The ultrastructures of small intestinal enterocytes, which line the villi and are continually renewed, mirror their main tasks in the absorption of digested food. Panels A and B show the apical cell domain of absorptive cells with the prominent brush border made up of numerous densely packed microvilli, which are the sites of final digestion and uptake of nutrients. Microvilli exist in an approximate number of 3,000 per cell, which greatly increases the luminal cell surface. They measure 1–2 μm in length and contain a filamentous core, composed of actin filaments, crosslinked by fimbrin and villin, and attached to the plasma membrane by myosin I and calmodulin. The rootlets of the filament bundles project into the terminal web located beneath the brush border (tw). They are interconnected by intestine-specific spectrin, attach to cytokeratin intermediate filaments of the terminal web, and are part of the apical cytoskeletal apparatus that is responsible for maintaining the upright positions of the microvilli and the overall organization of the brush border (cf. also Fig. 88). Components of the terminal web are associated with the junctional complex. In particular, actin filaments connected with the belt desmosome in the middle part of the junctional complex (panel B-2; cf. also Fig. 98) contribute essentially to the cells-cells expanding motion system. This is responsible for changes of the diameter of the apical cell domains, leading to a tilting of the brush border microvilli, which facilitates contact with the digested nutrients and supports absorption.

The luminal cell surface is lined by a 10 nm thick plasma membrane, clearly visible in panel B as a distinct three-lamellar structure covered by the fuzzy, filamentous network of the glycocalyx (cf. Fig. 94). Intramembraneous enzymes are responsible for the final breakdown of oligosaccharides and oligopeptides. The products, such as amino acids, di- and tripeptides, and sugars, are transported across the plasma membrane, which involves specific membrane channels and carrier systems.

Brush border enzymes and other membrane constituents have to be continually renewed and are inserted into the plasma membrane at areas located between the microvilli. These are the only membrane sites of the apical surface accessible for fusion and budding events and, from here, membrane invaginations deeply extend into the terminal web region. They appear as pleiomorphic compartments and apical tubules close to the microvilli rootlets (cf. Fig. 88, panels A, D and Fig. 98, panel B). Their membranes exhibit lipid microdomains different from those of the microvilli membranes and are considered to function as a membrane reservoir for adaptative changes at the apical cell surface.

Regular absorption requires a clear cell polarity with distinct boundaries between apical and basolateral cell surfaces, which are constituted by tight junctions forming the most apical zone of the junctional complex that connects adjacent cells. Panel A shows the apical part of an absorptive cell with numerous mitochondria, lysosomes (Ly), and the Golgi apparatus in typical supranuclear position (Golgi). Bars indicate the profiles of the junctional complex, which is shown at higher magnification in panel B. Close to the apical cell surface, tight junctions form an occluding belt, which defines cell polarity, seals the intercellular spaces, and controls the intercellular passage of substances (1; zonula occludens). The second and the third parts of the junctional complex are built up by adhering junctions forming a belt desmosome closely below the tight junctions (2; zonula adhaerens) and an additional circle of spot desmosomes (3; maculae adhaerentes). At more basal regions, adjacent cells are often interlocked by extended interdigitations (panel C).

Through the absorptive enterocytes, digested nutrients cross from the lumen of the gut to the connective tissue underlying the epithelium. After ingestion and specific processing by the absorptive cells (cf. Fig. 128), nutrient constituents are transported to the basolateral cell surfaces, where they again leave the cells and enter blood or lymph capillaries to be distributed in the body. A particular route across the absorptive cells is traveled by lipids, which in the majority are packed into lipoprotein particles, secreted by exocytosis, and moved into the lumen of lymphatics. Some stages of the transcellular and extracellular lipid pathways can readily be followed under the electron microscope.

In the gut lumen during fat breakdown, mixed micelles containing bile salts and the main products of fat digestion are assembled and have immediate access to the brush border through movements of villi and microvilli. Free fatty acids and monoglycerides liberated from the micelles diffuse into the microvilli and associate with fatty acid-binding proteins, to be transported into the apical cytoplasm. In the smooth endoplasmic reticulum, triglycerides and other lipids are resynthesized and packaged into lipoprotein particles (LPs). Subsequently exported out of the endoplasmic reticulum and transported to the Golgi apparatus, they are glycosylated and packaged into vesicles to be transported to the basolateral cell surfaces. Via exocytosis, they leave the cell into the intercellular spaces. Particularly large LPs, which are known as chylomicrons, are formed during postprandial lipid absorption. However, LPs are not only formed after intake of food. Lipids, mainly derived from the bile and shedded cells, are also absorbed during starvation. The LPs formed during starvation measure 50–80 nm in diameter and belong to the class of very low density lipoproteins (VLDLs; cf. Figs. 124 and 125).