Columnar Cells (Enterocytes)
These cells make up the vast majority of the epithelial sheet and are responsible for the digestion and absorption of nutrients.
Apical Pole Numerous microvilli are located on the surface facing the intestinal lumen. Under a light microscope, they appear as an oxyphilic "brush border". Overlying the microvilli is the glycocalyx—a layer rich in lipoproteins and glycosaminoglycans. Enzymes synthesized by the cell (glycosidases, aminopeptidases) via a well-developed RER and Golgi apparatus are anchored here. This zone is the site of membrane (surface) digestion, the final stage of nutrient breakdown.
Lateral Surface Adjacent enterocytes are firmly bound by lateral cell membranes to isolate the body's internal environment from the intestinal contents. Tight junctions and adherens junctions are located closer to the lumen for maximum sealing, followed by desmosomes and folded interdigitations more basally.
Mechanisms of Absorption
The pathways for substance penetration into the columnar cell depend on their nature:
- Amino acids are transported via active transport;
- Glucose enters via facilitated diffusion;
- Water and small polar molecules pass through the membrane via simple diffusion.
Water-soluble compounds leave the cell through the basement membrane, enter blood capillaries, and drain into the hepatic portal vein system.
Lipid Assimilation Fats are more complex to absorb. First, bile acids emulsify them in the intestinal lumen. The resulting small fragments penetrate the cell (often together with bile acids) via simple diffusion. Resynthesis occurs within the cisternae of the smooth ER—assembling new fats characteristic of the human body. Then, in the Golgi apparatus, these lipids are packaged into chylomicrons (a hydrophobic lipid core and a protein shell). Unlike carbohydrates and proteins, chylomicrons are secreted not into the blood, but into lymphatic capillaries, from where they enter the systemic circulation via lymph, bypassing the liver.
Goblet Cells
These are unicellular exocrine glands scattered among the columnar epithelial cells. They resemble a goblet in shape, and their cytoplasm appears reticular due to accumulated secretion.
Goblet cells produce polysaccharide-rich mucus. For this purpose, the agranular ER and Golgi apparatus are well-developed. Secretion occurs via the merocrine pathway (the cell remains intact) in response to the arrival of food (chyme). The resulting pericellular mucus layer protects the epithelium and creates an optimal environment for enzymes.
On standard histological preparations, these cells appear clear, but when stained with the PAS reaction (for polysaccharides), their cytoplasm becomes bright purple.
M Cells
M cells (microfold cells) are a specialized type of columnar epithelial cell involved in immune defense. They are located predominantly over lymphoid follicles, such as Peyer's patches.
Their apical surface has virtually no microvilli. The main feature of M cells is the presence of deep invaginations ( Pockets) on their basolateral side, which house lymphocytes and macrophages.
The function of an M cell is to capture antigen from the intestinal lumen, transport it across the cell without digestion (transcytosis), and deliver it to immune cells in the basal pocket. This triggers a cascade of immune reactions.