Platelike and globelike inclusion bodies associated with rough endoplasmic reticulum in alveolar type 2 cells of the sheep

Abstract

We studied the ultrastructure of alveolar type 2 cells of sheep. The inclusion bodies of rodlike structures or cisternal bodies derived from the endoplasmic reticulum were found in 13.6 +/- 7.0% (+/- SD) of type 2 cells. The length of the rodlike structure varied, the longest being 2.4 microns; but the width was usually 0.1 micron. Rod-like structures were usually straight and arranged in parallel stacks. An electron-dense line ran longitudinally at the center of the rodlike structures. Serial ultrathin sections revealed that the rodlike structures did not seem to be three-dimensionally rod-shaped structures but were actually plate-disc or cup-shaped, and the piled-up rodlike structures were connected to each other at their ends. This rodlike structure in type 2 cells of sheep seems rather to be a platelike inclusion body. The cisternal body, on the other hand, was circular or oval-shaped cisternae containing aggregated electron-dense materials distributed in an arabesque or speckled pattern. Serial ultrathin section examination revealed the cisternal body to be three-dimensionally a globe. Some plate bodies were arranged near the cisternal body, suggesting the transformation from the latter to the former. The functional significance of these findings has not yet been elucidated.

The plate body: 3D ultrastructure of a facultative organelle of alveolar epithelial type II cells involved in SP-A trafficking

“…The present study confirms the presence of a highly structured organelle consisting of various numbers of membranous cisterns with an electron dense middle bar in the adult goat and sheep lung. The ultrastructure of this organelle is consistent with previous reports (Stephens et al 1973; figures c, d, e and f) is only apparent in the regularly formed cistern but not in the vesicular network Shibamoto et al 1989). Due to their 3D characteristics, the individual cisterns had an appearance of plates or discs, the whole organelle reminded of a stack of plates, hence the name plate body.…”

“…Under normal conditions, plate bodies do not seem to be essential for AE2 cell biology as they are not present in all AE2 cells from one animal (which, however, may be a false interpretation due to 2D imaging), they do not occur in all animals of the same species and they have not been reported in newborn animals of species known to contain plate bodies so far. Their contribution to surfactant metabolism, particularly surfactant proteins, has been proposed by several authors (Stephens et al 1973;Manabe and Ikeda 1986;Miller et al 2018) and has been challenged by others (Shibamoto et al 1989). Plate bodies lack a clear connection with lamellar bodies or multi-vesicular bodies which raised the doubts about their importance for surfactant metabolism.…”

“…Nevertheless, there are species differences in AE2 cell ultrastructure which tend to be neglected if they are not present in the most widely used laboratory animals mouse and rat. One of these differences relates to an incidentally described organelle, in the beginning termed "bar-like structure" (Stephens et al 1973), later addressed as "plate-like body" (Shibamoto et al 1989) or "plate body" (Lakritz et al 1992) or "highly ordered granule" (Miller et al 2018). As the term "bar-like structure" was also used for a different type of organelle (Shimura et al 1980), likely widened cisterns of the endoplasmic reticulum, we follow the recommendation of Shibamoto et al (1989) to use the term "plate body" to address this organelle unambiguously.…”

“…One of these differences relates to an incidentally described organelle, in the beginning termed "bar-like structure" (Stephens et al 1973), later addressed as "plate-like body" (Shibamoto et al 1989) or "plate body" (Lakritz et al 1992) or "highly ordered granule" (Miller et al 2018). As the term "bar-like structure" was also used for a different type of organelle (Shimura et al 1980), likely widened cisterns of the endoplasmic reticulum, we follow the recommendation of Shibamoto et al (1989) to use the term "plate body" to address this organelle unambiguously. No matter what the name, this organelle was consistently described to be composed of a varying number of parallel, stacked membranous cisterns with a central electron dense line.…”