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Secretory vesicle refilling and toxin production in Ollotis nebulifer (Coastal Plain Toad) and Anaxyrus speciosus (Texas Toad) following transcutaneous electrical stimulation of the parotoid glands.

Article Excerpt
Abstract--Parotoid glands in two species of toads, Ollotis nebulifer and Anaxyrus speciosus were expressed using transcutaneous electrical stimulation. Toads were euthanized at various time stages following gland expression. Of the toads that had parotoid glands expressed, microscopic examination revealed the majority of the vesicles in the glands had emptied significantly with little to no contents remaining. General morphology of the vesicles was similar in both species with vesicles in A. speciosus somewhat smaller and more numerous than in O. nebulifer. Unexpressed vesicles were generally ovoid in shape with smooth cell walls. Emptied vesicles displayed numerous invaginations of the cell wall, resulting in the decrease of the vesicle volume. No gross injury or damage to the emptied vesicles or surrounding structures was noted. Hypertrophy of the inner (secretory) cell wall layer was evident at 24 h in both species. The majority of vesicles in both species had completely refilled and resumed normal size and shape within 5 to 7 days following gland expression.

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Amphibians possess two major types of integumentary glands that are widespread throughout the head, body, and limbs (Duellman & Trueb 1985; Zug et al. 2001). The mucous glands, which are more abundant dorsally, secrete a clear slimy fluid that maintains a thin protective layer over the skin to prevent dehydration of the animal. The granular glands, which produce a biochemical mixture of poisonous or noxious secretions, tend to be more concentrated on the head and shoulder regions (Duellman & Trueb 1985; Zug et al. 2001). Granular glands are commonly aggregated into large macrogland structures (e.g., parotoid gland; see Tyler et al. 2001 for an in-depth analysis on the nomenclature and spelling) and are present in various degrees throughout the anurans and occur in some salamanders (Duellman & Trueb 1985). Each granular gland consists of numerous alveoli or secretory vesicles that are surrounded by connective tissue (Hostetler & Cannon 1974; Cannon & Hostetler 1976; Alvarez et al. 2005). The vesicles produce and store toxic secretions and are the functional units of the granular glands. Microstructure of the vesicle is made up of a double cell layer consisting of an outer layer of myoepithelial cells that function as a contractile component and an inner layer of secretory cells that is continuous with the vesicle contents (Hostetler & Cannon 1974; Cannon & Hostetler 1976)....

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