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Functional Morphology of Epithelium of the Small Intestine

Full Article on Wiley Online Library



Abstract

The sections in this article are:

1 General Considerations
1.1 Supporting Elements of Epithelium
1.2 Compartmentalization of Epithelium
1.3 Contribution of the Paracellular Pathway to Secretion and Absorption
1.4 Polarity of Epithelial Cells
2 Structure of “Absorbing” Epithelial Cells
2.1 Villus Absorptive Cells
2.2 M Cells
3 Structure of “Secreting” Epithelial Cells
3.1 Undifferentiated Crypt Cells
3.2 Goblet Cells
3.3 Enteroendocrine Cells
3.4 Paneth Cells
4 Structure of Epithelial Cells not Currently known to have Absorptive or Secretory Function
4.1 Tuft Cells
4.2 Cup Cells
Figure 1. Figure 1.

Light micrograph (A) and labeled trace drawing (B) of small intestinal mucosa of monkey. Mucosa consists of muscularis mucosa, lamina propria, and epithelium. Villus projections of supporting lamina propria increase epithelial surface area. Between these epithelial‐lined villi are wells (crypts) also lined by epithelial cells. Whereas goblet cells and absorptive cells with prominent brush borders are the major cell types of the villus, undifferentiated cells, goblet cells, and basally located Paneth cells dominate crypts. Minor epithelial cell types such as enteroendocrine, tuft, and cup cells may be scattered throughout epithelium but are not well visualized here. Lamina propria contains mesenchymal, neural, lymphoid, and vascular elements, including small arteries, capillaries, and venules, but also a lymphatic lacteal in the center of each villus. A: × 180. [A from Madara and Trier .]

Figure 2. Figure 2.

Schematic illustration of some factors in microenvironment of intestinal epithelial cells that may influence their function. With exception of extra vascular polymorphonuclear leukocytes, all elements are normally present at this site.

Figure 3. Figure 3.

Light micrograph of epoxy‐embedded lymphoid follicle of human small intestine with overlying follicular dome epithelium. Lymphoid cell aggregates can be seen within the epithelial layer, presumably marking sites of M cells, × 300.

[From Owen and Jones .]
Figure 4. Figure 4.

Thin sections of junctional complexes in crypt (left) and on villus (right). Apical junctional complex includes occluding or “tight” junctions (brackets) and desmosomes (D). Organization and structure of occluding junctions cannot be fully appreciated in thin sections. × 52,700. Inset: villus occluding junction at higher magnification. Arrow, point of fusion of outer membrane leaflets of adjacent cells. Fusion points correspond to P‐face strands and E‐face grooves revealed by freeze fracture. × 78,200.

[From Madara and Trier .]
Figure 5. Figure 5.

Freeze‐fracture replicas of occluding junctions of villus (top) and crypt cells (bottom) in the fasting state. Occluding junctions of villus absorptive cells have, in general, greater depth (brackets) and are more uniformly organized than those of crypt cells. Unconnected lateral aberrant strands (double arrow) are common in crypt but rare on villus. Cells in both crypts and villi show ladderlike specializations at 3‐cell junctions (asterisk). Circled arrowheads, direction of platinum shadowing. × 56,100.

[From Madara et al. .]
Figure 6. Figure 6.

Freeze‐fracture replicas of occluding junction networks on P faces of 2 goblet cells on villi of monkey ileum. Depth and complexity of goblet cell junctions vary widely. Circled arrowheads, direction of platinum shadowing. × 73,100.

[From Madara et al. .]
Figure 7. Figure 7.

A: electron micrograph of adjacent villus absorptive cells from ileal mucosa exposed to 1 mM LaCl3 for 20 min prior to fixation. Although dense lanthanum precipitates are present in the brush border, none is apparent within occluding junction or intercellular space in the fasting state. × 61,500. B: electron micrograph of goblet cell from ileal mucosa exposed to 1 mM LaCl3 for 20 min prior to fixation. Dense lanthanum deposits are present within occluding junction (arrowhead) and intercellular space (arrows). × 53,500.

[From Madara and Trier .]
Figure 8. Figure 8.

Scanning electron micrographs of guinea pig ileal villus and crypt (inset) epithelium. Villus surface is covered by polygonal absorptive cells with estimated cell widths of 10 μm, which creates a honeycomb appearance on villus surface. × 1,225. Inset: crypt epithelial cells (arrowheads) are polygonal but have apical cell widths of only 3.5 μm. Thus high linear junctional density in crypts is not due to tortuous cell contours but to diminished apical cell widths. × 3,500. Bars, 10 μm.

[From Marcial et al. .]
Figure 9. Figure 9.

Illustrations of interplay between surface area and junctional density in determination of relative paracellular pathway contribution of ileal crypts and villi. As shown in A, crypt contributes substantially less than villus to total surface area. However, a standard unit of surface from each compartment (solid rectangles) magnified to examine tight junction density (B) shows that surface area dominance by villus is offset by greater linear junctional density in crypt. Enrichment of linear junctional density in crypt is related to relatively narrow surface diameter of crypt cells. Drawings are proportioned to approximate morphometrically determined variation in these parameters between crypts and villi.

[From Marcial et al. .]
Figure 10. Figure 10.

Electron micrographs of absorptive cell occluding junctions obtained from tissues perfused without substrates (A) and with 25 mM luminal glucose (B). Occluding junctions in glucose‐free, high‐impedance state revealed closely apposed lateral membranes (arrowheads). In contrast, dilatations within occluding junction zone (B, arrowheads) are induced by perfusion with glucose. Also, microfilaments of perijunctional ring appear to be condensed after glucose exposure (arrows).

[From Madara and Pappenheimer .]
Figure 11. Figure 11.

Freeze‐fracture electron micrographs of absorptive cell occluding junctions (OJ). A: after perfusion in absence of substrate, junction consists of netlike array of strands and/or grooves. B: in contrast, perfusion elicits focal dilatations of interstrand compartments (arrowheads) that often have concave surfaces and correspond to intrajunctional dilatations seen in thin sections. Some dilatations display a secondary prominent protuberance on their fracture faces (arrows). Such dilated interstrand compartments also distort the anatomy of the junction. For example, at sites where large dilatations exist, only 2 junctional strands separate the luminal from the paracellular space, whereas in glucose‐free preparations several junctional strands are always encountered separating these 2 compartments. Presumably this is the structural manifestation of increased permeability to hydrophilic solutes and decreased junctional resistance induced by glucose. Bars, 0.5 μm.

[From Madara and Pappenheimer .]
Figure 12. Figure 12.

Electron micrograph (left) and sketch (right) of naked cytoskeleton in zone of ideally sectioned absorptive cell occluding junction. Electron‐dense plaques intimately associate with junctional “kisses” on 1 side and with cytoskeletal elements on other. Specifically, in sections unlabeled with S1‐labeled actin probe, such cytoskeletal elements appear to be microfilaments (not shown) and in sections labeled with S1 (shown) such microfilaments are actin microfilaments with characteristic arrowhead label due to S1‐actin association. × ∼115,000.

[From Madara .]
Figure 13. Figure 13.

Transmission electron micrographs (left and inset) and labeled trace illustration (right) of villus absorptive cells. Inset: microvilli containing parallel arrays of actin microfilaments that plunge as a “rootlet” (arrowheads) into terminal web of structural proteins. Left panel, × 6,000; inset, × 69,000.

Figure 14. Figure 14.

Freeze‐fracture replicas of microvilli of villus absorptive cell (left) and undifferentiated crypt cell (right). Majority of intramembrane particles are associated with convex membrane half, which covers microvillus core (P, P face); fewer particles are associated with concave membrane half, which abuts the extracellular space (E, E face). P‐face particles are more numerous in microvillus membranes of villus cells than in those of crypt cells. E‐face particle density is comparable in both sites. Circled arrowheads, direction of platinum shadowing. × 97,750.

[From Madara et al. .]
Figure 15. Figure 15.

Electron micrographs (left and inset) and labeled trace illustration (right) of glycocalyx and microvillus tips of villus epithelial cells. Glycocalyx can occasionally be seen in thin sections prepared from tissues fixed in aldehydes (inset) but is better observed in freeze‐fixed, freeze‐fractured, deeply etched, rotary‐shadowed preparations as seen in left panel and diagram. Glycocalyx appears as multiple filaments that jut from microvillus tips to give a hair‐on‐end appearance. Left panel, × 98,000; inset, × 85,000.

Figure 16. Figure 16.

Transmission electron micrograph from nonvillus region of Peyer's patch epithelium, showing a cross‐sectional view of the apex of an M cell, associated microvillus‐covered epithelial cells, and at least 3 lymphoid cells (L). Arrowheads, borders of M cell. Attenuated cytoplasm of M cell bridges surface between microvillus‐covered cells, forming occluding junctions with them and producing a barrier between lymphoid cells and intestinal lumen. × 5,000.

[From Owen and Jones .]
Figure 17. Figure 17.

Unstained electron micrograph (left) and labeled trace illustration (right) of follicular dome epithelium exposed on luminal surface to macromolecule horseradish peroxidase. Dark reaction product indicating presence of this molecule can be seen within vesicles in M cell, within spaces surrounding M cell, and within vesicles of lymphoid cells under M cells. Comparable transport of this macromolecule across other types of epithelial cells is not known to occur in adult small intestine. × 12,000.

Figure 18. Figure 18.

Electron micrograph (left) and labeled trace diagram (right) of undifferentiated crypt cells. × 7,500.

Figure 19. Figure 19.

Electron micrograph (left) and labeled trace diagram (right) of villus goblet cell. × 6,000.

Figure 20. Figure 20.

Freeze‐fracture replica of apical cytoplasm of adjacent goblet and villus absorptive cells. Arrow, position of intercellular space separating goblet cell (GC) and absorptive cell (AC). P face of a microvillus from each cell is exposed. That of goblet cell (right) is particle poor, whereas that of absorptive cell (left) is particle rich. Mucin granules (MG) can be seen in goblet cell cytoplasm. Circled arrowhead, direction of platinum shadowing. × 105,400.

[From Madara et al. .]
Figure 21. Figure 21.

Electron micrographs of enteroendocrine cell. Above: low magnification of electron‐lucent cytoplasm and numerous secretory granules located predominantly in basal half of cell. × 5,000. Below: high magnification of membrane‐bound secretory granules showing characteristics of EC cell granules, pleomorphism and occasional electron‐dense cores (arrowhead). × 37,000.

Figure 22. Figure 22.

Electron micrograph of adjacent Paneth cells at base of intestinal crypt. Numerous secretory vesicles (SV) in apical portion of this cell are separated from each other by cytoplasmic elements. Other organelles such as rough endoplasmic reticulum (RER) are present between vesicles and at cell periphery, and nuclei (N) are present at basal pole. × 12,000.

Figure 23. Figure 23.

Electron micrograph (left) and labeled trace illustration (right) of tuft cell. × 6,000.

Figure 24. Figure 24.

Electron micrograph of cup cell (CC) interposed between 2 absorptive cells (AC). Cup cell has cuplike apical indentation, more lightly stained cytoplasm, and shorter microvilli than adjacent absorptive cells. In contrast to terminal web of absorptive cells, which is clearly demarcated basally by a dense accumulation of cytoplasmic organelles (arrowheads), basal border of cup cell terminal web is less clearly defined and small mitochondria are randomly and sparsely scattered in apical cytoplasm. Like absorptive cells, cup cells do have contact with basement membrane (BM). × 4,000. Inset: light micrograph of villus epithelium. Among tall absorptive cells, which stain darkly and have tall uniform brush borders, is a lightly stained cup cell. Cup cell brush border is relatively uniform in height but shorter than that of adjacent absorptive cells. Cuplike depression along luminal margin of cup cell brush border is clear (arrowhead). × 550.

[From Madara .]
Figure 25. Figure 25.

High magnification of replicated brush border. In contrast to absorptive cell microvilli (AC), which have randomly arranged P‐face (P) intramembrane particles, cup cell microvilli (CC) have P‐face intramembrane particles aligned in linear arrays with complementary E‐face (E) grooves (small arrowheads). Large arrowhead, site of intercellular space. × 61,000.

[From Madara .]


Figure 1.

Light micrograph (A) and labeled trace drawing (B) of small intestinal mucosa of monkey. Mucosa consists of muscularis mucosa, lamina propria, and epithelium. Villus projections of supporting lamina propria increase epithelial surface area. Between these epithelial‐lined villi are wells (crypts) also lined by epithelial cells. Whereas goblet cells and absorptive cells with prominent brush borders are the major cell types of the villus, undifferentiated cells, goblet cells, and basally located Paneth cells dominate crypts. Minor epithelial cell types such as enteroendocrine, tuft, and cup cells may be scattered throughout epithelium but are not well visualized here. Lamina propria contains mesenchymal, neural, lymphoid, and vascular elements, including small arteries, capillaries, and venules, but also a lymphatic lacteal in the center of each villus. A: × 180. [A from Madara and Trier .]



Figure 2.

Schematic illustration of some factors in microenvironment of intestinal epithelial cells that may influence their function. With exception of extra vascular polymorphonuclear leukocytes, all elements are normally present at this site.



Figure 3.

Light micrograph of epoxy‐embedded lymphoid follicle of human small intestine with overlying follicular dome epithelium. Lymphoid cell aggregates can be seen within the epithelial layer, presumably marking sites of M cells, × 300.

[From Owen and Jones .]


Figure 4.

Thin sections of junctional complexes in crypt (left) and on villus (right). Apical junctional complex includes occluding or “tight” junctions (brackets) and desmosomes (D). Organization and structure of occluding junctions cannot be fully appreciated in thin sections. × 52,700. Inset: villus occluding junction at higher magnification. Arrow, point of fusion of outer membrane leaflets of adjacent cells. Fusion points correspond to P‐face strands and E‐face grooves revealed by freeze fracture. × 78,200.

[From Madara and Trier .]


Figure 5.

Freeze‐fracture replicas of occluding junctions of villus (top) and crypt cells (bottom) in the fasting state. Occluding junctions of villus absorptive cells have, in general, greater depth (brackets) and are more uniformly organized than those of crypt cells. Unconnected lateral aberrant strands (double arrow) are common in crypt but rare on villus. Cells in both crypts and villi show ladderlike specializations at 3‐cell junctions (asterisk). Circled arrowheads, direction of platinum shadowing. × 56,100.

[From Madara et al. .]


Figure 6.

Freeze‐fracture replicas of occluding junction networks on P faces of 2 goblet cells on villi of monkey ileum. Depth and complexity of goblet cell junctions vary widely. Circled arrowheads, direction of platinum shadowing. × 73,100.

[From Madara et al. .]


Figure 7.

A: electron micrograph of adjacent villus absorptive cells from ileal mucosa exposed to 1 mM LaCl3 for 20 min prior to fixation. Although dense lanthanum precipitates are present in the brush border, none is apparent within occluding junction or intercellular space in the fasting state. × 61,500. B: electron micrograph of goblet cell from ileal mucosa exposed to 1 mM LaCl3 for 20 min prior to fixation. Dense lanthanum deposits are present within occluding junction (arrowhead) and intercellular space (arrows). × 53,500.

[From Madara and Trier .]


Figure 8.

Scanning electron micrographs of guinea pig ileal villus and crypt (inset) epithelium. Villus surface is covered by polygonal absorptive cells with estimated cell widths of 10 μm, which creates a honeycomb appearance on villus surface. × 1,225. Inset: crypt epithelial cells (arrowheads) are polygonal but have apical cell widths of only 3.5 μm. Thus high linear junctional density in crypts is not due to tortuous cell contours but to diminished apical cell widths. × 3,500. Bars, 10 μm.

[From Marcial et al. .]


Figure 9.

Illustrations of interplay between surface area and junctional density in determination of relative paracellular pathway contribution of ileal crypts and villi. As shown in A, crypt contributes substantially less than villus to total surface area. However, a standard unit of surface from each compartment (solid rectangles) magnified to examine tight junction density (B) shows that surface area dominance by villus is offset by greater linear junctional density in crypt. Enrichment of linear junctional density in crypt is related to relatively narrow surface diameter of crypt cells. Drawings are proportioned to approximate morphometrically determined variation in these parameters between crypts and villi.

[From Marcial et al. .]


Figure 10.

Electron micrographs of absorptive cell occluding junctions obtained from tissues perfused without substrates (A) and with 25 mM luminal glucose (B). Occluding junctions in glucose‐free, high‐impedance state revealed closely apposed lateral membranes (arrowheads). In contrast, dilatations within occluding junction zone (B, arrowheads) are induced by perfusion with glucose. Also, microfilaments of perijunctional ring appear to be condensed after glucose exposure (arrows).

[From Madara and Pappenheimer .]


Figure 11.

Freeze‐fracture electron micrographs of absorptive cell occluding junctions (OJ). A: after perfusion in absence of substrate, junction consists of netlike array of strands and/or grooves. B: in contrast, perfusion elicits focal dilatations of interstrand compartments (arrowheads) that often have concave surfaces and correspond to intrajunctional dilatations seen in thin sections. Some dilatations display a secondary prominent protuberance on their fracture faces (arrows). Such dilated interstrand compartments also distort the anatomy of the junction. For example, at sites where large dilatations exist, only 2 junctional strands separate the luminal from the paracellular space, whereas in glucose‐free preparations several junctional strands are always encountered separating these 2 compartments. Presumably this is the structural manifestation of increased permeability to hydrophilic solutes and decreased junctional resistance induced by glucose. Bars, 0.5 μm.

[From Madara and Pappenheimer .]


Figure 12.

Electron micrograph (left) and sketch (right) of naked cytoskeleton in zone of ideally sectioned absorptive cell occluding junction. Electron‐dense plaques intimately associate with junctional “kisses” on 1 side and with cytoskeletal elements on other. Specifically, in sections unlabeled with S1‐labeled actin probe, such cytoskeletal elements appear to be microfilaments (not shown) and in sections labeled with S1 (shown) such microfilaments are actin microfilaments with characteristic arrowhead label due to S1‐actin association. × ∼115,000.

[From Madara .]


Figure 13.

Transmission electron micrographs (left and inset) and labeled trace illustration (right) of villus absorptive cells. Inset: microvilli containing parallel arrays of actin microfilaments that plunge as a “rootlet” (arrowheads) into terminal web of structural proteins. Left panel, × 6,000; inset, × 69,000.



Figure 14.

Freeze‐fracture replicas of microvilli of villus absorptive cell (left) and undifferentiated crypt cell (right). Majority of intramembrane particles are associated with convex membrane half, which covers microvillus core (P, P face); fewer particles are associated with concave membrane half, which abuts the extracellular space (E, E face). P‐face particles are more numerous in microvillus membranes of villus cells than in those of crypt cells. E‐face particle density is comparable in both sites. Circled arrowheads, direction of platinum shadowing. × 97,750.

[From Madara et al. .]


Figure 15.

Electron micrographs (left and inset) and labeled trace illustration (right) of glycocalyx and microvillus tips of villus epithelial cells. Glycocalyx can occasionally be seen in thin sections prepared from tissues fixed in aldehydes (inset) but is better observed in freeze‐fixed, freeze‐fractured, deeply etched, rotary‐shadowed preparations as seen in left panel and diagram. Glycocalyx appears as multiple filaments that jut from microvillus tips to give a hair‐on‐end appearance. Left panel, × 98,000; inset, × 85,000.



Figure 16.

Transmission electron micrograph from nonvillus region of Peyer's patch epithelium, showing a cross‐sectional view of the apex of an M cell, associated microvillus‐covered epithelial cells, and at least 3 lymphoid cells (L). Arrowheads, borders of M cell. Attenuated cytoplasm of M cell bridges surface between microvillus‐covered cells, forming occluding junctions with them and producing a barrier between lymphoid cells and intestinal lumen. × 5,000.

[From Owen and Jones .]


Figure 17.

Unstained electron micrograph (left) and labeled trace illustration (right) of follicular dome epithelium exposed on luminal surface to macromolecule horseradish peroxidase. Dark reaction product indicating presence of this molecule can be seen within vesicles in M cell, within spaces surrounding M cell, and within vesicles of lymphoid cells under M cells. Comparable transport of this macromolecule across other types of epithelial cells is not known to occur in adult small intestine. × 12,000.



Figure 18.

Electron micrograph (left) and labeled trace diagram (right) of undifferentiated crypt cells. × 7,500.



Figure 19.

Electron micrograph (left) and labeled trace diagram (right) of villus goblet cell. × 6,000.



Figure 20.

Freeze‐fracture replica of apical cytoplasm of adjacent goblet and villus absorptive cells. Arrow, position of intercellular space separating goblet cell (GC) and absorptive cell (AC). P face of a microvillus from each cell is exposed. That of goblet cell (right) is particle poor, whereas that of absorptive cell (left) is particle rich. Mucin granules (MG) can be seen in goblet cell cytoplasm. Circled arrowhead, direction of platinum shadowing. × 105,400.

[From Madara et al. .]


Figure 21.

Electron micrographs of enteroendocrine cell. Above: low magnification of electron‐lucent cytoplasm and numerous secretory granules located predominantly in basal half of cell. × 5,000. Below: high magnification of membrane‐bound secretory granules showing characteristics of EC cell granules, pleomorphism and occasional electron‐dense cores (arrowhead). × 37,000.



Figure 22.

Electron micrograph of adjacent Paneth cells at base of intestinal crypt. Numerous secretory vesicles (SV) in apical portion of this cell are separated from each other by cytoplasmic elements. Other organelles such as rough endoplasmic reticulum (RER) are present between vesicles and at cell periphery, and nuclei (N) are present at basal pole. × 12,000.



Figure 23.

Electron micrograph (left) and labeled trace illustration (right) of tuft cell. × 6,000.



Figure 24.

Electron micrograph of cup cell (CC) interposed between 2 absorptive cells (AC). Cup cell has cuplike apical indentation, more lightly stained cytoplasm, and shorter microvilli than adjacent absorptive cells. In contrast to terminal web of absorptive cells, which is clearly demarcated basally by a dense accumulation of cytoplasmic organelles (arrowheads), basal border of cup cell terminal web is less clearly defined and small mitochondria are randomly and sparsely scattered in apical cytoplasm. Like absorptive cells, cup cells do have contact with basement membrane (BM). × 4,000. Inset: light micrograph of villus epithelium. Among tall absorptive cells, which stain darkly and have tall uniform brush borders, is a lightly stained cup cell. Cup cell brush border is relatively uniform in height but shorter than that of adjacent absorptive cells. Cuplike depression along luminal margin of cup cell brush border is clear (arrowhead). × 550.

[From Madara .]


Figure 25.

High magnification of replicated brush border. In contrast to absorptive cell microvilli (AC), which have randomly arranged P‐face (P) intramembrane particles, cup cell microvilli (CC) have P‐face intramembrane particles aligned in linear arrays with complementary E‐face (E) grooves (small arrowheads). Large arrowhead, site of intercellular space. × 61,000.

[From Madara .]
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James L. Madara. Functional Morphology of Epithelium of the Small Intestine. Compr Physiol 2011, Supplement 19: Handbook of Physiology, The Gastrointestinal System, Intestinal Absorption and Secretion: 83-120. First published in print 1991. doi: 10.1002/cphy.cp060403