Autoradiographical and morphological evidence for apocrine secretion of dipalmitoyl lecithin in the terminal bronchiole of mouse lung

Surfattante polmonare

La respirazione normale include circa 12 atti respiratori al minuto, ognuno dei quali di circa 0,500 l. Pertanto il volume di aria che transita nei polmoni al minuto (ventilazione minuto) è di circa: 12 × 0,500 = 6 l min. A sua volta, il volume inspirato è uguale all’incirca a quello espirato (volume corrente). Poiché, in tal caso, il flusso inspiratorio è uguale a − 0,5 l sec⁻¹ e quello espiratorio a + 0.5 l sec⁻¹, il flusso definitivo di un ciclo completo è pari a zero.

Comparative ultrastructure of Clara cells in neonatal and older cattle

Calf lungs were fixed with glutaraldehyde and examined by scanning (SEM) and transmission (TEM) electron microscopy to compare the ultrastructure of Clara cells in terminal bronchioles of neonatal calves and older cattle. In the neonatal calf, SEM revealed numerous smooth-surfaced Clara cells protruding above a similar number of ciliated cells, whereas in older animals the surface of Clara cells was lobulated. Thin sections examined by TEM revealed numerous cuboidal to columnar Clara cells with indented nuclei and a pale cytoplasm filled with faintly granular glycogen in the neonatal calf. Some cells were characterized by apical dense and/or pale membrane-bound granules or secretory droplets. Many cells had an apical tubular network of cisternae that were partly smooth and partly decorated with ribosomes. Ultrastructural comparison of Clara cells in a 2-day-old calf with those of 14- and 19-day-old, 4- and 5. 5-month-old, and 3.5-year-old cattle revealed a striking reduction in the amount of glycogen per cell after 14 days. The number of cells with apical granules was small at all ages, and the density of the secretory granules varied greatly in different cells. A variable amount of smooth endoplasmic reticulum (SER) was present but was less prominent than cisternae of ribosomal endoplasmic reticulum (RER). In older cattle, the limited amount of SER compared to the RER and secretory granules suggests that bovine Clara cells are more likely to be secretory than detoxifying.

CCSP deficiency does not alter surfactant homeostasis during adenoviral infection

Clara cell secretory protein (CCSP) deficiency in mice is associated with increased susceptibility to pulmonary inflammation after hyperoxia or viral infection. Because adenoviral exposure perturbs pulmonary surfactant homeostasis in vivo, we hypothesized that CCSP deficiency would influence surfactant metabolism after pulmonary infection. Alveolar and total lung saturated phosphatidylcholine pool sizes were similar in CCSP-deficient [CCSP(-/-)] and wild-type [CCSP(+/+)] mice before and 7 days after intratracheal administration of adenovirus. Radiolabeled choline and palmitate incorporation into saturated phosphatidylcholine was similar, and there was no alteration by previous infection 7 days before the incorporation measurements. Furthermore, CCSP deficiency did not influence clearance of [(14)C]dipalmitoylphosphatidylcholine and (125)I-labeled recombinant surfactant protein C. Increased persistence of alveolar capillary leak was observed in CCSP(-/-) mice after adenoviral infection. Surfactant lipid homeostasis was not influenced by CCSP before or after administration of adenovirus to the lung. Persistence of alveolar capillary leak in CCSP(-/-) mice after adenovirus provides further evidence for the role of CCSP in the regulation of pulmonary inflammation.

An ultrastructural study of pulmonary bronchiolar and alveolar epithelium in sheep

The cellular population lining the bronchiolar and alveolar epithelium of the lower respiratory tract of three clinically healthy, purebred Suffolk lambs of both sexes and 3-4 months old was studied using a transmission-electron microscope. The epithelium of primary and secondary bronchioles consisted of four cell types: basal cells, intermediate cells, ciliated cells, and non-ciliated (Clara) cells, whereas the epithelium of terminal and respiratory bronchioles consisted of only two kind of cells: ciliated and Clara cells. The alveolar wall, including alveolar ducts and saccules, was covered by type I and type II pneumocytes. In addition, the presence of pulmonary intravascular macrophages was a constant in most of the alveolar capillaries. The morphologic characteristics of all these cells are described in detail and discussed.

Ultrastructural localization of carbonyl reductase in mouse lung

The immunocytochemical localization of tetrameric carbonyl reductase in the mouse lung was determined by an electron-microscopical immunogold procedure using monospecific antibodies against the enzyme. The labelling of carbonyl reductase was observed within the mitochondria of the ciliated and non-ciliated cells of the bronchioles and the type II alveolar pneumocytes, and the density of labelling in the non-ciliated cells was higher than those in the other cells. No significant labelling was detected over other compartments of the epithelial cells. The labelling was undetectable in the type I alveolar cells, alveolar macrophages and connective tissue cells of the lung. These results clearly indicate the localization of carbonyl reductase to the mitochondrial matrix of these epithelial cells, of which the non-ciliated bronchiolar cells contained particularly high amounts of the enzyme.

Electron microscopic identification and morphologic preservation of enriched populations of lung cells isolated by laser flow cytometry and cell sorting: a new technique

There is increasing need to verify the identities of cell subpopulations enriched by laser flow cytometry and fluorescence-activated cell sorting (FACS). When cell subpopulations isolated from whole organs or tissues have similar characteristics (e.g., size, granularity, staining), light, phase contrast or fluorescence microscopy may not provide sufficient resolution to identify isolated cells accurately and many flow cytometric parameters (e.g., viability, fluorescence) require the cells to be live at the point of analysis where the cell transects the laser beam. In some studies, cells identified by fluorescence microscopy as a highly enriched subpopulation were found by electron microscopy to contain significant populations of other cell types. A technique, fixation-in-flow (FIF), has been developed to increase ability to correlate morphological and laser analyses of cell subpopulations. Sheath fluid is replaced by fixative, permitting fixation to be initiated immediately after laser beam analysis of live cells. This new procedure yields improved cytoarchitectural preservation of recovered cell subpopulation(s) for evaluation by transmission or scanning electron microscopy. This report presents results from applying the methodology to identify more accurately cell subpopulations of the distal lung, specifically type II pneumocytes, Clara cells and pulmonary macrophages. A modification of this procedure was employed to isolate fibroblast subpopulations from murine lung fibroblasts grown in vitro and the procedure is being used to determine the responses of cultured fibroblasts to other permutations (e.g., X-irradiation, cytokines).

The ultrastructure of epithelial cells of the distal lung

This review has focused on the structural and functional characteristics of those epithelial cells that line the walls of the lower respiratory bronchioles, alveolar ducts, and alveoli. In all, five cells types were considered: Clara cells, types I, II, and III pneumocytes, and alveolar macrophages. In addition, a very brief mention of the structure and influence of the basement membrane in alveolar development and repair was included, as well as a brief review of the role of epithelial cells in response to selected deleterious influences. No attempt was made to extend this review to cover the structure and functions of the epithelial lining of the conducting portions of the respiratory system, or the exciting and expanding complexities and interrelationships of the septal stroma. Since the volume of literature encircling this subject has virtually exploded during the last 15 years, it becomes almost impossible to review all reports. However, attempts were made to be selective in citations. Insofar as future developments are concerned, much remains to be understood concerning (1) the responses of all cell types to cytotoxic influences, including their respective abilities to repair induced damage, (2) cell-cell and cell-extracellular matrix relationships in response to injury, (3) the uniqueness of the basement membrane in the lung in controlling permeability and gaseous exchange, (4) the role(s) of alveolar macrophages in response to injury and their relationships to the septal macrophage population, (5) the aberrations in the respective cell types that can give rise to neoplastic growth, and (6) the role of the immune system in responding to the general defense of the lung. Indeed much has been learned in the past 2 decades, and it is expected that a review of this sort 1 or 2 decades hence will elucidate many of the functions and structural modifications of the lung.

Ion transport by rabbit nonciliated bronchiolar epithelial cells (Clara cells) in culture

The functions of epithelia that line small airways in mammalian lungs are unknown. To gain insight into the role of small-airway epithelia in lung liquid balance, Clara cells were isolated from excised rabbit lungs by enzymatic digestion, enriched by centrifugal elutriation and density centrifugation, and further purified by differential adherence to collagen matrices. The resulting cell population was composed of 85% Clara cells, 3% ciliated cells, and less than 1% macrophages. The remainder of the cells were not definitively identified. The transepithelial potential difference peaked on day 3 in culture. Preparations studied in Ussing flux chambers exhibited a potential difference of 8 mV (apical bath negative), a resistance of 500 ohm X cm2, and an equivalent short-circuit current (ISCeq) of 16 microA/cm2. Inhibition of the Na+/K+-ATPase by ouabain abolished ISCeq. Exposure of the apical surface to amiloride or replacement of Na+ in the apical bathing solution with an impermeant cation (N-methyl-D-glucamine) decreased ISCeq by 66% and 93%, respectively. Neither amiloride in the basolateral bathing solution, nor bumetanide, nor isoproterenol significantly altered basal ISCeq. These findings indicate that Clara cells in culture form polarized monolayers, Clara cells transport Na+ from the apical to the basolateral bathing solution, and the small airways of the rabbit may function in liquid absorption.

Presence of abundant filaments in apical caps of the nonciliated bronchiolar epithelial (Clara) cells

The nonionic detergent Triton X-100 has often been used for the extraction of cytoplasmic materials. We used the detergent in a vascular perfusion medium when preparing rat lung in order to observe the cytoskeleton of the nonciliated bronchiolar epithelial (Clara) cells. To eliminate some cytoplasmic materials selectively and to maintain good fine cell structure simultaneously, the lungs were perfused sequentially with the detergent (0.2% Triton X-100) alone for 2 min, with a mixture of low-concentration (0.1%) glutaraldehyde and detergent (0.2% Triton X-100) for 15 min, and finally with 2.5% glutaraldehyde for 5-10 min. After fixation, the nonciliated bronchiolar epithelial (Clara) cells were observed by scanning and transmission electron microscopy. At the apical region of the cells, there were central cytoplasmic protuberances (apical caps) filled with microfilaments. These filaments were bound at one end to the cytoplasmic side of the cell membrane and ran into the interior of the cytoplasm at the other end. As a control, the Clara cells were observed by transmission electron microscopy after perfusion with 2.5% glutaraldehyde solution. The luminal surfaces of the cells were covered with short, thick microvilli. The apical caps also had microvillus-like protrusions. These results suggest that the apical cap is not an apocrine droplet but rather is a stable structure involved in the function of the Clara cells.

Morphologic alteration of mouse Clara cells induced by glycerol: ultrastructural and morphometric studies

In our studies on activation of the Clara cell by biological substances and its relationship to pulmonary carcinogenesis, we found that large doses of glycerol induced drastic morphologic changes selectively in the Clara cell among distal airway epithelial cells in ddY mice. Subcutaneous injection of glycerol (7.2 g/Kg body weight) caused cytoplasmic edema with disruption of endoplasmic reticulum membranes at 1 and 3 hours, followed by hyperplasia of smooth endoplasmic reticulum (SER) at 12 and 24 hours. Concentric lamination of SER was observed at 48 and 96 hours. Oral administration of 5% glycerol in drinking water for 2 to 8 weeks induced more conspicuous hyperplasia and hypertrophy of SER in the Clara cells. Electron microscopic morphometry revealed a 3-fold increase in the profile area of SER in the Clara cells of the animals at 2 and 8 weeks. Both the profile area and the number of secretory granules increased significantly at 2 and 8 weeks, and those of mitochondria tended to increase with time of glycerol treatment. In both experiments, the mitochondria of the Clara cells exhibited marked elongation and distortion of the contour associated with appearance of prominent cristae. These results suggest that large doses of glycerol induce marked alteration in the functional activity of the mouse Clara cell.

Biosynthesis and release of proteins by isolated pulmonary Clara cells

The major proteins synthesized and released by Clara cells were identified and compared with those synthesized and released by mixed lung cells. Highly purified Clara cells (85.9 +/- 2.4%) and mixed lung cells (Clara cells 4%, Type II cells 33%, granulocytes 18%, macrophages 2.7%, ciliated cells 1.2%) were isolated from rabbit lungs, incubated with Ham's F12 medium in collagen/fibronectin-coated plastic culture dishes in the presence of 35S-methionine for periods of 4 and 18 hrs. Radiolabelled proteins were isolated from the cells and from the culture medium, electrophoresed on polyacrylamide gels in the presence of SDS under reducing conditions, and then autoradiographed. After 4 and 18 hr of incubation of the Clara cells the major radiolabelled cell-associated proteins were those with molecular weights of 6, 48, and 180 Kd. The major radiolabelled proteins released by Clara cells into the medium after 4 hrs of incubation had molecular weights of 6, 48, and 180 Kd, accounting for 42, 16, and 10%, respectively, of the total extracellular protein-associated radioactivity. After 18 hr of incubation the 6 and 48 Kd proteins represented 30 and 18% of the total released radioactivity, and the relative amount of the 180 Kd protein had decreased to 3%. With the mixed lung cells, the major proteins released into the medium had molecular weights of 6 and 48 Kd. Under nonreducing conditions the 6 Kd protein released by Clara cells had an apparent molecular weight of 12 Kd. Labelling isolated Clara cells with a mixture of 14C-amino acids also identified this low molecular weight protein as the major secretory product of the Clara cell. The 6 Kd protein did not label when the cells were incubated with 14C-glucosamine indicating that it was not a glycoprotein. These data demonstrate the release of several proteins from isolated Clara cells but the major protein had a molecular weight of 6 Kd.

The Clara cell and pulmonary surfactant: a study using selective chemical ablation

Administration of 3-hydroxymethylfuran-N-ethylcarbamate (HFC) to female hamsters via the jugular vein under pentobarbitone anaesthetic at 20 mg per kg body weight produced pronounced necrosis of the Clara cells without apparent morphological effect on other cell types as judged by transmission electron microscope examination. The surfactant material recoverable by minimal lavage followed by purification by sucrose gradient ultracentrifugation increased, reaching a maximum around 48 h after treatment. At this time static pressure/volume measurements on isolated lungs indicated an increase in airway surface compliance. Lavageable surfactant phospholipid composition was examined by 31P nuclear magnetic resonance (n.m.r.). The distribution of phospholipids between the various classes was unchanged by HFC treatment. No change in the total lung surfactant pool size was seen. These results are discussed in relation to the possible roles of the Clara cell in influencing airway surfactant levels.

Applications of electron microscopy to diagnostic pulmonary pathology

Viruses and other possible causative agents should be sought light and electron microscopically in all cases of ill-defined diseases including "sarcoid." Ideally, tissue should be prepared for electron microscopic examination as soon as a specimen is obtained; however, when this has not been done, tissue preserved in formalin solution can be used. Viruses, some bacteria, and other agents suspected on the basis of light microscopic findings can be verified electron microscopically by reprocessing paraffin-embedded tissue from areas that show smudge cells, focal necrosis with atypical cellular proliferation, and nuclear inclusions. Electron microscopically, all dying cells show swelling and rupture of cellular organelles and membranes; reactive changes include proliferation of branching tubules and paracrystalline and other types of proteinaceous precipitates (inclusions) in both the nucleus and cytoplasm. Qualitative and quantitative changes of cellular organelles, fibrils, microvilli, and intercellular junctions reflect hyperplasia, metaplasia, or dysplasia of the cell and may enable identification of the diseases, e.g., desquamative interstitial pneumonia. In various conditions, basal laminae become irregular, disruptive, or reduplicated following epithelial necrosis and regeneration. Electron microscopic evidence of immunologic damage to basal lamina and cells and immuno-electron-microscopic features of the lung in general require further studies. Electron microscopic features of transbronchial biopsy specimens may be diagnostic in cases of alveolar proteinosis, histiocytosis X, and amyloidosis. Ultrastructural abnormalities of cilia are common; primary ciliary defects are rare. Finally, light microscopic, scanning electron microscopic, and x-ray energy-dispersive spectrometric examinations of paraffin-embedded sections appear most practical for the pathologic evaluation of cases of pneumoconiosis.

Cytodifferentiation of the nonciliated bronchiolar epithelial (Clara) cell during rabbit lung maturation: an ultrastructural and morphometric study

The nonciliated bronchiolar epithelial (Clara) cell of adult lung is commonly defined by two cellular components: abundant agranular endoplasmic reticulum (AER) and electron-dense ovoid secretory granules. These reflect the Clara cell's proposed functions as the source of bronchiolar surface secretions and the site of xenobiotic metabolism via the cytochrome P-450 monooxygenase system. Since previous studies have indicated that Clara cells may not attain a fully functional state until some weeks after birth, the present study was undertaken to characterize systematically the differentiation of this cell type during lung maturation. Lungs were fixed by airway infusion with glutaraldehyde/paraformaldehyde (550 mOsm, pH 7.4) from at least three male rabbits at each of the following ages: 24, 27, and 30 days fetal, and 0-1 day, 3-4 days, 1, 2, 3, 4, 5, 8, 12, 15, 17, and 25 weeks postnatal; and pieces were processed for transmission electron microscopy by a selective embedding procedure. Quantitation was performed on electron micrographs (at 15,750 X) of cell profiles, which included the base, apex, and nucleus. Volume fractions of constituents of a minimum of 30 cells per animal (8 weeks and younger) and 10 per animal in older groups, were estimated by point counting with a Weibel 168-point test grid. Cell and nuclear size were estimated with a computerized digitizer (Zeiss Videoplan). Nonciliated cells of prenatal animals had large amounts of cytoplasmic glycogen (over 60% of the cell cytoplasm), few mitochondria (less than 15%), little granular endoplasmic reticulum (GER) (20%), minimal AER (less than 5%), and no granules. Postnatal animals 2 weeks of age and younger were similar, except for the presence of secretory granules and slightly more abundant AER (5 to 20%). By 4 weeks postnatal age, nonciliated cells resembled that of older animals with abundant apical AER (over 40%), secretory granules, little glycogen (11%), and GER (10%). We concluded that (1) the Clara cell is immature at birth; (2) differentiation occurs primarily during weeks 3 and 4 of postnatal life; (3) vast amounts of cytoplasmic glycogen are characteristic of the undifferentiated cell; and (4) four cellular constituents, AER, glycogen, mitochondria, and GER, undergo significant shifts in abundance during differentiation. These shifts appear to be in the sequence expected of a cell type undergoing the initiation of biosynthesis of secretory products and biogenesis of agranular endoplasmic reticulum.

Estimation of cell numbers and volumes of bronchiolar epithelium during rabbit lung maturation

To estimate the numbers and volumes of bronchiolar epithelial cells during lung maturation, we examined rabbits at three time points, 30 days gestation and 4 and 17 weeks postnatal age. Morphometric measures (mean caliper diameter, surface area, and volume) of nonciliated and ciliated bronchiolar cell nuclei, using computer modeling from serial sections, showed a significant decrease in nuclear size for both cell types and a significant increase in cell volume for the nonciliated bronchiolar cell during lung maturation. A shape coefficient (beta) proved to be the most efficient estimator of the number of cells per unit volume when it was used with estimates of the number of nuclei per unit area and the volumetric density of nuclei. Two-dimensional estimates of bronchiolar epithelial cell abundance (the number of nuclei per unit length or area) significantly underestimated the percentage of nonciliated bronchiolar cells as compared to three-dimensional estimates for rabbits 17 weeks of age. We have shown an inverse relationship between nonciliated and ciliated bronchiolar cell abundance during lung maturation. Nonciliated cells decreased while ciliated cells increased. We have confirmed that cytodifferentiation of the nonciliated bronchiolar cell occurs within the first 4 weeks of postnatal development. The volume of the nonciliated bronchiolar cell increased about twofold during development. Because of the concomitant decrease in nuclear volume, the cytoplasm of the cell showed an even greater increase in volume. Within the cytoplasm of the nonciliated bronchiolar cell, glycogen significantly decreased, and agranular endoplasmic reticulum (AER) and mitochondria significantly increased in volume during development. The biosynthesis of AER closely correlated with pharmacological studies of xenobiotic metabolism during rabbit lung maturation.

The Clara cell: an electron microscopy examination of the terminal bronchioles of rats and monkeys following inhalation of hexachlorocyclopentadiene

The effects of inhalation of hexachlorocyclopentadiene (Hex) up to 14 wk on the terminal bronchioles of rats and monkeys was examined by electronic microscopy. Exposed rats elicited a concentration-related increase in the incidence of electron-lucent inclusions in the bronchiolar Clara cells when compared to controls. The inclusions in the high-concentration (0.2 ppm) group were round and more abundant than the rod-shaped inclusions observed in the intermediate (0.05 ppm) and low-concentration (0.01 ppm) exposure group. No ultrastructural changes were observed that could be attributed to the inhalation of Hex vapor in exposed monkeys. The origin and significance of these inclusions is discussed in light of the literature.

1,1-Dichloroethylene-induced pulmonary injury

Oral administration of 1,1-dichloroethylene (1,1-DCE) produces acute injury to the lungs of C57Bl/6 mice. The bronchiolar epithelium is most severely affected with damage selective for Clara cells. After a 100 mg kg/dose of 1.1-DCE. Clara cells show extensive dilatation of cisternae and degeneration of the endoplasmic reticulum. At 6 hr after administration of 200 mg 1,1-DCE/kg, both ciliated and Clara cells are necrotic, and bronchiolar epithelial lining exfoliates. By 24 hr. pulmonary edema, hemorrhage, and focal atelectasis are also present. Pulmonary injury, at 24 hr after the high dose, is associated with a significant hypoxia as demonstrated through a decrease in the partial pressure of oxygen in arterial blood. In spite of the severe injury, recovery occurs and airways display an intact epithelial lining with normal parenchymal elements by 7 days.

Filamentary rod-shaped granules in Clara cells of rats

The bronchiolar Clara cells of rats contain characteristic rod-shaped granules always surrounded by a unit membrane. These granules contain thin filaments about 9 to 10 nm in diameter lying in a pale matrix. Our morphological results suggest that the filamentary rod-shaped granules originate from the common, round-to-oval, electron-dense Clara cell granules, as we found different intermediate structures between these two kinds of granules. The electron-dense granules are digested by pepsin, whereas the filamentary rod-shaped granules are apparently not affected. The biochemical nature and the possible function of the filamentary rod-shaped granules are also discussed in relation to the secretory activity of the Clara cells.

Phospholipids of the lung in normal, toxic, and diseased states

The highly pulmonary concentration of 1,2-dipalmitoyl-sn-glycerol-3-phosphorylcholine (dipalmitoyllecithin) and its implication as an important component of lung surfactant have promoted investigation of phospholipid metabolism in the lung. This review will set the contents including recent informations for better understanding of phospholipid metabolism of the lung in normal state (physiological significances of lung phospholipids, characteristics of phospholipids in lung tissue and alveolar washing, biosynthetic pathways of dipalmitoyllecithin, etc.) as well as in toxic states (pulmonary oxygen toxicity, etc.) and in diseased states (idiopathic respiratory distress syndrome, pulmonary alveolar proteinosis, etc.) Since our main concern has been to clarify the most important route for supplying dipalmitoyllecithin, this review will be focused upon the various biosynthetic pathways leading to the formation of different molecular species of lecithin and their potential significance in the normal, toxic, and diseased lungs.

The mode of secretion of the Clara cell in rat bronchiole: a freeze-fracture study

Freeze-fracture was used to study the nonciliated epithelial (Clara) cell of rat bronchiole. En face views of the P face of the luminal cell membrane reveal particle-free bulging and circular depressions at the presumptive sites of the membrane fusion between the cell membrane and secretory granule membrane. Cross fracture views of the cytoplasm reveal the membraneous continuity from the secretory granule to the luminal cell membrane with the decrease of the intramembrane particles. These findings indicate that the mode of secretion of the Clara cell is a merocrine secretion by exocytosis.

Ultrastructure of the nonciliated bronchiolar epithelial (Clara) cell of mammalian lung: I. A comparison of rabbit, guinea pig, rat, hamster, and mouse

The morphologic characteristics have been used to define the nonciliated bronchiolar epithelial: (1) abundance of agranular endoplasmic reticulum (AER) and (2) numerous membrane-bound ovoid granules. To quantitatively and qualitatively assess the ultrastructural homogeneity of this lung cell type among laboratory mammals used in lung research, we examined tissue from adult male rabbits, guinea pigs, rats, hamsters and mice. Following fixation by airway infusion at constant pressure (30 cm H2O), lungs were processed by a selective embedding technique and bronchioles of known anatomic location were examined by electron microscopy. Nonciliated bronchiolar epithelium of all five species contained avoid granules and abundant AER. Granules were most abundant in the rat (11.1 +/- 8.8 per cell) and least in the hamster (4.4 +/- 5.2 per cell). Granules were largest in hamster (0.72 +/- 0.25 micrometer) versus 0.44 micrometer or less in the other species. Granules were of uniform electron density except in the guinea pig. AER spherical mitochondria with few cristae were present only in mice, rabbit, and guinea pig. We concluded that there was considerable interspecies variation in abundance, size, and morphology of the granules, in abundance and distribution of AER; and in mitochondrial morphology.

Ultrastructure of the nonciliated bronchiolar epithelial (Clara) cell of mammalian lung: II. A comparison of horse, steer, sheep, dog, and cat

Two morphologic characteristics have been used to define the nonciliated bronchiolar epithelial cell: abundant agranular endoplasmic reticulum (AER) and membrane-bound avoid granules. To assess the ultrastructural homogeneity of this cell type in the lungs of large domestic mammals used as experimental models in pulmonary research, we evaluated lungs of horse, steer, sheep, dog, and cat. Bronchioles of known anatomic location were examined by electron microscopy following fixation by airway infusion at standard pressure and processing by selective embedding techniques. Nonciliated bronchiolar epithelium of the horse and sheep had numerous avoid granules (averaging above 15 per cell) and abundant AER. Granules were scarce (averaging less than 2 per cell) in steer and dog and absent in cat. AER was minimal in these species compared to horse and sheep. Glycogen was the dominant cytoplasmic feature in steer, dog, and cat, variable in sheep and rare in horse. Large mitochondria with few cristae and densely staining matrix were present only in cat. We concluded that nonciliated bronchiolar cells of horse and sheep were similar in essential features to this cell type in laboratory mammals, having granules and AER in abundance, while those of steer, dog, and cat were not.

Ultrastructure of the nonciliated bronchiolar epithelial (Clara) cell of mammalian lung. III. A study of man with comparison of 15 mammalian species

Two morphologic characteristics have been used to define the nonciliated bronchiolar epithelial cell: (1) abundance of agranular endoplasmic reticulum (AER) and (2) numerous membrane-bound ovoid granules. In this study, we examined lobectomy specimens from three nonsmoking humans: one male (9.5 yr) and two females (62 and 43 yr) for comparison with lung specimens from mammalian species used as experimental models in lung research. Following fixation by airway infusion at constant pressure (20 cm), lung tissue was processed by a selective embedding technique and bronchioles of known anatomic location were studied by electron microscopy. Nonciliated bronchiolar epithelial cells of man contained numerous membrane-bound granules (averaging 6 per cell) in the apical cytoplasm and abundant granular endoplasmic reticulum (GER), AER was not observed. Granules averaged 0.3 micrometer in diameter and contained a fine granular matrix and parallel tubular arrays. After comparing these features in man with those of fifteen other species, we concluded that there is a great deal of interspecies variability in the cellular morphology of the nonciliated epithelial cell of distal conducting airways in mammalian lung. Three categories of this cell were observed: (1) with abundant granules and AER, characteristic of rabbit, guinea pig, rat, hamster, mouse, horse, sheep, and pig; (2) with abundant granules of GER, but no AER, characteristic of man and other primates; and (3) with few granules, little AER, and abundant glycogen, characteristic of steer, dog, cat, and ferret.

The distribution and structure of cells in the tracheal epithelium of the mouse

The tracheal epithelium of the mouse is a single layer of columnar cells resting on a basement membrane. Many of the cell types resemble those of other species. However, goblet cells are rare and ciliated cells occur only in scattered patches. Submucosal glands are absent from all but the highest reaches of the airway. The major proportion of the epithelial cells are non-ciliated. These usually project into the lumen of the trachea. Large amounts of smooth endoplasmic reticulum and many secretory vesicles occur within the cytoplasm. Secretory activity of these cells may be either apocrine or merocrine and these cells may transform into other cell types. It is suggested that these non-ciliated cells are Clara cells and that the mouse tracheal epithelium may make a useful model for the study of this type of cell.

The morbid anatomy of high altitude

The morbid anatomical changes which take place in man and animals exposed to the chronic hypoxia of residence at high altitude are briefly reviewed.

Early bronchiolar damage following paraquat poisoning in mice

Paraquat causes focal intracellular oedema of the terminal bronchiolar epithelial cells and focal subpleural atelectasis with thickening of the interalveolar septa 1 hr after the administration of an LD50. These changes are progressive, and lead to panacinar atelectasis with necrosis plus sloughing of epithelial cells in many terminal bronchioles. Radioactive phosphatidyl choline (PC) is recoverable by lavage within 90 s of the administration of tritiated palmitate, which supports previous suggestions that one source of pulmonary surfactant is rapid secretion by the terminal bronchiole. Paraquat causes a reduction in the relative amounts of radioactive PC that are recoverable from the airways within 90 s of giving tritiated palmitate. A deficiency of pulmonary surfactant of bronchiolar origin is implicated, at least in part, in the pathogenesis of the acute phase of the paraquat lesion in mice.

Embryologic origin of various epithelial cell types in the thyroid gland of the rat

Ventral pharyngeal outpocketings and ultimobranchial outpocketings from the 14-day-old fetus of the Fischer rat were isolated before they fused to form thyroid glands. The outpocketings were implanted into different kidney capsules of adult male Fischer rats, and were allowed to grow for several months. Transplants were then excised and examined by electron microscopy. The ultimobranchial outpocketing gave rise to two types of follicles. One contained ciliated cells, cells with an abundant agranular reticulum, U cells with basal hemidesmosomes, and two types of cells with secretory vesicles. The other contained C cells separated from the follicular lumen by a single flat fiber-containing cell. The ventral outpocketing formed typical thyroid epithelium making up the usual thyroid follicles differing from follicles in the thyroid in situ by the absence of C cells. These follicles were functional as determined by autoradiographic studies with 125I but differed from thyroid follicles in situ with respect to size distribution. The results suggest that (1) in the adult thyroid gland the C cell in the usual follicle is an ultimobranchial contribution and (2) the so-called ultimobranchial follicle is ultimobranchial in origin but that the typical thyroid epithelium in mixed follicles of U cells and typical thyroid epithelium is a ventral contribution. The reason for the absence in the thyroid gland in situ of the variety of other cell types observed in ultimobranchial transplants is unknown.

Apical secretion from taste bud and other epithelial cells in amphibians

Taste buds of the axolotl, Ambystoma mexicanum, contain cells, previously undescribed in this species, which have a long apical process, and are similar to the Type III cells of mammalian taste buds, and to the gustatory cells in fish. In the supporting cells, there is evidence of periodic decapitation, in addition to secretion by exocytosis. Bilaminar fragments, which are leaf-shaped bodies formed of two dense laminae separated by a lucent gap, protrude from the apical microvilli of the supporting cells and are found detached in the extracellular secreted layer. Their form and dimensions suggest that they represent secreted lipo-protein material. Similar bilaminar fragments have been seen, in much smaller numbers, on some other epithelial cells in amphibians, and in fish. A unique case, in which rough endoplasmic reticulum was found in the extracellular layer of the axolotl oral epithelium, is reported; it had apparently been ejected from the cell. It is suggested that the axolotl produces a copious secretion at the taste bud pore, in order to wash the surface, and that the bilaminar fragments represent material aiding this cleansing process. The condition in the axolotl is compared with that in some other species, particularly Rana temporaria.

Secretory cells of the peripheral pulmonary epithelium as targets for toxic agents

The extracellular lining of the pulmonary peripheral airways is of vital importance to the lung. In this report, some aspects of the pulmonary extracellular lining and the epithelial cells believed to be responsible for its formation and secretion have been briefly reviewed. The influence of a number of toxic agents on the extracellular lining either directly or via those cells involved in its formation indicates that the extracellular lining may be important in understanding numerous toxic agent interactions with the lung.

The development of the human type II pneumocyte

Portions of lung from 25 human foetuses between 12 and 26 wk of gestational age were examined for ultrastructural changes during maturation of the epithelial cells. In 10 cases type I pneumocytes and in five cases type II cells were differentiated. The changes associated with the formation of lamellar inclusion bodies in type II cells are described. It is concluded that this is a synthetic process in which protein and lipid assembly are important. Changes in glycogen and participation of dense bodies are striking.

The destruction of type 2 pneumocytes by airborne influenza PR8-A virus; its effect on surfactant and lecithin content of the pneumonic lesions of mice

Influenzal pneumonia has been studied in mice subjected to sublethal doses of airborne PR8-A influenza virus. Electron microscopy revealed that the virus propagated in and at the same time destroyed the ciliated and nonciliated bronchial cells and the types 1 and 2 alveolar pneumocytes. The regenerating bronchial membranes were metaplastic and grew peripherally into the surrounding alveolar ducts and alveoli to form epithelial nodules which caused obstruction and collapse of the involved lobes. The development of the lung lesions was correlated with phospholipid (lecithin) levels in consolidated and unconsolidated infected and normal lungs. As the lungs became more and more consolidated, there was a corresponding and significant decrease in the amount of phospholipid (dipalmitoyl lecithin) compared to the amount of normal or unconsolidated infected tissue. The destruction of the type 2 pneumocytes by the influenza virus and their failure to regenerate is considered to be the reason for the low phospholipid levels in the involved lobes, and thus an important cause of post-influenzal collapse in mice. The above adds additional evidence to the view that the type 2 pneumocytes are a major source of surfactant in mammalian lungs.