Histo 3

• Study notes on the digestive system

Intestines

• Mammalian intestines feature a small intestine, consisting of the duodenum, jejunum, and ileum, and a large intestine, which is composed of the cecum, colon, rectum, and anal canal.
• Epithelium throughout both the small and large intestines is simple columnar with a striated border.
• Goblet cells are interspersed among the absorptive columnar cells.
• The tunica muscularis of the intestine consists of inner circular and outer longitudinal layers of smooth muscle.

Small Intestine

• Villi exist only in the small intestine.
• Villi morphology differs between species; ruminants have short, thick villi, while carnivores have long, slender villi.
• Intestinal glands are invaginations of the epithelium located at the bases of the villi.
• Mucosal epithelium is replaced via cell division, mainly within the invaginations.
• The lamina propria and submucosa of the small intestine, particularly the ileum, contain aggregations of lymphatic nodules known as Peyer's patches.
• Peyer’s patches provide immune surveillance and facilitate immune response within the intestinal lumen.

Brunner's Glands

• Brunner's glands, also known as duodenal glands, are compound tubular submucosal glands in the duodenum, often found within the lamina propria.
• These glands produce a mucus-rich alkaline secretion containing bicarbonate to protect the duodenum from acidic stomach content.
• They create an alkaline environment for the activity of intestinal enzymes, aiding absorption.
• Brunner's glands are limited to the initial or mid-region of the duodenum in carnivores, sheep, and goats, but extend into the jejunum in horses, pigs, and cows.

Large Intestine

• Large intestine structure is similar to the small intestine, but lacks villi.
• The submucosa contains prominent lymphatic nodules.
• Goblet cell numbers in the intestinal epithelium increase significantly to produce more mucus for lubrication as water is absorbed and the intestinal contents prep for defecation.
• Relative to carnivores, herbivores contain a well developed cecum, also it contains a simple stomach
• The outer tunica muscularis layer features longitudinal bands of smooth muscle, the taeniae ceci, which vary in number.
• Taeniae coli, are longitudinal bands of smooth muscle present in the tunica muscularis of pigs and horses.
• As the rectum shifts retroperitoneally, the tunica serosa transitions into a tunica adventitia.

Liver

• The liver is a large, lobed gland; each lobe is covered by a mesothelium with a thin connective tissue layer called the capsule of the liver.
• Each lobe contains multiple classic lobules consisting of sinusoids and plates of hepatocytes arranged radially around a central vein.
• Lobules are indistinctly separated in all but pigs, which have abundant connective tissue between lobules, clearly defining their boundaries.
• The interlobular connective tissue contains bile ducts and hepatic artery and portal vein branches, collectively known as the portal triad.
• Blood flow in the portal triad vessels occurs in one direction, while bile flows oppositely in the biliary system.
• Hepatic artery and portal vein blood flows into the sinusoids, percolates, exits through the central vein, enters the sublobular vein, and finally flows into the hepatic vein.
• The classical hepatic lobule features a hexagonal mass of liver cells and acts as the structural lobule of the liver.
• The portal lobule, centered around a portal triad, supports the nutritional function of the liver.
• The hepatic acinus of Rappaport, a diamond-shaped area of liver parenchyma, functions structurally and metabolically.
• The acinus is divided into zones 1 (well-oxygenated area around the vascular backbone), 2 (moderately oxygenated intermediate zone), and 3 (least oxygenated area close to the central vein), based on blood supply.

Pancreas

• Thin connective tissue surrounds the organ, extending between lobules of the parenchyma.
• Endocrine pancreatic islets are scattered among exocrine secretory units.
• Exocrine acinar cells produce trypsinogen and chymotrypsinogen.
• Acini contain pyramidal-shaped secretory cells connected to a duct system.
• Intercalated ducts connect with the intralobular ducts, both consisting of simple cuboidal epithelium.
• Larger interlobular ducts lie within the connective tissue between lobules and consist of simple columnar epithelium.

Notes on the mammalian intestines, including the small and large intestines, their structure, and function. Focus on epithelium, villi, and intestinal glands. Includes tunica muscularis and mucosal epithelium.