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The echidna and platypus have a crural/femoral gland that is linked by a large duct to a canalized, keratinous spur located on the medial side of the ankle. The echidna crural gland, like the femoral gland of the platypus, exhibits cyclic activity, being prominent in both monotremes when they are sexually active. In the present study, we compared the structure and histochemistry of these glands. During the active phase, the secretory epithelium forming the respective glands of both species increased in height and became packed with secretory granules that differed markedly in structure. Secretory granules of the echidna crural gland were electron dense and characterized by cores or areas of increased electron density. Those of the platypus were initially electron dense, but then became less dense and coalesced into irregular complexes of secretory material. Large cytoplasmic blebs extended from epithelial cell apices and appeared to be shed into the lumen, resulting in an apocrine mode of secretion. Exocytosis was also observed. A similar form of release of secretory product was not observed in the echidna. Secretory granules of both species were periodic acid-Schiff positive and stained for protein, suggesting that much of the secretory product was glycoprotein. Myoepithelial cells enveloped the secretory tubules of the platypus femoral gland, whereas they were not observed surrounding tubules comprising the echidna crural gland. During the quiescent phase, the epithelial cells of both species lost their secretory granules and decreased in height. As a result, the secretory tubules became smaller, intralobular connective tissue increased and the glands decreased in overall size.

Pathogenic strains of enteric bacteria secrete small heat-stable toxins (STs) that activate membrane guanylyl cyclase receptors found in the intestine. The intestinal peptide agonists, guanylin and uroguanylin, are structurally related to STs. Receptors for 125I-ST were found throughout the entire length of the intestinal tract of all the birds examined. These receptors were restricted to intestinal epithelial cells covering villi and forming intestinal glands and were not observed in other strata of the gut wall. The most intense labeling of receptors by !25I-ST occurred in the region of the microvillus border of individual enterocytes. There appeared to be a decrease in receptor density distally along the length of the small intestine, although labeling of receptors by l25I-ST was observed throughout the small intestine and colon. Cellular cGMP accumulation responses to Escherichia coli ST and rat guanylin in the domestic turkey and duck were greater in the proximal small intestine compared to the distal small intestine or colon. Brush border membranes (BBM) isolated from the mucosa of proximal small intestine of turkeys exhibited agonist-stimulated guanylyl cyclase activity. The rank order potency for enzyme activation was E. coli ST > uroguanylin > guanylin. Competitive radioligand binding assays using 125I-ST and turkey intestine BBM revealed a similar rank order affinity for the receptors that was exemplified by the Kd values of ST 2.5 nM, uroguanylin 80 nM and guanylin 2.6 µM. It may be concluded that functional receptors for the endogenous peptides, guanylin and uroguanylin, occur in the apical membranes of enterocytes throughout the avian intestine. The receptor-guanylyl cyclase(s) of proximal small intestine were preferentially activated by uroguanylin relative to guanylin, but both endogenous peptides were less potent than their molecular mimic, E. coli ST.

Numerous transitory intercellular attachments are observed between the central, lateral surfaces of adjacent glomerular epithelial cells in the differentiating renal corpuscle. The junctions are characterized by an increased electron density of the adjacent cell membranes and cytoplasm. The intervening intercellular space may contain an amorphous material of moderate electron density. The distribution and position of such temporary cell attachments, together with their modification and subsequent loss during the differentiation of podocytes, suggest that they play an important role in the histogenesis of the glomerular epithelium.

Cells immunoreactive for insulin, glucagon, somatostatin, bovine pancreatic polypeptide and 5-hydroxytryptamine are found in the pancreas of the newborn opossum and of all later stages examined. All immunoreactive cell types are present in primary and secondary islets and within elements of the exocrine pancreas. Cells immunoreactive for glucagon, bovine pancreatic polypeptide, somatostatin and 5-hydroxytryptamine generally are confined to the periphery of secondary (intralobular) islets, whereas insulin-immunoreactive cells occupy the central region. Endocrine cells within primary (interlobular) islets are randomly scattered. A small number of pancreatic-polypeptide-immunoreactive cells are reactive for the amine 5-hydroxytryptamine also, but the reverse is not observed.The endocrine pancreas continues to differentiate and develop throughout postnatal life and into adulthood. Little difference was observed between the head and tail regions of the opossum pancreas for the measurements made.

The human pyloric glands are simple or branched tubular glands which spiral through the connective tissue of the lamina propria. They are comprised of three cell types: the pyloric gland (mucous) cell, the parietal cell, and endocrine cells. The mucous cell is the most common cell type in the human pyloric glands and is characterized by an abundance of secretory granules. The secretory granules are usually round in shape and vary considerably in diameter. They are membrane-bound and comprise a heterogeneous population. The larger granules contain a light staining amorphous material and often possess an area of increased electron density near the limiting membrane of the granule giving them a mottled appearance. A smaller, more electron-dense secretory granule also is found both in relation to the Golgi complex and cell apex.The morphological observations indicate that the mechanism of mucin production by the mucous cells of the human pyloric glands may be similar to that reported in other mucin-secreting cells.

Development of the external musculature of the gastrointestinal tract has been studied in the stomach, small intestine and colon of the postnatal opossum. The muscle support is thin and poorly developed at birth, especially in the stomach and small intestine where only the inner layer is completely formed. The outer layer is discontinuous and formed by scattered myoblasts. The muscularis externa of the colon at birth is considerably thicker and both layers are present. Subsequent development of the muscularis externa consists of an early period of proliferative activity followed by hypertrophy. A low rate of mitotic activity continues throughout development and into the adult. Elements of the myenteric plexus are present at birth.

A 3-year study of general growth and development revealed a uniform increase in body length of young pouch opossums during the first 10 weeks of life. Throughout this period, growth was linear and constant for all animals regardless of sex, litter size, or whether animals were obtained from first, second or third litters. Body weights were somewhat more variable, but there were no significant sex differences. Various aspects of external gross morphology are presented and discussed as they relate to growth of the young opossum.