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

Abstract

The Clara cell of the mouse terminal bronchiole has been examined by high resolution phase contrast and electron microscopy, and the secretory route of surfactant in the mouse lung has been investigated using both light and electron microscopical autoradiography of a surf actant precursor (palmitic acid). Serial sections of terminal bronchioles examined by phase contrast microscopy demonstrated the appearances of apocrine secretion of amorphous droplets by Clara cells. Electron microscopy showed that the non-decapitating supranuclear region of Clara cells contained numerous rounded mitochondria with scanty achrestic cristae, whilst the apical portion was rich in smooth tubular membranes. Autoradiographical labeling of bronchiolar Clara cells exceeded that of all other cell types within three minutes of administration of the radioactive precursor, with location primarily over the endoplasmic reticulum. Subsequently there was heavy labeling of the bronchiolar lining layer. An hour later, silver grains were most numerous over the lamellated bodies of type I1 cells. It is suggested that palmitic acid is synthesized in the specialized mitochondria of the non-secretory region of the Clara cell and is incorporated into dipalmitoyl lecithin in the membranous apex. Subsequent secretion, possibly by the apocrine mechanism, provides a surface-active bronchiolar lining layer.The suggestion that surface tension is responsible for up to 75% of the elasticity of the mammalian lung was made by von Neergaard ('29) who compared the deflation pressurevolume curves of saline-filled lungs with air-filled lungs, and found marked hysteresis in the latter. Three decades elapsed before the presence of a highly surf ace-active extracellular alveolar lining layer became well documented. Therefore, there is much evidence that type 11 cells are responsible for surfactant secretion, but as Niden ('67) has indicated, the evidence is entirely circumstantial and could equally well support a phagocytic rather than secretory role for the type I1 cell. Finely divided thorium and carbon introduced via the trachea are incorporated into the lamellar bodies of type I1 cells in small amounts, implying either phagocytosis (Niden, '67), passive inflow of particles during the secretory process (Corrin, '69) or the possibility that type I1 cells represent a stage of lung macrophage development (Ladman and Finley, '66).Less attention has been given to Niden's suggested alternative site of surfactant production, namely the terminal bronchiole. The nonciliated epithelial cells were first described by Kolliker (1881) and later studied in detail by Clara ('37) after whom they are generally named. Clara concluded that these cells are specialized for the apocrine secretion of a non-mucoid substance, but further evidence to support an apocrine mode of secretion is sparse. and merocrine secretion to contain electron-dense secretory granules (von Spoendlin, '59; Wang et al., '71), abundant choline-based phospholipid, and many synthetic and energy-producing en...

A Morphological Study of Acute Respiratory Tract Lesions in a Lipopolysaccharide Instilled Rat Model.

“…The first change was a combination of apocrine-like protrusions and the marked growth of endoplasmic reticulum in Clara cells. The secretory mechanism in the Clara cell, whether apocrine (ETHERTON et al, 1973) or merocrine (exocytosis) (WANG et al, 1971;YONEDA, 1977) has not yet been determined. The present TEM observation demonstrates an accordance with the findings by ETHERTON et al (1973) that Clara cells possessing well-developed rough and smooth endoplasmic reticula simultaneously possess in an alveolus at 8h after LPS administration.…”

“…The secretory mechanism in the Clara cell, whether apocrine (ETHERTON et al, 1973) or merocrine (exocytosis) (WANG et al, 1971;YONEDA, 1977) has not yet been determined. The present TEM observation demonstrates an accordance with the findings by ETHERTON et al (1973) that Clara cells possessing well-developed rough and smooth endoplasmic reticula simultaneously possess in an alveolus at 8h after LPS administration. Note the nucleus of homogeneous appearance and moderate electron density, and phagocytic vesicles (arrows) and vacuoles (arrowheads) in the cytoplasm.…”

Fine structural aspects of the development and aging of the tracheal epithelium of mice.

“…By electron microscopy, it has become clear that the Clara cell is characterized by well developed sER (NIDEN and YAMADA, 1966). Though the function of this cell is not distinct, NIDEN (1967), AZZOPARDI and THURLBECK (1969), ETHERTON et al(1973) andSMITH et al (1974) considered it a secretor of pulmonary surfactant. CUTZ andCoNEN (1970, 1971) thought that Clara cells of human and rabbit make the hypophase component of the alveolar lining layer.…”

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

“…The culture model should also be useful for investigating other aspects of Clara cell function. The arrangement of the cells in a polarized monolayer is conducive to studies with labeled precursors similar to those previously performed in viv'o (30), and the presence of electron-dense granules in the cultured cells may enable investigators to define the secretory products and processes of this cell type.…”