Serous cutaneous glands in the paludiculine frog Physalaemus biligonigerus (Anura, Leptodactylidae): patterns of cytodifferentiation and secretory activity in premetamorphic specimens

The development of serous cutaneous glands is described in tadpoles of the leptodactylid frog Physalaemus biligonigerus , with attention to the cytodifferentiation processes of the secretory unit (adenomere). Secretory differentiation causes the adenomere to assume the structure of a syncytium and triggers biosynthesis of the serous product (poison), consisting of granules with peculiar repeating substructure. The secretory granules resemble glomerular-like aggregates of randomly orientated tubular subunits. This complex substructure derives from the rearrangement of the dense, compact material released by the Golgi apparatus in the form of small, membrane-bounded particles. Both early and advanced features of P. biligonigerus poison closely resemble those found during serous gland development in the Italian treefrog Hyla intermedia . The similar aspects described during serous biosynthesis in these leptodactylid and hylid frogs suggest they are phylogenetically informative. On the other hand, the peculiar features of their poisons may reflect the differential patternings of their secretory repertories along the divergent phylogenetic paths, which led these frogs to be currently included in different families.     

Serous gland polymorphism in the skin of<Emphasis Type="Italic"> Phyllomedusa hypochondrialis azurea</Emphasis> (Anura,Hylidae): response by different gland types to norepinephrine stimulation

Stimulation by norepinephrine in physiological concentration was used on the dorsal skin of the Argentine tree-frog Phyllomedusa hypochondrialis azurea to trigger contraction of myoepithelial cells encircling the serous glands and provoke secretory release. This hylid species possesses two kinds of serous cutaneous glands, producing secretory granules or vesicles (type Ia and Ib serous units, respectively), along with serous-derived glands which synthesise lipids and store them in complex aggregates (type II units). Structural and ultrastructural observations on myoepithelia, secretory units and gland products collected in saline after discharge, revealed consistent but different responses in the three types investigated. Type Ia glands reacted intensely to treatment, with both contractile and secretory responses, type Ib glands were only mildly affected in their myoepithelia and glands of type II were not affected at all. According to data available in the literature, these findings suggest that: (a) the dense (type Ia) granules are expelled as a phasic response through bulk (holocrine) discharge, (b) the secretory (type Ib) vesicles are released as a tonic response through a merocrine mechanism and (c) lipid (type II) aggregates are exuded as a secretory component of a complex behavioural response which tends to reduce transcutaneous water loss. Furthermore, these findings indicate that the use of pharmacological modulation of myoepithelial activity allows selective collection of skin products in species characterised by serous gland polymorphism.     

Morphology of the exocrine glands of the frog skin

Frog skin contains three distinct types of exocrine glands: granular (poison), mucous, and seromucous. The granular gland forms a syncytial secretory compartment within the acinus, which is surrounded by smooth muscle cells. The mucous and seromucous glands are easily identifiable as distinct glands. The serous and mucous secretory cells are arranged in a semilunar configuration opposite the ductal end and are filled with granules. Within the acinus, located at the ductal pole of the gland, are distinct groups of cells with few or no granules in the cytoplasm. In both the mucous and seromucous gland there is a cell type with abundant mitochondria; the one in the mucous gland is located in the region adjacent to the secretory cells. The duct of these glands is two-layered, with the individual cells appearing morphologically similar to the layers of the skin epithelium as the duct traverses the skin. The duct appears to be patent throughout its length. The morphological heterogeneity and distinct distribution of the cell types within the gland acinus may be indicative of a functional heterogeneity that allows the production of distinctly different types of secretion from the same gland type, depending on the type of stimulus.     

Three types of cutaneous glands in the skin of the salamandrid Pleurodeles waltl. A histological and ultrastructural study

Histological and ultrastructural investigations revealed three different multicellular skin gland types in the salamandrid Pleurodeles waltl. The mucous glands are small, with one layer of secretory cells surrounding a central lumen; they produce the viscous and slippery mucus film that has various functions in amphibians. The serous glands can be divided based on their histological and ultrastructural characters into the granular gland Type I (GGI) and the granular gland Type II (GGII). The first type (GGI) is moderately sized and distributed throughout the body surface, with higher concentrations in the parotoid and back regions. In contrast, the second type (GGII) is very large (for Pleurodeles) and was found only in the tail, with highest concentration in the tail dorsum. Both granular gland types contain mainly proteinaceous materials but differ in their morphological features including size, shape, cellular organization and vesicle distribution, vesicle size and vesicle shape. Both GGI and GGII are especially concentrated in body parts that are presented to an attacking predator and are hypothesized to produce repellent to poisonous substances to thwart potential aggressors. J. Morphol., 2009. © 2009 Wiley‐Liss, Inc.     

Serous gland dimorphism in the skin of Melanophryniscus stelzneri (Anura: Bufonidae)

Two serous gland types (I and II) in the skin of the Argentine toad Melanophryniscus stelzneri were discovered using light and electron microscopy. Glands of the two types differ in several traits: features of the products (both mature and immature), organelles involved in biosynthesis, and paths of serous maturation. No consistent differences, however, were detected between the myoepithelial sheaths encircling the secretory units. Type I glands manufacture vesicles containing a single dense body with a repeating substructure and conform to the fundamental secretory line of bufonid skin, a secretory line involved in biosynthesis of steroids. Type II glands synthesize granules of varying densities and seem to belong to a line of glands that secrete proteinaceous products. The occurrence of the two serous gland types in Melanophryniscus stelzneri is discussed in a comparison with current literature on the morphofunctional characteristics of anuran poison glands, which perform both regulative and defensive roles. It is suggested that di- or polymorphism in serous glands is an adaptive trait that allows differential release of active molecules on the body surface. J. Morphol. 237:19–32, 1998. © 1998 Wiley-Liss, Inc.     

New insights into sexually dimorphic skin glands of anurans: The structure and ultrastructure of the mental and lateral glands in Hypsiboas punctatus (Amphibia: Anura: Hylidae)

Many anuran species are characterized by sexually dimorphic skin glands. These glands often are concentrated on specific areas, such as the mental region, flanks, or the nuptial pads. We studied the histology and histochemistry of mental and lateral glands in Hypsiboas punctatus, and compared them to skin from other body regions. We describe four types of dermal glands, two types of mucous and two types of serous glands. The mucous glands are formed by a single layered epithelium. The mucocytes surrounding a central lumen are filled with polyhedral granules. Ordinary mucous glands are small sized glands with cubical epithelium, mucoid content, and small granules. Specialized mucous glands are characterized by a larger size, a columnar epithelium, a proteinaceous content and larger granules. Both types of serous glands are syncytial and share some structural features including size, shape, and morphology of secretory granules. However, ordinary and specialized serous glands differ in their histochemical properties, size and appearance of secretory granules, and glandular outlets. The specialized type of mucous glands in H. punctatus resembles most SDSGs described in anurans, whereas the presence of specialized serous glands that are sexually dimorphic is less common. Both specialized glands occur only in mental and lateral regions of males, whereas ordinary mucous and ordinary serous glands occur in males and females. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Distinct expression of calnexin in major human salivary glands

Calnexin (Cnx) has been characterized as a membrane-bound protein that transiently interacts in a unique chaperone system with newly synthesized glycoproteins in order to allow the establishment of their proper tertiary and, in most cases, quarternary structures. The aim of the study was to identify and to locate the expression of Cnx in the three major salivary glands of humans by different methods. Strong expression of Cnx protein and mRNA were generally found in serous salivary secretory units. With regard to mucous secretory units, expression of Cnx was only detectable at a low level in mucous acinar cells of sublingual glands, but not of submandibular glands. Expression of Cnx was always preserved in the surface epithelium of intralobar and interlobular duct segments. In addition, expression of Cnx was detected in sebaceous glands of parotid tissues, with a distribution pattern resembling that seen in sebaceous glands of the normal skin. In conclusion, production of saliva is associated with the expression of Cnx. Synthesis of molecules in mucous secretory units is not necessarily associated with a strong Cnx expression, whereas synthesis in serous secretory units apparently is. The tissue-specific Cnx expression is also paralleled by the observation that the secretions produced by the major salivary glands differ in their composition and amount.     

Serous cutaneous glands in new world hylid frogs: an ultrastructural study on skin poisons confirms phylogenetic relationships between Osteopilus septentrionalis and Phrynohyas venulosa

Transmission electron microscope investigations of the serous (poison) skin glands in the New World tree frogs Osteopilus septentrionalis and Phrynohyas venulosa revealed that they produce granules with closely similar substructures, namely, a dense cortex and pale medulla. In both species these features, that contrast the complex, sometimes repeating patterns described in other hylid frogs, derive from similar secretory and maturational processes starting from the Golgi phase of poison biosynthesis. Observations on secretory discharge showed that the two species share common release mechanisms, based on bulk discharge (holocrine) processes. Our data provide novel evidence of the extensive ultrastructural polymorphism of serous skin products in Hylidae and agree with phylogenies that regard this family as polyphyletic in origin. Assuming that ultrastructural features of cutaneous poison biosynthesis and maturation are adequate clues for tracking anuran phylogeny, the present findings also support a close relationship between Osteopilus and Phrynohyas taxa as previously suggested by osteological evidence.     

Vasculature of the parotoid glands of four species of toads (bufonidae: bufo)

The parotoid glands of toads (Bufonidae) consist of large aggregations of granular glands located between the otic region of the skull and the scapular region. To determine the circulatory pattern of these glands, we perfused the vascular systems of Bufo alvarius, B. marinus, B. terrestris, and B. valliceps with either India ink or Microfil, a fine latex. The perfused glands were studied by gross dissection, microscopic examination, and histology. The vascular patterns of the parotoid glands were compared to the arrangement of vessels in the dorsal skin of Rana sphenocephala (Ranidae), a frog that lacks parotoid glands. The parotoid glands of the four species of toads are supplied with blood by the lateral and dorsal cutaneous arteries and are drained by one or more branches of the internal jugular vein. The dorsal cutaneous artery supplies most of the blood to the parotoid glands in B. terrestris and B. valliceps. In B. alvarius and B. marinus, both the lateral and dorsal cutaneous arteries serve major roles in the blood supply of the glands. These patterns of blood flow have not been described previously for parotoid glands and conflict with earlier accounts for B. alvarius and B. marinus. The arteries and veins associated with the parotoid glands of toads are present in R. sphenocephala, but are arranged differently. In R. sphenocephala, the lateral cutaneous artery supplies the dorsal and lateral skin posterior to the shoulder region, whereas the dorsal cutaneous artery supplies the skin of the shoulder region. In toads, both the lateral and dorsal cutaneous arteries supply the skin of the shoulder region and ramify into subcutaneous capillaries that surround the secretory units of the parotoid glands. Extensive vasculature presumably is important for delivering cholesterol and other precursor molecules to the parotoid glands, where those compounds are converted into toxins.     

Carbonic anhydrase in the rat salivary glands: distribution and ultrastructural localization

Rat salivary glands were studied by Hanson's method to specify the ultrastructural localization of carbonic anhydrase (CA). Two different procedures were used: 1) The embedding of the tissues in water-soluble resins, followed by the incubation of the resin sections on the medium. 2) The embedding in epon-araldite of previously incubated frozen sections. Light and electron microscopy were used to observe the distribution and the ultrastructural localization of the cobalt precipitate. In parotid and mandibular glands, CA was localized in the secretion granules and the hyaloplasma of the secretory endpieces. The enzyme was also detected on the basal and lateral membranes of the striated duct cells in the three glands. In the convoluted granular duct cells of the mandibular gland CA was found in the hyaloplasma only. In the sublingual gland, CA was localized in the hyaloplasma of the serous crescents and no activity was detected in the mucous tubules. As regards the localization of the enzyme in the granules of the secretory endpieces of parotid and mandibular glands, it appears that CA has to be considered as a secretory product of these cells; this localization is consistent with the presence of the enzyme in rat saliva.     

Ultrastructure of poison glands of South American frogs: a comparison between Physalaemus albonotatus and Leptodactylus chaquensis (Anura: Leptodactylidae)

Serous (poison) cutaneous glands of the leptodactylid species Physalaemus albonotatus and Leptodactylus chaquensis were compared using light and transmission electron microscopy. Glands in the two species share structural traits common in anurans, including the peripheral contractile sheath (myoepithelium) and the syncytial secretory unit that produces, stores, and modifies the poison. At the ultrastructural level, early steps of poison production are also similar and fit the usual path of proteosynthesis, involving rough endoplasmic reticulum (RER) and Golgi stacks (dictyosomes) in the peripheral syncytial cytoplasm. However, several differences are obvious during the maturational processes that lead post-Golgian products to their ultimate ultrastructural traits. In P. albonotatus, the dense product released from the dictyosomes acquires a thick repeating substructure, which, however, becomes looser in the inner portion of the syncytium. In L. chaquensis, serous maturation involves gradual condensation, and opaque, somewhat "vacuolized" granules are formed. These different maturational paths expressed during poison manufacturing in the two species agree with the polyphyletic origin of the family Leptodactylidae. On the other hand, data collected for P. albonotatus fit previous findings from P. biligonigerus and stress the view that poisons produced by congeneric species share similar (or identical) ultrastructural features.     

Histochemical and biochemical determination of calcium in salivary glands of cat

Although feline salivary glands have been used in investigations on secretion and microlithiasis and both processes involve calcium, nothing is known about its distribution in these glands. Therefore we have demonstrated the presence of calcium by a histochemical technique using glyoxal bis(2-hydroxyanil) and a biochemical technique using dry ashing. The histochemical technique stained serous acinar cells weakly and rarely found mucous acinar cells strongly in the parotid gland, mucous acinar cells moderately to strongly and serous acinar cells weakly in the sublingual gland, and central and demilunar acinar cells moderately to strongly in the submandibular gland. The biochemical technique revealed less calcium in the parotid than in the submandibular and sublingual glands. Both techniques revealed a decrease of calcium in submandibular and sublingual glands following parasympathetic stimulation. The histochemical distribution of calcium, which corresponds to that of acinar secretory glycoprotein, and the loss of calcium following parasympathetic stimulation, which causes release of secretory granules, indicate the presence of calcium in secretory granules. The concentration of calcium in the different types of acinar cell corresponds to the acidity of the secretory glycoprotein and suggests that calcium is present as a cationic shield to allow the condensation of polyionic glycoprotein in secretory granules.     

Peptide secretion in the cutaneous glands of South American tree frog Phyllomedusa bicolor: an ultrastructural study

The development of the dermal glands of the arboreal frog Phyllomedusa bicolor was investigated by immunocytochemistry and electron microscopy. The 3 types of glands (mucous, lipid and serous) differed in size and secretory activity. The mucous and serous glands were apparent in the tadpole skin, whereas the lipid glands developed later in ontogenesis. The peptide antibiotics dermaseptins and the D-amino acid-containing peptide opioids dermorphins and deltorphins are abundant in the skin secretions of P. bicolor. Although these peptides differ in their structure and activity they are derived from precursors that have very similar preproregions. We used an antibody to the common preproregion of preprodermaseptins and preprodeltorphins and immunofluorescence analysis to show that only the serous glands are specifically involved in the biosynthesis and secretion of dermaseptins and deltorphins. Scanning and transmission electron microscopy revealed that the serous glands of P bicolor have morphological features, especially the secretory granules, which differ from those of the glands in Xenopus laevis skin.     

Development of Xenopus laevis skin glands producing 5-hydroxytryptamine and caerulein

Summary The granular glands in Xenopus laevis skin are known to contain large quantities of biogenic amines and bioactive peptides which closely resemble mammalian brain-gut peptides. We studied the development of glands producing 5-hydroxytryptamine (5-HT) and caerulein using immunohistochemistry, HPLC-fluorometric systems and RIA. The immunoreactivities of 5-HT and caerulein were first detected in the spherical gland rudiments in the stratum spongiosum at St. 58 (Nieuwkoop and Faber stage), or at the beginning of metamorphosis. Both immunoreactivities appeared in the same rudiment at the same time. Some of the gland rudiments have a small lumen filled with both immunoreactive materials at St. 58–59. During the rest of the metamorphic period, the glands grow in size, accumulating immunoreactive materials in the lumen. The concentrations of 5-HT and caerulein in the skin of tadpoles were below 1 ng per mg wet tissue at St. 58–59, increased as metamorphosis proceeded and reached 63 and 134 ng per mg wet tissue at St. 66, or at the end of metamorphosis, respectively. The amphibian granular glands where large quantities of biogenic amines and hormone-like peptides are rapidly synthesized may provide a useful model for the study of the development of amine- and peptide-producing cells including neurons and paraneurons.     

Distribution of S-100 protein and its subunits in bovine exocrine glands

 The distribution of S-100 protein and its α- and β-subunits in bovine exocrine glands was studied by indirect immunohistochemistry. The entire spectrum of salivary glands, glands of the respiratory tract, intestinal glands, male and female genital glands, and skin glands was examined. S-100 and its β-subunit were identified in most serous secretory cells of mixed salivary glands, although secretory acini in some serous glands remained unreactive for these antigens. Mucous cells were constantly negative; mucoid cells were positive in the lacrimal and Harderian gland. The α-subunit of S-100 protein was identified in serous cells but the staining reaction was faint. Subunits of S-100 showed a characteristic distribution along the excretory duct systems of compound glands: S-100 and the β-subunit were present in intercalated duct epithelium, while striated duct epithelium stained for S100-α. Therefore, it is suggested that S100-α is related to resorption and secretion in striated ducts, while S100-β may govern acinar exocytosis and probably regulates proliferation and differentiation of glandular cells. Differing staining intensities for S-100 and its subunits in secretory cells of exocrine glands most probably indicate functional differences with regard to secretory activity and the cell cycle. Accepted: 11 February 1997     

Skin peptides in Xenopus laevis: morphological requirements for precursor processing in developing and regenerating granular skin glands

The biosynthesis of the peptides caerulein and PGLa in granular skin glands of Xenopus laevis proceeds through a pathway that involves discrete morphological rearrangements of the entire secretory compartment. Immunocytochemical localization of these peptides during gland development indicates that biosynthetic precursors are synthesized in intact secretory cells, whereas posttranslational processing requires morphological reorganization to a vacuolated stage. The bulk of the processed secretory material is then stored in vacuolae- derived storage granules. In the mature gland, storage granules are still formed at a low level. However, in this case processing takes place in a distinct cytoplasmic structure, the multicored body, which we suggest to be functionally equivalent to vacuolae. When granular glands regenerate after having lost all their storage granules upon strong stimuli, another morphological pathway is used. 2 wk after gland depletion, secretory cells become arranged in a monolayer that covers the luminal surface of the gland. Storage granules are formed continuously within these intact secretory cells. Here, precursor processing does not require a vacuolated stage as in newly generated glands but occurs in multicored bodies. Most storage granules seem to be formed in the third week of regeneration. The high biosynthetic activity is also reflected by the high activity of the putative processing enzyme dipeptidyl aminopeptidase during this period of regeneration.     

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.     

Loss of traditional mucous glands and presence of a novel mucus-producing granular gland in the plethodontid salamander Ensatina eschscholtzii

All amphibians are thought to possess two distinct types of dermal gland: mucous and granular. Mucous glands typically produce an acidic, flocculent and carbohydrate positive secretion that keeps skin moist and helps maintain homeostasis. In this study, we use a battery of histochemical tests to identify these traditional mucous glands in the dermis of the plethodontid salamander Ensatina eschscholtzii . The periodic acid-Schiff, alcian blue, toluidine blue and carmine reactions reveal the absence of both acidic glycosaminoglycans and traditional mucous glands. Instead, E. eschscholtzii produces neutral mucus in specialized granular glands, which we name mucus-producing granular glands (MPGGs). The loss of traditional mucous glands is a derived trait for Ensatina .  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 469–477.     

Innervation of skin glands in the frog

Summary A comparative study was undertaken on the innervation of mucous and granular glands in frog skin. Results obtained by the Falck-Hillarp fluorescence technique and cholinesterase staining indicated that both types of glands receive exclusively adrenergic innervation. Electron microscopy was used to investigate the innervation pattern at the ultrastructural level. The distribution of nerve terminals was found to differ in the two types of glands. In the mucous gland, terminals were found at a distance of about 0.5 m from the basement membrane but never within the gland parenchyma. In the granular gland, the terminals were located between smooth muscle cells and also in direct contact with the secretory epithelium but never outside the basement membrane.This work was carried out in part at King Gustaf V:s Forskningsinstitut, Stockholm, and was supported by a grant from Karolinska Institutet     

Ultrastructure of the principal and accessory submandibular glands of the common vampire bat

The principal and accessory submandibular glands of the common vampire bat, Desmodus rotundus, were examined by electron microscopy. The secretory endpieces of the principal gland consist of serous tubules capped at their blind ends by mucous acini. The substructure of the mucous droplets and of the serous granules varies according to the mode of specimen preparation. With ferrocyanide-reduced osmium postfixation, the mucous droplets are moderately dense and homogeneous; the serous granules often have a polygonal outline and their matrix shows clefts in which bundles of wavy filaments may be present. With conventional osmium postfixation, the mucous droplets have a finely fibrillogranular matrix; the serous granules are homogeneously dense. Mucous cells additionally contain many small, dense granules that may be small peroxisomes, as well as aggregates of 10-nm cytofilaments. Intercalated duct cells are relatively unspecialized. Striated ducts are characterized by highly folded basal membranes and vertically oriented mitochondria. Luminal surfaces of all of the secretory and duct cells have numerous microvilli, culminating in a brush borderlike affair in the striated ducts. The accessory gland has secretory endpieces consisting of mucous acini with small mucous demilunes. The acinar mucous droplets contain a large dense region; the lucent portion has punctate densities. Demilune mucous droplets lack a dense region and consist of a light matrix in which fine fibrillogranular material is suspended. A ring of junctional cells, identifiable by their complex secretory granules, separates the mucous acini from the intercalated ducts. The intercalated ducts lack specialized structure. Striated ducts resemble their counterparts in the principal gland. As in the principal gland, all luminal surfaces are covered by an array of microvilli. At least some of the features of the principal and accessory submandibular glands of the vampire bat may be structural adaptations to the exigencies posed by the exclusively sanguivorous diet of these animals and its attendant extremely high intake of sodium chloride.