Nonciliated Bronchiolar Epithelial Cells Research Articles

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.

Chlorphentermine-induced neonatal and maternal pulmonary phospholipidosis in rats

Pregnant rats were treated on Days 16–20 of pregnancy with 30 mg/kg chlorphentermine hydrochloride (CP) or an equal volume of 0.9% NaCl. Females delivered on Day 21 and neonates were examined 12 hr postpartum. Electron micrographs showed the CP neonates to have lamellar inclusions, characteristic of drug-induced phospholipidosis in Type I and Type II cells, macrophages, and nonciliated bronchiolar epithelial cells of the lung. These inclusions were either unicentric, multicentric, or consisted of whorls of membranous material. The content of total phospholipids was increased in neonatal lung as a result of CP treatment. A decrease in the percentage of total phospholipid present as phosphatidylinositol plus phosphatidylserine and an increase in the content of disaturated phospphatidylcholine were found in neonatal lung as a result of maternal chlorphentermine treatment. In maternal lung, increases were seen in the content of total phospholipids, total phosphatidylcholine, disaturated phosphatidylcholine, and phosphatidylinositol plus phosphatidylserine. A decrease in the percentage of sphingomyelin was also found. The results of this study indicate that following prepartum maternal administration of CP morphological changes are accompanied by biochemical alterations which are consistent with the induction of phospholipidosis as observed in neonatal lung. These biochemical changes were different than those associated with the induction of this disorder in the maternal lung.

Ultrastructure of hamster bronchiolar epithelium in freeze fracture replicas.

The morphology of the plasma membrane of ciliated and nonciliated bronchiolar epithelial cells of the hamster lung was studied using freeze fracture electron microscopy and transmission electron microscopy of thin sections. Tight junction structure of bronchiolar epithelial cells was evaluated with respect to the total number of junctional strands, and the organization of tight junctional fibrils. Tight junctional complexes seen on the surfaces of nonciliated cells had from 4 to 9 junctional strands (mean: 6.0±0.1). Ciliated cells displayed 4 to 10 tight junctional strands (mean: 6.9±0.1). Nonciliated bronchiolar epithelial cell (NC) membranes showed aggregates of 6 nm particles organized into rectilinear arrays of 2 to 30 particles. These particle clusters were seen predominantly at the basal pole of cells in close association with areas of intercellular interdigitation. NC cells also displayed membrane areas of particle clearing and aggregation consistent with intermediate phases of membrane fusion and exocytosis.

Bronchiolar epithelial damage and impairment of pulmonary microsomal monooxygenase activity in mice by naphthalene

Injection of a single dose of naphthalene into C57BL/6J mice (225 mg/kg, ip) produced a significant (30–70%) and prolonged (8–15 days) impairment in pulmonary microsomal monooxygenase activities without altering these activities in liver microsomes. The time course of naphthalene-induced morphologic damage to bronchiolar epithelium paralleled compromises in pulmonary monooxygenase activity. No concomitant alterations in hepatic morphology were observed. Five microsomal enzymes were studied: benzphetamine N-demethylase, aryl hydrocarbon hydroxylase, NADPH cytochrome c reductase, 7-ethoxyresorufin O-deethylase (a cytochrome P-448-dependent enzyme), and styrene epoxide hydrolase (a cytochrome P-450-independent enzyme). In general, the time course of the inhibition of these pulmonary enzymes was similar but the magnitude of the inhibition varied somewhat. Maximum inhibition of enzyme activity occurred about 3 days after naphthalene administration; 7-ethoxyresorufin O-deethylase activity was reduced to about 30% of control values whereas benzphetamine N-demethylase declined to about 70% of control. The remaining enzymes clustered midway between these extremes at about 50% of control values. Inhibited activities remained at relatively constant levels between Days 3 and 8 and by Day 15, there was a clear trend returning toward controls. Despite this trend, three of the six pulmonary enzyme activities examined remained significantly below control levels 15 days after a single dose of the hydrocarbon. Histologically, the pulmonary nonciliated bronchiolar epithelial (Clara) cell was the primary target of naphthalene toxicity. At early time points and at low magnifications, it appeared as if the entire bronchiolar epithelium was undergoing necrosis and sloughing into the lumen. However, higher magnifications revealed residual ciliated epithelium. The distribution of Clara cell damage appeared to vary considerably. One could find bronchioles that appeared completely denuded of epithelium and others in the same section whose Clara cells had been spared or, alternatively, were in the process of regeneration. The results are discussed in relation to recent work which has shown selective covalent binding of naphthalene to pulmonary Clara cells.

Pulmonary toxicity of methylcyclopentadienyl manganese tricarbonyl: Nonciliated bronchiolar epithelial (Clara) cell necrosis and alveolar damage in the mouse, rat, and hamster

Methylcyclopentadienyl manganese tricarbonyl (MMT) was administered ip to young female BALB c mice (120 mg MMT/kg), S A albino rats (5 mg MMT/kg), or LV 6 LAK Syrian hamsters (180 mg MMT/kg). This administration resulted in lung cell damage followed by cellular proliferation, which was quantified by measuring increases in thymidine incorporation into DNA (mouse, rat, and hamster) and by labeling indices (LI) determined from cell kinetic studies (mouse and rat). Thymidine incorporation into pulmonary DNA was significantly elevated within 1 to 2 days following MMT treatment in all three species, with peak incorporation occurring on Day 2 in the rat and hamster, and Day 4 in the mouse. Both bronchiolar and parenchymal LI were elevated at this time. Alveolar damage and nonciliated bronchiolar epithelial (Clara) cell necrosis were evident within 1 day of injection. This finding was followed by type II epithelial and Clara cell proliferation. Ultrastructurally, in the mouse, mitochondrial swelling and degeneration preceded Clara cell necrosis. Bronchiolar damage was most severe in the mouse, whereas parenchymal damage was most severe in the rat. These results suggest that the mouse, rat, and hamster have different susceptibilities to MMT-induced injury.

In vivo studies on the target tissue metabolism, covalent binding, glutathione depletion, and toxicity of 4-ipomeanol in birds, species deficient in pulmonary enzymes for metabolic activation

Comparative studies on the target organ for covalent binding and toxicity by 4-ipomeanol, a furan derivative metabolically activated by and toxic to the nonciliated bronchiolar epithelial (Clara) cells in mammalian lung, were performed in birds. In birds, whose lungs lack the typical pulmonary ciliated and nonciliated bronchiolar cells and are deficient in enzymes necessary for the metabolic activation of 4-ipomeanol, the target organ for toxicity and covalent binding by 4-ipomeanol was the liver. Tissue damage was not observed in lungs or kidneys of either Japanese quail or roosters at any dose of 4-ipomeanol tested. Likewise, the levels of irreversibly bound 4-ipomeanol metabolites were highest in liver, with much lower levels in lungs, kidneys, and all other organs studied. In addition, toxic doses of 4-ipomeanol markedly depleted hepatic, but not pulmonary or renal glutathione in Japanese quail. The covalent binding of 4-ipomeanol metabolites and concomitant depletion of tissue glutathione in the liver were both time and dose dependent. In quail, over a dose range of 5 to 75 mg/kg, no dose threshold for covalent binding or hepatic damage which depended upon substantial glutathione depletion was observed. Pretreatment of quail with phenobarbital had no effect on liver cytochrome P-450 levels, nor did it have any effect on tissue covalent binding or toxicity by 4-ipomeanol. Although 3-methylcholanthrene pretreatment nearly tripled the microsomal cytochrome P-450 levels in quail liver, it had relatively little or no effect on the hepatic covalent binding or toxicity by 4-ipomeanol. Prior treatment with the drug metabolism inhibitor, piperonyl butoxide, markedly decreased the covalent binding, glutathione depletion, and the toxicity of 4-ipomeanol. Treatment of quail with diethylmaleate produced a dose-dependent depletion of hepatic, renal, and pulmonary glutathione, and it markedly increased hepatic covalent binding by 4-ipomeanol, but had no effect on renal or pulmonary covalent binding. Diethyl maleate markedly enhanced the hepatic necrosis by 4-ipomeanol. The results are consistent with the view that 4-ipomeanol is metabolized preferentially in quail liver to highly reactive metabolites which can be detoxified by glutathione. It appears unlikely however, that differences in detoxification via glutathione are the primary determinants of the remarkable tissue selectivity for covalent binding and toxicity of 4-ipomeanol in different animal species in vivo. The present studies support the view that the tissue selectivity of 4-ipomeanol resides primarily in the ability of the target tissue to rapidly metabolize 4-ipomeanol to its ultimate toxic metabolite(s). These investigations also suggest that the bird may be a potentially useful animal model in studies on the relationship of target organ toxicity to the formation of reactive metabolites by other chemicals requiring metabolic activation.

Metabolism of benzo(a)pyrene and benzo(a)pyrene 4,5-oxide in rabbit lung.

For several years our laboratory has been investigating the biotransformation of various environmental pollutants by lung. Studies have been performed with pulmonary subcellular fractions, purified monooxygenase and glutathione transferase enzymes, and preparations having intact cellular structure including the isolated perfused lung and cell fractions enriched in alveolar macrophages, Clara cells and alveolar type II cells. Collectively, these investigations have identified several metabolic factors which may contribute to the pulmonary toxicity mediated by certain polycyclic aromatic hydrocarbons (PAH). First, although lung has low overall cytochrome P-450-dependent monooxygenase activity for many substrates, relative to liver, this activity is localized in only a few cell types and specific activity in certain cell types, such as the non-ciliated bronchiolar epithelial (Clara) cell, can be high. Second, oxidative metabolites of benzo(a)pyrene tend to accumulate in pulmonary tissue due, at least in part, to the low ability of lung (relative to liver) to conjugate and detoxify phenolic, dihydrodiol and epoxide metabolites. Thus, products such as benzo(a)pyrene 7,8-dihydrodiol are available for further cytochrome P-450-dependent oxidation to ultimate carcinogens and cytotoxins. Moreover, the lung is efficient in removing benzo(a)pyrene 4,5-oxide and presumably other oxidized PAH metabolites, from the bloodstream. Consequently, the uptake of relatively stable electrophilic metabolites released by the liver may also contribute to pulmonary toxicity.

Isolation and identification of Clara cells from rabbit lung.

A procedure has been developed for the isolation of nonciliated bronchiolar epithelial cells (Clara cells) from rabbit lung. Following pulmonary lavage to eliminate macrophages, cells (5% Clara cells) were released by digestion with 0.1% Protease I in HEPES-buffered balanced salt solution containing 0.5 mM ethylene glycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid instilled through the trachea. These cells were then separated on the basis of size using the Beckman JE-6 elutriator rotor. The fourth fraction collected from the elutriator contained about 30% Clara cells. This fraction was then layered on a two-polymer aqueous phase system consisting of 5% dextran T500 (DT) and 3.8% polyethylene glycol 6000 (PEG) in sodium phosphate buffer. A cell fraction was obtained from the PEG phase, which included approximately 70% Clara cells. These cells were found to be greater than 90% viable by trypan blue dye exclusion. Identification of isolated Clara cells was confirmed by light microscopic observation of nitro blue tetrazolium staining and by ultrastructural characteristics as observed by electron microscopy.

Cytochrome p-450: localization in rabbit lung.

Cytochrome P-450-dependent monooxygenase systems, which metabolize endogenous as well as foriegn compounds, are found in hepatic and several extrahepatic tissues of mammals, including humans. A form of cytochrome P-450 is localized in the nonciliated bronchiolar epithelial cells (Clara cells) of the small airways of rabbit lung. The apparent high concentration of the cytochrome in this pulmonary cell type compared to liver may be an important determinant in the susceptibility of the lung to a number of toxic chemicals that undergo metabolic activation.

The histochemistry of the nonciliated bronchiolar epithelial cell.

The Embden-Meyerhof and Krebs cycles and the hexose monophosphate shunt are functional in nonciliated bronchiolar epithelial (Clara) cells, and the activity of their individual oxidative enzymes su...

The ultrastructure of mouse lung; a note on the fine structure of mitochondria and endoplasmic reticulum of the bronchiolar epithelium.

A few aspects of the fine structure of bronchiolar epithelium have already been presented (4). The two types of cells, ciliated and non-ciliated, differed in their cytoplasmic structure. The ciliated cells contained only a few mitochondria, and a few vesicles of the endoplasmic reticulum, whereas non-ciliated cells showed numerous large mitochondria and a very well developed endoplasmic reticulum. It was also shown (4) that the mitochondria of non-ciliated bronchiolar cells appeared either as long and filamentous or as spherical, even within one and the same cell, and that the endoplasmic reticulum could assume the form of paired double membranes as well as of vesicles or tubules. The difference in appearance of the endoplasmic reticulum in ciliated as compared with non-ciliated cells and the varying forms of the reticulum in the non-ciliated cells had been recognized earlier by other authors (1). The present brief report further illustrates the spherical mitochondria and the endoplasmic reticulum of non-ciliated bronchiolar epithelial cells. The methods of tissue processing and the technique used to select a small bronchiole for sectioning are the same as in earlier reports (2, 3, 4).