration, reprodu ction, thermoregulation and defense. The granular glands produce a tox ic or repellent secretion with an effect on various vertebrate species ; this is one of the principal elements in amphibian defense. The gran ular glands are generally made up of a gland alveolus formed by a secr etory layer covered externally by a myoepithelial layer. This contract ile layer is related with the extrusion of glandular products. Althoug h they are responsible for the production of noxious or toxic substanc es with a variety of pharmacological effects, the granular glands may also have other functions, so far not recognized. The granular glands in the Gymnophiona acid the Urodela are made up of a secretory layer w ith, in some cases, well-individualized cells. In the course of the se cretory cycle, these cells usually lose their limits and turn into a s yncytium. The venom of the Gymnophiona is chemically and pharmacologic ally unknown. Of note in the venom of the Urodela are tetrodotoxin and steroidal alkaloids such as samandarine. In the Anura, the granular g lands have a syncytial secretory layer. Depending on the chemical comp osition of the venom secreted, the organelles of this syncytium vary i n type and concentration. In general, the venoms contain peptides, gua nidine derivatives, biogenic amines, steroids, and alkaloids. In terms of pharmacological effect, these substances are cardiotoxic, haemotox ic, neurotoxic, myotoxic, hypotensive, hypertensive and anaesthetic. I n a number of amphibian species, clusters of granular glands are to be observed in certain regions of the body. These are known as macroglan ds, and may be divided into the following types: parotoid, paracnemid, lumbar, hedonic, and pectoral. They are fundamentally related to repr oduction and defense. Parotoid macroglands occur in a number of urodel an and anuran species. They consist principally of a large concentrati on of granular alveoli, which contain the venomous secretion. In the t oads, in general, this secretion basically contains steroids and bioge nic amines. In certain species, the venom contains non-bioactive subst ances such as glycosaminoglycans and catalase; the functional signific ance of these is as yet unknown. The paracnemid macroglands are cutane ous gland dusters situated around the level of the tibia in certain an urans. In Bufo, these glands are histologically similar to the parotoi d macroglands. In some lepto-dactylids, the gland clusters are situate d immediately posterior to the sacrum, and are known as lumbar or ingu inal macroglands. Their function is supposedly defense. Their form, pi gmentation and position in the body, furthermore, makes them look like two large eyes; this suggests a role in the intimidation of potential enemies. Substances with an antimicrobial action, such as samandarine , spinaceamine and the magainins were identified in the integument of amphibians. Certain elements of the venom have a cytotoxic effect; the y inhibit the cellular respiratory process, or provoke haemolysis, or cause injuries to the organelles of the muscle fibers and the neurones . The amphibians have also developed a set of defensive behavioural ac tivities in association with the secretion of venom. In the salamander s, rigid antipredator corporal positions are to be observed; these act synergistically with the cutaneous venom. As is the case with the anu rans, certain postures permit the exposure of a larger area of concent rations of granular glands to an enemy. The parotoid and inguinal macr oglands would seem to play an important part in these defensive postur es.