Digestive tract Li Zhong Jie（李仲杰）, Ph. D School of Medicine,Zhejiang University 20151106

---Digestive system function: Digestive tract Digestive gland ---Digestive system function: This system is responsible for the mechanical and chemical break down of food material, and for absorbing these digestive products into the blood for use as nutrients by the individual cells and tissues of the body The digestive system is a long, tortuous, hollow tube. It engages in many function such as propulsion, secretion, absorption, excretion ,immunologic protection, and hormone production.

The Digestive Process Ingestion Propulsion (movement of food) Taking in food through the mouth Propulsion (movement of food) Swallowing Peristalsis – propulsion by alternate contraction & relaxation Mechanical digestion Chewing Churning in stomach Mixing by segmentation Chemical digestion Carbohydrates, Fat, and Proteins are broken down by enzymes. Absorption Transport of digested end products into blood and lymph in wall of canal Defecation Elimination of indigestible substances from body as feces

Chemical digestion --- Complex food molecules (carbohydrates, proteins and lipids) broken down into chemical building blocks (simple sugars, amino acids, and fatty acids and glycerol) --- Carried out by enzymes secreted by digestive glands into lumen of the alimentary canal

Components of digestive tract ---oral cavity ---pharynx ---esophagus ---stomach ---small intestine ---large intestine ---rectum and anus

General plan of digestive tract ---Except for oral cavity and pharynx, all other organs share a similar histological plan Mucosa Submucosa Muscularis externa Adventitia from lumen (inside) out

Inner layer: the mucosa* (mucous membrane) Three sub-layers epithelium lamina propria (may contain glands) Muscularis mucosae (Smooth muscle) *

Mucosa 1. Epithelium -------two types stratified squamous & simple columnar epith. 2. Lamina propria LCT. contained small glands，blood & lymph capillaries. mucosa-associated lymphoid tissue 3. Muscularis mucosa inner circular / outer longitudinal layer of smooth muscle cells. surface depth

the submucosa* * LCT. with small blood / lymphatic vessels; * glands only in the esophagus and duodenum Submucosal ( Meissner’s autonomic) nerve plexus *

Plica *a fold of mucosa and submucosa *longitudinal/circular form. GI

the muscularis externa* Two layers of smooth muscle responsible for peristalsis and segmentation Inner circular layer (circumferential) Myenteric (Auerbach’s autonomic) nerve plexus Outer longitudinal layer: ---shortens gut *

Smooth muscle Muscles are spindle-shaped cells One central nucleus Grouped into sheets: often running perpendicular to each other Peristalsis No striations (no sarcomeres) Contractions are slow, sustained and resistant to fatigue Does not always require a nervous signal: can be stimulated by stretching or hormones

Adventitia *the outmost layer formed by CT with two types: fibrosa: CT blending with surrounding structure serosa: CT + mesothelium （for example, peritoneum）

Enteric nervous system: Mainly components: 100 million neurons! (as many as the spinal cord) unmyelinated nerve fibers controlling the muscles, glands and having sensory info Muscularis external: Myenteric nerve plexus Submucosa: Submucosal nerve plexus

* Myenteric (Auerbach’s) plexus : regulate the movement of SM inner circular Nerve plexus outer longitudinal

Esophagus mucosa submucosa muscularis adventitia

Esophagus Passage way for food from the pharynx to the stomach mucosa: epithelium: stratified squamous epithelium lamina propria: compact CT contain simple tubular glands muscularis mucosa: longitudinal arranged smooth muscle At EG junction – thin simple columnar epithelium submucosa: LCT esophageal gland: mucous gland ---acidic mucin

Muscularis externa: inner circular and outer longitudinal upper 1/3: skeletal muscle middle 1/3: mixed of skeletal muscle and smooth muscle lower 1/3: smooth muscle Tunica adventitia: a fibrous coat of loose connective tissue

When empty, mucosa and submucosa lie in longitudinal folds

Cardiac Junction Epithelial transition ---Stratified Squamous nonkeratinized to simple columnar It is clinically important, as it is the most site of esophageal carcinoma.

Stomach ---widest part of alimentary canal ---Temporary storage and mixing – 4 hours ---digest food partially to form a semi-fluid mass, termed chyme ---absorb part of water and ions Water, electrolytes, some drugs like aspirin and alcohol (absorbed through stomach)

Stomach (regions) Cardia Surrounds esophageal entrance Fundic stomach defined histologically includes Fundus Body Pylorus Pylorus is continuous with the duodenum

plica: longitudinal folds on internal surface (helps distensibility)

Structural Characteristic of Stomach Mucosa Epithelium (simple columnar mucus-secreting) Lamina propria (gastric glands of different types) Muscularis mucosae (Smooth muscle) Submucosa Loose C.T. no glands Muscularis externa inner oblique, middle circular, outer longitudinal Tunica adventitia Mostly serosa

mucosa plica Longitudinal folds of mucosa A mucosal fold contains submucosa Gastric pits: small depressions, 3-5 gastric gland open into the bottom Diffuse lymphoid tissue and nodules may be present Gastric pit

plica in the stomach Mucosa Muscularis mucosa Submucosa Muscularis externa plica

*gastric pits ①epithelium: simple columnar epithelium surface mucous cell: ---tall columnar ---ovoid, basally-located nuclei ---apical mucin granule ---tight junction *gastric pits

Simple columnar epithelium Gastric pit Laminia propria between pits Cross section of gastric pits Simple columnar epithelium Gastric pit Laminia propria between pits

②lamina propria: CT contains fibroblast, LC, plasma cell, mast cell and eosinophilic smooth muscle *Gastric gland: fundic gland:oxyntic gland cardiac gland: mucous gland pyloric gland: mucous gland

* Fundic gland ---long, straight, branched or unbranched gland

Three part of gland: The neck The body The base Five type cells are found: Chief cells Parietal cells Mucous neck cells Stem cells endocrine cells

Fundic gland

parietal cell or oxyntic cell ---structure: LM: large, pyramidal or spherical round centrally-located nuclei eosinophilic cytoplasm

intracellular secretory canaliculus (active stage) EM: intracellular secretory canaliculus (active stage) tubulovesicular system(resting stage) a few RER, Golgi apparatus and more mitochondria Trench –like infoldings of apical cell membranes form a ramifying network of narrow channels- secretory canaliculi. They occur throughout the cytoplasm and near the nucleus and open into glandular lumina. Cannliculi are lined by many densely packed microvilli. secreting stage Resting stage

Ultrastructural model of Parietal cell intracellular secretory canaliculus tubulovesicular system microvilli nucleus mitochondria Resting stage Secreting stage Ultrastructural model of Parietal cell GI

intracellular secretory canaliculus Secreting stage interconvert Resting stage GI tubulovesicular system

Function of parietal cell (oxyntic cell) *secreting hydrochloride acid ---providinng acidic condition for activation of the pepsinogen. ---killing bacteria except for helicobacter pylori . *producing intrinsic factor ---binding with vitamin B12, aiding its absorption in the small intestine. ---helping production of red blood cells. an deficiency of VB12 result in Addison’s anaemia GI

chief cell ( zymogenic cell) ---structure: LM: columnar Round, basally-located Nuclei cytoplasm: /basal-basophilic /apical-zymogen granules

EM: RER, Golgi apparatus ---function: secret pepsinogen(the precursor of pepsin)

Function of chief cell secreting pepsinogen (inactive) pepsin (active) protein--------peptides (acid environment) GI

Fundic gland 内分泌细胞 主细胞 颈粘液细胞 壁细胞

chief cell parietal cell secret hydrochloric acid secret intrinsic factor function: secret pepsinogen

mucous neck cell stem cell endocrine cell less, neck part pale stain in HE stain secrete mucus stem cell undifferentiated cell endocrine cell L cell: secreting histamine, promote secretion of parietal cell D cell: secreting somatostatin, inhibit the secretion of parietal cell

* Mucous-HCO3 barrier structure : mostly of glycoproteins rich in carbohydrates and bicarbonate ions 0.25-0.5mm thick mucous The pH of the surface is usually 0.9-1.5

* Mucous-HCO3 barrier function : The mucus( bicarbonate-buffered mucus) is secreted on to the epithelial surface to form a barrier layer which protects it from injury by ingested substance and the stomach’s own secretion of acid and enzymes. acid-base neutralization H+ + HCO3- → H2CO3 →H2O + CO2 ↑ bicarbonatase

Small intestine Duodenum – first region, only about 25cm long, Jejunum – second region is roughly 2.5m long Ileum – last region is roughly 3.5m long Primary functions Transport food from stomach to Large intestine Secretion of digestive enzymes to facilitate digestion of food substances Absorption of nutrient substances into blood and lymph vessels Secretion of certain hormones

Special structure of mucosa Plicae circulares Mucosa and submucosa are arranged in permanent, circular mucosal folds Intestinal villi Mucosal projections covered by epithelium and containing only lamina propria Crypt or intestinal glands Surrounded by lamina propria Extend to the muscularis mucosae

villi Plicae circulares

Plicae circulares Villi Plicae circulares, villi and microvilli are serve to increase the surface area of the small intestine by as much as 600-fold

Structural Characteristic of small intestine Mucosa Epithelium (simple columnar) Lamina propria (intestinal glands) Muscularis mucosae (Smooth muscle) Submucosa loose C.T. (contain duodenal glands in the duodenum) Muscularis externa inner circular, Myenteric (Auerbach’s) plexus , outer longitudinal Tunica adventitia serosa (except for the duodenum)

Lymph Nodules

Epithelium (Simple columnar) Absorptive cells A tall columnar cell with ovoid nucleus Numerous, regular microvilli form striated-border Well formed junctional complex *cell coat: a layer of glycoprotein film, protect the underlying cells from mucolytic and proteolytic agent Goblet cells :secrete mucus to lubricate and protect the epithelium Enteroendocrine cells: produce hormones

striated-border

simple columnar cells microvilli

lamina propria: LCT contains network of blood and lymph capillaries - transport sugar and amino acid into blood to liver via hepatic portal vein - delivers lipid into lymph *central lacteal: lymphatic vessel, absorb lipid

Small intestine glands Core of villus epithelium villus Smooth m.f. gland A gland L V Model H.E. staining Longitudinal section of small intestine

Crypt or small intestinal gland: the invagination of epithelium into lamina propria absorptive cell goblet cell endocrine cell stem cell Peneth cell

Peneth cell: Structure: LM: ---pyramidal in shape, locate in basal portion of the glands, in groups ---apical: acidophilic granules- contain defensin, lysozyme EM: ---protein-secreting cell feature Function: secreting defensin, lysozyme which involved in the control of infection.

Muscularis externa Inner circular SM Myenteric plexus Outer longitudinal SM

Large intestine ---Consists of: -cecum -ascending colon -transverse colon -descending colon -sigmoid colon -rectum -anal canal --- function: absorption of water and ions

Mucosa No villi or plicae circulares Glands are longer than in small intestine Simple columnar epithelium contain numerous goblet cells but absorptive cells are still present --- At anal junction there is an abrupt transition to stratified squamous non-keratinized epithelium.

Submucosa Muscularis externa Adventitia Similar to small intestine except nerve plexus are more easily found here Muscularis externa Inner circular layer is evident Outer longitudinal layer forms three bands, the taenia coli Adventitia Both serosa and fibrosa are found

Appendix

Appendix Mucosa ---Surface epith. with few goblet cells. Submucosa ---Numerous lymph nodules are present in in the lamina propria and the submucosa . ---rare intestinal glands and muscularis mucosa usually incomplete. Submucosa thin muscularis Serosa

Review and exercise 1. Compare the mucosal structure of stomach with that of esophagus, small intestine and large intestine. 2. Describe the fine structure and ultra-structure of the parietal cell and chief cell in the gastric gland. 3. Describe the different formation of villi and plicae in small intestine. 4. Describe the structure of mucosa of the digestive tract.

Thanks for your attention!