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With 8 figures Summary The secretory units and duct system of the echidna sublingual glands exhibit subtle architectural modifications to accommodate the viscous secretion produced by these glands. The glands are compound tubular glands, the secretory units of which are elongate with open lumina and consist only of mucous cells. Closely packed spindle-shaped myoepithelial cells invest the secretory units, but are absent around the ducts. The branched secretory tubules open into an abbreviated duct system characterized by wide lumina. Striated ducts normally associated with the second portion of the intralobular duct system are absent. The duct system shows the most obvious modification of general salivary gland architecture presumably to accommodate the viscous secretion propelled from the secretory units by surrounding myoepithelial cells.

The secretory units of the platypus and echidna mandibular glands consist of a single serous cell type. Secretory granules within the cells of the platypus mandibular gland stained intensely with the periodic acid-Schiff staining procedure but failed to stain with Alcian Blue, suggesting the granules contained neutral glycoproteins. Secretory granules within the mandibular glands of the echidna failed to stain with the methods used indicating little if any glycoprotein was associated with the secretory granules. Ultrastructurally, secretory granules of the platypus mandibular gland were electron dense with a central core of less electron-dense material and were membrane bound. In contrast, those of the echidna presented a lamellated appearance and also were limited by a membrane. These secretory granules appeared to form as a result of concentric layering of lamellae within cisternae of the Golgi membranes. The intralobular ductal system of the platypus was more extensively developed than that of the echidna. The striated ducts of both species were characterized by elaborate infoldings of the basolateral plasmalemma and an abundance of associated mitochondria.

Koala mandibular (submandibular) glands are compound tubuloacinar glands, the secretory units of which consist only of serous cells. Intercellular canaliculi occur between the serous cells, which are continuous with a minute lumen that courses through the centre of each secretory unit. Intercalated ducts are abundant and join striated ducts, the latter being characterized by elaborate basolateral infoldings of the plasmalemma. Secretory granules within the serous cells fail to stain with either the PAS or Alcian Blue (pH 2.5) staining procedures. Ultrastructurally, the secretory granules are membrane bound, and consist of a homogeneous electron lucent material with a fine filamentous texture. The granules tend to coalesce into irregular shaped complexes of secretory material. Discharge of secretory material into the canalicular lumen is a common observation.

Mucins of the gastroduodenal junction are secreted by the mucous surface and mucus-producing glandular cells in the stomach, and by goblet cells and Brunner's glands in the duodenum. Developmental studies have demonstrated that Brunner's glands can arise from undifferentiated gastric epithelium and/or intestinal epithelium in the proximal duodenum. The aim of this study was to investigate the carbohydrate composition of mucins from this region and compare it with that of mucins from Brunner's glands to evaluate the probable evolution of mucins from these glands. Toward that end, paraffin sections from 13 mammalian species were stained by classic carbohydrate histochemistry and treated with 13 lectins. In general, the mucous surface cells of the stomach, pyloric glands, duodenal goblet cells, and Brunner's glands secretory epithelium had different lectin-binding patterns. However, the lectin-binding profile of the secretory epithelium of Brunner's glands resembled that of pyloric glands more closely than that of duodenal goblet cells and mucous surface cells of the stomach. Mucins from Brunner's glands and pyloric glands showed a greater terminal carbohydrate residue diversity than those of gastric mucous surface cells or duodenal goblet cells. The lectin-binding profile argues for the evolution of similar mucins from the epithelia of Brunner's glands and pyloric glands. The greater diversity of carbohydrate residues in mucins secreted by Brunner's glands suggests that their mucus is more adaptable. This may explain why Brunner's glands metaplasia rather than goblet cell metaplasia is seen in the mucosa adjacent to chronic intestinal ulcers.

We studied aspirin-induced injury to the gastric mucosa in control rats pretreated with a solubilizer, pluronic F-68 (PL), and in rats pretreated with solubilized arachidonic acid (AA). Fasted male rats were pretreated intragastrically with 1 ml of either pluronic or AA and 1 h later acidified ASA (1 ml suspension of 200 mg kg-1 body weight) was administered intragastrically. Grossly apparent mucosal lesions developed 1 h after aspirin in pluronic-pre-treated rats, but were significantly reduced in AA-pretreated rats. Histology, scanning and transmission electron microscopy demonstrated that AA pretreatment did not prevent aspirin-induced initial damage to the surface epithelium but did significantly reduce extent of aspirin-induced deep mucosal necrosis at 1,4 and 18 h after aspirin. Initial aspirin-induced surface epithelial damage was rapidly restituted by two distinct types of re-epithelialization - vertical and horizontal. While the vertical type of re-epithelialization has been reported previously as the first stage of mucosal repair following injury by various noxious agents such as concentrated ethanol, the horizontal type of re-epithelialization, which is described for the first time in this paper, seems to be specific for the repair of aspirin-induced gastric mucosal injury. These studies suggest that dietary factors such as essential fatty acids may play a role in gastric mucosal protection against aspirin injury.

The gastric mucosa of both the echidna and platypus is aglandular and the lining epithelium is stratified squamous. The latter exhibits three principle layers: stratum germinativum, stratum spinosum, and stratum corneum. The cytoplasm of cells composing the first two strata of both species shows bundles of tonofibrils and numerous free ribosomes. Cells of the stratum spinosum in the platypus also show numerous dense granules limited to the peripheral cytoplasm. The stratum spinosum of both species is comprised of fusiform-shaped cells whose adjacent cell membranes show extensive interlocking. The stratum spinosum of the echidna in addition shows numerous intercellular bridges. Cells of the stratum corneum become flattened and elongate and in the echidna nuclei near the surface appear to degenerate. Cells comprising the stratum corneum of the platypus exhibit well preserved nuclei and contain scattered large granules of varying electron density. Prior to sloughing, cells near the surface of both species show a separation of adjacent cell membranes. True keratinization is not found in the gastric lining epithelium of either species and the epithelium lining of the stomach of the echidna more closely represents a form of parakeratosis. Delicate papillae containing capillaries extend considerable distances into the overlying epithelium of both species and are thought to contribute to its nutrition.

The duodenal glands of the species examined (Alces alces, Ovis canadensis, Cervus canadensis, Oreamnos americanus, Bison bison, Antilocapra americana, Odocoileus virginianas, Odocoileus heminous) are confined primarily to the submucosa of the small intestine. In one species, the moose, a significant population of secretory tubules also is observed in the mucosa. The ducts of the duodenal glands pierce the overlying muscularis mucosae to empty most often independently into the intestinal lumen. Those of the bison, unlike the other species examined, drain into intestinal glands. The duodenal glands consist primarily of a simple columnar epithelium, the cells of which contain basally positioned round or oval nuclei. The lumina of scattered duodenal glands in the pronghorn and to some extent those of the moose, white-tailed deer, and mule deer may be extremely dilated, and the surrounding epithelium thin and attenuated. Component cells of the duodenal glands of all the species examined show remarkably similar ultrastructural features. They exhibit scattered profiles of rough endoplasmic reticulum, dilated cisternae of which contain an electron-dense, amorphous material. Numerous well-developed Golgi complexes occupy the supranuclear region together with transport vesicles and forming secretory granules. Electron-dense, membrane-bound secretory granules generally are concentrated in the apical cytoplasm immediately subjacent to the cell membrane. The apical cell membrane exhibits short, scattered microvilli; and the basal cell membrane is smooth without apparent specialization. Histochemically, the duodenal glands of most species examined in this study consist of a heterogeneous population. The majority of the glands of the moose, elk, mountain goat, bison, pronghorn, and white-tailed deer elaborate a neutral mucin, whereas scattered individual glands, tubules or cells also produce acid mucins. Cells near the terminations of the ducts of the bighorn sheep are the only elements to produce acid mucins in the duodenal glands of this species. The duodenal glands of the bison are unusual in that only the peripheral portions of individual glands produce acid mucins. The remainder of the glands elaborate neutral mucins. Morphological differences between the two regions were not observed. The duodenal glands of the mule deer secrete both acid and neutral mucins. The structural and histochemical observations appear unrelated to the diet of individual species.

The majority of human cardiac glands that lie immediately distal to the termination of esophageal epithelium are compound or branched tubular glands. They empty into overlying gastric pits. The glands of this region are often organized into lobule-like complexes by the surrounding connective tissue of the lamina propria. The secretory tubules contain mucous cells, parietal cells and endocrine cells. The mucous cell is the most common cell type observed and appears to comprise two populations. The majority are pyramidal in shape and show numerous spherical, electron-dense secretory granules. Profiles of rough endoplasmic reticulum are observed scattered throughout the cytoplasm and Golgi complexes occupy a supranuclear position in relation to forming secretory granules. Morphologically this cell type appears similar to the mucous neck cell of the fundus. Secretory granules of a second mucous cell type are mottled in appearance and show an area of increased electron density near the limiting membrane. Parietal cells are observed and appear identical to those in the fundus. The large number of endocrine cells present raises questions concerning the traditional concept of the function of these particular glands.

The intestinal mucosa of the platypus takes the form of numerous transverse surface folds. These folds are made up of a lamina propria covered by pseudostratified epithelium which lies on a thick modified basement membrane. The cells of the intestinal epithelium consist of columnar cells which generally resemble typical intestinal epithelium and cuboidal cells, which are undifferentiated in appearance, show few organelles and possess an electron lucent cytoplasm. Numerous desmosomes are found between the adjacent cell membranes of both cell types. Villi are absent and appear to be represented by the large surface folds. Intestinal glands are composed of columnar epithelium similar to that found in the intestinal glands of other mammalian species. Groups of these glands drain into common tubular ducts which follow a tortuous course and empty into the intestinal lumen between the surface folds. The peculiar morphological features of the platypus intestinal mucosa raise questions concerning traditional concepts of intestinal gland formation as well as the origin and migration of intestinal epithelium with regard to this particular species.

The postnatal development of the respiratory system in the opossum has been traced, using 102 specimens divided into 11 groups according to snout-rump length. At birth, the lung is markedly underdeveloped and is represented by a pumitive system of branching airways that end in a number of terminal chambers or sacs. The airways, constituting the conducting portion of the lung, are lined by columnar epithelium that is devoid of cilia and goblet cells. The terminal air chambers contain an extensive capillary bed and are surfaced by squamous epithelium interspersed with scattered cuboidal cells. By the 3 cm stage, bands of smooth muscle have differentiated in relation to the bronchial epithelium and scattered cilia are present in the epithelium of the trachea and bronchi. Air chambers immediately adjecent to established bronchi differentiate further and become incorporated into the bronchial tree and new air chambers develop at the most distal extent of the bronchial system up to the 15.5 cm stage. Numerous spaces lined by squamous epithelium, which represent the first appearance of the arbor alveolaris, appear within the cellular stroma of the lung of the 7 cm opossum. By the 20 cm (juvenile) stage, mature alveoli, containing many vacuolated cells, are present and a thick collagenous lamina has developed between the pleura and the lung parenchyma.

A large gland patch is found in the submucosa of the lesser curvature of the koala stomach near the entrance of the esophagus. It measures approximately 3 cm in diameter and is comprised of numerous invaginations continuous with the surrounding mucosa. The glandular invaginations drain into the lumen of the stomach by 25 to 30 large orifices. The mucosa of the gland patch invaginations differs from the mucosal lining of the stomach chiefly in thickness. The elongate gastric glands that comprise the invaginations exhibit an increased number of parietal and chief cells. Ultrastructural examination of the gland patch illustrates that the parietal, chief, and mucous neck cells are similar in structure to those of other mammalian species.

Brunner's glands in the platypus form a lobulated, glandular collar confined to the submucosa of the most distal portion of the stomach. The glands end immediately proximal to the gastrointestinal junction and excretory ducts empty in the region where the stratified squamous epithelium lining the stomach changes abruptly to the intestinal lining epithelium of the duodenum. An individual gland of Brunner is composed of several elongate lobules drained by intralobular ducts which often join to form a single excretory duct. Light and electron microscopic studies have shown the secretory tubules to be comprised of large, pyramidal cells limited basally by a delicate basal lamina. The ergastoplasm, cisternae of which are dilated and contain amorphous material, is associated with numerous ribosomes. In basal and perinuclear regions intercisternal granules and smooth surfaced vesicles are found. Numerous small vesicles found in supranuclear areas apparently form from the smooth membrane portions of ergastoplasm located adjacent to Golgi complexes. Membrane-bound amorphous granules of varying electron density occupy the apical cytoplasm and show a tendency to coalesce before emptying their contents into the adjacent lumen. The intralobular duct system is lined initially by a columnar epithelium which changes to a simple squamous form before the ducts combine to form a short excretory duct lined by stratified squamous epithelium. The epithelium lining the duct system contains relatively few organelles but appears to be engaged in a limited amount of synthesis and release of secretory material. Histochemical studies indicate that both the secretory tubules and the duct system elaborate a neutral mucopolysaccharide.

The duodenal glands of the adult opossum form a lobed, glandular collar in the submucosa immediately distal to the pyloric sphincter. They empty into funnel-shaped mucosal depressions. The depressions, which form two or three irregular rows that run parallel to the pyloroduodenal junction, are lined by either pyloric or intestinal epithelium. An individual gland of Brunner has an extensive intralobular duct system which terminates in a single excretory duct just prior to entry into a depression. Histochemical studies indicate that the duodenal glands elaborate a neutral glycoprotein whereas the duct system appears to produce both neutral and acidic elements. Light and electron microscopic studies have shown the duodenal glands to consist of large, pyramidal cells which lie on a distinct basal lamina. Discrete, mottled or pale secretory granules are found in close association both with the apices of the cells and with the Golgi complexes. The ergastoplasm, cisternae of which are dilated and contain amorphous material, is associated with polysomes and, in the supranuclear region, often becomes markedly dilated, forming what appear to be large vacuoles containing fibrillar material. Blebbing of the ergastoplasm results in the formation of several small vesicles, many of which lie in close association with the Golgi network. Direct membrane continuity has been noted between the two elements.