What is it called when the membrane wraps around nutrients and pulls them into the cell?

Enterocytes as a part of the immune system

Enterocytes form a barrier that offers only a very limited passage of material in both directions. Cell-cell adhesion is ensured by tight junctions, desmosomes and adherens junctions. Epithelial goblet cells secrete mucins, which act as another physical barrier. Epithelial cells also secrete several microbicidal or antiviral agents and transfer immunoglobulins secreted from mucosal plasma cells to the lumen. Enterocytes can, upon certain stimuli, secrete large quatities of CP− -rich fluid that flushes the surface. Many intermediate and end products of the digestion process are also microbicidal (fatty acids, lysophospholipids, etc.), and digestive enzymes can digest, at least partly, microbe cell walls (Pitman and Blumberg, 2000).

Enterocytes communicate with regional immune cell populations and influence their growth, migration, and state of responsiveness to antigenic stimuli. They express surface factors, secrete chemokines, cytokines and prostanoids. By expressing cytokine receptors, enterocytes are also able to alter their own innate capacity for defence and their ability to interact with and influence local immune cell populations (Pitman and Blumberg, 2000). Enterocytes sample bacterial and other foreign antigens within the lumen and present them to cells of the mucosal immune system thus directing immune responses to potentially harmful antigens. Enterocytes have been shown to possess maybe several processing and presentation pathways for antigens. The solubility of antigens seems to be a key factor. Enterocytes preferentially take up soluble antigens, but specialised epithelial immune cells, M-cells, take up most particulate antigens (Shao et al., 2001). Byrne et al. (2002) suggest that healthy human duodenal epithelial cells process and present antigens, but they may induce anergy, rather than activation, of local T-cells.

Enterocytes may also enhance the level of antigen presentation by other cells and maintain the intestinal mucosa immune responsiveness (Pitman and Blumberg, 2000). Apparently, under normal circumstances, the presentation of antigens by intestinal epithelial cells leads to inactivation or suppression of the normal immune response (oral tolerance). The intestinal epithelial cell may regulate the mucosal immune system by controlling the transit of antigens (Shao et al., 2001).

Enterocytes

Enterocytes contain the intracellular organelles, such as mitochondria, lysosomes, and endoplasmic reticulum, common to all cells, and which support normal cellular functions. However, enterocytes also perform specific digestive and absorptive functions.19,20 Enzymes expressed on the surface of enterocytes perform terminal digestion of polysaccharides and peptides in conjunction with luminal hydrolysis of food polymers by pancreatic enzymes. The enterocytes then absorb the simple nutrients. These functions depend on the polarity of the enterocyte, involving a specialized portion of the cell membrane on the luminal surface, the microvillar membrane (MVM). The microscopic appearance of the MVM is the basis of its alternative name, the “brush-border” (see Figure 57-1, E and F). It consists of thousands of parallel cylindrical processes (microvilli) bearing the digestive enzymes and specific carrier proteins.

The MVM is a phospholipid bilayer that has specific proteins inserted into it. Enzymes responsible for the terminal stages of carbohydrate and protein digestion are usually anchored in the MVM by a small hydrophobic terminal and have an active site exposed to the intestinal lumen (see Figure 57-1, F). Specific carrier proteins traverse the MVM or basolateral membrane and, through conformational changes, shuttle nutrients into and out of the enterocyte across the cell membrane. The maximal brush-border enzyme and transport activities are expressed in the mid-villus region. Diseases damaging enterocytes often accelerate cell production and the more immature enterocytes are not as effective functionally.

Brush-border enzyme activities are highest in the proximal SI and decline in an aborad gradient. Digestive enzymes, especially disaccharidases and transport proteins, may be inducible in response to changes in the composition of the diet. This has been shown in dogs but not in cats, perhaps reflecting the obligate carnivore status of cats. Indeed dogs, which are omnivorous, show increased sucrase and maltase in response to increased dietary carbohydrate and, conversely, when fed a cereal-free diet develop reduced activities of brush-border sucrase and maltase but not lactase. Thus a sudden change in diet in dogs may cause diarrhea through transient intolerance until either existing enterocytes upregulate expression of specific enzymes and carriers, or new enterocytes expressing induced proteins differentiate, thus rendering the diarrhea self-limiting.

Enterocyte metabolism is geared toward the production of brush-border proteins and the transfer of nutrients and water from the lumen to the blood. Basolateral cell membranes export sodium from the cell via an energy-dependent N+-K+-ATPase. Water can follow osmotically, or compensatory sodium influx at the luminal surface can drive carrier-mediated nutrient absorption. Natural inhibition of glycolysis through expression of an alternate phosphofructokinase isoenzyme in enterocytes facilitates the transfer of glucose from the lumen to the blood. Gluconeogenesis is also inhibited, and so enterocytes can utilize ketone bodies. However their major energy source is actually glutamine (Figure 57-4). A surge in enterocyte glutamine metabolism during digestion is probably partly responsible for the postprandial rise in blood ammonia seen in some patients with hepatic dysfunction.

Both major nutrients for enterocytes (glutamine and ketone bodies) are largely derived from the lumen, which helps explain the decline in villus structure, epithelial integrity, immune function, and absorptive function in starvation and anorexia. Consequently, attempting to maintain enteral nutrition, often by using glutamine-containing products, may be of clinical benefit.

Digestive and carrier proteins are synthesized by enterocytes and inserted in the MVM. This mechanism has been demonstrated for the synthesis of the enzyme complex of sucrase–isomaltase in pigs, but a similar process is likely to occur for this and other enzymes in dogs and cats. The sucrase–isomaltase complex is synthesized as a single polypeptide by ribosomal translation of its messenger RNA (mRNA). A terminal signal peptide extension that is ultimately cleaved, directs the intracellular trafficking of the protein from the ribosome to the endoplasmic reticulum and Golgi apparatus, where glycosylation of the protein backbone occurs. The glycosylated polypeptide is directed to the brush-border where it is inserted. It then “flips” across the apical membrane so that the active sites are on the luminal surface, and the polypeptide is anchored in the membrane by an N-terminal hydrophobic chain. The parts of the exteriorized polypeptide containing the sucrase and isomaltase activity are cleaved by pancreatic proteases but remain in close association, and form a dimer with another sucrose–isomaltase molecule.

As enterocytes migrate to the villus tip, enzymes are cleaved from the brush-border by bacterial and pancreatic proteases and are released into the lumen where they comprise solubilized enzyme activities, commonly called digestive juice. However, this liquid, the succus entericus, is not a true intestinal secretion.

Gastrointestinal Tract

The enterocyte, the first entry portal for iron absorption, plays the predominant role in the iron metabolism. In the enterocytes, iron absorption is increased during iron deficiency and decreased during the body’s iron excess. The molecular circuitry signals that mediate iron absorption involve a network of signals. Hypoxia-inducible factors (HIFs), which are transcription factors, are involved in the enterocyte iron absorption. During hypoxic conditions, the HIF signaling cascade is upregulated, promoting iron entry into the enterocytes. Thus, in iron deficiency, anemia HIF promotes absorption. However, inappropriate HIF stimulation can lead to chronic iron accumulation [1]. It should also be noted that HIF upregulation stimulates erythropoietin production in renal cells, which promotes erythropoiesis in the bone marrow (see “Erythropoietin” in Chapter 26).

2.1 Enterocytes

The enterocyte is the most abundant epithelial cell lineage in both the small and the large intestines. Enterocyte membranes, as well as the tight junctions that form between the cells, present a significant physical barrier to microbial invasion. However, enterocytes also assume an active role in defending epithelial surfaces. This occurs in several ways, each of which will be discussed in detail in subsequent sections. First, enterocytes secrete a variety of antimicrobial proteins that directly attack and kill bacteria (discussed in detail in Section 5). Second, they support cellular processes, such as autophagy (Benjamin et al., 2013; Conway et al., 2013; Wlodarska et al., 2014), which defend against invading bacteria (discussed in Section 6). Third, enterocytes produce cytokines that coordinate responses from subepithelial immune populations (discussed in Section 7.2). Fourth, they transport secretory immunoglobulin A from the basolateral epithelial surface to the apical surface of the epithelium for discharge into the lumen (discussed in Section 7.1). This IgA plays an essential role in maintaining homeostasis between host tissues and intestinal microbial communities (Macpherson et al., 2000).

74.2.2 Transcellular Uptake of Protein Antigens

Enterocytes have developed several mechanisms to transport macromolecules from the intestinal lumen and into the underlying tissue via the transcellular pathway: through non-selective fluid-phase pinocytosis or through selective receptor-mediated endocytosis. The initial process of both the fluid-phase pinocytosis and receptor-mediated endocytosis involves membrane invagination with the formation of clathrin-coated pits at the base of the invagination.33–35 In the non-selective fluid-phase transport, macromolecules are mostly directed toward the degradative pathway.36–38 Fluid-phase macromolecular transport normally mediates the uptake of nutrients and soluble protein antigens. Non-selective macropinocytosis also involves the uptake of luminal contents such as bacterial antigens and is observed in M cells,39,40 but may occur in enterocytes in response to antigen stimulation.41,42 Conversely, receptor-mediated endocytosis is a highly specific process that involves the binding of ligands to their specific receptor.

Villus epithelium

Enterocytes are the major villus epithelial cell type. They are highly specialized tall and columnar cells, with an oval nucleus located basally (Fig. 3). The usual organelles are all represented: the endoplasmic reticulum is not prominent but the Golgi apparatus is, the mitochondria are usually long and filamentous, and a number of pinocytic vesicles may be seen near the apical region. The apical surface is composed of microvilli and glycocalyx. Microvilli are close projections of cytoplasm covered by the cell membrane. They are about 1 μm high and 0.1 μm in diameter, providing a 14- to 40-fold increase in the apical surface area. The glycocalyx is a thin, filamentous layer of mucopolysaccharide material, housing important enzymes that function in terminal digestive processes. Enterocytes (and the other epithelial cells) are surrounded at their apical side by typical 0.5- to 2-μm intercellular junctional complexes. They consist of two distinct structures, the tight junction and the adherens junction, that encircle the cells as a belt, allowing a tight and continuous contact between two adjacent cells. These two structures are associations of transmembrane and cytoplasmic proteins linked with the cytoskeleton.

Goblet cells are interspersed among the enterocytes. They are polarized, mucous-secreting cells, increasing in number distally. They are easily recognizable by their pear-shaped region containing the mass of mature mucigen granules. Often a small “puff” of mucous can be distinguished emerging from the apex. Apical microvilli are sparse and irregular in size and shape. Cytoplasmic organelles, including a well developed network of rough endoplasmic reticulum, free ribosomes, mitochondria, and lysosomes, lie around the mass of mucous granules. Last, intraepithelial lymphocytes, mainly T cells, lie between individual epithelial cells, usually just above the basement membrane. The ratio of epithelial cells to intraepithelial lymphocytes is about 6 : 1.

Intestinal Lipoprotein Secretion

Enterocytes secrete two major classes of lipoprotein. Bulk triglyceride transport is achieved through the assembly and secretion of chylomicrons, as detailed above. These are large (75–1200 nm) particles that are well adapted for the transport of triglyceride into the circulation and ultimately to the liver. By virtue of their size, a limited amount of apoB is capable of transporting a large amount of lipid (Fig. 1). ApoB is proposed to wrap itself around the lipid droplet, belt-like, and, together with the addition of other apolipoproteins, notably apoA-I, apoA-IV is secreted into the lymphatic circulation, where it acquires apoE (Fig. 1). Chylomicrons undergo hydrolysis of their core triglyceride in the peripheral circulation through the action of an enzyme, lipoprotein lipase, which is tethered within the capillary endothelium of muscle and adipose tissue beds. Following hydrolysis of the core triglyceride, the surface of the chylomicron shrinks and the particle becomes smaller, allowing more apoE to bind. This step is required for the receptor-dependent uptake of chylomicron remnants by the liver, through the chylomicron remnant receptor (Fig. 1). Enterocytes also secrete high-density lipoprotein (HDL). These are small (8–12 nm) particles composed mostly of phospholipid, apoA-I, and apoA-IV, together with a small amount of neutral lipid (Fig. 1). The importance of HDL secretion from the intestine is unknown although estimates suggest that ∼50% of the daily HDL input arises from this source. HDL plays a central role in “reverse cholesterol transport,” the process by which cholesterol is removed from the membranes of peripheral cells and delivered to the liver for excretion. This pathway represents an important mechanism of the body to rid itself of excess cholesterol and perhaps is the reason that HDL (the so-called good cholesterol) is protective against atherosclerosis. Intestinal cells express the transporter proteins ABC-A1, ABC-G5, and ABC-G8, recently identified as cholesterol and sterol transport proteins (Fig. 1). In addition, enterocytes express the macrophage scavenger receptor protein SR-B1, presumed to function as an HDL receptor. The precise role of these receptors and transporters in reverse cholesterol transport and their importance in the recognition and transport of cholesterol—both into and out of the enterocyte—are important questions for future investigation. Increased HDL levels in plasma are protective against the development of atherosclerosis in humans and factors that regulate intestinal secretion of HDL are under investigation. A further aspect of the ability of intestinal epithelial cells to secrete lipoproteins is that expression of the ABC-type transporters may allow these cells to discriminate between cholesterol and other, more toxic sterols. Rare genetic diseases are associated with the indiscriminate absorption of plant sterols, which are normally not absorbed by humans. Some of these defects have been linked to mutations in ABC-type transporters.

NUTRITION

Enterocytes will atrophy within 48 hours of lack of luminal nutrients. Nutritional support is one of the most important aspects of promoting healing. Inadequate enteral nutrition can delay wound healing and promote bacterial translocation from the gastrointestinal tract, and thus increase patient morbidity, length of hospital stay and, potentially, mortality. At the time of surgery, an esophagostomy, gastrostomy, or jejunostomy tube should be placed if an animal has been inappetent, or is at risk for inappetence during the postoperative period.12-16 A patient's daily caloric requirements (resting energy expenditure, or RER) can be calculated by the formula17

RER in kcal=(30×BWkg)+70

If enteral nutrition is impossible, parenteral nutrition can be provided through a central venous or intraosseous catheter.18 Supplemental feeding can be discontinued once the animal is able and willing to voluntarily consume its daily caloric requirements.

2.3.2.1 Enterocytes

Enterocytes are generally tall and narrow, with elongated nuclei located just below the middle of the cell, mitochondria located in both apical and basal regions, a well-developed brush border and lamellar structures running parallel to the lateral plasma membrane (Yamamoto 1966; Ezeasor and Stokoe 1981). The lamellar structures have anastomoses with the basolateral membrane and have been shown by Ruiter et al. (1985) using freeze fracture SEM to form a basal labyrinth that increases the basolateral surface area. It should be noted that fish generally lack the lateral interdigitations characteristic of mammalian enterocytes. In the posterior intestine absorptive cells with large vacuoles in the apical cytoplasm are present (Yamamoto 1966; Ezeasor and Stokoe 1981; Langille and Youson 1984b). In lamprey, in addition to the absorptive cell described in teleost fishes, the anterior intestine has a unique columnar cell type with a system of smooth reticulum continuous with the basolateral membrane similar to the tubular system of branchial chloride cells (Youson 1981; Langille and Youson 1984a). Based on this similarity, they have been suggested to be involved in ion regulation as well; however, this relationship has not been established (Youson 1981). In elasmobranchs, interdigitations of the lateral plasma membrane of neighboring enterocytes increase surface area (Teshima and Hara 1983). Enterocytes show strong basolateral expression of Na+/K+-ATPase which is essential in driving a number of transepithelial transport processes important for nutrient uptake and ion regulation (Figs 1.1H, 1.2F–H, 1.5C, D, 1.6C–E, J; Chapter 2 and 4Chapter 2Chapter 4). In the lamprey, the differences in the organization of the basolateral membrane are reflected in the staining patterns, with tubular system cells having a whole cell distribution of Na+/K+-ATPase immunostaining (Fig. 1.6c′), while in cells with a basal labyrinth, as in all other fishes, staining is limited to the cell periphery (Figs 1.5C, D, 1.6D′, E′).

The enterocyte apical membrane is characterized by the presence of microvilli that form a brush border that is evident with light microscopy. The brush border contributes greater than 90% to total intestinal surface area (Tilapia aurea and T. zilli; Frierson and Foltz 1992) and forms the critical digestive/absorptive interface, a functional microenvironment where enzymes involved in further food breakdown are located and where absorption and transport will occur (Crane 1968; Kuz’mina and Gelman 1997). The activities of a number of enzymes have been localized to the brush border membrane in vertebrates including fishes. This list includes alkaline phosphatase, disaccharidases, leucine-aminopeptidase, tri- and di-peptidases which are produced by the epithelial cells while pancreatic lipase and esterase, α-amylase, and carboxypeptidase are adsorbed to the microvilli (Kuz’mina and Gelman 1997; Chapter 2). The presence of these enzymes in the brush border has been demonstrated by enzyme histochemistry and immunohistochemistry. Glucose (SGLT1) and di-/tri-peptide transporters (pepT1) have also been localized to the brush border of enterocytes (Sala-Rabanal et al. 2004; Gonçalves et al. 2007; Yuen et al. 2007). The tight packing of the microvilli may also have a sieving effect, excluding large particles and aggregates from entering the microvillar space (Kuz’mina and Gelman 1997). The density and height of microvilli may vary with species and intestinal region (Frierson and Foltz 1992; German et al. 2010b). The density of microvilli is generally less in the posterior region of the intestine (e.g. Noaillac-Depeyre and Gas 1976; Stroband 1977; German et al. 2010b). Prolonged starvation markedly reduced microvilli density to the point at which they may completely disappear, although in the short term only microvilli height decreases (Gas and Noailliac-Depeyre 1976; German et al. 2010b).

The general appearance of the enterocyte indicates an absorptive function (Yamamoto 1966; Iwai 1968; Rombout et al. 1985; Noaillac-Depeyre and Gas 1976; Fig. 1.3F–I). Notably, the absorption pathways of lipids and macromolecular proteins were initially identified using morphological techniques. In the trout, lipid absorption in the anterior intestine and pyloric ceca have been demonstrated by the presence of 60–100 nm very low density lipoproteins (VLDL) (Sire et al. 1981), and 250–1200 nm lipid droplets, and 400 nm chylomicrons by TEM (Bauermeister et al. 1979). Feeding has a marked effect on the abundance of these features, which disappear with fasting (Sire et al. 1981). Enzymes localized by histochemistry include esterases and lipases secreted by the pancreas that hydrolyze wax esters and triacylglyerols forming free fatty acids and monoacylglycerols which are absorbed into enterocytes. Re-esterification of free fatty acids into triacylglyerols corresponds to the appearance of VLDL in the endoplasmic reticulum, Golgi apparatus, lamellar structures, interstitial spaces, and secondary circulation (Sire et al. 1981). In trout fed zooplankton, which is rich in wax esters, larger osmophillic lipid droplets are observed in these spaces and particles similar in size to chylomicrons released (Bauermeister et al. 1979). Lymph channels equivalent to the lacteals of mammals are lacking in fishes. Ultrastructural studies have also revealed VLDL in RER and Golgi apparatus of tench (Noaillac-Depeyre and Gas 1976), grasscarp (Stroband and Debets 1978) and lamprey (Langille and Youson 1984a, 1985). It is also interesting to note that lampreys are able to efficiently digest lipids without the need for bile salts, since the bile duct is not present in adults (Langille and Youson 1985).

In the distal intestine, columnar epithelial cells with large vacuoles have been identified as the site of macromolecular protein uptake (Fig. 1.3H, I). These cells are characterized by numerous microvilli, large numbers of apical pits or pinocytotic vesicles, a well-developed tubulovesicular network, and numerous vacuoles and lysosomes (Iwai 1968; Ezeasor and Stokoe 1981; Stroband and Kroon 1981; Rombout et al. 1985). Similar cells were also described in the posterior intestine of the lamprey (caveolated cells, Langille and Youson 1984b, 1985). Marker studies with horse radish peroxidise (HRP) and ferritin have demonstrated that intact proteins can be taken up via different routes for transcytosis to the intercellular space or for intracellular degradation in lysosomes, respectively (Noaillac-Depeyre and Gas 1976; Rombout et al. 1985; Langille and Youson 1985). H+-ATPase can be seen to localize to the apical cytoplasmic area of these cells in lamprey where vacuoles and lysosomes are present (Fig. 1.6E, E′, G, I), with a similar pattern seen in other fishes (J. M. Wilson unpublished).

However, it is important to keep in mind that the majority of protein uptake (80%) takes place in the anterior intestine (Fange and Grove 1979; Stroband and van der Veen 1981; Chapter 2), and that the quantitative significance of pinocytotic protein uptake for digestion is questionable. However, it has been proposed that this pathway may provide a reserve capacity for protein uptake or is of possible immunological significance (Rombout et al. 1985; Chapter 3).

Replacement of the Epithelium of the Small Intestine

The enterocyte cell cycle in laboratory rodents is only 10–17 hours and the entire epithelium is replaced every 2–3 days. Goblet cells hardly have time to refill after releasing their contents (Fawcett, 1994). Replacement is from the stem cells found in the depths of the crypts. The progeny of these cells differentiate into enterocytes and goblet cells as they migrate upwards. Effectively this means that the epithelium slides upwards as cells are lost from the tips of the villi. The process appears to be driven by the fibroblasts beneath the basement membrane, which seem to pull the surface along with them as they move, but more remains to be discovered about this process.