Digestive Tract Anatomy

• Digestive tract refers to the GI or alimentary canal

Digestive Tract

• Consists of the oral cavity, esophagus, stomach, and small & large intestines with the anus

Associated Glands

• Salivary glands, liver, and pancreas are associated with the digestive tract

General Structure of the Alimentary Canal Wall

• Mucosa is a layer with epithelium and lamina propria rich in blood, lymph vessels, and small glands
• Muscularis mucosa is a thin layer of smooth muscles that separates mucosa from submucosa and allows local movement
• Submucosa is a layer of denser connective tissue containing large blood and lymph vessels
• Submucosal (Meissner) plexus of autonomic nerves is also found in the submucosa, which may contain glands

Muscularis Externa

• Also known as muscularis propria
• It has two layers of smooth muscles, an inner circular layer and an outer longitudinal layer
• Myenteric (Auerbach) nerve plexus generates and coordinates muscle layer movement
• Connective tissue between muscle layers contains blood and lymph vessels
• Contraction, known as peristaltic contraction, mixes and propels luminal contents forward

Serosa

• It is a thin layer of loose connective tissue covered by simple squamous epithelium
• In the esophagus, the serosa gets replaced by adventitia: a layer of connective tissues without mesothelium

Esophagus

• The mucosa consists of non-keratinized stratified squamous epithelium
• Its lamina propria contains esophageal cardiac glands that secrete mucus and its muscularis mucosa is like other parts of the GI tract
• The submucosa has mucus-secreting esophageal glands
• Upper third of the muscularis is exclusively skeletal
• The middle third has a combination of skeletal and smooth muscles
• The lower third is exclusively smooth which begins with voluntary muscle action and finishes by involuntary peristalsis
• In the intra-abdominal part, the adventitia is replaced by serosa

Stomach Regions

• Cardia and pylorus produce mucus with similar histology
• Fundus and body contain gastric glands that secrete acidic juice

Gastric Rugae

• Large longitudinal folds in the mucosa and submucosa of the empty stomach
• These disappear when the stomach is full

Fundus & Body of Stomach

• The surface of the stomach is covered and lined by surface mucus cells

Epithelium

• Gastric pits lead to gastric glands
• It functions to secrete a thick, adherent, viscous mucous layer high in bicarbonate ions for stomach protection

Lamina Propria

• Contains glands

Gastric Glands

• They are long, branched tubular glands that fills the lamina propria in the mucosa
• Components include gastric pits functioning as ducts, isthmus and neck, and the body/base of the gland
• Secretory cells are mucus neck cells, parietal (oxyntic) cells, peptic (chief) cells, stem cells, and enteroendocrine cells
• Mucus neck cells are in the neck of the gastric glands and are columnar with apical secretory granules
• They secrete less alkaline mucus than surface cells
• Parietal (oxyntic) cells are along the whole gland, especially in the neck and body are large and pyramidal
• They have one, or occasionally two round nuclei; the cytoplasm is strongly acidophilic and rich in mitochondria

Parietal (oxyntic) cells

• They are ion-transporting cells with numerous mitochondria
• Intracellular canaliculi and microvilli lining the lumen of the canaliculi increases surface area
• Cytoplasmic tubules and vesicles are also present
• They are essential for HCL production and rough ER & Golgi apparatus aid synthesis and secretion of intrinsic factor (IF)

HCL Production

• CO2 and Cl- diffuse from the blood into the stomach cell
• CO2 combines with H2O to form H2CO3, which dissociates into bicarbonate (HCO3) and H+
• H+ combines with Cl- in the lumen of the gastric gland, forming HCL
• IF production is glycoprotein material essential for B12 absorption It is stimulated by parasympathetic innervation and gastrin hormone

Damage of oxyntic cells

• Achlorhydria
• Perinicious anemia

Peptic (chief, zymogenic) cells

• Protein- forming cells found in the body and base of gastric glands and are columnar
• The lower third contains abundant rough ER, the middle third has a round euchromatic nucleus & Golgi
• The upper third has numerous secretory granules (supra-nuclear)
• Secretion is Pepsinogen converted into pepsin, which is a proteolytic enzyme and Gastric lipase

Stem cells

• Located in the isthmus, neck, and base
• They renew cells of the fundic glands and migrate to replace surface cells or to form new oxyntic & chief cells

Entero-endocrine Cells

• Members of DNES
• Mainly scattered at the body & base and are stained by silver stain and IHC
• Secrete hormones of paracrine action such as Serotonin, Somatostatin, and Gastrin

Other Layers of the Fundus and Body of Stomach

• The muscularis has 3 layers: inner oblique, middle circular, and outer longitudinal
• In the pylorus, the circular layer is greatly thickened to form the pyloric sphincter

Small Intestine

• The small intestine consists of the duodenum, jejunum and ileum
• Increased surface area is due to plicae circulares (x3), which are permanent transverse mucosal and submucosal folds
• Also due to intestinal villi & crypts (x10) and microvilli (x30)

Intestinal Mucosa

• Villi are finger-like projections covered by epithelium
• Loose connective core of the villus contains a central lymphatic lacteal
• Epithelium has absorptive cells, goblet cells, and few enteroendocrine cells

Intestinal crypts

• Intestinal glands are the crypts of Leiberkühn
• Downward invagination of the mucosa
• Lining has cells covering the villi called Paneth cells, M-cells, and stem cells

Absorptive Cells

• They cover the villi and line the upper parts of the crypts and are tall columnar cells with basal oval nuclei
• They have a brush striated border

Apical Microvilli

• Contains a core of actin filaments
• Covered by a cell coat (glycocalyx), which contains enzymes for absorption
• Terminal web (microfilaments) is under the microvilli

Absorptive Cells (EM)

• Cytoplasm contains few small lipid droplets & well-developed smooth ER mitochondria

Function of Absorptive Cells

• They hydrolyze disaccharides into monosaccharides and dipeptides into amino acids that enter the cell by active transport towards blood capillaries
• They absorb lipids, which are surrounded by proteins forming chylomicrons that are released from the basolateral membrane of the cell to the central lacteal

Goblet Cells

• Covers the villi and lines the upper part of crypts; goblet-like shape and the apical part expanded and unstained because it's filled with mucinogen granules
• The basal part is constricted and contains nucleus and organoids
• Special stain used is PAS which appears a magenta color
• It's function it to synthesize and secrete mucus

Paneth cells

• A typical protein synthesizing cell found at the bottom of the crypts and are columnar
• Lower third is abundant with rough ER, middle third has round euchromatic nucleus and Golgi
• Upper third contains numerous secretory granules
• Functions in innate immunity by synthesizing secreting lysozyme & phospholipase A to break down membranes of micro-organisms and bacterial cell walls

M (microfold) cell

• APC antigen-presenting cell in the crypts of the ileum overlying Peyerʼs patches
• They are dome-shaped with basal pockets containing intraepithelial lymphocytes

Entero-endocrine cells

• Located scattered over the villi lining the crypts
• They secrete peptide hormones like Serotonin, Somatostatin, CCK

Other Layers of Small Intestine

• In the proximal part of the duodenum, the submucosa consists of duodenal or Brunner's glands that secrete alkaline mucus to neutralize chyme.
• In the ileum the lamina propria and submucosa contains Mucosa Associated Lymphoid Tissue consisting of large Peyer's patches
• Muscularis externa has two layers as the other parts of the digestive tract