Uptake of horseradish peroxidase from CSF into the choroid plexus of the rat,with special reference to transepithelial transport

Summary Protein uptake from cerebral ventricles into the epithelium of the choroid plexus, and transport across the epithelium were studied ultrastructurally in rats. Horseradish peroxidase (HRP, MW 40,000) was used as protein tracer. Steady-state ventriculo-cisternal perfusion with subatmospheric pressure (-10cm of water) in the ventricular system was applied. HRP dissolved in artificial CSF was perfused from the lateral ventricles to cisterna magna for various times, and ventriculo-cisternal perfusion, vascular perfusion or immersion fixation with a formaldehyde-glutaraldehyde solution was performed.Coated micropinocytic vesicles containing HRP were seen both connected with the apical, lateral and basal epithelial surface and within the cells. Heavily HRP-labeled vesicles were often fused with the lining membrane of slightly labeled or unlabeled intercellular spaces. Since the apical tight junctions of the epithelium never appeared open or never contained HRP in the spaces between the fusion points, and since the intercellular spaces between adjacent epithelial cells below the junctions only infrequently contained tracer after 5 min, by increasing amounts after 15–60 min of HRP perfusion, a vesicular transport of HRP from the apical epithelial surface to the intercellular spaces, bypassing the tight junctions, is suggested.In addition to the transepithelial transport, micropinocytic vesicles also transported HRP to the lysosomal apparatus of the epithelial cells. With increasing length of exposure to HRP, a sequence of HRP-labeled structures could be evaluated, from slightly labeled apical vacuoles and multivesicular bodies to very heavily labeled dense bodies.     

Endocytosis in the uterine luminal and glandular epithelial cells of mice during early pregnancy

We have localized horseradish peroxidase (HRP) in the mouse uterus after intravenous administration on days 1 and 5 of pregnancy in an effort to understand how serum proteins reach the uterine lumen. Direct movement of HRP into uterine and glandular lumina was blocked by the epithelial tight junctions on both days. In luminal and glandular epithelial cells at both times, HRP was localized in endocytic vesicles along the basolateral membranes, multivesicular bodies (mvb), elongated dense bodies below the nucleus (bdb), and many small vesicles near the apical surface of the cells. The uptake of HRP was most extensive in the luminal epithelium on day 1: the number of tracer-containing apical vesicles and bdb was largest, and there were also clusters of vesicles containing the tracer above the nucleus. Acid phosphatase was localized on day 1 in mvb and bdb in both cell types, indicating that these structures are lysosomes. It appeared that HRP followed two pathways after basolateral endocytosis by the epithelial cells: it was transported to the apical region of the cells, where it was present in small vesicles that may release their contents into the uterine or glandular lumina, or it was transported to lysosomes. To investigate whether macromolecules may be transported from the uterine lumen to the stroma, we also studied endocytosis at the apical pole of luminal epithelial cells after intraluminal injection of HRP. There was no detectable uptake of HRP from the lumen on day 1, and no tracer was detected in the intercellular spaces or basement membrane region. On day 5, a large amount of HRP was taken up from the lumen into apical endocytic vesicles, mvb, and dense bodies, but tracer was not present in the Golgi apparatus, lateral intercellular spaces, or the basement membrane region at the times studied. These observations indicate that there was no transport of luminal macromolecules to the uterine stroma on day 1, while the possibility of transport on day 5 requires further study.     

INTESTINAL TRANSPORT OF ANTIBODIES IN THE NEWBORN RAT

Evidence has been reported that the proximal small intestine of the neonatal rat selectively transports antibodies into the circulation. This study describes the morphology of the absorptive epithelial cells in this region of the intestine and their transport of several immunoglobulin tracers: ferritin-conjugated immunoglobulins (IgG-Ft) and antiperoxidase antibodies. Cells exposed to rat IgG-Ft bound the tracer on the membrane of tubular invaginations of the apical cell surface. Tubular and coated vesicles within the cell also contained the tracer, as did the intercellular spaces. Uptake of tracer was highly selective and occurred only with rat or cow IgG-Ft; when cells were exposed to chicken IgG-Ft, ferritin-conjugated bovine serum albumin, or free ferritin, tracer did not enter the cell or appear in the intercellular spaces. Experiments with rat and chicken antiperoxidase showed a similar selective uptake and transport of only the homologous antibody. When cells from the distal small intestine were exposed to the tracers, all tracers were absorbed nonselectively but none were released from the cells. Cells from the proximal small intestine of the 22-day-old rat failed to absorb even rat IgG-Ft. A model is presented for selective antibody transport in proximal cells of the neonatal rat in which antibodies are selectively absorbed at the apical cell surface by pinocytosis within tubular vesicles. The antibodies are then transferred to the intercellular space within coated vesicles. Distal cells function only to digest proteins nonselectively.     

Antidiuretic hormone response in the amphibian urinary bladder: time course of cytochalasin-induced vacuole formation, an ultrastructural study employing ruthenium red

Cytochalasin is known to inhibit the antidiuretic hormone-induced hydro-osmotic response (bulk water flow) in the amphibian urinary bladder without altering hormone-stimulated diffusional water permeability or short-circuit current. In addition, histological studies have shown that the mold metabolite induces the formation of large intracellular vacuoles or lakes in the epithelial cells. We report here a transmission electron microscopic time-course study which indicates that during the early phases of the ADH response cytochalasin causes the formation of numerous multivesicular bodies or aggregates derived from individual basolateral pinocytotic vesicles. Because of their apparent hypertonic nature, the vesicles, as well as the vesicular aggregates, accumulate water during hormone-stimulated hydro-osmotic flow. As a result, the multivesicular bodies dilate and fuse to form the large intracellular lakes characteristic of cytochalasin treatment in the presence of both an applied osmotic gradient and vasopressin. In the presence of mucosal ruthenium red, the luminal glycocalyx was heavily stained with this tracer. At no time, however, even in the presence of hormone, was there any evidence for the uptake of this dye at the apical epithelial border. In the presence of serosal ruthenium red, the lateral intercellular spaces, basolateral pinocytotic vesicles, basal lamina, and collagen, as well as other subepithelial structures, were ruthenium positive. With cytochalasin D, vasopressin, and serosal ruthenium red, both the pinocytotic vesicles and the multivesicular bodies demonstrated an apparent membrane associated ruthenium positive coat. The tracer data indicates that the basolateral pinocytotic vesicles, increased by the presence of hormone, are indeed endocytotic in nature. The mucopolysaccharide coat associated with these structures may be involved in ionic and/or fluid transport.     

FUNCTIONS OF COATED VESICLES DURING PROTEIN ABSORPTION IN THE RAT VAS DEFERENS

The role of coated vesicles during the absorption of horseradish peroxidase was investigated in the epithelium of the rat vas deferens by electron microscopy and cytochemistry. Peroxidase was introduced into the vas lumen in vivo. Tissue was excised at selected intervals, fixed in formaldehyde-glutaraldehyde, sectioned without freezing, incubated in Karnovsky's medium, postfixed in OsO₄, and processed for electron microscopy. Some controls and peroxidase-perfused specimens were incubated with TPP,¹ GP, and CMP. Attention was focused on the Golgi complex, apical multivesicular bodies, and two populations of coated vesicles; large (> 1000 A) ones concentrated in the apical cytoplasm and small (<750 A) ones found primarily in the Golgi region. 10 min after peroxidase injection, the tracer is found adhering to the surface plasmalemma, concentrated in bristle-coated invaginations, and within large coated vesicles. After 20–45 min, it is present in large smooth vesicles, apical multivesicular bodies, and dense bodies. Peroxidase is not seen in small coated vesicles at any interval. Counts of small coated vesicles reveal that during peroxidase absorption they first increase in number in the Golgi region and later, in the apical cytoplasm. In both control and peroxidase-perfused specimens incubated with TPP, reaction product is seen in several Golgi cisternae and in small coated vesicles in the Golgi region. With GP, reaction product is seen in one to two Golgi cisternae, multivesicular bodies, dense bodies, and small coated vesicles present in the Golgi region or near multivesicular bodies. The results demonstrate that (a) this epithelium functions in the absorption of protein from the duct lumen, (b) large coated vesicles serve as heterophagosomes to transport absorbed protein to lysosomes, and (c) some small coated vesicles serve as primary lysosomes to transport hydrolytic enzymes from the Golgi complex to multivesicular bodies.     

STUDIES ON THE PERMEABILITY OF LYMPHATIC CAPILLARIES

The passageway for interstitial fluids and large molecules across the connective tissue lymph interface has been investigated in dermal lymphatic capillaries in the ears of guinea pigs. Numerous endothelial cells overlap extensively at their margins and lack adhesion devices at many points. The observations suggest that these sites are free to move as a result of slight pressure changes. Immediately following interstitial injections of tracer particles (ferritin, thorium, carbon, and latex spheres), many of the overlapped endothelial cells are separated and thus passageways are provided between the interstitium and lymphatic lumen. Tracer particles also occur in plasmalemmal invaginations along both connective tissue and luminal fronts. All of the tracer particles accumulate within large autophagic-like vacuoles. Very few particles of ferritin are observed in the endothelium after 24 hr; however, the vesicles containing the nonprotein tracer particles (carbon, thorium, and latex) increase in size and content and remain within the lymphatic endothelial cells up to 6 months. The role of vesicles in the transport of large molecules and particles is discussed in relation to the accretion of tracer particles within large vesicles and autophagic-like vacuoles in the endothelial cytoplasm.     

Segregation of Ferritin in Glomerular Protein Absorption Droplets

Ferritin was used as a tracer to study the mechanism by which proteins are segregated into droplets by the visceral epithelium of glomerular capillaries. In glomeruli from both normal and aminonucleoside-nephrotic rats ferritin molecules introduced into the general circulation penetrated the endothelial openings and were seen at various levels in the basement membrane. Striking differences between nephrotic and controls were seen only in the amount of ferritin incorporated into the epithelium. In normal animals, a few ferritin molecules were seen in small invaginations of the cell membrane limiting the foot processes, within minute vesicles in the epithelium, or within occasional large vacuoles and dense bodies. In nephrotics, epithelial pinocytosis was marked, and numerous ferritin molecules were seen within membrane invaginations and in small cytoplasmic vesicles at all time points. After longer intervals, the concentration of ferritin increased in vacuoles and particularly within the dense bodies or within structures with a morphology intermediate between that of vacuoles and dense bodies. In nephrotic animals cleft-like cavities or sinuses were frequently encountered along the epithelial cell surface facing the urinary spaces. Some of these sinuses contained material resembling that filling the dense bodies except that it appeared less compact. The findings suggest that ferritin molecules—and presumably other proteins which penetrate the basement membrane—are picked up by the epithelium in pinocytotic vesicles and transported via the small vesicles to larger vacuoles which are subsequently transformed into dense bodies by progressive condensation. The content of the dense bodies may then undergo partial digestion and be extruded into the urinary spaces where it disperses. The activity of the glomerular epithelium in the incorporation and segregation of protein is similar in normal and nephrotic animals, except that the rate is considerably higher in nephrosis where the permeability of the glomerular basement membrane is greatly increased.     

A mechanism for rapid transport of colloidal particles by flounder renal epithelium

Large quantities of colloidal particles were rapidly transported around the junctional complex into the lateral intercellular spaces by flounder renal epithelial cells. Large invaginations containing particles developed in the apical cytoplasm of cells when tracer particles were injected into the tubular lumens. Some membranebounded profiles containing particles appeared close to the lateral intercellular spaces. Particles were then found in the lateral intercellular spaces, between the basal plasmalemma and the basement membrane, and within the basement membrane. It is suggested that this transport might operate in situ and provide a morphological mechanism to explain a type of protein transport noted in the renal tubules of another flounder species by Maack and Kinter ('67). It is interesting to consider that perhaps a similar mechanism for the transport of intact proteins might also operate in mammalian nephrons as well.     

FLUID TRANSPORT IN THE RABBIT GALLBLADDER : A Combined Physiological and Electron Microscopic Study

The fine structure of the rabbit gallbladder has been studied in specimens whose functional state was undetermined, which were fixed either in situ or directly after removal from the animal; in specimens whose rate of fluid absorption was determined, either in vivo or in vitro, immediately prior to fixation; and in specimens from bladders whose absorptive function was experimentally altered in vitro. Considerable variation was found in the width of the epithelial intercellular spaces in the bladders whose functional state was undefined. In bladders known to be transporting fluid, either in vivo or in vitro, the intercellular spaces were always distended, as were the subepithelial capillaries. This distension was greatest in bladders which had been functioning in vitro. When either Na⁺ or Cl^- was omitted from the bathing media, there was no fluid transport across the wall of the gallbladder studied in vitro. The epithelial intercellular spaces of biopsies taken from several bladders under these conditions were of approximately 200 A width except for minor distension at the crests of mucosal folds. The addition of the missing ion rapidly led to the reestablishment of fluid transport and the distension of the intercellular spaces throughout most of the epithelium of these bladders. Studies of sodium localization (by fixation with a pyroantimonate-OsO₄ mixture) showed high concentrations of this ion in the distended intercellular spaces. Histochemical studies of ATPase activity showed that this enzyme was localized along the lateral plasma membrane of the epithelial cells. The analogy is drawn between the structure of the gallbladder mucosa and a serial membrane model proposed by Curran to account for coupled solute-solvent transport across epithelia. It is concluded that the intercellular compartment fulfills the conditions for the middle compartment of the Curran model and that active transport of solute across the lateral plasma membrane into the intercellular space may be responsible for fluid absorption by the gall bladder.     

Histogenesis of Swiss white mouse secondary palate from nine and one-half days to fifteen and one-half days In utero. I. Epithelial-mesenchymal relationships—light and electron microscopy

Development of the secondary palate in Swiss white mouse embroyos was studied from age nine-and-one-half days in utero to the stage of mesenchymal coalescence in the secondary palate (approximately fifteen-and-one-half days). The greatest changes observed occur in the mesenchyme. At early stages, mesenchymal cells underlying oral ectoderm of the head are few and only occasionally contact the ectoderm. Electron micrographs show large intercellular spaces between the ectodermal cells. As embryogenesis continues, the mesenchymal cells become more numerous, closer to each other and closer to the epithelium. Just prior to horizontal transposition of shelves, the mesenchymal cells spread farther from each other and from the palatal epithelium and epithelium of the palatal tip becomes stretched. Ultrastructurally the intercellular spaces between epithelial cells of the palate tip have become much smaller. Some mitochondria in some epithelial cells are swollen and have clear matrices and distorted cristae. The shelves become horizontal and meet in the midpalate. Cells with degeneration bodies are seen in the epithelial seam. The seam undergoes autolysis and is replaced by mesenchyme. The morphological changes described, particularly in the mesenchyme, may play an important role in determining the effect of various teratogens at different stages of palatal development. The changes in both mesenchyme and epithelial cells in the later stages may constitute part of the process of preparing shelves for fusion as postulated by Pourtois ('66).     

Vesicular transport of cationized ferritin by the epithelium of the rat choroid plexus

We have studied the transport of ferritin that was internalized by coated micropinocytic vesicles at the apical surface of the choroid plexus epithelium in situ. After ventriculocisternal perfusion of native ferritin (NF) or cationized ferritin (CF), three routes followed by the tracers are revealed: (a) to lysosomes, (b) to cisternal compartments, and (c) to the basolateral cell surface. (a) NF is micropinocytosed to a very limited degree and appears in a few lysosomal elements whereas CF is taken up in large amounts and can be followed, via endocytic vacuoles and light multivesicular bodies, to dark multivesicular bodies and dense bodies. (b) Occasionally, CF particles are found in cisterns that may represent GERL or trans-Golgi elements, whereas stacked Golgi cisterns never contain CF. (c) Transepithelial vesicular transport of CF is distinctly revealed. The intercellular spaces of the epithelium, below the apical tight junctions, contain numerous clusters of CF particles, often associated with surface-connected, coated vesicles. Vesicles in the process of exocytosis of CF are also present at the basal epithelial surface, whereas connective tissue elements below the epithelium are unlabeled. Our conclusion is that fluid and solutes removed from the cerebrospinal fluid by endocytosis either become sequestered in the lysosomal apparatus of the choroidal epithelium or are transported to the basolateral surface. However, our results do not indicate any significant recycling via Golgi complexes of internalized apical cell membrane.     

A comparative study of the desquamation of urothelial cells during gestation and in adults mice following moderate stress or endotoxin treatment

The present paper deals with the comparative EM study of the detachment of mouse urinary bladder epithelial cells under various physiological conditions, namely, during gestation and in adult mice following induction by endotoxin or by exposure to moderate stress. It has been shown that desquamation during gestation involves two distinct modes, shedding of single cells and formation of apoptotic bodies. Moderate stress in adult female mice induced by constant illumination for 72 or 96 hours, results in desquamation of superficial and intermediate cells. Application of LPS is followed by desquamation of single cells and whole sheets of cells. Cell detachment involves interruption of tight junctions between neighbouring cells and formation of numerous cup shaped vesicles, multivesicular bodies and vacuoles at the base of desquamating cells. LPS induces desquamation entails delivery of lysosomal enzymes extracellulary into the intercellular space. In the areas of sloughing the bladder epithelium lack permeability barrier. Desquamated cells in the bladder lumen are mostly alive. These results clearly demonstrate the existence of specific adhesion mechanisms of detachment and desquamation of uroepithelial cells.     

Electron microscopic study of colonic epithelial cells from the grass frog Rana temporaria under different intensity of water absorption

Three cell types have been revealed in the epithelium of the frog large intestine: granular, mitochondria-rich, and mucosal cells. Under a low water permeability (0.12 +/- 0.10 mkl/(min.cm2)) the distribution of intramembrane particles (IMP) in the apical cell membrane was the same as in the most cell plasma membranes studied with freeze-fracture method. Under rising osmotic permeability and water absorption (0.43 +/- 0.05 mkl/(min.cm2)) the IMP distribution did not change. In these conditions, the quantity of fusion sites between granule membranes and the apical membrane increased, and the intercellular spaces in basolateral epithelial region were diluted. A a low water permeability, in addition to usual microtubules, bundles of noncentrosomal microtubules with associated osmiophilic globules were revealed. A comparative analysis has been made of the present evidence and previously obtained data on the frog urinary bladder epithelium.     

STRUCTURE OF THE TOAD'S URINARY BLADDER AS RELATED TO ITS PHYSIOLOGY

The structure of the urinary bladder of the toad Bufo marinus was studied by light and electron microscopy. The epithelium covering the mucosal surface of the bladder is 3 to 10 microns thick and consists of squamous epithelial cells, goblet cells, and a third class of cells containing many mitochondria and possibly representing goblet cells in early stages of their secretory cycle. This epithelium is supported on a lamina propria 30 to several hundred microns thick and containing collagen fibrils, bundles of smooth muscle fibers, and blood vessels. The serosal surface of the bladder is covered by an incomplete mesothelium. The cytoplasm of the squamous epithelial cells, which greatly outnumber the other types of cells, is organized in a way characteristic of epithelial secretory cells. Mitochondria, smooth and rough surfaced endoplasmic reticulum, a Golgi apparatus, "multivesicular bodies," and isolated particles and vesicles are present. Secretion granules are found immediately under the plasma membranes of the free surfaces of the epithelial cells and are seen to fuse with these membranes and release their contents to contribute to a fibrous surface coating found only on the free mucosal surfaces of the cells. Beneath the plasma membranes on these surfaces is an additional, finely granular component. Lateral and basal plasma membranes are heavily plicated and appear ordinary in fine structure. The cells of the epithelium are tightly held together by a terminal bar apparatus and sealed together, with an intervening space of only 0.02 mµ near the bladder lumen, in such a way as to prevent water leakage between the cells. It is demonstrated in in vitro experiments that water traversing the bladder wall passes through the cytoplasm of the epithelial cells and that a vesicle transport mechanism is not involved. In vitro experiments also show that the basal (serosal) surfaces of the epithelial cells are freely permeable to water, while the free (mucosal) surfaces are normally relatively impermeable but become permeable when the serosal surface of the bladder is treated with neurohypophyseal hormones. The permeability barrier found at the mucosal surface may be represented, structurally, either by the filamentous layer lying external to the plasma membrane, by the intracellular, granular component found just under the plasma membrane, or by both of these components of the mucosal surface complex. The polarity of the epithelial sheet is emphasized and related to the physiological role of the urinary bladder in amphibian water balance mechanisms.     

Fine structure of the rectal pads in Grylloblatta compodeiformis (walker) (Orthoptera : Grylloblattidae) with reference to fluid transport

The rectal pads of the primitive insect Grylloblatta compodeiformis (Orthoptera : Grylloblattidae) were studied using light and electron microscopy. In this species, the rectal epithelium is thickened to form 6 prominent rectal pads, each of which is composed of tall columnar epithelial cells and laterally placed slender junctional cells, but is devoid of secondary cells. The rectal pads are interconnected by simple rectal epithelium, and are lined by a thin cuticular intima. They are surrounded by an extensive connective tissue space, which contains bundles of delicate connective tissue fibers, neurosecretory axons, and tracheae and tracheoles, which do not penetrate into the pads. The epithelial cells exhibit extensive infoldings of the apical plasma membranes that are closely associated with mitochondria. The lateral membranes are also highly folded around large mitochondria that possess longitudinally oriented cristae. These membrane folds form mitochondrial-scalariform junctional complexes and enclose intercellular channels and spaces. The apical cytoplasm of the epithelial cells contains numerous coated vesicles, dense tubular elements, multivesicular bodies and lysosomes, which suggests receptor-mediated endocytosis of macromolecules. The presence of large whorls of rough endoplasmic reticulum and abundant free ribosomes in the cytoplasm and nuclei with multiple, well-developed nucleoli indicate that the epithelial cells are actively engaged in protein synthesis. The ultrastructural features were examined in relation to their role in fluid transport in a cold habitat.     

Light and electron microscopic study of the degeneration and early regeneration of olfactory epithelium in the mouse.

Degeneration and early regeneration of olfactory epithelium from two strains of mice was studied at the light and electron microscopic levels from 12 hours to 3 days following nasal irrigation with 1% aqueous solution of zinc sulfate (ZnSO4) (a compound known to selectively damage olfactory epithelium). Distinct patterns of degeneration and stages of regeneration were evident following treatment. During the first 24 hours after treatment three progressive manifestations of the degenerative process were seen: (1) a relatively mild condition which was characterized by surface irregularities produced by cell protrusions, highly vacuolated cytoplasm, presence of large lysosome-like bodies and prominent intercellular spaces, (2) a more severe condition in which large areas of the epithelium were detached from the basement membrane cellular debris was present in the nasal chamber, and (3) a condition of total or near-total denudation of the epithelium of olfactory mucosa. The basal lamina was continuous and intact in most regions and the integrity of the subadjacent connective tissue was mostly well-preserved. Nerve bundles of the fila olfactoria were noted in varying degrees of degeneration during the course of the experiment. The most advanced neural degeneration was seen 24 to 72 hours after treatment. Onset of regeneration was suggested by the appearance of a simple squamous layer of cells above the basement membrane 48 to 72 hours after treatment. In addition to the simple epithelium a stratified epithelium consisting of two to four cell layers was also observed at this time. Glandular cells, containing secretory granules identical to those in Bowman's glandular cells, were noted in an apparent process of migration from the lamina propria into the the stratified epithelial layer. The last mentioned observation supports the proposition that new supportive epithelial cells originate from cells of Bowman's gland.     

THE FINE STRUCTURE OF THE GALL BLADDER EPITHELIUM OF THE MOUSE

Sections of mouse gall bladder epithelium fixed by perfusion with buffered osmium tetroxide have been studied in the electron microscope as an example of simple columnar epithelium. The free surface presents many microvilli, each presenting a dense tip, the capitulum, and displaying a radiating corona of delicate filaments, the antennulae microvillares. Very small pit-like depressions, representing caveolae intracellulares, are encountered along the cell membrane of the microvilli. The free cell surface between microvilli shows larger cave-like depressions, likewise representing caveolae intracellulares, containing a dense material. The lateral cell borders are extensively folded into pleats, which do not interdigitate extensively with corresponding folds of the adjacent cell membrane. The terminal bars are shown to consist of thickened densities of the cell membrane itself in the region of insertion of the lateral cell wall with the free cell surface. This thickening is associated with an accumulation of dense cytoplasmic material in the immediate vicinity. The terminal bar is thus largely a cytoplasmic and cell membrane structure, rather than being primarily intercellular in nature. The basal cell membrane is relatively straight except for a conical eminence near the center of the cell, projecting slightly into the underlying tunica propria. The basal cell membrane itself is overlain by a delicate limiting membrane, which does not follow the lateral contours of the cell. Unmyelinated intercellular nerve terminals with synaptic vesicles have been encountered between the lateral walls of epithelial cells. A division of the gall bladder epithelial cell into five zones according to Ferner has been found to be convenient for this study. The following cytoplasmic components have been noted, and their distribution and appearance described: dense absorption granules, mitochondria, Golgi or agranular membranes, endoplasmic reticulum or ergastoplasm, ring figures, and irregular dense bodies, perhaps lipoid in nature. The nucleus of these cells is also described.     

Ultrastructural modifications of frog urinary bladder epithelium under the influence of hypertonic media

Summary The frog urinary bladder undergoes a marked increase in its water permeability when incubated in hypertonic media. Many similarities are found between this effect and the hydrosmotic action of antidiuretic hormone. The ultrastructural modifications of the epithelium observed under the influence of serosal hypertonicity (the intercellular spaces are dilated while the tight junctions remain closed) lead us to assume that the pathways of water movement across the epithelium could be the same in this case and in hydrosmotic response to the hormone. In contrast, when the mucosal medium is made hypertonic, the ultrastructure is differently altered: the intercellular spaces are closed, the tight junctions show small vesicles and numerous large vacuoles appearing in the cytoplasm.     

SCHIZOGENY AND ULTRASTRUCTURE OF DEVELOPING LATEX DUCTS IN MAMMILLARIA GUERRERONIS (CACTACEAE)

The schizo-lysigenous latex ducts of Mammillaria guerreronis, sect. Subhydrochylus, were examined by optical and electron microscopy. These ducts are complex having a distinct outer epithelium without intercellular spaces and an inner epithelium in which schizogenous spaces arise. Schizogeny begins with formation of bulbous pockets and/or production of dark streaks in certain walls. Spaces formed by these processes ultimately contain a combination of electron dense materials, vesicles, and numerous thin, convoluted wall layers. Schizogeny may be responsible for initial formation of lumen and latex and may also separate some inner epithelial cells from the surrounding layers. Lysigeny of the inner epithelial cells contributes materials to the latex and allows enlargement of the duct. The inner epithelial cells contain mitochondria, dictyosomes, apparently functional nuclei, and numerous vesicles. The outer epithelium has fewer vesicles than the inner epithelium. Schizo-lysigeny in the members of sect. Subhydrochylus is considered to be ancestral to the strict lysigeny of the members of sect. Mammillaria. The inner and outer epithelia of M. guerreronis are thought to be homologous to the lumen and epithelium respectively of M. heyderi.     

Uptake and intracellular transport of cationic ferritin in the bronchiolar and alveolar epithelia of the rat

Summary Cationic ferritin was used as a marker to reveal the processes of endocytosis and intracellular transport in bronchiolar and alveolar epithelia. The marker was injected into the lung via the trachea, and ultrastructural observation of the distribution of ferritin particles in bronchiolar and alveolar epithelial cells was carried out at intervals of 5, 15, 30 and 60 min after the injection. The luminal surface of the airway and the alveolar epithelium showed diffuse labeling with cationic ferritin. In general, ferritin particles were observed in vesicles and vacuoles of the bronchiolar and alveolar epithelial cells within 5 min of injection; they appeared in multivesicular bodies within 15 min. Multivesicular bodies and secondary lysosomes containing ferritin particles, some of which showed a positive reaction for acid phosphatase, were seen in the basal cytoplasm within 30 min; ferritin particles appeared in the basal lamina below the Clara cells, ciliated cells and type 2 alveolar cells within 30 min. Ferritin particles were seen in ovoid granules of some Clara cells and in lamellar inclusion bodies of many type 2 alveolar cells. Brush cells and type 1 alveolar cells took up only a small quantity of ferritin particles.