• Unit 2 – Digestion Module

Histology of the small and large intestine

Safaa El Bialy (M.D, PhD)

Ottawa University

Objectives

• Describe the histology of the duodenum, small intestine and colon

• Describe the principal musculature, vascular supply and innervation of the small and large intestines

• Describe the different layers making up the walls of the intestines and their corresponding cell types

Transition from the stomach to the duodenum

pyloric sphincter

duodenum

stomach

Small intestine• divided into 3 parts

– duodenum– jejunum – ileum

• Principal site of absorption of products of digestion

• Functions:• completion of digestion• absorption of nutrients

– lipids, amino acids, carbohydrates, etc.• absorption of water and electrolytes

Small intestine

Small intestine

circular folds

villi

serosa

Mucosa of the small intestine

• absorption of nutrients requires an increase in the absorptive surface between the mucosa and products of digestion

+ the small intestine is very long (4 to 6 metres) – folds of the mucosa and submucosa = circular folds (= valvulae

conniventes = valves of Kerckring) • increase the absorptive surface ~ 3x

– villi• increase the absorptive surface ~ 10x

+ Microvilli on the luminal surface of enterocytes (≈ 3000 per cell)

• glands (crypts) are present between the villi (crypts of Lieberkühn)

Circular fold and villi

lymphoid islet

submucosa

villus

lamina propria

Mucosa

submucosa

lacteal

smoothmuscle

simple columnarepithelium

crypts of Lieberkühn

Villi covered by enterocytes

enterocytes

brush border

goblet cell

PAS stain (for demonstration of carbohydrates)

enterocytes

goblet cells (mucus is PAS +)

brush border (glycocalyx is PAS +)

Epithelium of the small intestine

• simple columnar epithelium• epithelial cell types

– enterocytes– goblet cells– Paneth cells– enteroendocrine cells– stem cells

• epithelium composition differs by region (villi vs glands, duodenum vs jejunum vs ileum)

• Tall columnar cells – predominant cell type on the villi• Functions:

• absorption of water, electrolytes and products of digestion• participate in digestion (see below)• enterocytes in the glands secrete water and electrolytes

• Apical end contains abundant (~3000) microvilli• increase the absorptive surface by at least 20x• microvilli make up the brush border• microvilli are covered by glycocalyx = layer of carbohydrates• brush border contains transport proteins and digestive enzymes (e.g.

lactase, sucrase, peptidase)

Enterocytes

Goblet cells

• found in the glands and also on the villi interspersed among enterocytes

• contain globules of mucin in the apical end of the cell – mucus secretion lubricates and protects the intestinal mucous membrane

• these cells are increasingly abundant along the length of the small intestine (i.e., more numerous in the ileum than in the duodenum)

Crypts of Lieberkühn

• Have the same cells as the villi but those 2 cells are situated at their base

• Stem cells • Paneth cells

Goblet cells and Paneth cells

http://anatomy.iupui.edu/courses/histo_D502/D502f04/Labs.f04/Lab2/s9540x1.jpg

goblet cells

Paneth cells

Paneth cells

• found in the lower part of the glands • contain prominent eosinophilic granules in the

apical portion of the cell • secrete proteins

– granules contain antimicrobial peptides (defensins) and protective enzymes (e.g. lysozyme)

– Longer life span than enterocytes and goblet cells

Enteroendocrine cells and Stem cells

Enteroendocrine cells

• found in the lower part of the glands• also contain eosinophilic granules, but these

granules are smaller and lie below the nucleus• resemble gastric enteroendocrine cells• secrete various hormones (e.g. cholecystokinin,

secretin)

Lamina propria of the small intestine

• loose connective tissue• a few smooth muscle fibres• immune cells• rich in blood and lymphatic

vessels (lacteals) for absorption of products of digestion

http://medical-dictionary.thefreedictionary.com/_/viewer.aspx?path=dorland&name=villus_villi-intestinales.jpg

lacteal

blood vessel

The intestine is part of the immune system

• many cells of the immune system are found in the lamina propria of the intestine, including:– lymphocytes (part of GALT - gut-associated lymphoid

tissue)• predominantly T cells• B cells (plasmocytes) secrete IgA

– macrophages– eosinophils– mastocytes

Submucosa, muscularis and tunica externa of the small intestine

Submucosa, muscularis and tunica externa of the small intestine

• submucosa– connective tissue– blood and lymphatic vessels– Meissner’s plexus

• muscularis – 2 layers of smooth muscle

• inner circular layer• outer longitudinal layer

– between the 2 layers is the myenteric plexus (Auerbach’s plexus)

• tunica externa – serosa (exception: most of the duodenum is situated in the

retroperitoneum so its tunica externa consists primarily of adventitia)

Regional specializations• Duodenum• Is almost entirely retroperitoneal• Principal function is to neutralize the gastric acid and pepsin

and initiate further digestive mechanisms• submucosa contains mucous glands (Brünner’s glands)

• secrete an alkaline mucous substance in the duodenal lumen (stimulated by the presence of chyme in the duodenum)

– protects the duodenum against the acid contents of the stomach

– increases pH to a more optimum level for pancreatic digestive enzymes

• The mucosa contains neuroendocrine cells secreting secretin and cholecystokinin (CCK) under the effect of chyme

• Those two hormones stimulate the exocrine secretion of the pancreas and CCK induces the contraction of the gall bladder

Duodenum

muscularis

submucosa with Brünner’s glands

Brünner’s glands

submucosamuscularis mucosae

goblet cells

Regional specializations

• jejunum– principal site of absorption – greatest development of circular folds

• ileum– greatest development of lymphoid tissue forming

large nodules called Peyer’s patches that occupy the lamina propria and submucosa

Peyer’s patchesand M cells

• epithelium covering these patches contains M cells (MALT)– have invaginations of the cell

membrane of lymphocytes and antigen-presenting cells

– trap antigens in the lumen and and carry them to immune cells

http://www.siumed.edu/~dking2/erg/GI196b.htm

http://www.ims.u-tokyo.ac.jp/EnMen/science/m_cells_e.html

ileum

Regional differences of the small intestine

• The villi are longer at the level of the duodenum and become shorter towards the ileum

• The lymphoid tissue, scarce in the duodenum is more developed in the ileum

• The proportion of goblet cells in the epithelium increases from the duodenum towards the ileum

• The circular folds generally absent at the level of the proximal duodenum and the distal ileum, are more developed and more numerous in the jejunum and the proximal ileum

Large intestine• subdivisions:

– cecum – ascending, transverse, descending and sigmoid colon– rectum

• principal function is reabsorption of water and electrolytes

• secretes mucus to facilitate elimination of stools and protect its mucous membrane

• colonized by numerous bacteria that break down food residues

• appendix– cecal diverticulum– same basic structure as the large intestine– abundant lymphoid tissue

Mucosa of the large intestine

N.B.: absence of villi

gland

(crypt)

Epithelium of the large intestine

• epithelium is organized into simple invaginations of the surface extending to the muscularis mucosae (invaginations = glands/crypts)

• epithelial cell types– abundant goblet cells (mucus-secreting)– columnar cells (absorptive)– stem cells– enteroendocrine cells (few)– Paneth cells in the ascending colon

Lamina propria of the large intestine

• similar to that of the small intestine• loose connective tissue• blood and lymphatic vessels (but no lacteals)• immune cells, including scattered small lymphoid

islets • muciphages (macrophages containing mucin

granules)

Muciphages

H&E Alcian blue

Submucosa, muscularis and tunica externa of the large intestine

• submucosa – similar to that of the small intestine

• muscularis– similar to that of the small intestine (2 layers of smooth muscle,

between which lies Auerbach’s plexus), but the external layer is formed of 3 longitudinal bands named:

– taenia coli• separate muscle bands along the length of the colon• responsible of propulsion of the food bolus

• tunica externa – posterior aspect of some parts of the colon contain an adventitia;

elsewhere it is a serosa

Large intestine

taenia coli

Large intestine

Large intestine (Haustra)

Appendix

lumen

submucosamuscularis(underdeveloped)

mucosa

lymphoid islets

Anorectal junction

rectum anal canal