J. Harry Cutts

Small Intestine and Colon

The endoderm of the 9-day opossum forms the innermost layer of the embryo and also lines the interior of the chorion (Krause and Cutts 1985b). At this stage, the embryo appears as a flattened disk and consists of ectoderm, mesoderm, and endoderm, Cell boundaries between individual endodermal cells are distinct, and microvilli on the apical surfaces of the cells are short and scattered (Fig. 26). No obvious differences are seen between the endodermal cells that line the interior of the yolk sac chorion and those that form the third layer of the opossum embryo (Krause and Cutts 1985b,c). The endoderm consists of a single layer of cuboidal cells which are joined by junctional complexes and often bear elongated processes that unite with similar processes of adjacent endodermal cells. The intercellular space formed between the processes usually appears empty. The cytoplasm of the endodermal cells is characterized by numerous free ribosomes, occasional profiles of granular endoplasmic reticulum, and scattered mitochondria. There is little morphological evidence to suggest absorptive activity by the endoderm at this time in Didelphis. Similar observations have been reported in the Philander opossum (Enders and Enders 1969).

Development of the digestive system in the North American opossum (Didelphis virginiana)

1 Introduction.- 1.1 Endoderm Formation.- 1.2 State of Visceral and Other Structures at Birth.- 1.3 The Scope of This Review.- 2 Oral Cavity.- 2.1 Prenatal Development.- 2.2 Postnatal Development.- 2.2.1 Tongue.- 2.2.2 Salivary Glands.- 2.2.3 Development of Dentition.- 2.3 Adult Teeth.- 2.4 Adult Tongue.- 2.5 Jaws and Associated Musculature.- 2.6 Adult Salivary Glands.- 3 Esophagus.- 3.1 Prenatal Development.- 3.2 Postnatal Development.- 3.2.1 Esophageal Glands.- 3.2.2 Quantitative Data on the Development of Esophageal Epithelium.- 3.2.3 Quantitative Data on Mitotic Activity in the Subepithelial Layers.- 3.3 Structure of Adult Esophagus.- 3.3.1 Histochemistry of Esophageal Glands.- 3.3.2 Muscularis Externa.- 4 Stomach.- 4.1 Prenatal Development.- 4.2 Postnatal Development.- 4.2.1 Fundic Mucosa.- 4.2.2 Glands of the Pylorus and Cardia.- 4.2.3 Mucin Histochemistry.- 4.2.4 Immunohistochemistry of Gastric Proteinases.- 4.2.5 Enteroendocrine Cells.- 4.2.6 Submucosa.- 4.2.7 Muscularis Externa.- 4.2.8 Interaction Between Exocrine and Endocrine Components of the Stomach.- 5 Small Intestine and Colon.- 5.1 Prenatal Development of Small Intestine and Colon.- 5.2 Postnatal Development of Small Intestine.- 5.2.1 Intestinal Mucosa.- 5.2.2 Enteroendocrine Cells.- 5.2.3 Lamina Propria.- 5.2.4 Submucosa.- 5.2.5 Submucosal (Meissners) Plexus.- 5.2.6 Duodenal (Brunners) Glands.- 5.2.7 Muscularis Externa.- 5.2.8 Myenteric (Auerbachs) Plexus.- 5.3 Postnatal Development of Colon.- 5.3.1 Enteroendocrine Cells.- 5.3.2 Lamina Propria.- 5.3.3 Submucosa.- 5.3.4 Submucosal (Meissners) Plexus.- 5.3.5 Muscularis Externa.- 5.3.6 Myenteric (Auerbachs) Plexus.- 5.4 Caecum.- 5.5 Comparison of Enteroendocrine Cells in the Gastrointestinal Tract of the Adult.- 5.6 Interaction Between Enteroendocrine and Exocrine Components of the Gut.- 6 Pancreas.- 6.1 Prenatal Development.- 6.2 Postnatal Development.- 7 Liver.- 7.1 Prenatal Development.- 7.2 Postnatal Development.- 7.3 Gallbladder and Common Bile Duct of the Adult Opossum.- 7.4 Opossum Bile.- 8 Factors That Influence the Differentiation and Growth of Glands Associated with the Gastrointestinal Mucosa.- 9 Concluding Remarks.- References.

Factors that Influence the Differentiation and Growth of Glands Associated with the Gastrointestinal Mucosa

Various factors influence the differentiation, growth, and function of the glands associated with the gastrointestinal mucosa and also play an important role in regulating and maintaining the balance between epithelial cell proliferation and loss of cells from the gastrointestinal mucosa. Although the nature of the activities of such substances in Didelphis is unknown, there appears to be no reason to believe that they function in any way different to those in other mammals. Differentiation and growth, as well as epithelial cell proliferation and loss, are influenced by luminal, dietary, neural, and hormonal factors. Preprogrammed intrinsic factors also may play an important role in the regulation of the processes. The effect of such factors on gastrointestinal differentiation and growth have been summarized in elegant reviews by Klein and McKenzie (1983a,b), and by Johnson (1987) and Henning (1987). These factors include hypophysial hormones or tissues under hypophysial control, endogenous gastrointestinal hormones from enteroendocrine cells, neurotransmitters from the enteric nervous system, factors present in luminal content (amniotic fluid, milk, or dietary factors), and intrinsic mechanisms such as prostaglandins and chalones. Gastrin-, CCK-, somatostatin-, BPP-, glucagon-, secretin, motilin-, GIP-, neurotensin-, and 5-HT-immunoreactive cells are present in the gastrointestinal mucosa of the newborn opossum (Krause et al. 1986, 1989b). Cells with neurotensin immunoreactivity are unique in that, although present in the proximal colon of the newborn, the cells disappear and a significant population is not seen again in the intestinal mucosa until the 74th postnatal day, just prior to weaning.