Columnar cells (enterocytes)
These cells make up the vast majority of the epithelial layer and are responsible for the digestion and absorption of nutrients.
Apical pole The surface facing the intestinal lumen bears numerous microvilli. 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 itself via the well-developed rough endoplasmic reticulum and Golgi apparatus are anchored here (glycosidases, aminopeptidases). This is the site of membrane digestion—the final stage of food nutrient breakdown.
Lateral surface The lateral walls of adjacent enterocytes are tightly bound to isolate the body's internal environment from the intestinal contents. Closer to the lumen lie tight junctions and adherens junctions for maximum seal integrity, followed basally by desmosomes and folded interdigitations.
How absorption works
The pathways for substances entering the columnar cell depend on their chemical nature:
- Amino acids are transported via active transport;
- Glucose enters through facilitated diffusion;
- Water and small polar molecules cross the membrane by 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 harder to absorb. First, bile acids emulsify them in the intestinal lumen. The resulting small fragments enter the cell (often together with bile acids) by simple diffusion. In the cisternae of the smooth endoplasmic reticulum, resynthesis occurs—the assembly of new fats specific to the human body. Then, in the Golgi apparatus, these lipids are packaged into chylomicrons (a hydrophobic lipid core surrounded by a protein coat). Unlike carbohydrates and proteins, chylomicrons are secreted not into the blood, but into lymphatic capillaries, from which they enter the systemic circulation via lymph, bypassing the liver.
Goblet cells
These are unicellular exocrine glands scattered among the columnar epithelial cells. They truly resemble a goblet in shape, and their cytoplasm appears reticular due to accumulated secretion.
Goblet cells produce a polysaccharide-based mucus. To accomplish this, they feature a well-developed agranular endoplasmic reticulum and Golgi apparatus. Secretion occurs via a merocrine mechanism (the cell remains intact) in response to the arrival of food (chyme). The resulting surface mucus layer protects the epithelium and provides an optimal environment for enzymes.
On standard histological preparations, these cells appear clear, but when stained with the PAS reaction (for polysaccharides), their cytoplasm turns bright purple.
M cells
M cells (microfold cells) are a specialized type of columnar epithelial cells involved in immune defense. They are located predominantly over lymphoid follicles, such as Peyer's patches.
Their apical surface has almost no microvilli. The main feature of M cells is the presence of deep invaginations (pockets) on their basal 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 the immune cells in the basal pocket. This triggers a cascade of immune responses.
Briefly: answers to topical questions
Where are Paneth cells located in the epithelium of the small intestine?
Paneth cells are located exclusively at the base of the small intestinal crypts.
Features of their localization:
- Topography — found in the lower parts of crypts (in tubular invaginations).
- Lack of migration — unlike other epithelial cells, they differentiate and remain at the crypt base.
- Absence on villi — these exocrine cells are completely absent from the villus epithelium.
What is the lifespan of columnar enterocytes in days?
The average lifespan of an intestinal enterocyte is 5–6 days. The entire epithelial population is completely renewed every 4–6 days.
During this short period, the cells go through a complete life cycle: they originate from stem cells at the crypt base, migrate upward along the villus walls while gradually differentiating, and, upon reaching the villus tip, are shed into the intestinal lumen.
Which hormones are secreted by enteroendocrine cells of the small intestine?
Enteroendocrine cells of the small intestine secrete hormones into blood capillaries or act on neighboring cells in a paracrine manner.
According to the provided sources for the small intestine:
- EC cells — serotonin and motilin.
- S cells — secretin.
- I cells — cholecystokinin (pancreozymin).
Class and exam questions
Where are membrane digestion enzymes located?
They are fixed in the glycocalyx layer that covers the microvilli on the apical surface of columnar cells.
How to identify a goblet cell on a histological preparation?
On standard staining, these are clear, goblet-shaped cells. For precise identification, the PAS reaction is used, which stains their mucous secretion purple.
What is the main function of M cells?
They capture antigens from the intestinal lumen and transport them via transcytosis to macrophages and lymphocytes, initiating an immune response.
What else to study on this topic
- Structure and functions of the glycocalyx
- Mechanisms of specific fat resynthesis in the smooth endoplasmic reticulum
- Organization of intercellular junctions in columnar cells
- Structure and composition of chylomicrons
- Interaction of M cells with lymphoid follicles
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