Abstract
BackgroundOwing to their incomplete adaptation to the terrestrial environment, amphibians possess complex cutaneous glandular systems. The skin glands not only regulate water loss and respiratory gas and salt exchange, but are also involved in defense against predators and microorganisms, social communication, and reproduction. These glands are distributed throughout the integument, but can accumulate in specific regions, forming visible outgrowths known as macroglands. Some macroglands are sexually dimorphic and mediate intersexual communication and reproductive success. The postaxillary gland is a sexually dimorphic macrogland in Nidirana pleuraden. Its biological function and its morphological and histochemical characteristics are unclear. In the present study, we describe the structure and ultrastructure of the postaxillary gland, and explore its main function.ResultsThe postaxillary gland has a thinner epidermis than the dorsal region of N. pleuraden. In addition to ordinary serous glands (OSG), type I and II mucous gland (I MG & II MG), a type of specialized mucous gland (SMG) is also found to constitute the postaxillary gland. The SMG is larger than other gland types, and consists of high columnar mucocytes with basal nuclei arranged radially toward a lumen. SMGs are positive to periodic acid-Schiff stain and stained blue in Masson’s trichrome stain. A discontinuous myoepithelial sheath lacking innervation encircles SMG mucocytes, and the outlets of such glands are X- or Y-shaped. Transmission electron microscopy reveals abundant secretory granules in SMG, which are biphasic, composed of an electron-opaque outer ring and a less electron-dense core. Lipid droplets, and organelles, such as rough endoplasmic reticulum and Golgi stacks, are located in the supranuclear cytoplasm of the mucocytes in SMG. Female N. pleuraden exhibits chemotaxis toward homogenate of the postaxillary gland, but male does not. On treatment with trypsin, this sexual attraction disappears.ConclusionsThe postaxillary gland of N. pleuraden is a male-specific macrogland that consists primarily of SMGs, together with OSGs, I MGs and II MGs. Other than their extremely large size, SMGs structurally and histochemically resemble many reported specialized gland types in amphibian sexually dimorphic skin glands. Secretions of the postaxillary gland are proteinaceous sexual pheromones, which are believed to attract females at male calling intermissions.
Highlights
Owing to their incomplete adaptation to the terrestrial environment, amphibians possess complex cutaneous glandular systems
Macroscopic structure of the postaxillary glands in N. pleuraden The postaxillary glands are male-specific, comprising a pair of well-defined and enlarged skin macroglandular structures in N. pleuraden, which lie behind bases of the forelimbs (Fig. 1a, b, c)
Structural and histochemical observations Comparing the transverse sections of the postaxillary gland with those of the dorsal skin, we found that they share the usual structure in anurans: A layer of keratinized cells covers the epidermis, below which is the dermis
Summary
Owing to their incomplete adaptation to the terrestrial environment, amphibians possess complex cutaneous glandular systems. Each type of amphibian skin gland exerts certain biological functions: for the serous glands, antipredatory defense [9, 10], nutrient storage [11], antimicrobial activities [12] and chemical communication [13, 14]; for the mucous glands, saline regulation, gas exchange, water loss control, friction reduction [1], immunoreactivity [15], pheromone production [16, 17], amplexus facilitation [18] and parental care [19]; the lipid or wax glands have been observed only in anuran species, their secretions serve to reduce abrasion and dehydration [20]; the mixed glands synergistically produce mucous and serous secretions as an adaptation to environmental and social constraints in urodele larvae [21], but probably represent a transient stage in serous gland development or restoration in anurans [18]. All amphibian skin glands involve the integration of four structural components: a myoepithelial sheath, a secretory unit, an intercalated tract (or neck), and a duct [1, 18]
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