Absorption of phosphorus from the spermatophore through the cuticle of the bursa copulatrix of the butterfly, Calpodes ethlius

In lepidoptera the spermatophore is deposited in a bursa copulatrix, which is lined by a cuticle through which solubilized components of the spermatophore must be absorbed by the underlying epithelium. This study follows the route taken by the phosphorus which, with calcium, forms a significant portion of the spermatophore of Calpodes ethlius in the form of homogeneous, amorphous concretions. Lead nitrate was used in the primary fixative to precipitate the phosphorus in order to follow the pathway taken by it through the cuticle and bursal epithelium. The epicuticle-free pits, which cover the bursal cuticle, are identified as the points of entry, and the epicuticular filaments as major pathways for transport of phosphorus and presumably soluble molecules. Lead phosphate deposits also occurred in the epithelium extracellularly between the apical microvilli, lateral membranes and basal folds, in calcium granules and in large compound structures which appear to be made up, at least in part, of calcium granules. The confinement of the phosphorus to extracellular compartments of the epithelium suggests that its function is to be found elsewhere.     

The formation of type-I concretions in Drosophila Malpighian tubules studied by electron microscopy and X-ray microanalysis

There are two types of concretions in Drosophila Malpighian tubules: Type-I concretions originate in the distal segments of the anterior tubules, type-II concretions in the adjacent transitional segment between the apical microvilli. Type-I concretions are formed with the aid of carbonic anhydrase within intracellular vesicles, which migrate to the apical cell membrane where they are discharged into the lumen by exocytosis. The carbonic anhydrase inhibitors acetazolamide or hydrochlorothiazide prevent the formation of concretions by interruption of bicarbonate supply. In addition, the formation of concretions can be reduced by feeding with sodium cellulose phosphate.     

Pulse discharge of calcium through a demoralizing epithelium in the crustacean Orchestia: Ultrastructural cytochemistry and X-ray microanalysis

During the post-exuvial period, the posterior caeca of the terrestrial crustacean Orchestia quickly reabsorb the calcareous concretions stored during the pre-exuvial period through successive generations of spherules which appear, grow, then disappear from the apex to the base of a typical network of extracellular channels. The caecal epithelium is thus alternatively calcium-loaded and unloaded. Ultrastructural cytochemistry, using direct precipitating methods (potassium pyroantimonate, oxalic acid and sodium fluoride) or indirect substitution method (lead acetate) reveals that this extracellular pathway could be the main route for a massive transport down a concentration gradient of ionic or ionizable calcium which temporarily precipitates into instable spherules basally solubilized. Quantitative microanalysis of both frozen dry sections and anhydrous thin sections of cryosubstituted resin-embedded material indicates maximal paracellular calcium concentrations during the loading phases, which may be responsible not only for the calcification of the spherules but also for the abnormally high calcium content within the cytoplasm and the mitochondria.     

Developmental changes in Malpighian tubule cell structure.

Structural changes which occur in the Malpighian tubule yellow region primary cells during larval-pupal-adult development of the skipper butterfly Calpodes ethlius are described. The developmental changes in cell structure are correlated with functional changes in fluid transport (Ryerse, 1978a) in a way which supports osmotic gradient models of fluid secretion. Larval tubules are specialized for fluid secretion with deep basal infolds and elongate mitochondria-containing apical microvilli which provide channels in which osmotic gradients could be set up. The Malpighian tubule cells are extensively remodelled at pupation when fluid transport is switched off, but they persist intact through metamorphosis. At this time, the basement membrane doubles in thickness, the mitochondria are retracted from the microvilli and are isolated for degradation in autophagic vacuoles, and both apical and basal plasma membranes are internalized via coated vesicles for degradation in multivesicular bodies, which results in the shortening of the microville and the disappearance of the basal infolds. Mitochondria are re-inserted into the microvilli, and the basal infolds re-form in pharate adult stage Malpighian tubules when fluid secretion resumes. Adult tubules are similar in general structure to larval tubules and contain mitochondria in the microvilli and basal infolds. However, they differ from larval tubules in that they are capable of very rapid fluid transport, have a reduced tubule diameter and tubule wall thickness, a much thicker basement membrane and peripherally associated tracheoles. Mineral concretions of calcium phosphate accumulate in larval tubules, persist through metamorphosis and decline in number in adults, suggesting they serve some anabolic role.     

Ultrahistochemical localization of Na+−K+ ATPase,Ca2+-ATPase and alkaline phosphatase activity in a calcium-transporting epithelium of a crustacean during moulting

Summary Periodical changes in Na+–K+-ATPase, Ca2+–ATPase and non-specific alkaline-phosphatase activity were observed using cytochemical techniques in the posterior caeca of the crustacean amphipod, Orchestia cavimana, during the moult cycle. These changes were considered in relation to the calcium transport mechanisms in the posterior caecal epithelium. For both ATPases as well as alkaline phosphatase, the specific reaction products were most intense during the pre-exuvial period, i.e. when calcium is slowly transported against a concentration gradient: the localization of Na+–K+-ATPase activity in microvilli and the upper extracellular channels strongly supports the hypothesis that this enzyme is involved in an indirect, sodium-dependent mechanism for the transport of calcium. The detection of Ca2+-ATPase activity in microvilli would seem to indicate that this enzyme plays a role in the direct, active extrusion of Ca2+ at this level. Although the role of alkaline phosphatase in the transport of calcium remains unclear, the histochemical detection of this enzymatic activity throughout the apical part of the caecal epithelium suggests that this enzyme may be involved in calcium secretion. In post-exuvial period, we found only weak specific reaction products, thus indicating a reduced active calcium transport as these ions are rapidly reabsorbed down the concentration gradient.     

An ultrastructural study of Dirofilaria immitis infection in the Malpighian tubules of Anopheles quadrimaculatus

An ultrastructural study was conducted of the Malpighian tubules of Anopheles quadrimaculatus, both uninfected and following infection with Dirofilaria immitis. The Malpighian tubules in Anopheles are composed of primary and stellate cells. The primary cells are the predominant cell type and are characterized by the presence of membrane-bound, intracellular, mineralized concretions and large apical microvilli containing mitochondria. Following the infective blood meal, the microfilariae enter the primary cells of the Malpighian tubules and reside in the cytoplasm in a clear zone without a delimiting membrane. Cells in infected tubules differ from those in uninfected tubules in that the membranes of the vacuoles surrounding the concretions are disrupted in many specimens. The apical and basal cell membranes and the mitochondria associated with these are not disrupted during the first 6-8 days of infection. These observations differ sharply from those previously described in Aedes taeniorhynchus infected with D. immitis. The observations are consistent with the hypothesis that the extended transport capacity observed in previous physiological studies of An. quadrimaculatus infected with D. immitis are dependent on the prolonged normal ultrastructure of the apical microvilli, mitochondria, and basal membranes.     

Ultrahistochemical localization of Na+-K+ ATPase, Ca2+-ATPase and alkaline phosphatase activity in a calcium-transporting epithelium of a crustacean during moulting

Periodical changes in Na+-K+-ATPase, Ca2+-ATPase and non-specific alkaline-phosphatase activity were observed using cytochemical techniques in the posterior caeca of the crustacean amphipod, Orchestia cavimana, during the moult cycle. These changes were considered in relation to the calcium transport mechanisms in the posterior caecal epithelium. For both ATPases as well as alkaline phosphatase, the specific reaction products were most intense during the pre-exuvial period, i.e. when calcium is slowly transported against a concentration gradient: the localization of Na+-K+-ATPase activity in microvilli and the upper extracellular channels strongly supports the hypothesis that this enzyme is involved in an indirect, sodium-dependent mechanism for the transport of calcium. The detection of Ca2+-ATPase activity in microvilli would seem to indicate that this enzyme plays a role in the direct, active extrusion of Ca2+ at this level. Although the role of alkaline phosphatase in the transport of calcium remains unclear, the histochemical detection of this enzymatic activity throughout the apical part of the caecal epithelium suggests that this enzyme may be involved in calcium secretion. In post-exuvial period, we found only weak specific reaction products, thus indicating a reduced active calcium transport as these ions are rapidly reabsorbed down the concentration gradient.     

Ultrastructure changes associated with osmoregulation in the hindgut cells of a saltwater insect, Ephydrella sp. (Ephydridae: Diptera)

Ephydrella larvae strictly regulate their blood osmotic pressure and Na⁺ content over a wide range of environmental salinities (7 mM to 3000 mM NaCl). They can survive in distilled water and 6000 mM NaCl for several days. In the hindgut the ileum is concerned with the regulation of urine composition whilst the rectum has a purely mechanical function. In the ileum there are large cells which have long basal channels and short apical microvilli, and small cells which have long apical leaflets and short basal channels. It is suggested that the large cells reabsorb water and that the small cells either reabsorb or secrete ions. The configurations of the channels, and the spacing of leaflets and microvilli change with alterations in environmental salinity. Fixation experiments using fixatives of different osmotic pressures show that the configuration of extracellular space in the cells has a marked dependence on both the osmotic pressure of the fixative and upon the environmental salinity. It is suggested that the osmotic response of the cells to fixatives indicates that the osmotic pressure of the cells increases with increasing environmental salinity. It is suggested, in general, that correlation of changes in the volume of extracellular space with changes in ion and water transport must be regarded with caution.     

Calcium-activated chloride channels in the apical region of mouse vomeronasal sensory neurons

The rodent vomeronasal organ plays a crucial role in several social behaviors. Detection of pheromones or other emitted signaling molecules occurs in the dendritic microvilli of vomeronasal sensory neurons, where the binding of molecules to vomeronasal receptors leads to the influx of sodium and calcium ions mainly through the transient receptor potential canonical 2 (TRPC2) channel. To investigate the physiological role played by the increase in intracellular calcium concentration in the apical region of these neurons, we produced localized, rapid, and reproducible increases in calcium concentration with flash photolysis of caged calcium and measured calcium-activated currents with the whole cell voltage-clamp technique. On average, a large inward calcium-activated current of -261 pA was measured at -50 mV, rising with a time constant of 13 ms. Ion substitution experiments showed that this current is anion selective. Moreover, the chloride channel blockers niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid partially inhibited the calcium-activated current. These results directly demonstrate that a large chloride current can be activated by calcium in the apical region of mouse vomeronasal sensory neurons. Furthermore, we showed by immunohistochemistry that the calcium-activated chloride channels TMEM16A/anoctamin1 and TMEM16B/anoctamin2 are present in the apical layer of the vomeronasal epithelium, where they largely colocalize with the TRPC2 transduction channel. Immunocytochemistry on isolated vomeronasal sensory neurons showed that TMEM16A and TMEM16B coexpress in the neuronal microvilli. Therefore, we conclude that microvilli of mouse vomeronasal sensory neurons have a high density of calcium-activated chloride channels that may play an important role in vomeronasal transduction.     

Cell volume regulation in frog urinary bladder

We have studied the problem of cell volume homeostasis in toad and frog urinary bladder by using electrophysiological measurements and an optical measure of cell volume. After osmotically induced swelling, urinary bladder cells spontaneously regulate their volume through a net loss of potassium, chloride, and water. During inhibition of sodium transport by amiloride the cells swell to the same extent as controls, but the volume-regulatory process is blocked. Electrophysiological results under isosmotic conditions indicate that basolateral membrane resistance increases simultaneously with the amiloride-induced rise in apical membrane resistance during transport inhibition. These independent observations indicate that inhibition of apical membrane sodium entry results in a secondary decrease in basolateral membrane potassium permeability. When cells are exposed to calcium-free, hyposmotic Ringer's solution, cell volume regulation is blocked; subsequent addition of the calcium ionophore A23187 is ineffective in restoring the regulatory process. The ionophore does induce volume regulation, however, in amiloride-inhibited, osmotically swollen cells in the presence of external calcium. Calcium thus seems to control basolateral membrane potassium permeability and may be the intracellular mediator of apical and basolateral membrane interactions.     

Tormogen cell and receptor-lymph space in insect olfactory sensilla

Summary (1) The basiconic sensilla on the antennae of Calliphora resemble other insect epidermal sensilla; one or several bipolar sense cells are surrounded by three non-neural cells. (2) The apical cell membrane of the tormogen cell(one of the three accessory cells) forms microvilli coated internally with particles. (3) In the (extracellular) outer receptor-lymph space hyaluronic acid can be demonstrated histochemically. (4) Demonstration of non-specific alkaline phosphatase, Mg²⁺-activated ATPase, and the presence of mitochondria in the apical part of the tormogen cell suggest active transport processes through these cells into the outer receptor-lymph space.Supported by the Deutsche Forschungsgemeinschaft     

Prey Capture and Phagocytosis in the Choanoflagellate Salpingoeca rosetta

Choanoflagellates are unicellular and colonial aquatic microeukaryotes that capture bacteria using an apical flagellum surrounded by a feeding collar composed of actin-filled microvilli. Flow produced by the apical flagellum drives prey bacteria to the feeding collar for phagocytosis. We report here on the cell biology of prey capture in rosette-shaped colonies and unicellular “thecate” or substrate attached cells from the choanoflagellate S. rosetta. In thecate cells and rosette colonies, phagocytosis initially involves fusion of multiple microvilli, followed by remodeling of the collar membrane to engulf the prey, and transport of engulfed bacteria into the cell. Although both thecate cells and rosette colony cells produce ∼70 nm “collar links” that connect and potentially stabilize adjacent microvilli, only thecate cells were observed to produce a lamellipod-like “collar skirt” that encircles the base of the collar. This study offers insight into the process of prey ingestion by S. rosetta, and provides a context within which to consider potential ecological differences between solitary cells and colonies in choanoflagellates.     

Shell formation and calcium transport in the barnacle Chthamalus fragilis

The mantle epithelium of the barnacle Chthamalus fragilis (Darwin) exhibits several ultrastructural features which may serve to regulate the calcification process. At the basis-mural plate and intermural plate junctions where rapid shell growth occurs, cells are characterized by long apical cytoplasmic projections and large intercellular spaces. These features may increase the functional surface area of the epithelium and enable more rapid deposition of calcium. The cells underlying the general shell surfaces contain numerous electron-dense inclusion bodies and show frequent cellular disintegration near the growing shell interface. Release of the granular contents of these inclusion bodies has been observed in both disintegrating and non-disintegrating cells. X-ray microanalysis revealed significantly higher calcium levels in the inclusion bodies than in the surrounding cytoplasm. This suggests a calcium transport role for these inclusion bodies. Cellular debris produced as a result of the disintegration of the mantle cells near the shell may play some role in the formation of the organic matrix of the shell. The presence of large numbers of mitochondria and well-developed apical microvilli in the cells of the inner mantle epithelium suggest that these cells serve to transport calcium into the mantle from the ambient sea water.     

Cytochemistry and X-ray microprobe analysis of the midgut of Tomocerus minor lubbock (Insecta,Collembola) with special reference to the physiological significance of the mineral concretions

Summary Histochemical and cytochemical analyses have been made on the mineral concretions within the midgut cells of Tomocerus minor. The classical histochemical methods are not specific and precise enough and have been supplemented with cytochemical techniques on ultrathin sections. The most interesting of these was the K-pyroantimonate technique combined with glutaraldehyde-osmium fixation. This technique shows the distribution of cations such as Ca⁺⁺, K⁺, Mg⁺⁺ and Na⁺ on the concentric layers of the concretions. Chloride ions can be detected by means of the silver lactate technique. The action of calcium chelators such as E.D.T.A. shows an important distribution of calcium ions in the concretions. The spectra obtained by electron probe microanalysis from areas of fresh, dried and carbon coated midguts as well as from carbon coated semithin or ultrathin sections reveal the presence of Ca, K, Mg, S, Cl and P principally. Other elements such as aluminium, silicon and manganese have also been detected. Iron is not always present. The chemical and X-ray analytical investigations indicate that the midgut concretions are mainly built up of calcium, potassium, magnesium and sodium phosphates, perhaps associated with chlorides and carbonates. An organic matrix formed by polysaccharides seems to join the different mineral layers. These concretions may be formed within the vesicles of rough endoplasmic reticulum. The midgut cells are highly differentiated and very active in transport. Extensive basal infoldings and apical microvilli as well as lateral membranes are a site of small cationic deposits. The possible pathway of ion transport in the cell and the physiological significance of the concretions are discussed. The principal function of these concretions seems to be the maintenance of the mineral balance and to trap foreign and excess ions.

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Résumé L'analyse chimique des sphérocristaux de l'intestin moyen de Tomocerus minor a été réalisée. Les méthodes histochimiques courantes manquant souvent de spécificité et de sensibilité ont été complétées avec des méthodes cytochimiques sur coupes ultrafines. La plus intéressante a été la technique du pyroantimonate de K montrant la distribution des cations Ca⁺⁺, K⁺, Mg⁺⁺, Na⁺ sur les couches concentriques des sphérocristaux. La technique au lactate d'argent permet de déceler les ions Cl^-. L'action d'agents chélateurs du Ca tels l'E.D.TA. montre une importante distribution du calcium dans les sphérocristaux. L'analyse spectrographique d'étalements de mésentérons séches, carbonés et de coupes semi-fines ou ultrafines carbonées montre la présence de Ca, K, Mg, S, Cl, P, Na. D'autres éléments tels l'Al et le Si ont pu être détectés. Le Fe n'est pas toujours présent. Les sphérocristaux semblent formés essentiellement de phosphates de calcium, de potassium, de magnésium, de sodium associés peut-être à des chlorures ou des carbonates. Une matrice organique constituée essentiellement par des polysaccharides semble lier les différentes couches minérales. Ces sphérocristaux prennent naissance à l'intérieur des vésicules de l'ergastoplasme. Les cellules de l'intestin moyen sont très différenciées et sont le siège de nombreux transports actifs. Les replis basaux de la membrane plasmique, les microvillosités apicales, de même que les membranes latérales sont le siège de dépôts de cations. Le transport des ions dans les cellules ainsi que le rôle physiologique des sphérocristaux sont discutés. Le maintien de la balance hydrique ainsi que le piégeage d'ions étrangers ou en surplus semblent être la principale fonction des sphérocristaux.

     

Vasopressin-induced morphological changes in polarized rat hepatocyte multiplets: dual calcium-dependent effects

Calcium-mobilizing hormones and neurotransmitters are known to affect cell morphology and function including cell differentiation or division. In this study, we examined vasopressin (AVP)-induced morphological changes in a polarized system of rat hepatocytes. Light and electron microscope observations showed that AVP induced microvilli formation and a remodeling of the isolated hepatocyte F-actin submembrane cytoskeleton, these two events being correlated. We showed that these effects were rapid, reversible, observed at nanomolar AVP concentration and mediated by the V(1a) receptor. On polarized multicellular systems of hepatocytes, we observed a rapid reduction of the bile canaliculi lumen at the apical pole and micovilli formation at the basolateral domain with an enlarged F-actin cytoskeleton. Neither activation of protein kinase C nor A via phorbol ester or dibutyryl cAMP induced such rapid morphological changes, at variance with ionomycin, suggesting that AVP-induced intracellular calcium rise plays a crucial role in those effects. By using spectrofluorimetry and cytochemistry, we showed that calcium release from intracellular stores was involved in bile canaliculus contraction, while calcium entry from the extracellular space controlled microvilli formation. Taken together, AVP and calcium-mobilizing agonists differentially regulate physiological hepatocyte plasma membrane events at the basal and the apical domains via topographically specialized calcium-dependent mechanisms.     

Regional specialization in the malpighian tubules of the new zealand glow-worm Arachnocampa luminosa (diptera: Mycetophilidae). The structure and function of type I and II cells

The Malpighian tubules of the glow-worm Arachnocampa luminosa are divided into four morphologically distinct regions (Parts 1--4) each comprised of a different cell type (Types I--IV). The ultrastructure of Type II cells is indicative of a transport function. The basal cell surface is highly invaginated and at the apical surface the lumen is lined with microvilli about 80% of which contain mitochondria. Spherites contained in these cells are formed from small vesicles produced by the Golgi apparatus. They have a central uric acid core enclosed by laminations of phosphates of calcium and magnesium. Cells of Part 2 of the tubule secrete a fluid high in potassium (173 mM) and low in sodium (18 mM). The cell is 30 mV negative and the lumen 44 mV positive to the bathing solution. This is consistent with the proposal of an apical cation pump. The secretion produced by Part 2 of the tubules is modified by the Type I cells by the reabsorption of potassium (162 mM) and the addition of sodium (24 mM) to the primary excretory fluid. Type I cells are 20 mV negative and the lumen 22 mV positive with respect to the bathing medium. From ultrastructural observations, Type I cells exhibit features characteristic of transporting cells thought to have an absorptive function. The basal and apical cell surfaces are extensively folded, and mitochondria are found in bands above the basal infoldings and below the microvilli. Mitochondria do not penetrate the microvilli. On comparative grounds, the fine structure of Type I cells suggest that they reabsorb ions from the tubule lumen. Energy for these processes may come from the breakdown of lipids by microperoxisomes contained within these cells. Alternatively, the fluid produced by Part 2 of the tubule may be modified passively by diffusional processes across Type I cells.     

Ultrastructural and functional aspects of the spermatheca of the African Migratory Locust Locusta migratoria migratorioides (Reiche and Fairmaire) (Orthoptera: Acrididae)

The ultrastructure of the spermathecal epithelium of the African Migratory Locust Locusta migratoria migratorioides R. & F. (Orthoptera: Acrididae) was investigated with the aid of transmission and scanning electron microscopic methods. The unpaired spermatheca can be subdivided into a multiple coiled tube and a terminal bulb region with vestibule, small apical and extensive pre-apical diverticulum. The wall of the spermatheca consists of a chitin intima, a layer of epithelial cells with a distinct apical microvilli border and a layer of gland cells, whereby slender projections of the epithelial cells extend between the gland cells. Through extensive folding, the basal plasma membrane of the gland and epithelial cells form a huge labyrinth, which is bounded by a basal lamina. Extending into the above mentioned projections there are bundles of parallel-arrayed microtubules, which run perpendicular to the microvilli border of the epithelial cell. They end in the base region of the microvilli and in the basal labyrinth on hemidesmosomes and serve to provide a mechanically stressable anchorage for the epithelium. The gland cells show structures typical for the production of export proteins: ribosomes, rER, dictyosomes, as well as vesicles of different size and electron-density. Every gland cell contains an extracellular cavity, arising through invagination, which is coated with a microvilli border. Over an end-apparatus and a ductule joining onto it (also with chitin intima) the lumen of the extracellular cavity is connected with the spermathecal lumen. The release of secretions and other substances from the epithelium into the spermatheca lumen is as possible as the uptake of substances from the latter into the epithelium. Regional differences in the fine structure of the cuticular intima, epithelial and gland cells point to different functions of the epithelium in these regions.     

Potassium modulation of taurine transport across the frog retinal pigment epithelium

Net taurine transport across the frog retinal pigment epithelium-choroid was measured as a function of extracellular potassium concentration, [K+]o. The net rate of retina-to-choroid transport increased monotonically as [K+]o increased from 0.2 mM to 2 mM on the apical (neural retinal) side of the tissue. No further increase was observed when [k+]o was elevated to 5 mM. The [K+]o changes that modulate taurine transport approximate the light-induced [K+]o changes that occur in the extracellular space separating the photoreceptors and the apical membrane of the pigment epithelium. The taurine-potassium interaction was studied by using rubidium as a substitute for potassium and measuring active rubidium transport as a function of extracellular taurine concentration. An increase in apical taurine concentration, from 0.2 mM to 2 mM, produced a threefold increase in active rubidium transport, retina to choroid. Net taurine transport can also be altered by relatively large, 55 mM, changes in [Na+]o. Apical ouabain, 10(-4) M, inhibited active taurine, rubidium, and potassium transport; in the case of taurine, this inhibition is most likely due to a decrease in the sodium electrochemical gradient. In sum, these results suggest that the apical membrane contains a taurine, sodium co-transport mechanism whose rate is modulated, indirectly, through the sodium pump. This pump has previously been shown to be electrogenic and located on the apical membrane, and its rate is modulated, indirectly, by the taurine co-transport mechanism.     

Trans-epidermal Transport and Storage of Calcium in Holthuisana transversa (Brachyura; Sundathelphusidae) During Premoult

Holthuisana transversa reabsorbs much of its exoskeletal calcium in the last 3 days before ecdysis and stores it in circulating granules in the haemocoel and in non-circulating granules in the subepidermal connective tissue. Calcium enters the epidermal cells from the moulting fluid, probably through their apical microvilli and is either incorporated into intracellular calcium granules or exits the cell via the basolateral membranes to be used in formation of two other granule types. Intracellular granules (0.4–2 μm long) form in large masses in the apical cytoplasm of the epidermal cells. They are formed as membrane-bound vesicles by the Golgi, and calcium and organic matrix material are added from the surrounding cytoplasm. As development proceeds, lamellae appear and calcium carbonate is deposited in the matrix. Granule masses move basally and are stored in the connective tissue. Calcium is also incorporated into extracellular large granules (0.8–3.8 μm long) which are formed in narrow intercellular channels between epidermal cells. A third granule type (small granules, 0.26 μm diameter) is formed in subepidermal connective tissue cells and released into the haemolymph in very large numbers. Calcium was identified in the two larger granule types using X-ray microanalysis and significant amounts of phosphorus and potassium were also present in the large granules. A model for ion cycling between the exoskeleton and granules is presented.     

A model of epithelial water transport. The corneal endothelium.

To try to understand how an epithelial tissue can transport water between bathing solutions of equal tonicity and how intracellular solute and protein concentration are related to the structural specialization of the cell membrane at its apical, basal, and lateral margins, we have formulated and solved, using approximate analytical techniques, a new model which combines the detailed transport of local osmotic flow in extracellular channel with the multicompartment approach of thermodynamic models requiring the overall conservation of water and solute for the entire cell layer. Thus, unlike most previous models, which dealt exclusively with either the average properties of the cell layer or the local transport in the extracellular channel, we are able to solve simultaneously for the interaction of the cell with its environments across its apical, basal, and lateral cell membranes as well as the detailed transport in the extracellular channel. The model is then applied to corneal endothelium to obtain new insight into the water flow movement in this tissue under in vitro and in vivo conditions. Then in vitro solution shows that the cell at 297 mosmol/liter is slightly hypotonic to the 300-mosmol/liter external bathing solutions which drive water equally out both the aqueous (apical) and stromal (basal) cell faces. This water is replaced from the extracellular channel. There is a net flow of water because more water enters the channel through its open stromal end than through the higher resistance tight junction. In vivo, the solution predicts that the stromal swelling pressure forces water through the tight junctions towards the stroma so that there is no net flow. The interesting new features of our solution are the water recirculation pattern and the role of the osmotically active proteins in making the cell hypertonic relative to the channel.