Overview of Gastrointestinal Tract Histology Radial organization of gastrointestinal tract wall of tubular gastrointestinal tract consists of 4 concentric layers: mucosa epithelium typically a simple cuboidal or a simple columnar epithelium secretory function in stomach secretory and absorptive functions in small intestine and large intestine lamina propria supports avascular mucosal epithelium contains blood and lymphatic vessels muscularis mucosae typically a double layer of smooth muscle inner layer of circularly oriented smooth muscle outer layer of longitudinally oriented smooth muscle contraction causes local movement in mucosa submucosa a layer of connective tissue contains large blood vessels and large lymphatic vessels contains submucosal (Meisnner's) nerve plexus anchors the mucosa to the muscularis externa muscularis externa a double layer of smooth muscle inner layer of circularly oriented smooth muscle outer layer of longitudinally oriented smooth muscle contains myenteric (Auerbach's) nerve plexus in between double layer of smooth muscle contraction causes peristalsis adventitia / serosa a layer of connective tissue Distinctive Features of Gastrointestinal Tract Histology Esophagus esophageal mucosa non-keratinizing, stratified squamous epithelium muscularis mucosae is a single layer of longitudinally oriented smooth muscle esophageal muscularis externa upper one third of esophagus striated muscle middle one third of esophagus striated muscle and smooth muscle lower one third of esophagus smooth muscle Stomach gastric mucosa gastric glands occupy gastric mucosa simple, branched, tubular glands that extend from muscularis externa to bottom of gastric pits consist of mucus neck cells, parietal cells, chief cells, and G cells elaborate gastric secretions into lumen of stomach via gastric pits Small Intestine overview small intestinal mucosa exhibits numerous projections, or villi, that protrude from epithelial layer of mucosal surface villi increase surface area over which digestion and absorption occurs epithelial layer of small intestinal mucosa is heterogeneous, composed of: mucus-secreting cells (goblet cells) absorptive cells (enterocytes) exhibit numerous projections, or microvilli, that protrude from apical border microvilli increase surface area over which digestion and absorption occurs microvilli are responsible for characteristic striated border, or brush border, of enterocytes frequency of villi and of microvilli in small intestine jejunum > duodenum and ileum frequency of goblet cells in small intestine increases as you progress down the small intestine duodenum < jejunum < ileum duodenum duodenal mucosa crypts of Lieberkühn, or intestinal glands, occupy duodenal mucosa simple tubular glands that extend from muscularis externa to base of villi elaborate small intestinal secretions into lumen of duodenum duodenal submucosa Brunner's glands, or submucosal glands, occupy duodenal submucosa elaborate alkaline (basic pH) secretions likely function to neutralize acidic chyme propelled from stomach to duodenum of small intestine peptic ulcer disease presents with hypertrophy of Brunner's glands jejunum jejunal mucosa crypts of Lieberkühn, or intestinal glands, occupy jejunal mucosa jejunal submucosa plicae circulares are circularly arranged transverse folds containing a core of submucosa that extend partially around jejunal lumen ileum ileal mucosa Peyer's patches, or aggregations of nodules of unencapsulated lymphatic tissue, occupy ileal lamina propria (and ileal submucosa) M cells, overlying Peyer's patches, function as antigen-transporting cells take up microorganisms and macromolecules deliver antigens to antigen-processing macrophages macrophages present processed antigen to lymphocytes triggers secretory immunity stimulates B cells in germinal centers of Peyer's patches to differentiate into IgA-secreting plasma cells that reside in ileal lamina propria crypts of Lieberkühn, or intestinal glands, occupy ileal mucosa ileal submucosa plicae circulares are circularly arranged transverse folds containing a core ofsubmucosa that extend partially around ileal lumen (proximal ileum) increase surface area over which absorption occurs Large intestine colon colonic mucosa "smooth" surface devoid of villi crypts of Lieberkühn, or intestinal glands, occupy colonic mucosa colonic muscularis externa outer layer of longitudinally oriented smooth muscle is organized into 3 bundles, or teniae coli anal canal anal canal mucosa keratinizing, stratified squamous epithelium