Morphological factors influencing transepithelial permeability: A model for the resistance of theZonula Occludens

Summary Epithelial cells are joined at their apical surfaces byzonulae occludentes. Claude and Goodenough (1973) demonstrated a correlation between the structure of thezonula occludens as seen in freeze-fracture preparations and the passive electrical permeability of several simple epithelia. In epithelia with high transepithelial resistance, thezonula occludens consisted of many strands. In epithelia with low transepithelial resistance thezonula occludens was much reduced, sometimes consisting of only one strand.Evidence is reviewed here that indicates that in a number of simple epithelia the structure of thezonula occludens is largely responsible for the magnitude of transepithelial conductance. An equation is derived relating transepithelial junctional resistance to the number of junctional strands:R=R _min p ^–n whereR is the transepithelial resistance of thezonula occludens,R _min is the minimum resistance of the junction (as when there areno strands in the zonula occludens),p is the probability a given strand is open andn is the number of strands in the junction. Using published experimental values ofR andn for different epithelia, the calculated value ofp was found to be as high as 0.4, which suggests that the strands in thezonula occludens are remarkably labile.Other morphological parameters relevant to transepithelial permeability are also considered, such as the width and depth of the intercellular spaces, and the size of the epithelial cells themselves.     

Freeze-fracture and morphometric analysis of occluding junctions in rectal glands of elasmobranch fish

Summary The structure of occluding junctions in secretory and ductal epithelium of salt-secreting rectal glands from two species of elasmobranch fish, the spiny dogfishSqualus acanthias and the stingrayDasyatis sabina, was examined by thin-section and freeze-fracture electron microscopy. In both species, occluding junctions between secretory cells are shallow in their apical to basal extent and are characterized by closely juxtaposed parallel strands. Average strand number in the dogfish was 3.5±0.2 with a mean depth of 56±5 nm; in the stingray a mean of 2.0±0.2 strands encompassed an average depth of 18±3 nm. In contrast, the linear extent of these junctions was remarkably large due to the intermeshing of the narrow apices of the secretory cells to form the tubular lumen. Morphometric analysis gave values of 66.8±2.5 and 74.9±4.6 m/cm² for the length of junction per unit of luminal surface area in the dogfish and stingray, respectively. This junctional morphology is similar to that generally described for leaky epithelia. In comparison, the stratified ductal epithelium which carries the NaCl-rich secretion to the intestine is characterized by extensive occluding junctions which extend 0.6–0.8 m in depth and consist of a mean of 12 strands arranged in an anastomosing network, an architectural pattern typical of tight epithelia. The length density of these junctions in the dogfish rectal gland was 7.6±0.1 m/cm².The junctional architecture of the rectal gland secretory epithelium (few strands, large junctional length densities) is similar to that described for several other hypertonic secretory epithelia [20, 34] and is compatible with the recent model for salt secretion in rectal glands [39] and in other Cl^– secretory epithelia which posits a conductive paracellular pathway for transepithelial Na⁺ secretion from intercellular space to the lumen to form the NaCl-rich secretory product.     

Immunolocalization of Na/K–ATPase and Na/K/2Cl cotransporter in the tubular epithelia of sea snake salt glands

The sublingual salt gland is the primary site of salt excretion in sea snakes; however, little is known about the mechanisms mediating ion excretion. Na⁺/K⁺–ATPase (NKA) and Na⁺/K⁺/2Cl⁻ cotransporter (NKCC) are two proteins known to regulate membrane potential and drive salt secretion in most vertebrate secretory cells. We hypothesized that NKA and NKCC would localize to the basolateral membranes of the principal cells comprising the tubular epithelia of sea snake salt glands. Although there is evidence of NKA activity in salt glands from several species of sea snake, the localization of NKA and NKCC and other potential ion transporters remains unstudied. Using histology and immunohistochemistry, we localized NKA and NKCC in salt glands from three species of laticaudine sea snake: Laticauda semifasciata, L. laticaudata, and L. colubrina. Antibody specificity was confirmed using Western blots. The compound tubular glands of all three species were found to be composed of serous secretory epithelia, and NKA and NKCC were abundant in the basolateral membranes. These results are consistent with the morphology of secretory epithelia found in the rectal salt glands of marine elasmobranchs, the nasal glands of marine birds and the gills of teleost fishes, suggesting a similar function in regulating ion secretion.     

THE DEVELOPMENT OF SURFACE SPECIALIZATION IN THE SECRETORY EPITHELIUM OF THE AVIAN SALT GLAND IN RESPONSE TO OSMOTIC STRESS

Cell surface specialization, a characteristic common to most ion-transporting epithelia, was studied in the salt (nasal) gland of the domestic duck in relation to osmotic stress. Three days after hatching, experimental ducklings were given 1% NaCl to drink for 12 hr and freshwater for the remainder of each day. Control ducklings were maintained exclusively on freshwater. The fine structure of the secretory epithelium was examined on various days of the regimen. The nasal gland epithelium of the secretory lobule is composed of several types of cells. Peripheral cells, lying at the blind ends of the branched secretory tubules, are similar in both control and experimental animals at all stages of glandular development. These generative cells contain few mitochondria and have nearly smooth cell surfaces. Partially specialized secretory cells predominate in the secretory tubules of control animals and appear as transitional cells in the tubular epithelium of salt-stressed animals. These cells contain few mitochondria and bear short folds along their lateral cell surfaces. Fully specialized cells dominate the secretory epithelium of osmotically stressed ducklings. The lateral and basal surfaces of these cells are deeply folded, forming complex intra- and extracellular compartments. This vast increase in absorptive surface area is paralleled by an increase in the number of mitochondria that pack the basal compartments. The development of this fully specialized cell is correlated with the marked increase in (Na⁺-K⁺)-ATPase activity in the glands of osmotically stressed birds.     

Bracelet salt glands of the recretohalophyte Limonium bicolor: Distribution,morphology, and induction

Halophytes complete their life cycles in saline environments. The recretohalophyte Limonium bicolor has evolved a specialized salt secretory structure,the salt gland, which excretes Na⁺to avoid salt damage. Typical L. bicolor salt glands consist of 16 cells with four fluorescent foci and four secretory pores. Here, we describe a special type of salt gland at the base of the L. bicolor leaf petiole named bracelet salt glands due to their beaded-bracelet-like shape of blue auto-fluoresc...     

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 (~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 µ; 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 (~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.     

Study on pathway and characteristics of ion secretion of salt glands of Limonium bicolor

Recretohalophytes with specialized salt-secreting structures, including salt glands and salt bladders, can secrete excess salts from plant tissues and enhance salinity tolerance of plants. However, the pathway and property of salt secretion by the salt gland has not been elucidated. In the article, Limonium bicolor Kuntze was used to investigate the pathway and characteristics of salt secretion of salt gland. Scanning electron microscope micrographs showed that each of the secretory cells had a pore in the center of the cuticle, and the rice grain-like secretions were observed above the pore. The chemical composition of secretions from secretory pores was mainly NaCl using environmental scanning electron microscope technique. Non-invasive micro-test technology was used to directly measure ion secretion rate of salt gland, and secretion rates of Na⁺ and Cl^? were greatly enhanced by a 200-mmol/L NaCl treatment. However, epidermal cells and stoma showed little secretion of ions. In conclusion, our results provide evidence that the salt glands of L. bicolor have four secretory pores and that NaCl is secreted through these pores of salt gland.     

VARIATIONS IN TIGHT AND GAP JUNCTIONS IN MAMMALIAN TISSUES

The fine structure and distribution of tight (zonula occludens) and gap junctions in epithelia of the rat pancreas, liver, adrenal cortex, epididymis, and duodenum, and in smooth muscle were examined in paraformaldehyde-glutaraldehyde-fixed, tracer-permeated (K-pyroantimonate and lanthanum), and freeze-fractured tissue preparations. While many pentalaminar and septilaminar foci seen in thin-section and tracer preparations can be recognized as corresponding to well-characterized freeze-fracture images of tight and gap junction membrane modifications, many others cannot be unequivocally categorized—nor can all freeze-etched aggregates of membrane particles. Generally, epithelia of exocrine glands (pancreas and liver) have moderate-sized tight junctions and large gap junctions, with many of their gap junctions basal to the junctional complex. In contrast, the adrenal cortex, a ductless gland, may not have a tight junction but does possess large gap junctions. Mucosal epithelia (epididymis and intestine) have extensive tight junctions, but their gap junctions are not as well developed as those of glandular tissue. Smooth muscle contains numerous small gap junctions The incidence, size, and configuration of the junctions we observed correlate well with the known functions of the junctions and of the tissues where they are found.     

Secretory capacity of the lachrymal salt gland of hatchling sea turtles,Chelonia mydas

Summary The lachrymal salt glands ofChelonia mydas were functional when hatchlings emerged from the nest. Osmotic concentrations up to 720 mosmol kg^–1 were recorded in spontaneously produced tears (salt gland secretions). When injected with a Na⁺ load (1500–2700 mol (100 g)^–1) newly emerged hatchlings produced tears ranging in osmotic concentration from 1000–1900 mosmol kg^–1 with Na⁺ secretion rates from single glands of 200–475 mol (100 g·h)^–1. In these circumstances the rate of sodium excretion, via the salt glands, was equivalent to the sodium content of 0.2 to 0.5 ml of sea water per hour. Since the apparent drinking rate of hatchlings within the first two days of entering sea water was approximately 0.5 to 1.7 ml per day, the excretion of Na⁺ imbibed by drinking is well within the secretory capacity of the lachrymal salt glands.In feeding hatchlings extraordinarily high Na⁺ secretion rates were induced by Na⁺ loading. Hatchlings which were loaded with Na⁺ by injection (1500–5400 mol (100 g)^–1) produced tears having osmotic concentrations between 1500 and >2000 mosmol kg^–1. The Na⁺ secretion rates from single glands were 750–4185 mol (100 g·h)^–1 with extremely high short term rates of 10700 mol (100 g·h)^–1 (50 mol min^–1 for 28 g hatchlings).In terms of gland mass the highest long term secretion rate translates into 21 mmol of Na⁺ per gram of salt gland per hour and is the highest secretion rate yet recorded for a reptilian salt gland. This rate is almost three times the highest rate recorded for sea snakes (8 mmol g·h^–1) and is similar to rates commonly observed in avian salt glands (25 mmol g·h^–1).Secretion by the lachrymal salt glands was initiated by increased blood concentrations of Na⁺ or K⁺, K⁺ being as effective as Na⁺ but with the composition of the teras being virtually unchanged compared to tears from Na⁺ stimulated hatchlings. Preliminary experiments indicated that secretion was not initiated by increased Cl^– concentration in the blood or by increased volume or osmotic concentration of the blood.Abbreviation O.P. osmotic pressure     

Structural diversity of occluding junctions in the low-resistance chloride-secreting opercular epithelium of seawater-adapted killifish (Fundulus heteroclitus)

The structural features of the chloride-secreting opercular epithelium of seawater-adapted killifish (Fundulus heteroclitus) were examined by thin-section and freeze-fracture electron microscopy, with particular emphasis on the morphological appearance of occluding junctions. This epithelium is a flat sheet consisting predominantly of groups of mitochondriarich chloride cells with their apices associated to form apical crypts. These multicellular groups are interspersed in an otherwise continuous pavement cell epithelial lining. The epithelium may be mounted in Ussing-type chambers, which allow ready access to mucosal and serosal solutions and measurement of electrocal properties. The mean short-circuit current, potential difference (mucosal-side negative), and DC resistance for 19 opercular epithelia were, respectively, 120.0 +/- 18.2 microA/cm2, 12.3 +/- 1.7 mV, and 132.5 +/- 26.4 omega cm2. Short-circuit current, a direct measure of Cl- transport, was inhibited by ouabain (5 micron) when introduced on the serosal side, but not when applied to the mucosal side alone. Autoradiographic analysis of [3H]-ouabain-binding sites demonstrated that Na+,K+-ATPase was localized exclusively to basolateral membranes of chloride cells; pavement cells were unlabeled. Occluding junctions between adjacent chloride cells were remarkably shallow (20-25 nm), consisting of two parallel and juxtaposed junctional strands. Junctional interactions between pavement cells or between pavement cells and chloride cells were considerably more elaborate, extending 0.3-0.5 micron in depth and consisting of five or more interlocking junctional strands. Chloride cells at the lateral margins of crypts make simple junctional contacts with neighboring chloride cells and extensive junctions with contiguous pavement cells. Accordingly, in this heterogeneous epithelium, only junctions between Na+,K+-ATPase- rich chloride cells are shallow. Apical crypts may serve, therefore, as focal areas of high cation conductivity across the junctional route. This view is consistent with the electrical data showing that transmural resistance across the opercular eptihelium is low, and with recent studies demonstrating that transepithelial Na+ fluxes are passive. The simplicity of these junctions parallels that described recently for secretory cells of avian salt gland (Riddle and Ernst, 1979, J. Membr. Biol., 45:21-35) and elasmobranch rectal gland (Ernst et al., 1979, J. Cell Biol., 83:(2, Pt. 2):83 a[Abstr.]) and lends morphological support to the concept that paracellular ion permeation plays a central role in ouabain-sensitive transepithelial NaCl secretion.     

Fracture faces of zonulae occludentes from "tight" and "leaky" epithelia

Epithelia vary with respect to transepithelial permeability. In those that are considered "leaky", a large fraction of the passive transepithelial flux appears to follow the paracellular route, passing across the zonulae occludentes and moving down the intercellular clefts. In "tight" epithelia, the resistance of the paracellular pathway to passive flux is greatly increased. To see whether differences in the morphology of the zonula occludens could contribute to this variability in leakiness among epithelia, replicas of zonulae occludentes in freeze-fractured material from a variety of tight and leaky epithelia were examined. The junctions appear as a branching and anastomosing network of strands or grooves on the A and B membrane fracture faces, respectively. It was found that the zonula occludens from a "very leaky" epithelium, the proximal convoluted tubule of the mouse kidney, is extremely shallow in the apical-basal direction, consisting in most places of only one junctional strand. In contrast, the "very tight" frog urinary bladder exhibits a zonula occludens that is relatively deep (>0.5 µm) in the apical-basal direction, and consists of five or more interconnected junctional strands interposed between luminal and lateral membrane surfaces. Epithelia of intermediate permeabilities exhibited junctions with intermediate or variable morphology. Toad urinary bladder, mouse stomach, jejunum, and distal tubule, rabbit gallbladder, and Necturus kidney and gallbladder were also examined, and the morphological data from these epithelia were compared to physiological data from the literature.     

Morphological changes in the zonula adhaerens during embryonic development of chick retinal pigment epithelial cells

Summary Retinal pigment epithelial cells from chicks at various stages of development were examined by transmission electron microscopy to determine how the adult form of the zonula adhaerens, composed of subunits termed zonula adhaerens complexes, is acquired. During early stages of development, between embryonic day 4 and embryonic day 7, the intermembrane discs of zonula adhaerens complexes appear to be formed from material already present between the junctional membranes of the zonulae adhaerentes. In contrast, the cytoplasmic plaque material of the zonulae adhaerentes is difficult to detect before hatching; it is seen as a dense band along the junctional membranes at hatching and as individual subunits in register with the intermembrane discs in adult retinal pigment epithelial cells. After embryonic day 16, when the zonulae adhaerentes increase dramatically in size, single zonula adhaerens complexes are also present basal to the zonulae adhaerentes along the lateral cell membrane. This suggests that, during later stages of development, the junctions grow in size and/or turn over by the addition of pre-assembled zonula adhaerens complexes.Abbreviations CMB Circumferential microfilament bundle - ZA Zonula adhaerens - ZAC Zonula adhaerens complex - RPE Retinal pigment epithelium     

CELL JUNCTIONS IN AMPHIBIAN SKIN

Cell junctions have been investigated in the amphibian epidermis, a stratified squamous epithelium, and compared to those described previously in simple columnar epithelia of mammalian cavitary organs. In adult frogs and toads, and in larvae approaching metamorphosis, belts of membrane fusion or zonulae occludentes of considerable depth are regularly found between adjoining cells of the outermost layer of the stratum corneum, binding the cells together into a continuous, uninterrupted sheet. Another set of occluding zonules appears in the second cornified layer (when such a layer is present), and a third set usually occurs in the outermost layer of the stratum granulosum. Specialized elements described as "modified" and "composite" desmosomes are encountered along the lateral and basal aspects, respectively, of the cornified cells; ordinary desmosomes and maculae occludentes (i.e., spots of membrane fusion) are found in all other strata. The usual 200 A intercellular gap is generally maintained between the cells of the stratum germinativum at the basal ends of the intercellular spaces. Hence, the intercellular spaces of the epidermis form a largely continuous network, closed to the external medium and open to the dermal interstitia. The situation is comparable to that found in columnar epithelia, except that the intercellular spaces are much more extensive, and an extracellular subcompartment (or two) apparently exists in the stratum corneum and between the latter and the stratum granulosum. The last subcompartment is usually filled with a dense substance, probably derived from discharged secretory granules. The tripartite junctional complex characteristic of lumen-lining epithelia (i.e., a zonula occludens followed by a zonula adhaerens, and desmosome) is seen only in early larvae; in adults and in larvae approaching metamorphosis, the occluding zonule is followed directly by a series of modified desmosomes. Interpreted in the light of current physiological data, these findings suggest that the diffusion of water, ions, and small, water-soluble molecules is impeded along the intercellular spaces of the epidermis by zonulae occludentes while it is facilitated from cell to cell within the epidermis by zonulae and maculae occludentes.     

The ultrastructure of the epithelium of the ciliary body

Summary The fine structure of the human and rabbit ciliary body epithelium has been studied with the electron microscope, both under normal conditions and after paracentesis of the anterior chamber. The disposition of the junctional complexes in the two layers of the ciliary epithelium is described in detail. Junctional complexes appear particularly developed between the apical surfaces of the cells of the two layers, but are present, as in other monolayered epithelia, also between the lateral surfaces of adjacent cells of each layer. The junctional complexes connecting the apical surfaces of the cells of the two layers are represented by zonulae occludentes, zonulae adhaerentes and desmosomes following each other irregularly, with interposition of rounded dilatations of the intercellular space called ciliary channels. The zonulae occludentes and adhaerentes found along the lateral surfaces of the epithelial cells probably form discontinuous and overlapping fasciae. Moreover, the existence of a peculiar dove-tail arched junction called macula occludens is suggested. Few differences were encountered when comparing the arrangement of junctional complexes in the ciliary epithelium of man with that of the rabbit. Many desmosomes connecting the basal portion of lateral surfaces of the non-pigmented cells were found only in human ciliary bodies.The study of the modifications of the junctional apparatus of the ciliary epithelium following paracentesis of the anterior chamber, confirms the functional hypotheses on junctional complexes previously suggested: in particular only zonulae occludentes cause a real block of the intercellular spaces. On the basis of the present work, the close relationships between the number, kind and disposition of junctional complexes in epithelia and the functional possibilities of these epithelia are stressed.Dedicated to Professor W. Bargmann on his 60th birthday.Dr. Orzalesi is the recipient of a research grant from Ministero della Pubblica Istruzione for 1964.     

Adhesive glands in the Pterastericolidae (Plathelminthes,Rhabdocoela)

Summary The ultrastructure of anteroventral gland cells with processes penetrating the epidermis inPterastericola bergensis, P. fedotovi, P. pellucida and the undescribedP. (sp. Rottnest) was studied with transmission and scanning electron microscopy. Specimens ofP. pellucida were shock frozen in situ in the epithelium of their asteroid host to study the function of the glands. Secretory products released from the gland cell processes fan out towards the host epithelium. The glands are concluded to have an adhesive function. They are compared with similar structures in Neodermata and other rhabdocoel taxa. The phylogenetic significance of the glands is discussed.Abbreviations b basal lamina - c cilium - cr ciliary rootlets - d septate desmosome - g gland cell process - gc gland cell - h host epithelium - m mitochondria - mc muscle cell - mv microvilli - mt microtubules - n nucleus - o ootype - pm plasma membrane - s secretory granule - sm secretory material released from dissolving secretory granules     

Ultrastructure of the ocular ciliary epithelium of the common frog

The ultrastructure of the adult frog ciliary epithelium cells has definite regional differences. Cells of ciliary epithelium folds near the iris display morphological features characterizing its barrier and secretory functions which lead to the formation of aqueous humor. These are junctional complexes with tight junctions (zonula occludents) in the apical parts of contacting sides of cells of the inner leaf: a great quantity of mitochondria, ribosomes and various vesicles, well developed endoplasmic reticulum in the cytoplasm, much folded basal surface, gap junctions between cells of external and internal leaflets. In the mammalian inner epithelial layer different cell junctions are known to be arranged in a fixed spatial fashion. Unlike, in the frog's epithelium both zonula adherent and desmosomes may be found in any sequence. Tight junctions are formed during metamorphosis, on the place of focal junctions, whereas gap junctions, referred to earlier as "extended", start functioning between cells just on the very early stages of eye morphogenesis (Dabagyan et al., 1979). The epithelium of the posterior part of the ciliary fold and pars plana of the ciliary body have, in addition, the number of morphological sign indicating the cell involvement in the accomodational function of any eye (i. e. a majority of desmosomes binding all cells together and of zonulae adherentes, well developed intracellular skeleton of tonofilament bundles). These features are characteristic of the whole distal part of ciliary epithelium rather than of the place of attachment of zonula fiber only.     

Ultrastructure of bovine parotid glands

Bovine parotid glands exhibit outstanding structural differences when compared with those of non-ruminant mammals. The acini are tortuous, branched and lined with cells of different heights, imparting a scalloped appearance to acinar lumina. Numerous microvilli, ca. 1.5 μ in length, extend into the lumina and intercellular canaliculi. Intercellular canaliculi measure ca. 3 μ in diameter and interweave in close association with intercellular tissue spaces. Intercellular tissue spaces are separated from the extraacinar spaces across a basal lamina only, whereas junctional complexes guard canaliculi from direct continuity with tissue spaces and/or extraacinar spaces. Flattened cytoplasmic lamellae extend from adjacent acinar cells and loosely interdigitate with one another across the tissue spaces. Acinar cells contain more mitochondria and less granular endoplasmic reticulum than parotid glands of non-ruminant mammals. Two types of secretory material, in the form of inclusions which vary in size and electron density, are present in the acinar cells. Intercalated ducts connect acini with striated ducts which in turn, empty into collecting ducts located between gland lobules. In terms of frequency of “basal infoldings” and numbers of mitochondria, striated ducts of calf parotid glands are not as well developed as those of certain other salivary glands. Myoepithelial cells are most often present at junctions of acini and intercalated ducts where they may attach to both acinar and ductal epithelium. Nerve “terminals” were not observed on the epithelial side of basement membranes in relation to the secretory cells.     

Partitioning of paracellular conductance along the ileal crypt-villus axis: A hypothesis based on structural analysis with detailed consideration of tight junction structure-function relationships

Summary Current models of intestinal transport suggest cells which absorb ions are located on the villus while secretory cells are located in the crypt and putatively have paracellular pathways which are highly conductive to Na⁺. One approach to assess possible variation in small intestinal paracellular conductance along the crypt-villus axis is to morphometrically analyze the structural aspects of crypt and villus tight junctions (TJs) which relate to paracellular resistance. Such detailed analysis of junctional structure in this heterogeneous epithelium would permit one to compare intestinal TJ structure-function relationships with those in a structurally simpler epithelium such as that of toad urinary bladder. This comparison would also be of considerable interest since previous similar comparisons have failed to consider in detail the geometric dissimilarity between these two epithelia. We applied light, electron microscopic, and freezefracture morphometric techniques to guinea pig ileal mucosa to quantitatively assess, for both crypts and villi, linear TJ density, relative surface contributions, and TJ strand counts. Mean linear TJ densities were 76.8 m/cm² for crypt cells and 21.8 m/cm² for villus absorptive cells. Mean TJ strand counts were 4.45 for undifferentiated crypt cell TJs and 6.03 for villus absorptive cell TJs. The villus constituted 87% and the crypt 13% of total surface. We utilized these data to predict paracellular conductance of cryptsvs. villi based on equations derived from those of Claude (P. Claude,J. Membrane Biol. 39:219–232, 1978). Such analysis predicts that 73% of ileal paracellular conductance is attributable to the crypt. Furthermore, we obtained literature values for paracellular resistance in mammalian ileum and toad urinary bladder and for toad bladder TJ structure and linear density and constructed a relationship which would allow us to more accurately compare TJ structure-function correlates between these two epithelia. Such a comparison, which considers both surface amplification and TJ structure and distribution in these epithelia, shows that one would predictin vitro measured values for paracellular resistance should be approximately two orders of magnitude less in mammalian ileum than in toad urinary bladder. This predicted discrepancy (115-fold) correlates well with the observed difference (100-fold). These findings suggest that highly similar TJ structure-function relationships apply to these geometrically dissimilar tissues and that, in mammalian ileum, the crypt compartment may be responsible for the majority of net ileal paracellular conductance. We speculate that high crypt linear TJ density and low crypt TJ strand counts may serve as the structural basis of massive paracellular Na⁺ movement which is coupled to active Cl^– secretion and appears to originate from the crypt following exposure to intestinal secretagogues.     

Intracellular and luminal ion concentrations in sea turtle salt glands determined by x-ray microanalysis

Summary The lachrymal salt glands of hatchlings of the green sea turtle (Chelonia mydas) secrete a hyperosmotic (up to 2000 mosmol·kg^–1) NaCl solution. X-ray microanalysis of frozen-hydrated glands showed that during secretion intracellular Na⁺ concentration in the principal cells increased from 13 to 34 mmol·l^–1 of cell water, whilst Cl^– and K⁺ concentrations remained unchanged at 81 mmol·l^–1 and 160–174 mmol·l^–1, respectively. The high Cl^– concentration and the change in Na⁺ concentration are consistent with the prevailing paradigm for secretion by the structurally and functionally similar elasmobranch rectal gland. Concentrations of Na⁺, Cl^– and K⁺ in the lumina of secretory tubules of secreting (Na⁺ 122, Cl^– 167, K⁺ 38 mmol·l^–1) and non-secreting (Na⁺ 114, Cl^–1 174, K⁺ 44 mmol·l^–1) glands were similar and the fluid was calculated to be approximately isosmotic with blood. In the central canals Na⁺ and Cl^– concentrations were similar but K⁺ concentration was lower (11–15 mmol·l^–1). It is concluded that either a high transepithelial NaCl gradient in secretory tubules and central canals is very rapidly dissipated during the short time between gland excision and freezing, or that ductal modification of an initial isosmotic secretion occurs.