Structure and Function of the Small Intestine

Small Intestine Structure and Function

Overview

  • The small intestine is the primary site for the completion of digestion and absorption of nutrients.
  • It is approximately 5 meters long and consists of three segments:
    • Duodenum
    • Jejunum
    • Ileum

Mucosa

  • Macroscopic Features:
    • Plicae circulares: Permanent circular (or semilunar) folds, most prominent in the jejunum.
    • Villi: Mucosal outgrowths (0.5–1.5 mm) projecting into the lumen, densely covering the entire mucosa.
    • Intestinal glands (Crypts of Lieberkühn): Located between villi.

Villi Structure

  • Finger- or leaf-like projections.
  • Lined by:
    • Enterocytes (absorptive cells)
    • Goblet cells
  • Core contains:
    • Loose connective tissue
    • Fibroblasts, lymphocytes, plasma cells
    • Capillaries and a central lacteal
  • The mucosa and submucosa are the innermost two layers of the gut and handle absorption.
  • Together, they form circular folds (plicae circulares) covered with fingerlike projections called villi, which increase the surface area for absorption.
  • Each villus has a surface lining of enterocytes (absorptive cells) and goblet cells (mucus-secreting).
  • This lining dips down to form intestinal glands (crypts) containing stem cells and Paneth cells.
  • Inside each villus is connective tissue with small blood vessels and a lacteal (lymphatic capillary).
  • A microscope image shows the villi lined with simple columnar epithelium made up of enterocytes and goblet cells.
  • Under high magnification, the enterocytes display a dense brush border of microvilli that greatly increase their surface area for absorption.

Intestinal Glands (Crypts of Lieberkühn)

  • Short tubular glands between villi.
  • Lined by:
    • Differentiating cells
    • Pluripotent stem cells
  • Give rise to:
    • Enterocytes
    • Goblet cells
    • Paneth cells
    • Enteroendocrine cells
    • M (microfold) cells

Enterocytes

  • Tall absorptive cells with basal nuclei.
  • Striated (brush) border: dense microvilli covered by glycocalyx.
  • Increase surface area:
    • Plicae: 3×3 \times
    • Villi: 10×10 \times
    • Microvilli: 20×20 \times
    • Total absorptive surface ≈200m2\approx 200 m^2

Nutrient Absorption by Enterocytes

  • Carbohydrates & Proteins:
    • Hydrolyzed by disaccharidases & peptidases.
    • Absorbed as monosaccharides & amino acids.
  • Lipids:
    • Digested by lipases.
    • Emulsified by bile salts into micelles.
    • Absorbed into enterocytes → re-esterified into triglycerides.
    • Form chylomicrons → lacteals.

Cells Covering Villi

  • The mucosa and submucosa (SM) of the small intestine form distinct projecting folds called plicae (P).
  • On each fold, the mucosa forms a dense covering of projecting villi (V).
  • In this longitudinal section, the two layers of the muscularis (M) are clearly distinguished.
  • The outer layer runs lengthwise just inside the serosa (S), the gut’s outer layer.

Goblet Cells

  • Scattered among enterocytes.
  • Secrete:
    • Mucins (form mucus layer)
    • Antibacterial peptides
  • Function:
    • Protect and lubricate intestinal lining.

Additional Cell Details

  • The columnar epithelium that covers intestinal villi consists mainly of the tall absorptive enterocytes (E). The apical ends of these cells are joined and covered by a brush border of microvilli.
  • Mucus-secreting goblet cells (G).
  • At higher magnification, individual microvilli of enterocytes (E).
  • An enteroendocrine cell (EC).

Paneth Cells

  • Located at the base of intestinal crypts.
  • Contain eosinophilic secretory granules.
  • Secrete:
    • Lysozyme
    • Phospholipase A2
    • Defensins
  • Role:
    • Protect crypts.
    • Regulate microbial environment (innate immunity).

Location Details

  • Between villi (V) throughout the small intestine, the covering epithelium invaginates into the lamina propria (LP) to form short tubular glands called intestinal glands or intestinal crypts (IC).
  • Paneth cells (P).
  • The submucosa (S).
  • Enteroendocrine cell (EC).
  • Differentiating goblet cell (G).

Enteroendocrine Cells

  • Scattered throughout the intestine.
  • Secrete various peptide hormones.
  • Mostly “open” type:
    • Apical ends contact lumen.
    • Detect nutrients.
    • Regulate hormone release basally.

Enteroendocrine Cell Function

  • TEM of an “open” enteroendocrine cell in the epithelium of the duodenum shows microvilli at its apical end in contact with the lumen.
  • Activation of these cells by nutrients triggers the release at the basolateral membranes of peptide factors, including satiation peptides, which diffuse through extracellular fluid to enter capillaries (endocrine) or to bind receptors on nearby nerve terminals, smooth muscle fibers, or other cells (paracrine).

M (Microfold) Cells

  • Specialized for:
    • Transport of particles and microorganisms.
  • Located over Peyer’s patches.
  • Contain:
    • Large basolateral pockets with lymphocytes and macrophages.
  • Important in mucosal immunity.

Other Layers

  • Lamina propria:
    • Loose connective tissue.
    • Blood & lymph capillaries.
    • Smooth muscle fibers.
    • Lymphoid tissue.
  • Smooth muscle in villi aids absorption.

Microvasculature

  • Diagrammatic representations of the small intestine villi blood microvasculature (left), lymphatic capillaries called lacteals (center), and both innervation and smooth muscle fibers (right).

Submucosa

  • Contains:
    • Larger blood & lymph vessels.
    • Submucosal (Meissner’s) plexus.
  • Duodenum:
    • Brunner’s glands (alkaline mucus, pH 8.1–9.3).
  • Ileum:
    • Peyer’s patches (MALT).

Tissue Layers

  • Epithelium
  • Lamina Propria
  • Muscularis Mucosa
  • Muscularis Propria
  • Mucosa
  • Submucosa

Brunner's Glands

  • Brunner's glands are located in the submucosa of the duodenum. They secrete an alkaline fluid containing mucin, which protects the mucosa from the acidic stomach contents entering the duodenum.

Duodenal (Brunner) Glands

  • Large masses of compound tubular mucous glands, the duodenal glands (DG).
  • With many lobules that occupy much of the submucosa and may extend above the muscularis mucosae (MM) into the mucosa.
  • Small excretory ducts (D).

Peyer's Patches

  • Peyer's patches or aggregated lymphoid nodules are organized lymphoid follicles, named after the 17th-century Swiss anatomist Johann Conrad Peyer.
  • Peyer's patches are aggregations of lymphoid tissue in the lamina propria and submucosa of the GI tract opposite the mesentery. These develop throughout the small intestine but become more prominent in the adult in the terminal ileum.

Muscularis & Serosa

  • Muscularis:
    • Inner circular layer
    • Outer longitudinal layer
    • Myenteric (Auerbach’s) plexus between them
  • Serosa:
    • Thin layer with mesothelium

Plexus Locations

  • Circular Muscle
  • Myenteric Plexus
  • Submucosal Plexus
  • Mucosa

General Organization of the Gastrointestinal Tract

  • Epithelium
  • Mucosa
  • Lamina Propria
  • Submucosa
  • Muscularis Mucosa
  • Meissner's (Submucosal) Plexus
  • Muscularis Propria
  • Serosa or Adventitia
  • Circular Muscle
  • Auerbach's (Myenteric) Plexus
  • Longitudinal Muscle

Leiomyomas

  • Benign tumors of smooth muscle cells represent the most common type of tumor in the stomach and small intestine and may become large. Autopsy records suggest that the muscularis of the stomach may include leiomyomas in up to 50% of the population older than 50 years.

Celiac Disease

  • Celiac sprue is a disorder of the small intestine mucosa that causes malabsorption and can lead to damage or destruction of the villi. The cause of celiac disease includes an immune reaction against gluten or other proteins in wheat and certain other types of grain. The resulting inflammation affects the enterocytes, leading to reduced nutrient absorption.

Celiac Disease Details

  • a.k.a. CELIAC SPRUE / GLUTEN-SENSITIVE ENTEROPATHY
  • IMMUNE-MEDIATED
  • RESPONSE to GLUTEN
    • protein in wheat, barley, rye
    • against gliadin & tissue transglutaminase
  • MALABSORPTION of VITAMINS, MINERALS, & other NUTRIENTS
  • GLIADIN
    • after 6 months
    • when caregivers introduce gluten

Crohn's Disease

  • Is a chronic inflammatory bowel disease that occurs most commonly in the ileum or colon, resulting from a poorly understood combination of immune, environmental, and genetic factors. Excessive lymphocytic activity and inflammation occur in any or all layers of the tract wall, producing pain, localized bleeding, malabsorption, and diarrhea.

Crohn's Disease Symptoms and Features

  • Thickened Wall
  • Narrowed Lumen
  • Cobblestone Appearance of Mucosal Surface
  • Fat wrapping or Creeping Fat
  • Abscess
  • Most Commonly Involves the End of Small Intestine and the Beginning of the Colon
  • Fever
  • Weight Loss
  • Decreased Appetite
  • Normal Lumen
  • Normal Wall
  • Healthy Intestine
  • MOUTH
    • Stomatitis
  • EYE
    • Inflammation
  • SKIN
    • Ulcers and Sores
  • STOMACH
    • Vomiting
  • JOINT
    • Pain and Swelling
  • LIVER AND BILE DUCT
    • Inflammation
  • INTESTINES
    • Diarrhea
    • Abdominal pain
    • Cramping
    • Ulcers
    • Rectal Bleeding