Morphology and glycoconjugate histochemistry of the palpebral glands of the adult newt, Notophthalmus viridescens

An insight into the skin glands, dermal scales and secretions of the caecilian amphibian Ichthyophis beddomei

The caecilian amphibians are richly endowed with cutaneous glands, which produce secretory materials that facilitate survival in the hostile subterranean environment. Although India has a fairly abundant distribution of caecilians, there are only very few studies on their skin and secretion. In this background, the skin of Ichthyophis beddomei from the Western Ghats of Kerala, India, was subjected to light and electron microscopic analyses. There are two types of dermal glands, mucous and granular. The mucous gland has a lumen, which is packed with a mucous. The mucous-producing cells are located around the lumen. In the granular gland, a lumen is absent; the bloated secretory cells, filling the gland, are densely packed with granules of different sizes which are elegantly revealed in TEM. There is a lining of myo-epithelial cells in the peripheral regions of the glands. Small flat disk-like dermal scales, dense with squamulae, are embedded in pockets in the dermis, distributed among the cutaneous glands. 1-4 scales of various sizes are present in each scale pocket. Scanning electron microscopic observation of the skin surface revealed numerous glandular openings. The skin gland secretions, exuded through the pores, contain fatty acids, alcohols, steroid, hydrocarbons, terpene, aldehyde and a few unknown compounds.

Light and transmission electron microscopic structure of skin glands and dermal scales of a caecilian amphibian Gegeneophis ramaswamii, with a note on antimicrobial property of skin gland secretion

Amphibian skin secretions contain a variety of bioactive compounds that are involved in diverse roles such as communication, homeostasis, defence against predators, pathogens, and so on. Especially, the caecilian amphibians possess numerous cutaneous glands that produce the secretory material, which facilitate survival in their harsh subterranean environment. Inspite of the fact that India has a fairly abundant distribution of caecilian amphibians, there has hardly been any study on their skin and its secretion. Herein, we describe, using light microscopy and electron microscopy, two types of dermal glands, mucous and granular, in Gegeneophis ramaswamii. The mucous glands are filled with mucous materials. The mucous-producing cells are located near the periphery. The granular glands are surrounded by myoepithelial cells. A large number of granules of different sizes are present in the lumen of the granular gland. The granule-producing cells are present near the myoepithelial lining of the gland. There are small flat disk-like dermal scales in pockets in the transverse ridges of the posterior region of the body. Each pocket contains 1-4 scales of various sizes. Scanning electron microscopic (SEM) study of the skin surface showed numerous funnel-shaped glandular openings. The antibacterial activity of the skin secretions was revealed in the test against Escherichia coli, Klebsiella pneumoniae, and Aeromonas hydrophila, all gram-negative bacteria. SEM analyses confirm the membrane damage in bacterial cells on exposure to skin secretions of G. ramaswamii.

Form and Function of the skin glands in the Himalayan newt Tylototriton verrucosus

BACKGROUND

Amphibians have evolved a remarkable diversity of defensive mechanisms against predators. One of the most conspicuous components in their defense is related to their ability to produce and store a high variety of bioactive (noxious to poisonous) substances in specialized skin glands. Previous studies have shown that T. verrucosus is poisonous with the potential to truly harm or even kill would-be predators by the effect of its toxic skin secretions. However, little is known on form and function of the skin glands responsible for production and release of these secretions.

RESULTS

By using light- and scanning electron microscopy along with confocal laser scanning microscopy, we show that T. verrucosus exhibits three different multicellular skin glands: one mucous- and two granular glands. While mucous glands are responsible for the production of the slippery mucus, granular glands are considered the production site of toxins. The first type of granular glands (GG1) is found throughout the skin, though its average size can vary between body regions. The second type of granular glands (GG2) can reach larger dimensions compared with the former type and is restricted to the tail region. Despite their different morphology, all three skin gland types are enwrapped by a distinct myoepithelial sheath that is more prominently developed in the granular (i.e. poison-) glands compared to the mucous glands. The myoepithelial sheath consists of one layer of regularly arranged slender myoepithelial cells that run from the gland pore to the basal gland pole.

CONCLUSIONS

This study shows that the skin in the Himalayan newt T. verrucosus displays one mucus- and two poison gland types enwrapped by a myoepithelial sheath. Contraction of the myoepithelium squeezes the glands and glandular content is released upon the skin surface where the secretion can deploy its defensive potential.

A comparative study of gland cells implicated in the nerve dependence of salamander limb regeneration

Limb regeneration in salamanders proceeds by formation of the blastema, a mound of proliferating mesenchymal cells surrounded by a wound epithelium. Regeneration by the blastema depends on the presence of regenerating nerves and in earlier work it was shown that axons upregulate the expression of newt anterior gradient (nAG) protein first in Schwann cells of the nerve sheath and second in dermal glands underlying the wound epidermis. The expression of nAG protein after plasmid electroporation was shown to rescue a denervated newt blastema and allow regeneration to the digit stage. We have examined the dermal glands by scanning and transmission electron microscopy combined with immunogold labelling of the nAG protein. It is expressed in secretory granules of ductless glands, which apparently discharge by a holocrine mechanism. No external ducts were observed in the wound epithelium of the newt and axolotl. The larval skin of the axolotl has dermal glands but these are absent under the wound epithelium. The nerve sheath was stained post-amputation in innervated but not denervated blastemas with an antibody to axolotl anterior gradient protein. This antibody reacted with axolotl Leydig cells in the wound epithelium and normal epidermis. Staining was markedly decreased in the wound epithelium after denervation but not in the epidermis. Therefore, in both newt and axolotl the regenerating axons induce nAG protein in the nerve sheath and subsequently the protein is expressed by gland cells, under (newt) or within (axolotl) the wound epithelium, which discharge by a holocrine mechanism. These findings serve to unify the nerve dependence of limb regeneration.

Description of the postcloacal glands of Plethodon cinereus, the red-backed salamander, during bouts of scent marking

Plethodon cinereus, the red-backed salamander, is a small territorial vertebrate that defends refugia located on the forest floor. As a component of territorial defense, these animals use scent marks to advertise their refugia. Behavioral evidence indicates that scent marks are produced by the postcloacal glands located on the ventral surface of the tail just posterior to the cloaca. We placed animals on unmarked substrates and recorded changes in serous acini from the postcloacal and shoulder region over a 48-h period. Within the first hour there was an increase in the number of acini filled with secretory product in the postcloacal region. At 12 h the number of full acini decreased and the number of empty acini increased. By 24 h the number of empty acini had decreased and the number of renewing acini containing secretory cells producing product had increased. By 48 h the ratio of full to empty to renewing acini was similar to those observed at the start of the study. In the shoulder region, no significant changes in the ratio of full to empty to renewing acini were observed. Observations of the serous acini within the postcloacal region and the shoulder region indicate that the mode of secretory production is holocrine. These findings are additional evidence that the postcloacal glands are the site of scent mark production.

Seasonal variations in integumental glycoconjugates of Rana rugosa

BACKGROUND

Although environmental conditions can influence the expression of glycoconjugates (GCs) in the epidermis and cutaneous gland of amphibians, seasonal features regarding GCs in frogs have rarely been studied. In the present study, we report the seasonal variations of the integumental GCs in Rana rugosa.

METHODS

GCs were investigated at the light microscopic level using conventional and lectin histochemistry.

RESULTS

During the pre- through posthibernating periods, stronger periodic acid-Schiff (PAS)-positive GCs were observed in the stratum corneum and transitional layer of the epidermis. At the same time, alcian blue and PAS-positive GCs filled the lumen of the mucous gland. The affinity for PNA in the transitional layer, UEA-1 in stratum germinativum, and SBA in the mucous glands increased significantly during the same periods. In addition, new GCs, which are specific for SBA in the stratum corneum, BSL-1, RCA-1, sWGA, and LCA in some cells of the transitional layer, were detected.

CONCLUSIONS

Elevated GCs and a diversity of lectin affinities may be a result of seasonal adaptations related to hibernation. An affinity for PNA, UEA-1, and SBA in the frog skin may be useful as a marker of seasonal changes.