An electron microscopic study of the intestinal villus. I. The fasting animal

Observations on the fine structure of mature herpes simplex virus and on the composition of its nucleoid

The fine structure and composition of mature herpes virus have been investigated in thin sections by electron microscopy. The virus was grown in cultured HeLa cells and was collected with them. Tests for the biological activity of the infected cultures were included in the first half of the work. Preparations were fixed with both osmium and permanganate, and were embedded either in methacrylate or in aquon, a water-miscible fraction of a commercially available epoxy resin. In further experiments material fixed with permanganate was subjected to the action of specific nucleases or control medium before embedding. All the preparations showed numerous uniform particles around and between the cells and this was paralleled by considerable biological activity where tests were made on samples of the culture fluids. Mature herpes virus has been found to be round and to measure, when dehydration shrinkage is avoided, about 165 mmicro in diameter. The particle contained an eccentric round-ended rod-shaped, electron-opaque nucleoid lying in an inner zone of low density. A dense outer zone or viroplasm surrounded this, no membrane being present between the two zones. After permanganate fixation the particle was found to have an outer limiting membrane showing a triple-layered structure morphologically indistinguishable from that of the plasma membrane of the HeLa cells. The results of the digestion experiments show that herpes virus contains nucleic acid of deoxyribose type and that this is localized in the dense nucleoid. Both the findings and the methods used are discussed.

Junctional complexes in various epithelia

The epithelia of a number of glands and cavitary organs of the rat and guinea pig have been surveyed, and in all cases investigated, a characteristic tripartite junctional complex has been found between adjacent cells. Although the complex differs in precise arrangement from one organ to another, it has been regularly encountered in the mucosal epithelia of the stomach, intestine, gall bladder, uterus, and oviduct; in the glandular epithelia of the liver, pancreas, parotid, stomach, and thyroid; in the epithelia of pancreatic, hepatic, and salivary ducts; and finally, between the epithelial cells of the nephron (proximal and distal convolution, collecting ducts). The elements of the complex, identified as zonula occludens (tight junction), zonula adhaerens (intermediary junction), and macula adhaerens (desmosome), occupy a juxtaluminal position and succeed each other in the order given in an apical-basal direction. The zonula occludens (tight junction) is characterized by fusion of the adjacent cell membranes resulting in obliteration of the intercellular space over variable distances. Within the obliterated zone, the dense outer leaflets of the adjoining cell membranes converge to form a single intermediate line. A diffuse band of dense cytoplasmic material is often associated with this junction, but its development varies from one epithelium to another. The zonula adhaerens (intermediate junction) is characterized by the presence of an intercellular space ( approximately 200 A) occupied by homogeneous, apparently amorphous material of low density; by strict parallelism of the adjoining cell membranes over distances of 0.2 to 0.5 micro; and by conspicuous bands of dense material located in the subjacent cytoplasmic matrix. The desmosome or macula adhaerens is also characterized by the presence of an intercellular space ( approximately 240 A) which, in this case, contains a central disc of dense material; by discrete cytoplasmic plaques disposed parallel to the inner leaflet of each cell membrane; and by the presence of bundles of cytoplasmic fibrils converging on the plaques. The zonula occludens appears to form a continuous belt-like attachment, whereas the desmosome is a discontinuous, button-like structure. The zomula adhaerens is continuous in most epithelia but discontinuous in some. Observations made during experimental hemoglobinuria in rats showed that the hemoglobin, which undergoes enough concentration in the nephron lumina to act as an electron-opaque mass tracer, does not penetrate the intercellular spaces beyond the zonula occludens. Similar observations were made in pancreatic acini and ducts where discharged zymogen served as a mass tracer. Hence the tight junction is impervious to concentrated protein solutions and appears to function as a diffusion barrier or "seal." The desmosome and probably also the zonula adhaerens may represent intercellular attachment devices.

The identification of chylomicra and lipoproteins in tissue sections and their passage into jejunal lacteals

The electron microscopic appearances of chylomicra and lipoproteins have been investigated. The particles were isolated from rat chyle by differential flotation in an ultracentrifuge. Various fixing and embedding media were used. The two kinds of particles were then identified in thin sections of the jejunum of rats. The chylomicra had diameters of from 1,000 A to 1 µ; the lipoproteins ranged from 100 to 1,000 A. They were identified by their sizes and their similarities to the isolated particles after the various fixing and embedding procedures. In addition, the relative amounts of the two kinds of particle varied greatly under different dietary conditions. The chylomicra had a thin rim, probably of phospholipid. Section B records the passage of the two kinds of particle into the lacteals in the villi of the jejunum. Both chylomicra and lipoproteins were seen passing through many open junctions. From permeability considerations it would seem that this is the most important route. These open junctions appear to act as "inlet valves" which prevent backflow as the contractions of the villi pump material out of the lacteals. Both chylomicra and lipoproteins were also seen entering the endothelial cells and lying inside them. The lipoproteins entered via "normal" caveolae and were seen in "normal" vesicles (∼500 A); the chylomicra necessarily occupied much larger organelles. Both kinds of particles were also seen in caveolae on the luminal surface of the endothelium, but it was impossible to be certain that these were not just particles entering the cells from the lumen. The chylomicra often seemed to be washed out of these caveolae as many large, empty ones were seen on the luminal sides of the cells. Frequently, these caveolae had dark membranes.

The anatomy of secretion in the follicular cells of the thyroid gland. I. The fine structure of the gland in the normal rat

The paper contains a description of the fine structure of the thyroid gland of the normal rat. The follicular colloid, a homogeneous substance of faintly granular texture, is bounded by cuboidal or low columnar epithelial cells. Numerous pleomorphic microvilli, often permeated by small vesicles extend from the apices of the epithelial cells into the colloid. Many small, membrane-limited vesicles lie in the superficial cytoplasmic layer just below the apical plasmalemma. The ergastoplasmic sacs of the follicular cells are dilated and contain a substance resembling colloid. They are of irregular outline, and the larger sacs tend to be located in the base of the cells. The Golgi apparatus lies in the vicinity of the nucleus and consists primarily of numerous small, membrane-bound droplets with a homogeneous content. Droplets, similar to the Golgi vesicles but larger, lie in the same vicinity and are tentatively identified as colloid droplets. The colloid droplets contain an extremely fine, dense particulate material. Other droplets with a denser, more heterogenous content are also present. Both the follicular cells and the perifollicular capillaries are bounded by a continuous basement membrane. The capillary endothelium is in certain regions extremely attenuated and is pierced by numerous patent pores, 450 A in diameter. The marked similarity between the presumptive colloid droplets and vesicles of the Golgi apparatus suggests that the droplets arise from this organelle. On morphological grounds alone no relation can be established between any of the organelles of the follicular cell and the process of colloid resorption.

An electron microscopic study of the passage of colloidal particles from the blood vessels of the ciliary processes and choroid plexus of the rabbit

The thinnest areas of the capillaries of the choroid plexus and ciliary processes in the eye of the rabbit are characterized by the presence of fenestrae. When various colloidal particles opaque to the electron beam (thorotrast, gold sol, and saccharated iron oxide) were injected into the blood stream, none were found in fenestrae or in areas that might suggest their having passed through fenestrae. The passage of marker particles from the lumen to the surrounding connective tissue does take place on occasion in the areas of thicker walls in the capillaries and venules rather than in the attenuated and fenestrated endothelial walls. The pathway taken by these markers may be either through the cytoplasm of the endothelial cells via membrane-bounded vesicles and vacuoles or through the intercellular spaces of the vessels. An altered aqueous humor (cloudy and plasmoid) was produced by endotoxin injection or by making a draining fistula in rabbit cornea. Both methods gave rise to the same changes in the blood vessels of the ciliary processes. Under such conditions of inflammation the passage of colloidal particles through the thicker walls of the capillaries and venules was greatly increased and occurred primarily as an intercellular passage between the endothelial cells. The attenuated and fenestrated areas of the endothelium of the small capillaries remained unchanged with no particles passing through them. These results on the altered vessels of the ciliary processes parallel those of Majno and Palade (26) on the rat cremaster muscle.

THE ISOLATION AND CHARACTERIZATION OF SUBCELLULAR COMPONENTS OF THE EPITHELIAL CELLS OF RABBIT SMALL INTESTINE

1. Homogenization of the epithelial cells of rabbit small intestine in 0.3m-sucrose-5mm-EDTA, pH7.4, maintains intact the microvillus sheets that form the lumenal surface of the cells, the nuclei, the mitochondria and the vesicles (microsomes) formed from the endoplasmic reticulum. 2. These particulate components of the cell, and the cell-sap fraction, have been isolated by differential centrifuging of cell homogenates. 3. The nuclei and microvillus sheets sediment together and it has been impossible to separate these subcellular components by centrifugal methods. 4. The isolated subcellular fractions have been identified by a combination of light-microscopic examination, electron-microscopic examination, chemical analysis and assay for selected enzyme activities.

ADIPOSE TISSUE. MORPHOLOGICAL CHANGES ASSOCIATED WITH LIPID MOBILIZATION

Morphological changes associated with mobilization of lipid were studied in epididymal adipose tissue from fasted and from alloxan diabetic rats. In both groups of animals a decrease in lipid content was accompanied by the formation of complex frond-like cytoplasmic processes and of loops and folds of basement membrane which extended from cell surfaces. These changes, evident after 1 day of fasting, increased in magnitude with increasing weight loss. As the lipid content of the cell decreased further, lipid-cytoplasmic interfaces became irregular and convoluted. Cytoplasmic microvesicles were prominent and appeared to be greatly increased in number. Rosette-like structures composed of microvesicles were observed in both lipid-depleted fat cells and endothelium. The interpretation of these changes and their physiological significance are discussed in terms of the physical and chemical properties of lipids and lipid metabolism. It is postulated that microvesicles may represent the mechanism of transport of free fatty acids in fat cells and in endothelium. Hypotheses are proposed and illustrated schematically for the mode of formation of microvesicular rosettes, for the mobilization and uptake of lipids by fat cells, and for the transport of lipids through endothelium.

FINE STRUCTURE OF CELL SURFACE SPECIALIZATIONS IN THE MATURING DUODENAL MUCOSA OF THE CHICK

Cell surfaces in the duodenal mucosa have been studied in maturing tissue of the chick from incubation until hatching. Changes in the distribution of mitoses in this tissue give an indication of its rate of maturation. This rate is paralleled in developmental changes in microvilli and junctional complexes. Concentration of mitotic figures towards the base of villous folds is gradual from day 9 to day 16, then rapid to day 19, after which the mature pattern is acquired. By day 11, microvilli appear in a regular pattern which does not alter through hatching. Their height remains the same to day 16 when it increases gradually to day 19, then very sharply. The angle formed between the microvilli and the cell surface increases gradually to day 16, giving evidence of advancing internal structure. Changes in cell adhesion also occur at day 16. Thereafter, following trypsin treatment cells are held together in patches by the tight junctions of the terminal bar, although the desmosomes are separated. The timing of these morphological changes is compared with that of alkaline phosphatase accumulation in this tissue as reported by Moog (13). Increase in the surface area of the microvilli parallels the increase in the activity of the enzyme.

AGE VARIATIONS IN CORTICAL MEMBRANES OF ROTIFERS

Electron micrographs were made of rotifers of known ages embedded in Vestopal and stained with lead hydroxide. In epithelial cells lining the gut of the rotifer, anatomical continuity is seen between the cell membrane, the cortical membranes, and the rough endoplasmic reticulum at the basal region of the cells. At their luminal surface, these cells possess a terminal web, and colloidal gold is taken up via food vacuoles at this surface. Such pinocytosis does not occur on the peripheral or distal surface (pseudocoelomic) of these cells. Exposure of rotifers to adenine inhibits the formation of cortical infoldings of the cell membrane and the formation of the rough reticulum, whereas the ribosomes and the Golgi apparatus appear to be unaffected. The suggestion is made that the rough reticulum may be derived from modifications of the cell membrane and that the rate of formation of the rough reticulum in adult and elderly rotifers is lower than in the actively growing animal.

STUDIES ON SMALL INTESTINAL CRYPT EPITHELIUM. I. THE FINE STRUCTURE OF THE CRYPT EPITHELIUM OF THE PROXIMAL SMALL INTESTINE OF FASTING HUMANS

Small intestinal crypt epithelium obtained from normal fasting humans by peroral biopsy of the mucosa was studied with the electron microscope. Paneth cells were identified at the base of the crypts by their elaborate highly organized endoplasmic reticulum, large secretory granules, and small lysosome-like dense bodies within the cytoplasm. Undifferentiated cells were characterized by smaller cytoplasmic membrane-bounded granules which were presumed to be secretory in nature, a less elaborate endoplasmic reticulum, many unattached ribosomes and, in some cells, the presence of glycogen. Some undifferentiated cells at the base of the crypts contained lobulated nuclei and striking paranuclear accumulations of mitochondria. Membrane-bounded cytoplasmic fragments, probably originating from undifferentiated and Paneth cells, were frequently apparent within crypt lumina. Of the goblet cells, some were seen actively secreting mucus. In these, apical mucus appeared to exude into the crypt lumen between gaps in the microvilli. The membrane formerly surrounding the apical mucus appeared to fuse with and become part of the plasma membrane of the cell, suggesting a merocrine secretory mechanism. Enterochromaffin cells were identified by their location between the basal regions of other crypt cells and by their unique intracytoplasmic granules.

FINE STRUCTURAL CHANGES IN THE INTESTINAL EPITHELIUM OF THE BULLFROG DURING METAMORPHOSIS

The fine structural changes occurring in the columnar absorbing cells of the intestinal epithelium during metamorphosis of the bullfrog, Rana catesbeiana, have been examined by phase contrast and electron microscopy. Tissue samples taken just posterior to the entrance of the hepatopancreatic duct were fixed in veronal acetate-buffered osmium tetroxide and embedded in methacrylate. Under the action of the metamorphic stimulus (thyroid hormone), specific and characteristic responses were given by differentiated larval cells and undifferentiated basal cells within the same epithelium. The functional larval cells underwent degenerative changes and were retained for a time within the metamorphosing epithelium. Dense bodies appeared and increased in number in association with the loss of normal cell structure. Because of their morphology and time of formation, these bodies have been tentatively identified as lysosomes. Early in metamorphosis the basal cells did not change, but they subsequently proliferated to form a new cell layer beneath the remaining degenerating cells that lined the lumen. After the dying cells were sloughed into the gut, the new epithelium differentiated to form the adult tissue. The columnar epithelial cells of the mature animal differed in their fine structural organization from their larval precursors. Therefore, their adult configuration was molded by the action of the metamorphic stimulus.

THE FINE STRUCTURE OF EPENDYMA IN THE BRAIN OF THE RAT

The ciliated ependyma of the rat brain consists of a sheet of epithelial cells, the luminal surface of which is reflected over ciliary shafts and numerous evaginations of irregular dimensions. The relatively straight lateral portions of the plasmalemma of contiguous cells are fused at discrete sites to form five-layered junctions or zonulae occludentes which obliterate the intercellular space. These fusions occur usually at some distance below the free surface either independently or in continuity with a second intercellular junction, the zonula adhaerens. The luminal junction is usually formed by a zonula adhaerens or, occasionally, by a zonula occludens. The finely granular and filamentous cytoplasm contains supranuclear dense bodies, some of which are probably lysosomes and dense whorls of perinuclear filaments which send fascicles toward the lateral plasmalemma. The apical regions of the cytoplasm contain the basal body complexes of neighboring cilia. These complexes include a striated basal foot and short, non-striated rootlets emanating from the wall of each basal body. The rootlets end in a zone of granules about the proximal region of the basal body, adjacent to which may lie a striated mass of variable shape. All components of the basal body complex of adjacent cilia are independent of each other.

EXPERIMENTAL STUDIES IN ACUTE RENAL FAILURE. II. FINE STRUCTURE CHANGES IN TUBULES ASSOCIATED WITH RENAL FAILURE INDUCED BY GLOBIN

When purified human globin is injected intravenously into rats it produces acute renal failure characterized by tubular casts and oliguria. The globin is identifiable within vesicles and channels in the cytoplasm of the proximal tubules, through which it passes from lumen to basal side with no apparent serious effect on the cells. When a very minimal amount of globin is taken up by cells of the distal limb of Henle's loop or distal tubules (lower nephron), a markedly deleterious effect is apparent and the cells die within a short time. The mixture of cell debris and precipitated globin forms plugs within the confines of the basement membranes of the former distal limbs and distal tubules. After a number of lower nephrons are plugged a disruption of proximal tubules is found, which apparently results from the effect of back pressure in the obstructed nephrons. We suggest that any amount in excess of a low threshold of globin, either alone or combined with heme or related material, has a toxic effect on lower nephron cells. Once initiated, the toxic effect is not reversible and the resulting plug of debris and precipitate will occlude the lumen. If a sufficient number of nephrons are made non-functional the animal becomes anuric; otherwise it is oliguric. A high rate of urine flow will protect against the excess absorption of material and thus against acute renal failure.

The long-term effect of pinealectomy on the crypts of the rat gastrointestinal tract

Previously it has been found that rat small bowel crypt cell hyperplasia occurred several weeks after pinealectomy. To determine if this effect was longer-lasting (because of the possible role of the pineal in bowel malignancy) the crypt cell proliferation rate was determined in rat small bowel and colon 6 months after pinealectomy, using a stathmokinetic technique. Although the hyperproliferative effect of pinealectomy was well maintained in the small bowel crypts after 6 months, the hyperproliferative effect in the colonic crypts was much less marked. There is no obvious explanation for these findings, although it is possible that regional differences in levels of gut neuropeptides or melatonin are involved. The mechanism of the effect of pinealectomy on the crypts remains unexplained--in particular, why the effect is so prolonged.

Secretion of lipoprotein particles by the cells of the kidney proximal segment in the migrating arctic lamprey, Lampetra japonica

The cells of the kidney proximal segment of the migrating arctic lamprey, Lampetra japonica, contain particles of the same size, electron-density and intracellular location as particles identified by others as very low-density lipoproteins (VLDL) in the liver and intestine of teleost fishes and lampreys. These particles are synthesized within the cisternae of the smooth endoplasmic reticulum and elements of the Golgi complex. They are transferred to the lateral intercellular space and lamina propria by way of the Golgi vesicles and an intracellular channel system. Some particles are discharged into the lumina of the sinusoidal capillaries of the lamina propria. Although the physiological role of lipoprotein secretion in the renal proximal segment cells is unknown, the present observations provide morphological evidence that the kidney of the arctic lampreys synthesizes lipoproteins and releases them into the circulation at the time when they are undertaking their anadromous migration.

Three-dimensional organization of lymphatics and its relationship to blood vessels in rat small intestine

The lymphatic organization and its relationship to the vascular system in the rat small intestine was studied by scanning electron microscopy of corrosion casts and freeze-fractured tissues, and by light microscopy of injected preparations. The villus possessed 3-10 or more central lacteals depending upon the villous width. The lacteals in each villus possessed interconnections between adjacent ones and were surrounded externally by the villous capillary network. At the villous base, the lacteals fused and formed a wide sinus, from which 2 or 3 lymphatics descended and led into the submucosal ones. In the muscularis externa there was a coarse lymphatic network which, together with the submucosal one, drained into collecting lymphatics continuous with the mesenteric ones. The central lacteals and the sinus were lined with thin endothelial cells with cytoplasmic leaves interdigitating with those of adjacent ones. There were tissue channels in the villous interstitial space, which opened through the gaps between the lymphatic endothelial cells into the central lacteals. The voluminous lacteals in the villi suggest their great potential for lymph formation. The existence of collecting lymphatics with valves in the muscularis externa suggests that contraction of the layer is involved in transporting lymph towards the efferent lymphatics.

Occurrence, absorption and metabolism of short chain fatty acids in the digestive tract of mammals

Short chain fatty acids (SCFA) also named volatile fatty acids, mainly acetate, propionate and butyrate, are the major end-products of the microbial digestion of carbohydrates in the alimentary canal. The highest concentrations are observed in the forestomach of the ruminants and in the large intestine (caecum and colon) of all the mammals. Butyrate and caproate released by action of gastric lipase on bovine milk triacylglycerols ingested by preruminants or infants are of nutritional importance too. Both squamous stratified mucosa of rumen and columnar simple epithelium of intestine absorb readily SCFA. The mechanisms of SCFA absorption are incompletely known. Passive diffusion of the unionized form across the cell membrane is currently admitted. In the lumen, the necessary protonation of SCFA anions could come first from the hydration of CO2. The ubiquitous cell membrane process of Na+-H+ exchange can also supply luminal protons. Evidence for an acid microclimate (pH = 5.8-6.8) suitable for SCFA-protonation on the surface of the intestinal lining has been provided recently. This microclimate would be generated by an epithelial secretion of H+ ions and would be protected by the mucus coating from the variable pH of luminal contents. Part of the absorbed SCFA does not reach plasma because it is metabolized in the gastrointestinal wall. Acetate incorporation in mucosal higher lipids is well-known. However, the preponderant metabolic pathway for all the SCFA is catabolism to CO2 except in the rumen wall where about 80% of butyrate is converted to ketone bodies which afterwards flow into bloodstream. Thus, SCFA are an important energy source for the gut mucosa itself.

Morphologic and biochemical evidence for a contractile cell network within the rat intestinal mucosa

Subepithelial and pericryptal fibroblastlike cells form a two-dimensional network immediately subjacent to the epithelial basal lamina in the small intestine and colon in several mammalian species. Stellate-shaped cells with similar, but not identical characteristics, form a three-dimensional network deep within the villar lamina propria. Electron microscopic studies indicate that these cells contain a putative contractile apparatus, are attached to each other and to apparently organized elements of the extracellular matrix by typical adhesive devices, and form gap junctions with each other. Comparative in situ immunoperoxidase localization studies document the presence in these cells of four contraction-associated proteins (smooth muscle isotropomyosin, cyclic guanosine monophosphate-dependent protein kinase, both nonmuscle and smooth muscle isomyosin, and actin) in amounts generally greater than those found in connective tissue fibroblasts, but less than in smooth muscle cells. Taken together, these results strongly suggest a smooth muscle-like, contractile function for these cells and indicate that this cellular network may provide a supportive tonus for the epithelium, as well as provide the force needed for active movement of the villus, expulsion of crypt secretion products, and propulsion of absorption products in the lamina propria, the microvasculature, and lacteals of the intestinal villus.

Tritiated thymidine radioautographic study on the origin and renewal of secretin cells in the rat duodenum

The origin and renewal of secretin cells in the duodenum were investigated using the unlabeled antibody peroxidase-antiperoxidase technique and radioautography in rats killed at various times after single or multiple injections of [3H]thymidine. Secretin cells were spatially distributed from the upper crypt to the villus tip, being particularly numerous in the upper two-thirds of the duodenal villi. After a single injection of [3H]thymidine, there were no labeled secretin cells, indicating a lack of self-replicating activity. After repeated injections of the isotope, labeled secretin cells appeared and increased in number. They first occurred at the upper part of the crypt and the lower part of the villus, and later at the villus tip. All these cells were found to be labeled after continuous labeling for 120 h, which is considered to be the renewal time for this cell population.

Lamina propria of intestinal mucosa as a typical reticular tissue. A scanning electron-microscopic study of the rat jejunum

The three-dimensional architecture of the lamina propria in the jejunal mucosa of the rat was studied by scanning electron microscopy. The lamina propria is a typical reticular tissue with fine nets of reticular fibers and free and fixed cells lying among them. However, the lamina propria in the core of villi displayed structural features closer to other reticular or lymphoid tissues than that among the crypts. In the villi, the tissue was supported by a spongy framework of interconnecting fibroblasts, and densely infiltrated by free cells. Among the crypts, cellular elements were rather sparsely dispersed in a complex network of reticular fibers. A thin layer of reticular fibers lined the basal lamina of the epithelium. This layer contained a network of flattened cells which anastomosed with each other via slender processes. The basal lamina and the fibrous layer were perforated with round pores, through which free cells or basal processes of the epithelial cells passed. Many macrophages were found resting on the reticular framework of the lamina propria, frequently in close association with immunoblast-like cells. This paper further includes scanning electron-microscopic observations on the central lacteal with special reference to its luminal projections and trabeculae.

Anatomical study of the intestine of the insect-feeder bat, Myotis frater kaguae

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Role of macrophage in the defense against intestinal cancers

The capability of activated macrophages to kill tumor cells in vitro is now well documented. The tumoricidal activation of macrophages against intestinal tumor cells by different agents is described and the main hypothesis on the mechanisms of tumor cell killing in vitro are discussed. These in vitro results suggest that the macrophage can constitute an efficient effector cell in the defense against intestinal tumors. The distribution and ratio of macrophages in normal intestine and intestinal tumors is described. At the moment, potent activators of macrophages studied in vivo on experimental and human intestinal tumors give poor results or even enhance the growth of tumors. Macrophages may also interfere with the specific immune response in two directions by enhancing the immune response or decreasing it by elaboration of mediators such as prostaglandins.

The microvillus in adult jejunal mucosa--an electron microscopic study

The increase in surface area contributed by microvilli per square unit mucosal surface area in the jejunal villus, i.e. amplification factor, has been studied in four normal adults and three adults with treated coeliac disease. Villi were split into three regions, photomicrographs of microvilli were taken from each region and the microvillus amplification factor calculated. The mid-villus region contributed a significantly greater microvillus amplification factor than either the low or upper regions in all patients studied. In addition surface cell heights were greatest in the low- and mid-villus regions. This suggests that the enterocytes in the mid-villus region are best adapted for absorption, and that the enterocytes in the upper-villus regions are undergoing an ageing process.

Intestinal changes caused by DL-alpha-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase

Subacute (2 week) oral or intravenous administration of DL-alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase (ODC), caused diarrhea and frequent emesis as early as 4 to 5 days in dogs (dose greater than or equal to 200 mg/kg/day). Diarrhea also occurred in monkeys after 1 week of treatment with an intravenous dose of 1000 mg/kg/day. Especially evident in the treated dogs with diarrhea were fluid loss, hemoconcentration, and decreased serum sodium and chloride which were findings totally reversible about 2 weeks after cessation of dosing. As a result of treatment with the highest intravenous dosage (1000 mg/kg/day), villous atrophy of the mucosa was observed by light and scanning electron microscopy in the canine small intestine. Transmission electron microscopy demonstrated that the most significant alterations of the canine intestinal tract involved the microvilli of epithelial cells which became shorter and were frequently less numerous or absent along focal areas of the plasma membrane. Intestinal mucosal levels of putrescine, especially in the duodenum and jejunum, were decreased as demonstrated in the monkeys following intravenous treatment with 100, 300, or 1000 mg/kg/day of DFMO. The results of this investigation are consistent with the hypothesis that the inhibition of ODC activity and subsequent altered polyamine metabolism may lead to delayed maturation of the intestinal epithelial cells and the impaired development of their microvilli, causing fluid loss due to reduced absorptive surface area.

Structural and functional relationship between the membrane and the cytoskeleton in brush border microvilli

Electron microscopic and biochemical studies have described the organization and composition of microvilli from chicken intestinal brush borders. An actin-based cytoskeleton, composed of a paracrystalline core of bundled microfilaments, maintains the finger-like shape of the membrane through a helical array of membrane-microfilament linkages. Two proteins, fimbrin and villin, are components of the core bundle in situ and can independently bundle the actin filaments in vitro. Structural studies comparing microvillar core bundles with villin bundles and fimbrin bundles suggest that fimbrin, and not villin, is the major actin-filament-bundling protein in the microvillus core. These points, together with the capability of villin to sever actin filaments when activated by Ca2+, raise questions about villin's function in the microvillus. One possible explanation is that villin induces vesiculation of the membrane by disassembling the underlying cytoskeleton.

The spectrin-related molecule, TW-260/240, cross-links the actin bundles of the microvillus rootlets in the brush borders of intestinal epithelial cells

Previous studies have shown that molecules related to erythrocyte spectrin are present in the cortical cytoplasm of nonerythroid cells. We report here the localization by immunoelectron microscopy of one such molecule, TW-260/240, in the brush border of intestinal epithelial cells. Using highly specific antibodies against TW-260 and TW-240 as well as antibodies against fodrin, another spectrinlike molecule, we have found that the TW-260/240 molecules are displayed between rootlets at all levels of the terminal web. Occasionally, extended structures appear labeled suggestive of the fine filaments known to cross-link actin bundles. These results are in line with previous in vitro studies showing that TW-260/240 binds to, and cross-links, actin filaments. The results are discussed in terms of a model in which rootlets are immobilized in the terminal web in a matrix of TW-260/240.

The distribution, ontogeny and origin in the rat of Ia-positive cells with dendritic morphology and of Ia antigen in epithelia, with special reference to the intestine

Ia antigens were localized in cryostat sections of rat intestine and other tissues by an indirect immunoperoxidase technique using monoclonal antibodies that recognize the rat antigens homologous to the gene products of the I-A and I-E subregions of the mouse major histocompatibility complex (MHC). Two categories of Ia+ cells were characterized, namely epithelial cells and bone marrow-derived cells with dendritic morphology. In the small intestine Ia antigen was present in the distal 2/3 of the absorptive epithelium but absent from the bases of the villi, the crypts and the epithelium covering the Peyer's patches. The distribution in nude rats was similar, indicating that T lymphocytes are not obligatory for its expression. In ontogeny Ia antigen was absent in the epithelium of neonatal gut, appearing at about 4 weeks of age and reaching adult levels at about 6 weeks. Different rat strains showed large differences in the amount of Ia antigen expressed by villus epithelium and the traits for the level of expression were shown to map outside the MHC. The levels of expression of Ia antigen in the proximal tubules of the kidney followed that of the gut epithelium in the different strains and in both tissues was mostly intracellular. Studies with chimeras showed that the Ia antigen in epithelial cells was not acquired from bone marrow-derived cells. The second category of cell studied had a characteristic dendritic morphology and was present in large numbers in the lamina propria of the villi and in the crypts. In the Peyer's patches these cells were present both in the subepithelial dome region and within the epithelium itself. These Ia+ dendritic cells were present in nude rat jejunum and appeared in normal fetal gut by 18 days gestation and were also shown to migrate into antigen-free grafts of fetal gut. This suggests that they do not require stimulation from antigens, bacterial products or T lymphocytes in order to localize in the gut or to express Ia antigen. Studies with other cell surface markers suggest that the Ia+ cells with dendritic morphology represent a range of cell types, some with similarities to macrophages and others to nonphagocytic dendritic cells.

The fenestrations of the choriocapillaris in the presence of choroidal melanoma

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Quick-freeze, deep-etch visualization of the cytoskeleton beneath surface differentiations of intestinal epithelial cells

The cytoskeleton that supports microvilli in intestinal epithelial cells was visualized by the quick-freeze, deep-etch, rotary-replication technique (Heuser and Salpeter. 1979. J. Cell Biol. 82: 150). Before quick freezing, cells were exposed to detergents or broken open physically to clear away the granular material in their cytoplasm that would otherwise obscure the view. After such extraction, cells still displayed a characteristic organization of cytoskeletal filaments in their interiors. Platinum replicas of these cytoskeletons had sufficient resolution to allow us to identify the filament types present, and to determine their characteristic patterns of interaction. The most important new finding was that the apical "terminal web" in these cells, which supports the microvilli via their core bundles of actin filaments, does not itself contain very much actin but instead is comprised largely of narrow strands that interconnect adjacent actin bundles with one another and with the underlying base of intermediate filaments. These strands are slightly thinner than actin, do not display actin's 53A periodicity, and do not decorate with myosin subfragment S1. On the contrary, two lines of evidence suggested that these strands, could include myosin molecules. First, other investigators have shown that myosin is present in the terminal web (Mooseker et al. 1978. J. Cell Biol. 79: 444-453), yet we could find no thick filaments in this area. Second, we found that the strands were removed completely in the process of decorating the core filament bundles with the myosin subfragment S1, suggesting that they had been competitively displaced by exogenous myosin. We conclude that myosin may play a structural role in these cells, via its cross-linking distribution, in addition to whatever role it plays in microvillar motility.