Regeneration of the cell wall in protoplasts of Candida albicans

To assess the dynamics of synthesis of the wall by regenerating Candida albicans protoplasts deposition of chitin and mannoproteins were investigated ultrastructurally using wheat germ agglutinin conjugated with either horseradish peroxidase or colloidal gold, and Concanavalin A coupled to ferritin respectively.Freshly prepared protoplasts lacked wheat germ agglutinin receptor sites but after 1–2 h of regeneration, they were detected. After 4–5 h of regeneration, the cell wall showed a discrete structure which was only labelled with wheat germ agglutinin in thin sections. At this stage of regeneration the outermost layer of the wall was labelled with clusters of Concanavalin A-ferritin particles.After 8 h regeneration, the cell wall appeared compact, and homogenously marked with wheat germ agglutinin whereas only the surface layers appeared consistently labelled with Concanavalin A-ferritin.From these observations we conclude that C. albicans protoplasts are able to regenerate in liquid medium a cell wall consisting of a network of chitin fibrils and mannoproteins at least (glucan polymers were not determined in the present cytological study). The former are the fundamental component of the inner layers at early stages of regeneration, whereas the latter molecules are predominant in the outer layers of the wall.Abbreviations WGA-HRP wheat germ agglutinin conjugated with horseradish peroxidase - WGA-Au wheat germ agglutinin conjugated with colloidal gold - Con A-ferritin Concanavalin A coupled to ferritin     

The Choroid Plexuses and the Barriers Between the Blood and the Cerebrospinal Fluid

1. The fluid homeostasis of the brain depends both on the endothelial blood–brain barrier and on the epithelial blood–cerebrospinal fluid (CSF) barrier located at the choroid plexuses and the outer arachnoid membrane.2. The brain has two fluid environments: the brain interstitial fluid, which surrounds the neurons and glia, and the CSF, which fills the ventricles and external surfaces of the central nervous system.3. CSF acts as a fluid cushion for the brain and as a drainage route for the waste products of cerebral metabolism.4. Recent findings suggest that CSF may also act as a third circulation conveying substances secreted into the CSF rapidly to many brain regions.     

Ultrastructural localization of ependymins in the endomeninx of the brain of the rainbow trout: possible association with collagen fibrils of the extracellular matrix

In the rainbow trout, ependymins represent the predominant protein constituents of the cerebrospinal fluid (CSF) and perimeningeal fluid (PMF). Synthesis of these glycoproteins occurs exclusively in the endomeninx. Generally, ependymins share characteristics with proteins mediating cell-contact phenomena. Here, we show that the endomeninx of the rainbow trout is composed of three different layers, viz. an outer layer, an arachnoid-like intermediate barrier layer and an inner layer. This structure is in agreement with a meningeal barrier concept separating the PMF from the CSF. Furthermore, by immuno-electron microscopy, we have localized the majority of intracellular ependymins to the rough endoplasmic reticulum of fibroblast-like cells of the inner layer and to cells to the intermediate barrier layer. This pattern is compatible with the observed distribution of ependymins in both the PMF and CSF. In addition to their intracellular localization, an extracellular association of ependymins with bundles of collagen fibrils is demonstrated; this is particularly pronounced around all blood vessels of the brain.     

Barriers to sodium movement across frog skin

Summary The aim of this paper is to obtain information on the number, nature and location of the barriers to Na movement across the frog skin, and on the size and location of the Na-pool that might be contained between these barriers. On the basis that Na penetrates passively across an outer barrier, and is actively extruded across an inner barrier which is impermeable to passive movements of Na, we expected to detect at least the Na-pool of a single cell layer containing some 10^–8 moles per cm² of epithelium (i.e., in a cell layer 5 thick and with 21mm Na). Yet no Na-pool with these characteristics was found. The method employed could have detected a Na-pool at least an order of magnitude smaller than the one expected. It is concluded that either a Na-pool does not exist (except for the Na bound to the mechanisms operating the translocation), or else that the Na-pool is contained between barriers with different characteristics than the ones assumed above. In the first case, Na transportacross the epithelium would consist of a translocation across a single asymmetrical functional barrier. In the second case, the experimental results would require that ouabain either directly (by inhibiting an active step) or indirectly (through a mediated decrease of the Na permeability of the outer barrier) prevents Na penetration at the outer border.     

Breakdown of the blood–brain barrier to proteins in white matter of the developing brain following systemic inflammation

Compromised blood–brain barrier permeability resulting from systemic inflammation has been implicated as a possible cause of brain damage in fetuses and newborns and may underlie white matter damage later in life. Rats at postnatal day (P) 0, P8 and P20 and opossums (Monodelphis domestica) at P15, P20, P35, P50 and P60 and adults of both species were injected intraperitoneally with 0.2–10 mg/kg body weight of 055:B5 lipopolysaccharide. An acute-phase response occurred in all animals. A change in the permeability of the blood–brain barrier to plasma proteins during a restricted period of postnatal development in both species was determined immunocytochemically by the presence of proteins surrounding cerebral blood vessels and in brain parenchyma. Blood vessels in white matter, but not grey matter, became transiently permeable to proteins between 10 and 24 h after lipopolysaccharide injection in P0 and P8 rats and P35–P60 opossums. Brains of Monodelphis younger than P35, rats older than P20 and adults of both species were not affected. Permeability of the blood–cerebrospinal fluid (CSF) barrier to proteins was not affected by systemic inflammation for at least 48 h after intraperitoneal injection of lipopolysaccharide. These results show that there is a restricted period in brain development when the blood–brain barrier, but not the blood–CSF barrier, to proteins is susceptible to systemic inflammation; this does not appear to be attributable to barrier immaturity but to its stage of development and only occurs in white matter.This work was supported by NIH grant number R01 NS043949-01A1.     

The gas bladder of Argentina silus L., with special reference to the ultrastructure of the gas gland cells and the countercurrent vascular bundles

Summary The orifice between the two chambers of the gas bladder in Argentina silus is surrounded by a sphincter muscle. Gas analyses of the gas bladder contents of fish from 400 meters depth give 0–1% carbon dioxide and 9–72% oxygen. Micro-retia mirabilia form a countercurrent vascular system, and the arterial component has peripherally a sphincter mechanism. The function of the glandular layer of the anterior chamber remains uncertain, but the structure indicates secretion into blood capillaries. The lining epithelium of the anterior chamber may secrete some substance into blood or directly into the lumen, which may be involved in a secretory mechanism. This conclusion is not supported by our histochemical tests. The posterior chamber has no micro-retia and the blood vessels have a different origin from those of the anterior chamber. The blood vessels form a plexus of capillaries or sinuses in contact with the flat lining epithelium, thus allowing gases to pass freely by diffusion. — The muscular layers of both chambers are innervated by catecholamine-containing nerve fibres.The investigation was supported by grants from the Swedish Natural Research Council (No. 99-35) and by the Faculty of Mathematics and Science, University of Lund.     

Potential Role of Cerebral Glutathione in the Maintenance of Blood-Brain Barrier Integrity in Rat

Using the model of glutathione (GSH) depletion, possible role of GSH in the maintenance of blood-brain barrier (BBB) integrity was evaluated in rats. Administration (ip) of GSH depletors, diethyl maleate (DEM, 1–4 mmol/kg), phorone (2–3 mmol/kg) and 2-cyclohexene-1-one (CHX, 1 mmol/kg), to male adults was found to deplete brain and liver GSH and increase the BBB permeability to micromolecular tracers (sodium fluorescein and [¹⁴C]sucrose) in a dose-dependent manner at 2h. However, BBB permeability to macromolecular tracers such as horseradish peroxidase and Evan's blue remained unaltered. It was also shown that observed BBB permeability dysfunction was associated with brain GSH depletion. A lower magnitude of BBB increase in rat neonates, as compared to adults, indicated a possible bigger role of GSH in the BBB function of mature brain. The treatment with N-acetylcysteine, methionine and GSH provided a partial to full protection against DEM-induced brain (microvessel) GSH depletion and BBB dysfunction; however, the treatment with -tocopherol, ascorbic acid and turmeric were not effective. Our studies showed that cerebral GSH plays an important role in maintaining the functional BBB integrity.     

Morphogenesis of rat cranial meninges

Summary The meninges of albino Wistar rat embryos, aged between the 11th embryonic day (ED) and birth, were sectioned using a specially constructed device. This technique permits optimal microanatomical preservation of all tissues covering the convexity of the brain: skin, muscle, cartilage or bone, and the meninges. At ED11, the zone situated between the epidermis and the brain is occupied by a mesenchymal network. At ED12, part of this delicate network develops as a dense outer cellular layer, while the remainder retains its reticular appearance, thus forming an inner layer (the future meningeal tissue). At ED13, the dura mater starts to differentiate. At ED14, the bony anlage of the skull can be identified, and along with the proceeding maturation of dura mater some fibrillar structures resembling skeletal muscle fibers appear in the developing arachnoid space. At ED15–17, a primitive interface zone — dura mater/ arachnoid — is formed, comprised by an outer electronlucent and an inner electron-dense layer marking the outer aspect of the arachnoidal space. At ED18–19, the innermost cellular row of the inner durai layer transforms into neurothelium, which is separated from the darker arachnoidal cells by an electron-dense band. The arachnoidal trabecular zone with the leptomeningeal cells is formed at ED19. By the end of the prenatal period (ED20–21), its innermost part organizes into an inner arachnoidal layer and an outer and inner pial layer. The results from this study indicate (i) that dura mater and leptomeninges develop from an embryonic network of connective tissue-forming cells, and (ii) that the formation of cerebrospinal fluid (CSF)-containing spaces accompanies the differentiation of the meningeal cellular layers.     

An electron microscopic investigation of the surface coat of the electrocyte of Electrophorus electricus

Summary The surface coat of the electrocyte of the main electric organ of Electrophorus electricus was studied using cytochemical methods (periodic acid-silver methenamine, periodic acid-chromic acid-silver methenamine, periodic acid-thiosemicarbazide-silver proteinate, Concanavalin A — horseradish peroxidase, ruthenium red, Alcian-blue lanthanum nitrate, colloidal iron hydroxide and cationized ferritin). The surface of the electrocyte presents perpendicularly oriented tubular invaginations of the cell membrane. The fibrous coat 50–100 nm thick, penetrates into the lumen of the invaginations. It is also observed in the synaptic clefts existent in the posterior face of the electrocyte. The coating of the surface membrane gives a positive reaction with all techniques used. Binding of colloidal iron hydroxide particles was observed only in the outer layer of the coat. With the Alcian-blue lanthanum nitrate technique, microtubules were observed in the cytoplasm of the electrocyte.The results indicate that the surface coat of the electrocyte contains mucopolysaccharides, glycoproteins, acid mucopolysaccharides and anionic sites detected at low (colloidal iron hydroxyde) and neutral (cationized ferritin) pH.This work has been supported by Conselho Nacional de Desenvolvimento Cientifico e Tecnológico (CNPq), Conselho de Ensino e Pesquisa da UFRJ (CEPG) and Banco Nacional de Desenvolvimento Econômico     

Barriers in the Immature Brain

1. The term blood–brain barrier describes a range of mechanisms that control the exchange of molecules between the internal environment of the brain and the rest of the body.2. The underlying morphological feature of these barriers is the presence of tight junctions which are present between cerebral endothelial cells and between choroid plexus epithelial cells. These junctions are present in blood vessels in fetal brain and are effective in restricting entry of proteins from blood into brain and cerebrospinal fluid. However, some features of the junctions appear to mature during brain development.3. Although proteins do not penetrate into the extracellular space of the immature brain, they do penetrate into cerebrospinal fluid by a mechanism that is considered in the accompanying review (Dziegielewska et al., 2000).4. In the immature brain there are additional morphological barriers at the interface between cerebrospinal fluid and brain tissue: strap junctions at the inner neuroependymal surface and these and other intercellular membrane specializations at the outer (pia–arachnoid) surface. These barriers disappear later in development and are absent in the adult.5. There is a decline in permeability to low molecular weight lipid-insoluble compounds during brain development which appears to be due mainly to a decrease in the intrinsic permeability of the blood–brain and blood–cerebrospinal fluid interfaces.     

Ultrastructure of the osphradium of Aplysia californica

Summary The osphradium of Aplysia californica, a sensory organ, is a small yellow-brown epithelial patch located in the mantle cavity immediately anterior to the rostral attachment of the gill. Scanning electron microscopy reveals a round ellipsoid structure of 0.6–1 mm in diameter with a central, occasionally folded, sensory epithelium. The central area is covered with microvilli and surrounded by a densely ciliated epithelium. Transmission electron micrographs show that the columnar supporting cells in the sensory epithelium contain an abundance of apical pigment granules and microvilli. Between the epithelial-supporting cells, the putative sensory elements consist of thin neurites (0.4–1.5 m in diameter) that reach the sea-water side of the osphradium. The neurites contain many neurotubules, mitochondria, vesicles and cilia in their apices. The nerve endings originate from cell bodies up to 40 m below the epithelium or in the osphradial ganglion itself, as revealed by electron microscopy and retrograde labeling with Lucifer yellow. There appear to be two populations of putative sensory cells, a large population of heavily stained cell bodies 4–10 m in diameter and a few scattered cells of large diameter (25–60 m). Following lanthanum impregnation, septate junctions can be seen between all types of cells in the epithelium, 3–5 m below the sea-water surface. This study provides new information for further investigation of osmo- and mechanosensation in Aplysia californica.     

Compartments in the organum vasculosum laminae terminalis of the rat and their delineation against the outer cerebrospinal fluid-containing space

Summary Using intravenously injected horseradish peroxidase (HRP) as tracer, we demonstrate, that — in contrast to other neurohemal regions — the organum vasculosum laminae terminalis (OVLT) is composed of two functionally different divisions. Both parts of the OVLT are endowed with fenestrated capillaries which, however, obviously differ in their permeability for HRP. In one of these portions the neurohemal region remains unlabeled under the experimental conditions used, while the other portion, in analogy to the majority of neurohemal regions, is labeled by the tracer. The functionally different divisions of the OVLT are separated from one another by tanycytic processes and meningeal cells establishing a barrier between the two hemal compartments. The meningeal elements penetrate the organ in the form of an uninterrupted layer; they are continuous with the pia mater and produce large amounts of basal lamina-like material. Furthermore, they provide the delineation of the OVLT against the outer cerebrospinal fluid-containing compartment, a structural feature that is characteristic of both divisions of the OVLT and corresponds to the arrangement of meninges in all other portions of the brain where a blood vessel penetrates its surface.Supported by the Deutsche Forschungsgemeinschaft (Grant Nr. Kr 569/5-2) and the Stiftung Volkswagenwerk     

Architecture of the cell wall of a green alga,Oocystis apiculata

Summary The cell wall of a green alga,Oocystis apiculata, was visualized by electron microscopy after preparation of samples by rapid-freezing and deep-etching techniques. The extracellular spaces clearly showed a random network of dense fibrils of approximately 6.4 nm in diameter. The cell wall was composed of three distinct layers: an outer layer with a smooth appearance and many protuberances on its outermost surface; a middle layer with criss-crossed cellulose microfibrils of approximately 15–17 nm in diameter; and an inner layer with many pores between anastomosing fibers of 8–10 nm in diameter. Both the outer and the inner layer seemed to be composed of amorphous material. Cross-bridges of approximately 4.2 nm in diameter were visualized between adjacent microfibrils by the same techniques. The cross-bridges were easily distinguished from cellulose microfibrils by differences in their dimensions.     

Über den Peroxydasetransport im Darmepithel der Ratte

Zusammenfassung Bei Ratten tritt 3 min nach intravenöser Injektion von Peroxydase elektronenmikroskopisch ein entsprechendes Reaktionsprodukt im Kapillarlumen der Lamina propria des Dünndarms und an der Basalmembrangrenze der Saumepithelzellen auf. 5 min nach der Injektion finden sich im basalen Abschnitt des Darmepithels pinozytotische Bläschen mit dem Peroxydase-Reaktionsprodukt. — 10–30 min nach der Injektion erreichen die Partikel die apikalen Teile der Zelle. Sie dringen in den interzellulären Spalten bis zu den Haftplatten vor, erreichen jedoch nie das Darmlumen. Im Dünndarm existiert vermutlich auch ein der Resorption entgegengesetzter Saftstrom, der durch Peroxydase markiert werden kann.

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The transport of horseradish peroxidase in the epithelium of the small intestine

Summary In rats, 3 minutes after intravenous injection of peroxidase the reaction product can be observed electronmicroscopically in the lumina of the capillaries of the small intestine as well as at the border of the basement membrane of the epithelial border cells. Pinocytotic vesicles containing peroxidase particles occur in the basal portion of the epithelium of the small intestine 5 minutes after injection. 10–30 minutes later, the peroxidase reaches the apical region of the cell. The particles infiltrate into the intercellular spaces as far as the tight junctions but never reach the intestinal lumen. In the small intestine there probably exists a flow of fluid in opposite direction to the resorption, which can be marked by peroxidase.

     

The Nature and Composition of the Internal Environment of the Developing Brain

1. The fetal brain develops within its own environment, which is protected from free exchange of most molecules among its extracellular fluid, blood plasma, and cerebrospinal fluid (CSF) by a set of mechanisms described collectively as brain barriers.2. There are high concentrations of proteins in fetal CSF, which are due not to immaturity of the blood–CSF barrier (tight junctions between the epithelial cells of the choroid plexus), but to a specialized transcellular mechanism that specifically transfers some proteins across choroid plexus epithelial cells in the immature brain.3. The proteins in CSF are excluded from the extracellular fluid of the immature brain by the presence of barriers at the CSF–brain interfaces on the inner and outer surfaces of the immature brain. These barriers are not present in the adult.4. Some plasma proteins are present within the cells of the developing brain. Their presence may be explained by a combination of specific uptake from the CSF and synthesis in situ. 5. Information about the composition of the CSF (electrolytes as well as proteins) in the developing brain is of importance for the culture conditions used for experiments with fetal brain tissue in vitro, as neurons in the developing brain are exposed to relatively high concentrations of proteins only when they have cell surface membrane contact with CSF.6. The developmental importance of high protein concentrations in CSF of the immature brain is not understood but may be involved in providing the physical force (colloid osmotic pressure) for expansion of the cerebral ventricles during brain development, as well as possibly having nutritive and specific cell development functions.     

Sensory cells in the head skin of pond snails

Summary Several types of receptor endings were identified with scanning electron microscopy and silver-impregnation techniques in the skin of the tentacles, lips, dorsal surface of the head and mouth region of the pond snails Lymnaea stagnalis and Vivipara viviparus. Sensory endings at the tips of dendrites of primary receptor neurones, scattered below the epithelium, differ in structure, i.e., the endings exposed to the surface of the skin possess different proportions of cilia and microvilli, which vary in number, length, and packing. Type-I endings have microvilli and a few (1–5) cilia, 5–12 m in length. Type-2 endings have abundant (20–40), interwoven long (9–12 m) cilia and random microvilli. Type-3 endings show typical packing of 10–25 cilia in the form of bundles or brushes. They may be composed either of long (9–18 m) or short (2–7 m) cilia, or of both long and short ones. Microvilli here are absent. Type-4 endings have only microvilli. Two other types of skin receptors do not extend their sensory endings to the surface and can be indentified only in silver-stained preparations. Type-5 endings are branching dendrites of skin receptors cells that terminate among epithelial cells. In type-6, the sensory endings also terminate among epithelial cells but their cell bodies are located outside of the skin. In both species all skin regions examined possess the receptors of all six types differing only in their relative proportion. Possible functional roles of different receptors are discussed.     

Ultrastructural observations on the lung of the emperor penguin (Aptenodytes forsten)

Summary Of all avian species the emperor penguin is the best adapted bird to attain the greatest diving depths and diving durations. Therefore the lung of this bird was investigated with electron-microscopic, i.e., freeze-fracture and thin-section methods. The parabronchi are surrounded by bundles of smooth muscle cells innervated by varicosities of autonomic nerves. The parabronchial epithelium is flat, bears a few microvilli and does not show any conspicuous ultrastructural specializations; only individual cells contain secretory granules. The atrial epithelial cells bear apical microvilli and are interconnected by adhering and tight junctions (5–10 sealing strands), the latter presumably forming an effective barrier against paracellular fluid movements. The cells contain lamellar inclusions of two types: (i) round membrane-bounded granules, the lamellar content of which is fixation-labile, and (ii) large polymorphic compact deposits of well-preserved lamellae. In both types of inclusions the individual lamellae can be of trilaminar appearance, whereas their fracture faces are smooth. Lamellar material also covers the epithelium of atria, infundibula and air capillaries. In thin areas the diameter of the morphological blood-air barrier measures 220–330 nm. Usually the endothelium of the blood capillaries is thicker (40–180 nm) than the air capillary epithelium (25–150 nm). Both epithelium and endothelium are interconnected by tight junctions, which seem to be more extensive and presumably tighter in the epithelium than in the endothelium. Frequently the common basal lamina is the thickest individual component of the blood-air barrier, measuring between 170–230 nm. Often collagen fibrils occur in this area of the barrier. In comparison with that of other birds the entire blood-air barrier of the emperor penguin is relatively thick, probably owing to an adaptation of the lung tissue which must resist high hydrostatic pressure during diving excursions.     

Versatility of anti-horseradish peroxidase antibody-gold complexes for cytochemistry and in situ hybridization: preparation and application of soluble complexes with streptavidin-peroxidase conjugates and biotinylated antibodies

Summary In previous studies we have employed a gold-labelled, affinity-purified polyclonal antibody against horseradish peroxidase (anti-HRP — gold) in the avidinbiotin peroxidase complex (ABC) technique and indirect labelled avidin-biotin methods. The gold-labelled antibody was used as final revealing reagent to replace the 3,3-diaminobenzidine (DAB) reaction by immunogold silver staining. The anti-HRP — gold reagent proved to be advantageous since blocking of endogenous peroxidase activity in the tissue sections was not further required and staining of superior contrast and resolution could be achieved in paraffin sections. In the present study we have optimized this technique by combining the last two incubation steps, i.e. HRP-conjugated streptavidin and anti-HRP — gold. Different ratios of the two reagents were tested empirically to establish the conditions for the formation of a soluble complex with optimal staining properties. Quantitative evaluation by densitometry of the staining intensity showed that the soluble streptavidin-HRP/anti-HRP — gold complex and the indirect labelled avidin-biotin method employing the gold-labelled anti-HRP antibody performed equally well. Thus, the availability of this complex simplifies the streptavidin-biotin immunogold technique for immunohistochemistry, lectin histochemistry and in situ hybridization and further demonstrates the versatility of anti-HRP — gold complexes.     

Macromolecules can pass through occluding junctions of rat ileal epithelium during cholinergic stimulation

Summary Crypt, but not villus, goblet cells in the ileum accelerate their secretion of mucus within 5 min following cholinergic stimulation. This study was done to determine whether the macromolecular permeability and structure of occluding junctions in the ileum are altered during accelerated secretion. Rats were injected intravenously with horseradish peroxidase followed by carbachol (250 g/kg, subcutaneous) and the intestinal mucosa was fixed 3–12 min later. In control mucosa (saline-injected), peroxidase filled lateral intercellular spaces up to the occluding junctions of both crypt and villus epithelium, but did not enter occluding junctions or pass into the lumen. In 3 of 8 carbachol-stimulated rats, peroxidase was present within occluding junctions in crypt epithelium and in the crypt lumen, although all intermembrane junctional fusion sites appeared intact. Villus epithelial occluding junctions, in contrast, continued to exclude peroxidase. In freeze-fracture replicas of crypt cells prepared after carbachol stimulation, we detected no structural changes in strand networks of occluding junctions that could account for increased paracellular permeability.     

The permeability of nonhuman primate vaginal epithelium: a freeze-fracture and tracer-perfusion study

The lining of the vaginal mucosa in primates is a keratinized stratified squamous epithelium. As in other structurally similar epithelia, one function of the vaginal epithelium is to provide a barrier between the external environment and the underlying tissues. The vaginal lining is aglandular and the source of true vaginal fluid has been suggested to be the intercellular channels of the epithelium. On the other hand, other structurally similar epithelia have been shown to have a barrier to the movement of water-soluble molecules through these channels. In the present study, we have examined the permeability of rhesus monkey vaginal epithelium to lanthanum and horseradish peroxidase. Both tracer molecules penetrated the intercellular channels in the lower layers of the epithelium, but were excluded from the channels at and above the granular layer. Neither tracer penetrated significantly between cells at the free surface of the epithelium and usually did not penetrate between cells in the upper layers to any degree from the cut edges of the biopsy. These results are consistent with tracer studies in other structurally similar epithelia and strongly suggest that the upper layers of vaginal epithelium present a barrier to the movement of water-soluble molecules through the intercellular channel system. Freeze-fracture analysis of the epithelium revealed gap junctions and desmosomes between cells in the lower layers, but the former disappear in the upper layers. Unlike other keratinizing epithelia that have been described, random intramembranous particles do not disappear from the plasma membranes of the fully differentiated cells. Fracture planes through the upper layers reveal particle-free lamellae in the intercellular spaces, supporting the idea that intercellular lipids may be one of the components that limits the permeability of the intercellular spaces in this epithelium.