Innervation of skin glands in the frog

The adaptive microbiome hypothesis and immune interactions in amphibian mucus

The microbiome is known to provide benefits to hosts, including extension of immune function. Amphibians are a powerful immunological model for examining mucosal defenses because of an accessible epithelial mucosome throughout their developmental trajectory, their responsiveness to experimental treatments, and direct interactions with emerging infectious pathogens. We review amphibian skin mucus components and describe the adaptive microbiome as a novel process of disease resilience where competitive microbial interactions couple with host immune responses to select for functions beneficial to the host. We demonstrate microbiome diversity, specificity of function, and mechanisms for memory characteristic of an adaptive immune response. At a time when industrialization has been linked to losses in microbiota important for host health, applications of microbial therapies such as probiotics may contribute to immunotherapeutics and to conservation efforts for species currently threatened by emerging diseases.

The importance of antimicrobial peptides (AMPs) in amphibian skin defense

Antimicrobial peptides (AMPs) are produced for defense in nearly all taxa from simple bacteria to complex mammalian species. Some amphibian families have developed this defensive strategy to a high level of sophistication by loading the AMPs into specialized granular glands within the dermis. Enervated by the sympathetic nervous system, the granular glands are poised to deliver an array of AMPs to cleanse the wound and facilitate healing. There have been a number of excellent review publications in recent years that describe amphibian AMPs with an emphasis on their possible uses for human medicine. Instead, my aim here is to review what is known about the nature of amphibian AMPs, the diversity of amphibian AMPs, regulation of their production, and to provide the accumulated evidence that they do, indeed, play an important role in the protection of amphibian skin, vital for survival. While much has been learned about amphibian AMPs, there are still important gaps in our understanding of peptide synthesis, storage, and functions.

Frog Skin Innate Immune Defences: Sensing and Surviving Pathogens

Amphibian skin is a mucosal surface in direct and continuous contact with a microbially diverse and laden aquatic and/or terrestrial environment. As such, frog skin is an important innate immune organ and first line of defence against pathogens in the environment. Critical to the innate immune functions of frog skin are the maintenance of physical, chemical, cellular, and microbiological barriers and the complex network of interactions that occur across all the barriers. Despite the global decline in amphibian populations, largely as a result of emerging infectious diseases, we understand little regarding the cellular and molecular mechanisms that underlie the innate immune function of amphibian skin and defence against pathogens. In this review, we discuss the structure, cell composition and cellular junctions that contribute to the skin physical barrier, the antimicrobial peptide arsenal that, in part, comprises the chemical barrier, the pattern recognition receptors involved in recognizing pathogens and initiating innate immune responses in the skin, and the contribution of commensal microbes on the skin to pathogen defence. We briefly discuss the influence of environmental abiotic factors (natural and anthropogenic) and pathogens on the immunocompetency of frog skin defences. Although some aspects of frog innate immunity, such as antimicrobial peptides are well-studied; other components and how they contribute to the skin innate immune barrier, are lacking. Elucidating the complex network of interactions occurring at the interface of the frog's external and internal environments will yield insight into the crucial role amphibian skin plays in host defence and the environmental factors leading to compromised barrier integrity, disease, and host mortality.

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.

Amphibian immunity-stress, disease, and climate change

Like all other vertebrate groups, amphibian responses to the environment are mediated through the brain (hypothalamic)-pituitary-adrenal/interrenal (HPA/I) axis and the sympathetic nervous system. Amphibians are facing historically unprecedented environmental stress due to climate change that will involve unpredictable temperature and rainfall regimes and possible nutritional deficits due to extremes of temperature and drought. At the same time, amphibians in all parts of the world are experiencing unprecedented declines due to the emerging diseases, chytridiomycosis (caused by Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans) and ranavirus diseases due to viruses of the genus Ranavirus in the family Iridoviridae. Other pathogens and parasites also afflict amphibians, but here I will limit myself to a review of recent literature linking stress and these emerging diseases (chytridiomycosis and ranavirus disease) in order to better predict how environmental stressors and disease will affect global amphibian populations.

Odorous secretions in anurans: morphological and functional assessment of serous glands as a source of volatile compounds in the skin of the treefrog Hypsiboas pulchellus (Amphibia: Anura: Hylidae)

Serous (granular or venom) glands occur in the skin of almost all species of adult amphibians, and are thought to be the source of a great diversity of chemical compounds. Despite recent advances in their chemistry, odorous volatile substances are compounds that have received less attention, and until now no study has attempted to associate histological data with the presence of these molecules in amphibians, or in any other vertebrate. Given the recent identification of 40 different volatile compounds from the skin secretions of H. pulchellus (a treefrog species that releases a strong odour when handled), we examined the structure, ultrastructure, histochemistry, and distribution of skin glands of this species. Histological analysis from six body regions reveals the presence of two types of glands that differ in their distribution. Mucous glands are homogeneously distributed, whereas serous glands are more numerous in the scapular region. Ultrastructural results indicate that electron-translucent vesicles observed within granules of serous glands are similar to those found in volatile-producing glands from insects and also with lipid vesicles from different organisms. Association among lipids and volatiles is also evidenced from chemical results, which indicate that at least some of the volatile components in H. pulchellus probably originate within the metabolism of fatty acids or the mevalonate pathway. As odorous secretions are often considered to be secreted under stress situations, the release of glandular content was assessed after pharmacological treatments, epinephrine administrated in vivo and on skin explants, and through surface electrical stimulation. Serous glands responded to all treatments, generally through an obvious contraction of myoepithelial cells that surround their secretory portion. No response was observed in mucous glands. Considering these morpho-functional results, along with previous identification of volatiles from H. pulchellus and H. riojanus after electrical stimulation, we suggest that the electron-translucent inclusions found within the granules of serous glands likely are the store sites of volatile compounds and/or their precursors. Histochemical and glandular distribution analyses in five other species of frogs of the hylid tribe Cophomantini, revealed a high lipid content in all the species, whereas a heterogeneous distribution of serous glands is only observed in species of the H. pulchellus group. The distribution pattern of serous glands in members of this species group, and the odorous volatile secretions are probably related to defensive functions.

Skin peptides protect juvenile leopard frogs (Rana pipiens) against chytridiomycosis

One issue of great concern for the scientific community is the continuing loss of diverse amphibian species on a global scale. Amphibian populations around the world are experiencing serious losses due to the chytrid fungus, Batrachochytrium dendrobatidis. This pathogen colonizes the skin, leading to the disruption of ionic balance and eventual cardiac arrest. In many species, antimicrobial peptides secreted into the mucus are thought to contribute to protection against colonization by skin pathogens. Although it is generally thought that antimicrobial peptides are an important component of innate immune defenses against B. dendrobatidis, much of the current evidence relies on correlations between effective antimicrobial peptide defenses and species survival. There have been few studies to directly demonstrate that antimicrobial peptides play a role. Using the northern leopard frog, Rana pipiens, we show here that injection of noradrenaline (norepinephrine) brings about a long-term depletion of skin peptides (initial concentrations do not recover until after day 56). When peptide stores recovered, the renewed peptides were similar in composition to the initial peptides as determined by MALDI-TOF mass spectrometry and in activity against B. dendrobatidis as determined by growth inhibition assays. Newly metamorphosed froglets depleted of their peptide stores and exposed to B. dendrobatidis died more rapidly than B. dendrobatidis-exposed froglets with their peptides intact. Thus, antimicrobial peptides in the skin mucus appear to provide some resistance to B. dendrobatidis infections, and it is important for biologists to recognize that this defense is especially important for newly metamorphosed frogs in which the adaptive immune system is still immature.

Norepinephrine depletion of antimicrobial peptides from the skin glands of Xenopus laevis

The dermal granular glands of the South African clawed frog, Xenopus laevis, contain antimicrobial peptides (AMPs) that are secreted following local nerve stimulation. These natural antibiotics are active against bacteria and fungi including Batrachochytrium dendrobatidis, a fungal pathogen that causes the skin disease chytridiomycosis. Granular gland secretion can be stimulated in the laboratory by norepinephrine injection. We found that two injections of 80nmol/g norepinephrine were necessary to fully deplete the AMP stores. One injection resulted in the secretion of most of the stored peptides. A second injection, 2 days later, released a small amount of additional AMPs that are not compositionally different from those released by the first injection. A third injection, 4 days after the first, did not result in further AMP release. Mass spectrometry and histology confirmed that glands are depleted after two injections. Periodic acid-Schiff staining indicated that mucus gland secretion was also induced by norepinephrine.

Immune defenses against Batrachochytrium dendrobatidis, a fungus linked to global amphibian declines, in the South African clawed frog, Xenopus laevis

Batrachochytrium dendrobatidis is a chytrid fungus that causes the lethal skin disease chytridiomycosis in amphibians. It is regarded as an emerging infectious disease affecting diverse amphibian populations in many parts of the world. Because there are few model amphibian species for immunological studies, little is known about immune defenses against B. dendrobatidis. We show here that the South African clawed frog, Xenopus laevis, is a suitable model for investigating immunity to this pathogen. After an experimental exposure, a mild infection developed over 20 to 30 days and declined by 45 days postexposure. Either purified antimicrobial peptides or mixtures of peptides in the skin mucus inhibited B. dendrobatidis growth in vitro. Skin peptide secretion was maximally induced by injection of norepinephrine, and this treatment resulted in sustained skin peptide depletion and increased susceptibility to infection. Sublethal X-irradiation of frogs decreased leukocyte numbers in the spleen and resulted in greater susceptibility to infection. Immunization against B. dendrobatidis induced elevated pathogen-specific IgM and IgY serum antibodies. Mucus secretions from X. laevis previously exposed to B. dendrobatidis contained significant amounts of IgM, IgY, and IgX antibodies that bind to B. dendrobatidis. These data strongly suggest that both innate and adaptive immune defenses are involved in the resistance of X. laevis to lethal B. dendrobatidis infections.

Serous cutaneous glands of the Pacific tree-frog Hyla regilla (Anura, Hylidae): Patterns of secretory release induced by nor-epinephrine

The serous (poison) cutaneous glands of the Pacific tree-frog Hyla regilla were induced to release their product by 10(-3)M nor-epinephrine stimulation. After discharge structural and ultrastructural features of the cutaneous glands involved in release were observed. Furthermore, the discharged product, consisting of discrete, secretory granules, was collected and processed for transmission electron microscope analysis. As indicated by patterns found in the myoepithelium encircling the syncytial secretory unit, gland discharge is caused by contraction of the peripheral myocytes. Muscle cell compression dramatically affects the syncytium and results in degenerative changes, including expulsion of the secretory unit nuclei. Therefore, the structural collapse in depleted glands has been ascribed to the mechanical activity performed by the myoepithelium during discharge, rather than cytoplasm involution described in conventional, holocrine glands. TEM investigation revealed that the secretory granules collected after discharge maintain their peculiar traits: they consist of recurrent patterns of thin subunits, acquired during serous maturation and provided with remarkable structural stability.

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.

In vitro relation between preganglionic sympathetic stimulation and activity of cutaneous glands in the bullfrog

1. Activation of cutaneous glands was studied by measuring changes in transepithelial potentiation (TEP) after pre- and postganglionic sympathetic stimulation in the bullfrog, Rana catesbeiana. 2. In normal Ringer solution, TEP was 20-90 mV with the basolateral (inside) surface positive. Single shocks to the preganglionic B pathway decreased TEP by up to 3 mV. Cutaneous depolarizations had a latency of 1.2 s, a rise time of 2.5 s, and decayed with an exponential time constant of 15 s. Similar depolarizations were evoked by postganglionic stimulation. 3. Cutaneous depolarizations summed during repetitive stimulation and > 0.05 Hz. For trains of three stimuli, peak amplitude increased with frequency and saturated at 2 Hz. In some preparations, longer trains evoked polyphasic changes in TEP. Preganglionically evoked cutaneous responses were abolished by (+)-tubocurarine. Postganglionically evoked cutaneous depolarizations were antagonized by phentolamine, but not propranolol. 4. Repetitive preganglionic stimulation of the C pathway (> 100 at 20 Hz) evoked little change in TEP and did not modulate depolarizations evoked through the B pathway. In nicotine, peptidergic cotransmission was enhanced in the ganglia, and repetitive C pathway stimulation evoked cutaneous depolarizations whose time course mirrored that of the postganglionic peptidergic after-discharge. The after-discharge and associated cutaneous depolarization were blocked by a luteinizing hormone-releasing hormone antagonist. 5. The results show cutaneous glands are selectively innervated by B neurones and respond to low levels of neural activity. Asynchronous postganglionic firing mediated by peptidergic cotransmission can provide a basis for heterosynaptic interactions between the B and C pathways.

Biosynthesis of frog skin mucins: cysteine-rich shuffled modules, polydispersities and genetic polymorphism

1. Frog integumentary mucins (FIM-A.1, FIM-B.1 and FIM-C.1) consist of typical threonine-rich highly O-glycosylated (semi)repetitive domains, and cysteine-rich modules, i.e. the P-domain, the short consensus repeat and a region with high similarity to the C-terminal end of von Willebrand factor (designated here CC29-motif). 2. These modules are thought to be involved in protein-protein interactions and they have been observed in a variety of extracellular proteins. In FIMs, these modules may be involved in oligomerization processes leading to an entangled mucin network. 3. Polydispersities have been detected in FIM-B.1 and FIM-C.1 within single individuals. Multiple transcripts are probably generated by alternative splicing of a huge array of different (semi)repetitive cassettes encoding the threonine-rich domains. 4. Furthermore, genetic polymorphism is observed between different individuals, probably due to allelic variations in the number of (semi)repetitive cassettes.

Expression of spasmolysin (FIM-A.1): an integumentary mucin from Xenopus laevis

In the past, a unique type of precursor for a secretory protein was discovered. It contains a central repetitive domain rich in threonine residues and terminal cysteine-rich domains. Due to striking homologies of these terminal domains with pancreatic spasmolytic polypeptide, originally the name "prepro-spasmolysin" was proposed. Here we show that the mature protein has a MW of about 130 kDa, consisting of about 70% carbohydrate and 30% protein. Similar O-linked glycoproteins have been found in mucins from human intestine. For this and numerous other reasons we decided to rename this glycoprotein "frog integumentary mucin A.1" (FIM-A.1). Furthermore, analysis of the protein with specific antibodies against the predicted C-terminal end indicates that FIM-A.1 is probably not processed at pairs of basic residues. In situ hybridization as well as immunofluorescence studies revealed that FIM-A.1 is expressed and stored exclusively in mature mucous glands of Xenopus laevis skin. Only cone cells at the proximal part of these glands do not synthesize FIM-A.1. In contrast, all other physiologically active peptides from X. laevis skin investigated so far are synthesized in granular glands. A hypothetical function of FIMs for defense against microbial infections is discussed.

Differential projections of B and C sympathetic axons in peripheral nerves of the bullfrog

Accumulating evidence indicates that electrophysiologically distinct subsets of sympathetic neurons selectively innervate different classes of targets. The organization of this system may therefore be reflected in the sympathetic fiber contents of peripheral nerves. To test this possibility, we have mapped the pathways followed by three groups of postganglionic sympathetic axons in the bullfrog by recording compound action potentials and by retrograde tracing with horseradish peroxidase (HRP). The axons that were studied arise from fast B, slow B, and C-type neurons in ganglia 9 and 10 at the lumbar end of the paravertebral sympathetic chain. They project to peripheral targets primarily by way of the sciatic nerve and can be distinguished by the velocities with which they conduct action potentials. Action potentials were recorded with suction electrodes from isolated preparations composed of paravertebral chain ganglia 7-10, the sciatic nerve, and branches of the sciatic nerve that supply striated muscles, skin, and the bladder. Preganglionic B fibers were selectively activated by stimulating the paravertebral chain rostral to ganglion 7, and preganglionic C fibers were selectively activated by stimulating spinal nerves 7 and 8 at points central to their rami communicantes. Compound action potentials recorded from the sciatic, peroneal, tibial, and sural nerves and from the primary trunk of the pelvic nerve were each found to contain three components produced, respectively, by fast B, slow B, and C-type sympathetic axons. Similarly, action potentials recorded from cutaneous branches of the sciatic tree were found to contain three sympathetic components. By contrast, when compound action potentials were recorded from branches of the sciatic tree that directly enter and innervate striated muscles and also the bladder, the sympathetic responses were found to arise solely from C-type axons. HRP was used to label the sympathetic neurons that project to the sartorius muscle and into the cutaneous lateral crural nerve. Retrograde transport of HRP from the sartorius muscle labeled 17 +/- 4 (mean +/- s.d.) sympathetic neurons and 27 +/- 3 spinal motoneurons while transport from the lateral crural nerve labeled 68 +/- 47 sympathetic neurons but no spinal neurons. The average somatic diameter of ganglion cells projecting to the sartorius muscle was significantly smaller than that of cells projecting to the lateral crural nerve. The electrophysiological results indicate that fast B and slow B sympathetic axons in the sciatic trunk and its primary branches project selectively into cutaneous nerves while sympathetic C axons project into all peripheral nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Sympathetic modulation of mechanoreceptor sensitivity in frog skin

1. Sympathetic effects on the mechanical sensitivity of frog cutaneous mechanoreceptors were examined in vivo. 2. Functionally identified units were tested with repetitive mechanical stimuli of threshold intensity during electrical stimulation of the sympathetic trunk. 3. Sympathetic activity resulted in increased sensitivity for three classes of afferents; slowly adapting compression receptors, slowly adapting stroke receptors, and rapidly adapting stroke receptors. Decreased sensitivity was produced in the fourth class, rapidly adapting compression receptors. 4. Preliminary tets of several possible modes of sympathetic influence indicated that blood flow changes, changes in probe-skin coupling and changes in tissue compliance could not account for the observed changes in receptor sensitivity. Na+ and Cl- ions, secreted by cutaneous mucous glands were found to be possible contributors to the decreased sensitivity of rapidly adapting compression receptors. Direct neurotransmitter action on the receptors, a likely mechanism of sympathetic action, was not tested. 5. The data indicate that systematic changes in cutaneous sensibility occur with modest changes in sympathetic efferent activity. Possible mechanisms of these sympathetic effects are discussed.

In vitro studies of frog mucous glands

The ionic outflow, mainly consisting of Na+ and Cl-, from the mucous glands in an excised nerve-skin preparation of frog has been determined by recording the conductance changes occurring in a fluid layer covering a small area of the skin surface. In the main series of experiments the glands were activated by stimulation of sympathetic nerve fibers in the skin nerve. The relationship between the ionic outflow and the number of nerve volleys was studied over a wide range. The outflow per impulse was found to be fairly constant during the first tens of impulses but diminished gradually with increasing number of stimuli up to a certain maximum value--varying in different preparations--after which the outflow ceased completely. During the initial phase of stimulation the outflow is most likely caused by an ejection of performed secretion due to the contractions of the glandular myoepithelium. The continued outflow in the later stages of the stimulation periods must be due to production of new secretion. Since the glandular epithelium is devoid of nerve terminals a nervous control of the ionic secretion can only be explained by an indirect influence mediated either by transmitter diffusion from the myoepithelial nerve endings or by a close electric coupling between the contractile and the secretory gland cells. Adrenaline and noradrenaline induce ionic outflows which like those evoked by nerve stimulation are inhibited by the beta-adrenoreceptor blocker propranolol, alpha-adrenoreceptor blockers being without effect. A serendipitous finding of tonus changes in the frog skin during nerve stimulation is also described.

In vivo studies of individual mucous glands in the frog

Individual mucous glands in the toe web were studied in curarized decerebrate frogs using vital microscopy in combination with still or motion photomicrography. By changing the focus position to different levels various structures in the gland could be identified and their changes during glandular activation studied. The first visible effect of nerve stimulation was a contraction of the myoepithelium and probably also structural changes of the secretory epithelium resulting in a narrowing of the glandular lumen. Following this, tricuspid valve opened and secretion was ejected. The latency and time course of the contractile response to nerve stimulation were determined and the influence of the number of stimuli on the duration of the contraction and relaxation phases was analyzed. Comparisons were made with reflex activation of the gland as well as with neurohormonal stimulation. The myoepithelial contraction was found to be under adrenergic control. Of the smooth-muscle stimulants tested only Substance P induced contractions. The time course of the ionic outflow from the toe web was determined by conductance measurements in the fluid surrounding the web and compared with the visually observed phenomena. The initial outflow was concomitant with the phasic myoepithelial contraction but a continued secretion could also be observed and recorded from glands kept in a steady state of contraction by iterative nerve stimulation. The functions of the toe web glands were found to be critically dependent on a maintained circulation in the surrounding capillary network.

Monoaminergic fluorescence in frog skin

The Falck-Hillarp fluorescence technique was employed in an attempt to determine the distribution of sympathetic innervation in frog skin. No evidence was found of a direct monoaminergic nerve supply to the cells of the non-glandular epithelium in the epidermis. Instead, specific fluorescence was mainly confined to the vicinity of the skin glands. Fluorescent fibers were observed surrounding the mucous type of gland. The secretory content of this gland was not fluorescent. In the granular type of gland the main source of fluorescence was the secretory granules filling the lumen. These developed a fluorescence in the spectral range of 5-hydroxytryptamine. The brightness of the fluorescence indicated a very high content of this amine. Fluorimetric analysis showed that no catecholamines were present in the secretion. In glands devoid of secretory granules there were some indications of a monoaminergic innervation of the secretory epithelium, but this was hard to determine because of the abundant nonspecific fluorescence. Sparse dots of specific fluorescence were found close to the surrounding smooth muscle cells. -- These findings rule out the possibility of a direct sympathetic nervous control of the non-glandular epithelium in frog skin but indicate that this is instead confined to the skin glands.