Fine structure of the gastric mucous and endocrine cells of the toad,Bufo marinus

Summary The ultrastructure of the mucous and endocrine cells of the gastric mucosa of the cane toad (Bufo marinus) has been examined. Surface mucous cells line the entire gastric mucosa and pits. Many of their secretory granules contain an electron-dense core that remains unreactive after cytochemical testing for glycoproteins. A second spatially and structurally discrete population of mucous cells is present in the gastric glands. These glandular mucous cells are probably homologous with the antral gland and mucous neck cells of mammals; their secretory granules also contain non-glycoprotein cores. Three distinct populations of endocrine cells show structural homologies with gastric hormone-storing cells of higher vertebrates.This study was supported by grants from N.H. & M.R.C. (Australia) and the Clive and Vera Ramaeiotti Foundations     

Morphology and ultrastructure of the venom glands of the northern pacific rattlesnake Crotalus viridis oreganus

The venom gland of Crotalus viridis oreganus is composed of two discrete secretory regions: a small anterior portion, the accessory gland, and a much larger main gland. These two glands are joined by a short primary duct consisting of simple columnar secretory cells and basal horizontal cells. The main gland has at least four morphologically distinct cell types: secretory cells, the dominant cell of the gland, mitochondria-rich cells, horizontal cells, and “dark” cells. Scanning electron microscopy shows that the mitochondria-rich cells are recessed into pits of varying depth; these cells do not secrete. Horizontal cells may serve as secretory stem cells, and “dark” cells may be myoepithelial cells. The accessory gland contains at least six distinct cell types: mucosecretory cells with large mucous granules, mitochondria-rich cells with apical vesicles, mitochondria-rich cells with electron-dense secretory granules, mitochondria-rich cells with numerous cilia, horizontal cells, and “dark” cells. Mitochondria-rich cells with apical vesicles or cilia cover much of the apical surface of mucosecretory cells and these three cell types are found in the anterior distal tubules of the accessory gland. The posterior regions of the accessory gland lack mucosecretory cells and do not appear to secrete. Ciliated cells have not been noted previously in snake venom glands. Release of secretory products (venom) into the lumen of the main gland is by exocytosis of granules and by release of intact membrane-bound vesicles. Following venom extraction, main gland secretory and mitochondria-rich cells increase in height, and protein synthesis (as suggested by rough endoplasmic reticulum proliferation) increases dramatically. No new cell types or alterations in morphology were noted among glands taken from either adult or juvenile snakes, even though the venom of each is quite distinct. In general, the glands of C. v. oreganus share structural similarities with those of crotalids and viperids previously described.     

The Harderian gland of the Dhub lizard Uromastyx microlepis of the Kuwaiti desert: an ultrastructural approach

Harderian glands exist in the orbits of most terrestrial vertebrates. The basic function of the gland is the lubrication of the eye. The present study was carried out to shed some light on the ultrastructure of the still enigmatic Harderian gland of the lizard Uromastyx microlepis, a common species in Kuwait and other Gulf areas. The Harderian gland of Uromastyx microlepis is well developed, relatively large in size and lingual in shape. The epithelial cells of the anterior part of the gland are characterized by the presence of membrane bound granules of almost homogeneous consistency. These secretory granules are gathered in compartments and separated by membranes and stacks of granular endoplasmic reticulum (GER). Most of the lumina were empty. Moderate amounts of GER, free ribosomes and pleiomorphic mitochondria were observed in the perinuclear area of the epithelial cells. The medial and caudal parts of the gland were rich in special secretory granules, GER, free ribosomes and pleiomorphic mitochondria. The anterior part of the gland could represent the future lacrimal gland of mammals. A network of myoepithelial cells was recognized around the gland tubules. While no melanocytes or lymphocytes were observed in the scarce interstitial tissue, macrophages, that might have an immune function in the gland, were observed.     

Morphology of the Harderian gland of some Australian geckos

This study of the morphology, histology, histochemistry, and ultrastructure of the Harderian gland in Geckos (Squamata, Gekkota) revealed previously unreported variation. The gecko Harderian gland is unlike that of other squamates in that each cell of the secretory epithelium has both lipid and protein secretory granules. Lipid secretion has not been reported previously for the squamate Harderian gland. The structure of the protein granules resembles that described for a scincomorph lizard (Podarcis, Lacertidae). Differences between representatives of the subfamilies Gekkoninae and Diplodactylinae suggest possible phylogenetic constraints in the structure or function of Harderian glands within gekkotan lineages. The structural relationship between the Harderian gland and the lacrimal duct supports previous suggestions of a possible functional link between the Harderian gland and the vomeronasal organ. J Morphol 231:253–259, 1997. © 1997 Wiley-Liss, Inc.     

Ultrastructure of the cardiac and pyloric glands of the gastric mucosa of the South African hedgehog,Atelerix frontalis

The cardiac and pyloric glands in the gastric mucosa of the South African hedgehog, Atelerix frontalis, are described. The cardiac area of the stomach contains proper cardiac glands and lacks undifferentiated fundic glands. The cardiac glands are simple tubular, coiled, and lined with columnar cells ultrastructurally similar to those of the gastric surface epithelium. Secretory granules with varying electron densities fill the apical cytoplasm of these cells. In contrast to other mammals, these glands lack mucous neck cells. The neck of the pyloric glands contains only a single cell type, whereas the basal regions of these glands contain “light” and “dark” cells. The secretory granules in the “dark” cells and the pyloric neck cells have a moderate electron density and often contain an electron dense core. An electron-lucent cytoplasm with numerous polysomes is characteristic of the “light” cells. Some “light” cells contain electron-dense granules in the apical cytoplasm. The presence of only neutral mucins in the cardiac gland cells denotes the absence of mucous neck cells. The acidic mucins within the pyloric neck cells seem to indicate that these cells are mucous neck cells, whereas the neutral mucins within the basally located pyloric gland cells show at least a partial functional difference from the pyloric neck cells. © 1993 Wiley-Liss, Inc.     

Ultrastructural study of the bacillary, granular and mucoid proboscidial gland cells of Riseriellus occultus (Nemertini, Heteronemertini)

Junoy, J., Montalvo, S., Roldán, C. and García‐Corrales, P. 2000. Ultrastructural study of the bacillary, granular and mucoid proboscidial gland cells of Riseriellus occultus (Nemertini, Heteronemertini). — Acta Zoologica (Stockholm) 81 : 235–242. The ultrastructure of six types (G₅‐G₁₀) of proboscidial gland cells whose cell necks emerge independently on the epithelium surface is analysed and compared with data from other nemerteans. These types differ in cytological features, as well as in the morphology of their respective secretory granules. Secretory granules of the types G₅ and G₆ have a bacillary shape, and differ from each other based on their contents and dimensions. Secretory granules of the types G₇ and G₈ are spherical to ovoid; type G₈ gland cells are monociliated, and their secretory granules contain a paracrystalline material. Types G₉ and G₁₀ gland cells are typically goblet‐shaped; secretory granules in the type G₉ have a spherical shape, contain a homogeneous electron dense material and maintain their individuality, whereas those of the G₁₀ type are elongate and have fibrillar contents, showing a tendency to fuse before they are extruded. The mucus sheet of the proboscis is responsible for lubrication of its epithelial surface. Secretion products of type G₁₀ gland cells form the background substance of this mucus, and those of the G₅ type confer stickiness to it. Type G₉ gland cells could provide the toxic component to the mucus, and type G₇ and G₈ gland cells could be concerned with the production of enzymatic secretions.     

Microscopic structures of the ovary and female genital ducts of Supachai's caecilian,Ichthyophis supachaii Taylor, 1960 (Amphibia: Gymnophiona)

The structures of the female reproductive system (ovary, oviduct and cloaca) of Ichthyophis supachaii were investigated by dissection, histology and light microscopy. Paired, elongated, sac‐like ovaries are parallel to the gut and fat bodies. Follicle stages include germinal nests of oogonia and primary oocytes, early and late previtellogenic follicles, early and late vitellogenic follicles and atretic follicles. Germinal nests of oogonia comprise oogonia and prefollicular cells. Nests of primary oocytes contain clusters of synchronously developing primary oocytes enclosed by connective tissue. Primary oocytes are associated with follicular cells. Previtellogenic follicles initially form the vitelline envelope, theca cell layers and patches of ooplasmic glycoproteins. Vitellogenic follicles contain heterogeneously sized spherical yolk granules. Atresia is present in several stages of developing follicles. The oviduct is divided into the anterior, middle and posterior parts. All oviductal parts are lined by non‐ciliated epithelium. A small number of mucous cells are present in the middle part. The cloaca of female I. supachaii is divided into the anterior and posterior chambers. The anterior chamber is lined by glandular stratified columnar epithelium, while the posterior chamber has stratified cuboidal epithelium with less mucus production. Our results contribute to useful information on the reproductive biology of caecilians.     

Mucin histochemistry of submandibular and parotid salivary glands of man: light and electron microscopy

Light-microscopy showed parotid serous acinar cells to contain neutral mucin, serous and mucous acinar cells of submandibular gland and intercalary ductal cells of both glands to contain acid and neutral mucins, and cells of striated ducts and excretory ducts to contain neutral mucin. Mucins were demonstrated ultrastructurally in a portion of the components of secretory granules of acinar cells and intercalary ductal cells, and in secretory granules of striated and excretory ductal cells. The mucins were all stained by techniques that reveal 1,2-glycols. Secretory granules of submandibular mucous and serous acinar cells and intercalary ductal cells were stained variably by the low iron-diamine technique for acid mucin, and those of mucous acinar cells by the high iron-diamine technique for sulphomucins mucin and possibly consisted of protein. The results suggest that one type of cell may be able to produce a range of secretory products and to package them variously into secretory granules.     

Immunofluorescence-microscopic demonstration of myosin and actin in salivary glands and exocrine pancreas of the rat

Summary Actin and myosin were localized in various salivary glands (parotid, submandibular, sublingual, lingual and Harderian gland) and the exocrine pancreas of rats by indirect immunofluorescence microscopy using specific rabbit antibodies against chicken gizzard myosin and actin. A bright immunofluorescent staining with both antibodies was observed at three main sites: (1) In myoepithelial cells of all salivary glands, (2) in secretory gland cells underneath the cell membrane bordering the acinar lumen (except Harderian and mucous lingual gland), and (3) in epithelial cells of the various secretory ducts (of all glands) in similar distribution as in acinar cells. The present immunohistochemical findings in acinar cells could lend further support to a concept suggesting that myosin and actin are involved in the process of transport and exocytosis of secretory granules.Supported by grants form Deutsche Forschungsgemeinschaft (Dr. 91/1, Ste. 105/19 and U. 34/4). We thank Mrs. Ursula König, Mrs. Christine Mahlmeister and Miss Renate Steffens for excellent technical assistance.     

Morphology,histochemistry and physiology of the major salivary glands in the echidna Tachyglossus aculeatus (Monotremata)

The three major salivary glands of the monotreme echidna are described. The parotid is a typical serous gland with tubulo-acinar secretory endpieces and a well-developed system of striated ducts. The mandibular gland, although light microscopically resembling a mucous gland, secretes very little glycoprotein. Its cells are packed instead with serous granules, resembling in fine structure the “bull's eye” granules in the mandibular gland of the European hedgehog Erinaceus europaeus. The sublingual glands secrete an extremely viscous mucous saliva. Expulsion of this saliva through the narrow ducts is probably aided by contraction of the extensive myoepithelial sheaths surrounding the secretory tubules. Application of the glyoxylic acid induced fluorescence method failed to demonstrate adrenergic innervation in any of the glands.     

Presence of cells combining features of two different cell types in the colonic crypts and pyloric glands of the mouse.

A small number of epithelial cells which combine features of two cell types were observed in the descending colon and pyloric stomach of the mouse. In the descending colon, where the base of the crypts is mainly composed of poorly differentiated "vacuolated" cells, a few of these cells contain, besides the characteristic "vacuoles," mucous globules identical to those in mucous cells or, less frequently, dense granules such as are found in entero-endocrine cells. Because there is evidence that the poorly differentiated vacuolated cells give rise to the other cells of the epithelium, those which also contain mucous globules or dense granules are likely to be differentiating into mucous cells or entero-endocrine cells respectively. In the pyloric stomach, where the glands are mainly composed of mucous cells, some of which are poorly differentiated, a few of the latter exhibit, besides the mucous globules, entero-endocrine type granules or features of caveolated cells. It is likely that the poorly differentiated mucous cells give rise to the other gland cells; and, therefore, those mucous-containing cells which also display dense granules or caveolated cell features are taken to be differentiating into entero-endocrine or caveolated cells respectively. Most of the cells containing two kinds of secretory materials are believed to be stem cells which initially contain a few vacuoles (colon) or mucous globules (pylorus) but are differentiating into a cell containing a different type of secretion. Rare observations of two kinds of secretory materials in a mature cell suggest that the transitional period may be prolonged, perhaps indefinitely.     

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.     

Ultrastructure of the adenohypophysis in the larval anadromous sea lamprey,Petromyzon marinus L.

The ultrastructure of the adenohypophysis (AH) in the larval anadromous sea lamprey, Petromyzon marinus L., was examined. The AH is subdivided into three regions, the pro-, meso-, and meta-AH. Cells of the nasopharyngeal stalk extend directly beneath the pro- and meso-AH to form the ventral surface of the gland. Some cells in the pro- and meso-AH are arranged into small follicles. Each region of the AH is characterized by a single granulated (secretory) cell type. Granulated cells constitute 80–90% of the pro-AH and contain secretory granules that range from 800 to 2400 Å in diameter. Only 10–20% of the cells in the meso-AH are granulated and they contain much smaller secretory granules (400 to 1250 Å diameter) than those in the pro-AH. Granulated cells constitute 80–90% of the meta-AH and contain only a few secretory granules, ranging from 1000 to 2500 Å in diameter, and many vesicles containing either a loose flocculent or dense granular material. Nongranulated (stellate) cells are found in all regions. They are characterized by their long cell processes, abundant cytoplasmic filaments, and variable electron density. The appearance of organelles in these cells suggests they are nonsecretory. They may play a role in maintaining the structural integrity of the gland and the regulation of granule release in the pro-AH. Two types of nongranulated cells make up 80–90% of the meso-AH. Type I are stellate cells, type II may be undifferentiated cells. The functional significance of the secretory cells in the larval AH is discussed.     

Autophagy facilitates secretion and protects against degeneration of the Harderian gland

The epithelial derived Harderian gland consists of 2 types of secretory cells. The more numerous type A cells are responsible for the secretion of lipid droplets, while type B cells produce dark granules of multilamellar bodies. The process of autophagy is constitutively active in the Harderian gland, as confirmed by our analysis of LC3 processing in GFP-LC3 transgenic mice. This process is compromised by epithelial deletion of Atg7. Morphologically, the Atg7 mutant glands are hypotrophic and degenerated, with highly vacuolated cells and pyknotic nuclei. The mutant glands accumulate lipid droplets coated with PLIN2 (perilipin 2) and contain deposits of cholesterol, ubiquitinated proteins, SQSTM1/p62 (sequestosome 1) positive aggregates and other metabolic products such as porphyrin. Immunofluorescence stainings show that distinct cells strongly aggregate both proteins and lipids. Electron microscopy of the Harderian glands reveals that its organized structure is compromised, and the presence of large intracellular lipid droplets and heterologous aggregates. We attribute the occurrence of large vacuoles to a malfunction in the formation of multilamellar bodies found in the less abundant type B Harderian gland cells. This defect causes the formation of large tertiary lysosomes of heterologous content and is accompanied by the generation of tight lamellar stacks of endoplasmic reticulum in a pseudo-crystalline form. To test the hypothesis that lipid and protein accumulation is the cause for the degeneration in autophagy-deficient Harderian glands, epithelial cells were treated with a combination of the proteasome inhibitor and free fatty acids, to induce aggregation of misfolded proteins and lipid accumulation, respectively. The results show that lipid accumulation indeed enhanced the toxicity of misfolded proteins and that this was even more pronounced in autophagy-deficient cells. Thus, we conclude autophagy controls protein and lipid catabolism and anabolism to facilitate bulk production of secretory vesicles of the Harderian gland.     

Autophagy facilitates secretion and protects against degeneration of the Harderian gland

The epithelial derived Harderian gland consists of 2 types of secretory cells. The more numerous type A cells are responsible for the secretion of lipid droplets, while type B cells produce dark granules of multilamellar bodies. The process of autophagy is constitutively active in the Harderian gland, as confirmed by our analysis of LC3 processing in GFP-LC3 transgenic mice. This process is compromised by epithelial deletion of Atg7. Morphologically, the Atg7 mutant glands are hypotrophic and degenerated, with highly vacuolated cells and pyknotic nuclei. The mutant glands accumulate lipid droplets coated with PLIN2 (perilipin 2) and contain deposits of cholesterol, ubiquitinated proteins, SQSTM1/p62 (sequestosome 1) positive aggregates and other metabolic products such as porphyrin. Immunofluorescence stainings show that distinct cells strongly aggregate both proteins and lipids. Electron microscopy of the Harderian glands reveals that its organized structure is compromised, and the presence of large intracellular lipid droplets and heterologous aggregates. We attribute the occurrence of large vacuoles to a malfunction in the formation of multilamellar bodies found in the less abundant type B Harderian gland cells. This defect causes the formation of large tertiary lysosomes of heterologous content and is accompanied by the generation of tight lamellar stacks of endoplasmic reticulum in a pseudo-crystalline form. To test the hypothesis that lipid and protein accumulation is the cause for the degeneration in autophagy-deficient Harderian glands, epithelial cells were treated with a combination of the proteasome inhibitor and free fatty acids, to induce aggregation of misfolded proteins and lipid accumulation, respectively. The results show that lipid accumulation indeed enhanced the toxicity of misfolded proteins and that this was even more pronounced in autophagy-deficient cells. Thus, we conclude autophagy controls protein and lipid catabolism and anabolism to facilitate bulk production of secretory vesicles of the Harderian gland.     

The epidermis of Timarete filigera (Polychaeta, Cirratulidae): histochemical and ultrastructural analysis of the gland cells

The aim of this study was to verify whether different living conditions of Polychaeta are correlated with morphological and functional differences in the organization of the integument. For this purpose, we decided to study the epidermis of Timarete filigera, a non-tubicolous polychaete. With this objective in mind, we have identified the various cellular types responsible for mucous secretion in the epidermis of this species and defined the histochemical composition of the mucus produced by different types of gland cells. Three types of gland cells have been identified by histochemical and ultrastructural studies in the epidermis of this polychaete. The histochemistry was carried out using standard techniques and peroxidase-labelled lectins. In type 1 cells, the secretory granules contain neutral glycoproteins with glucosidic residues of GalNAc, Galbeta 1,3 GalNAc, glucosidic and/or mannosidic residues. In type 2 cells, the secretory granules contain acid glycoproteins mainly sulphated with glucosidic residues of GalNAc, Galbeta 1,3 GalNAc, glucosidic and/or mannosidic residues, and some terminal sialic acid. In type 3 cells, the residual granules have the same chemical composition as that of granules present in type 2 cells. The secretion of these glandular mucous cells consists of mainly sulphated acidic glycoproteins and GAG resistant to testis jaluronidase. In these cells, the residual granules have the same chemical composition as that of their secretion. The heterogeneity of mucus composition may be correlated with its different functions.     

Morphological and Histochemical Study of Cephalic Glands of Bothrops jararaca (Ophidia,Viperidae)

Morphological and histochemical studies of the cell types in the cephalic glands of Bothrops jararaca have been performed. It is concluded: 1) mucous cells are found in the salivary labial, accessory glands; mucous-serous cells are found in the salivary labial, accessory and Harderian glands; serous-mucous cells are found only in the venom gland; 2) neutral mucosubstances and protein were found in the salivary labial, venom, accessory and Harderian glands; 3) hyaluronic acid was detected in the Harderian gland; 4) of the to sulfated acid mucosubstances, only chondroitin sulfate B was detected in the salivary labial and accessory glands; 5) sialic acid was detected in the salivary labial, accessory and Harderian glands.     

Ultrastructure and cytochemistry of the gastric mucosa of a reptile,Tiliqua scincoides

Summary The gastric mucosa of a reptile, the lizard Tiliqua scincoides, has been examined by light and electron microscopy. The gastric pits lead into glands that are extensively coiled in the proximal stomach but become progressively shorter and straighter in the distal stomach. The following epithelial cell types have been identified: (i) Surface mucous cells (SMC) line the entire lumenal surface as well as the pits. They contain mucus granules that stain with periodic acid-Schiff and, like the granules of mammalian SMC, commonly contain an electron dense core that appears not to be mucus (periodic acid-chromic acid-silver methenamine nonreactive). (ii) Glandular mucous cells are present in glands throughout the mucosa. They are probably homologous with the mucous neck and antral gland cells of mammals; like SMC their mucus granules contain nonglycoprotein cores. (iii) Oxynticopeptic cells (OPC) are the predominant cell type in the proximal glands but become infrequent distally. Their fine structure resembles that of OPC in other nonmammalian vertebrates, with features like those of both parietal cells and zymogen cells of mammals, (iv) Endocrine cells of three different types have been identified. Two of these show close similarities to the EC and ECL cells of mammals.The authors thank Mrs. D. Flavell for technical assistance. This study was supported by a grant from the Clive and Vera Ramaciotti Foundations     

Microscopic anatomy of the integument in the ribbon worm Micrura bella (Stimpson, 1857) (Pilidiophora,Nemertea)

The integument of ribbon worms in the order Heteronemertea is distinct from the integuments in the other taxa of nemerteans due to the presence of a special subepidermal glandular layer, the cutis. Among heteronemerteans, the ultrastructure of the cutis has been studied only in the Lineus ruber species complex. In the current study, ultrastructural (transmission electron microscopy) and histochemical studies of the epidermis and the cutis of Micrura bella from the basal Lineage A of the family Lineidae were performed. The epidermis consisted of ciliated and serous gland cells and is separated from the cutis by a layer of the subepidermal extracellular matrix; the basal lamina was not detected. The cutis comprised musculature, two types of mucous and four types of granular gland cells, and pigment cells with four types of granules. In the cutis of juvenile worms, type II granular gland cells and type II mucous cells were not observed. The integument of the caudal cirrus consisted of ciliated and serous gland cells and two intraepidermal lateral nerve cords; the cutis was absent. The compositions of the integument glands of M. bella and the L. ruber species complex are similar, except for the presence of type IV granular gland cells with narrow rod-shaped and lamellated granules exhibiting an alternating dark and light transverse layers and type II mucous cells found only in M. bella.     

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.