Large Intestine

• The layers of the wall of the digestive tract include the mucosa, submucosa, muscularis externa, and serosa.
• The structure of the Large intestine includes the cecum with its projecting vermiform appendix, the colon, the rectum, and the anal canal.
• The primary function of the columnar absorptive cells is reabsorption of water and electrolytes.
• Elimination of semisolid to solid waste materials is facilitated by the large amounts of mucus secreted by the numerous goblet cells of the intestinal glands.
• The rectum and anal canal have 3 zones of different epithelium.
• The appendix contains lymphoid aggregates.
• The histologic layers of the large Intestine include Mucosa, submucosa, muscularis mucosa, and serosa/adventitia.
• The large intestine contains straight tubular intestinal glands (crypts of Lieberkühn) - smooth surface that are longer than that of the small intestine that extend through the full thickness of mucosa to the muscularis mucosa.
• intestinal glands extend through the full thickness of the mucosa to the muscularis mucosa.
• The mucosal epithelium of the large intestine contains the same cell types as the small intestine except for Paneth cells.
• The mucosa of the large intestine has a smooth surface and does not have villi or plicae circulares.
• The mucosa of the large intestine is lined by simple columnar epithelium called colonocytes or enterocytes, with irregular microvilli and dilated intercellular spaces indicating active fluid absorption.

  goblet cells, enteroendocrine and stem cells
• Goblet cells are also present in the mucosa of the large intestine.
• Enteroendocrine cells are also present in the mucosa of the large intestine.
• Stem cells for epithelial and enteroendocrine cells are scattered among enterocytes and goblet cells.
• The lower third of the gland constitutes the intestinal stem cell niche.
• Enterocytes have an apical striated border of short microvilli that project into the lumen.
• These cells occur with goblet cells, which have apical mucin granules.
• The turnover times of the epithelial cells about 6 days for absorptive cells and goblet cells and up to 4 weeks for enteroendocrine cells.
• The ratio of goblet cells to enterocytes, is greater in the colon than in the small intestine.
• The structure of the GALT includes the lamina propria, submucosa, muscularis externa, and serosa.
• Lamina propia:

  CT stroma

  GALT is continuous with that of the terminal ileum.
• GALT is more extensively developed; large lymphatic nodules distort the regular spacing of the intestinal glands and extend into the submucosa.
• The mucosa of the large intestine contains the same cell types as the small intestine except for Paneth cells.
• The external anal sphincter is formed by skeletal muscle.
• Colorectal cancer is associated with risk factors such as age above 50, history of adenomatous polyps, diet related factors like alcohol use, obesity, high red meat diet, cooking meat at high temperature, vitamin D deficiency, and disease related factors like type 2 DM, inflammatory bowel diseases, and family history.
• The mucosa and submucosa of the anal canal form several longitudinal folds.
• The rectum is the dilated distal portion of the alimentary canal, distinguished from the rest of the large intestine by the presence of folds called transverse rectal folds.
• Outer layer is condensed into promninent bands of muscle called teniae coli which are responsible for the formation of haustra coli.
• Immediately above the anal valves, the mucosa forms longitudinal folds called columns of Morgagni.
• Colorectal polyps are asymptomatic and usually arise from the mucosa.
• The rectal mucosa lined by simple columnar epithelium, lacks villi and resembles that of the colon but glands are somewhat longer. consists solely f goblet cells and adventitia replaces serosa
• The rectal mucosa has a smooth surface with periodic invaginations equivalent to intestinal crypts.
• The anal canal has longitudinal folds called anal columns.
• This epithelium changes to keratinized stratified squamous epithelium at the anus proper.
• Ganglionic megacolon, also known as Hirschsprung disease, is characterized by the absence of enteric neurons within the myenteric and submucosal plexus of the rectum and/or colon, and is seen in approximately 1 per 5000 live births.
• The myenteric plexus, also known as Auerbach’s plexus, is present between the outer and inner muscle layers of the large intestine.
• The muscularis externa of the large intestine differs from the colon in having a uniform layer of longitudinal muscle.
• Recto-anal juntion The simple columnar epithelium with tubular intestinal glands in the rectum changes abruptly, at the pectinate line, to stratified cuboidal/squamous epithelium in the anal canal - which gets keratinsed at the anus proper
• The outer layer of the large intestine, part of which is condensed into prominent longitudinal bands of muscle called teniae coli, is present on the cecum and colon and absent on the rectum.