Session 2 Histology and embryology

• Label the layers of oesophagus

  A) Mucosa

  B) Submucosa

  C) Muscularis externa

  D) Adventitia

  
• Four layers of GI tract?

  1. Mucosa

  • Epithelial lining
  • Lamina propria
  • Muscularis mucosae (smooth muscle)
  • 2. Submucosa Connective tissue with blood, lymph vessels and fat Meissner’s nerve plexus Muscularis Circular internal layer (Auerbach’s nerve plexus) Longitudinal external layer Serosa
• Four layers of GI tract?

  • Mucosa - epithelium, lamina propia, muscularis mucosae
  • Submucosa - glands, blood vessels, Meissner's plexus
  • Muscularis - Inner circular layer, Auerbach's plexus, Outer longitudinal layer
  • Serosa/ Adventitia (for retroperitoneal organs)
• The epithelium of the oesophagus is stratified squamous non-keratinised.
• Layers of stomach?

  • Mucosa - Simple columnar epithelium, lamina propia, muscularis mucosae
  • Submucosa - meisnner's plexus, blood vessels
  • Muscularis - Oblique, circular, longitudinal
  • Serosa
• Surface mucous cells secrete alkaline fluid containing mucin.
• Mucous neck cells secrete acidic fluid containing mucin.
• Cells of fundic/gastric glands?

  • Surface mucus cells
  • Mucus neck cells
  • Parietal/oxyntic cells
  • Chief/peptic cells
  • Enteroendocrine/G cells
• Parietal (oxyntic) cells are pyramid shaped cells that produce HCl and gastric intrinsic factor (B 12 absorption).
• Chief (zymogenic/peptic) cells synthesise pepsinogen and gastric lipase.
• Cells of the diffuse neuroendocrine system (DNES) secrete hormones such as gastrin (which promotes acid secretion).
• Brunner's glands of duodenum?

  Brunner’s gland are compound tubular glands that secrete alkaline mucus to counteract acidic chyme. This protects the mucous membrane and creates the optimal pH for enzymatic digestion.
• Plicae circularis is present in jejunum of small intestine
• Microvilli on the surface of enterocytes increase the surface area for absorption of nutrients a further 20-fold.
• Lacteals are dilated lymphatic vessels important in fat absorption
• Paneth cells located in crypts (with characteristic red cytoplasmic granules) produce defensins and lysozyme.
• Enteroendocrine cells in the duodenum and jejunum (K cells) produce gastric inhibitory peptide (which suppresses acid secretion).
• The mesothelium produces a lubricating fluid to allow movement within the peritoneal cavity, and is continuous with the mesentery.
• Peyer’s patches are large aggregations of lymphoid follicles found in the ileum (particularly the distal region), providing immune surveillance.
• M cells of Peyer's patches?

  • M (microfold) cells form a pocket that encloses T and B lymphocytes and dendritic cells.
  • Transcytosed antigens are taken up by dendritic cells and presented to Th-cells.
  • These, in turn, activate B-lymphocytes which mature and secrete IgA molecules.
  • IgAs are transported into the gut to neutralise invaders
• Epithelium of rectum?

  Simple columnar
• Epithelium of anus?

  Stratified squamous
• Nerve plexus involved in peristalsis?

  Auerbach's (myenteric) plexus
• The major salivary glands?

  • Parotid: mainly serous
  • Submandibular: serous/ mucous
  • Sublingual: mainly mucous
• • The endocrine pancreas (islets of Langerhans) produces insulin (β-cells), glucagon (α-cells), somatostatin (γ-cells) and pancreatic polypeptide (PP cells).

  • The exocrine pancreas releases proteases, amylases, lipases and DNAses as zymogens into the duodenum via the major duodenal papilla.
• Function of liver?

  The main digestive function of the liver is the production of bile, which emulsifies fats.
• The gallbladder contracts in response to cholecystokinin.
• Bile is transferred to the duodenum - via the common bile duct and the major duodenal papilla (papilla of Vater) where it emulsifies fats.
• Bile contains water, electrolytes, bile salts (derived from cholic acid), fatty acids and bilirubin and cholesterol.
• The ducts of the biliary tract are lined with cholangiocytes.
• Epithelium of gall bladder?

  Simple columnar epithelium
• Dietary fats are hydrolysed to FFAs by lipases, which are emulsified by bile salts to form micelles that are taken up by enterocytes where triglycerides are reformed and transported in the blood as chylomicrons.
• Embryo folding occurs in two directions:?

  1. Longitudinal direction: the cephalo-caudal flexion
  2. Transversal direction: the lateral flexion
• Embryo folding establishes tube-within-a-tube body plan in which embryo consists of two main tubes:

  • An outer ectodermal tube that forms the skin
  • An inner endodermal tube that forms the gut
  • The space between the two tubes is filled mostly by mesoderm, the lateral plate mesodermal part of which splits to form the body cavity or coelom.
• During the 4th week, dorsal mesentery is formed by the condensation of splanchnic mesoderm that surrounds the gut tube. Dorsal mesentery is thin, bi-layered membrane.
• By the 5th week, the caudal portion of the septum transversum thins to form the ventral mesentery connecting the stomach and developing liver to the ventral body wall.
• Dorsal mesentery derivatives?

  • dorsal mesogastrium or greater omentum in the region of the stomach
  • dorsal mesoduodenum in the region of the duodenum
  • dorsal mesocolon in the region of the colon
  • mesentery proper
• Ventral mesentery derivatives?

  • the lesser omentum, extending from the lower portion of the oesophagus, the stomach, and the upper portion of the duodenum to the liver
  • the falciform ligament
• The mesentery suspending some intraperitoneal organs disappears as both the mesentery and the organ fuse with the body wall- these organs are then called secondarily retroperitoneal.
• As with the rest of the gut tube, the lumen of the oesophagus becomes temporarily occluded around the 5th week of development and recanalises by around 9 th week.