The plate body: 3D ultrastructure of a facultative organelle of alveolar epithelial type II cells involved in SP-A traff
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ORIGINAL PAPER
The plate body: 3D ultrastructure of a facultative organelle of alveolar epithelial type II cells involved in SP‑A trafficking Christian Mühlfeld1,2,3 · Christoph Wrede1,2,3 · Viktor Molnár4 · Alexandra Rajces1 · Christina Brandenberger1,2 Accepted: 21 August 2020 © The Author(s) 2020
Abstract Plate bodies are facultative organelles occasionally described in the adult lungs of various species, including sheep and goat. They consist of multiple layers of plate-like cisterns with an electron dense middle bar. The present study was performed to elucidate the three-dimensional (3D) characteristics of this organelle and its presumed function in surfactant protein A (SP-A) biology. Archived material of four adult goat lungs and PFA-fixed lung samples of two adult sheep lungs were used for the morphological and immunocytochemical parts of this study, respectively. 3D imaging was performed by electron tomography and focused ion beam scanning electron microscopy (FIB-SEM). Immuno gold labeling was used to analyze whether plate bodies are positive for SP-A. Transmission electron microscopy revealed the presence of plate bodies in three of four goat lungs and in both sheep lungs. Electron tomography and FIB-SEM characterized the plate bodies as layers of two up to over ten layers of membranous cisterns with the characteristic electron dense middle bar. The membranes of the plates were in connection with the rough endoplasmic reticulum and showed vesicular inclusions in the middle of the plates and a vesicular network at the sides of the organelle. Immuno gold labeling revealed the presence of SP-A in the vesicular network of plate bodies but not in the characteristic plates themselves. In conclusion, the present study clearly proves the connection of plate bodies with the rough endoplasmic reticulum and the presence of a vesicular network as part of the organelle involved in SP-A trafficking. Keywords Surfactant protein A · Plate body · Alveolar epithelial type II cell · Electron tomography · Focused ion beam scanning electron microscopy · Immunocytochemistry
Introduction Alveolar epithelial type II (AE2) cells are an essential part of the mammalian alveolar epithelium: they serve as progenitor cells of the terminally differentiated type I cells and produce the surface-tension lowering surfactant (Fehrenbach 2001). The latter is stored in and secreted from characteristic * Christina Brandenberger brandenberger.christina@mh‑hannover.de 1
Institute of Functional and Applied Anatomy, Hannover Medical School, Carl‑Neuberg‑Str. 1, 30625 Hannover, Germany
2
Biomedical Research in Endstage and Obstructive Lung Research (BREATH), Member of the German Center for Lung Research (DZL), Hannover, Germany
3
Research Core Unit Electron Microscopy, Hannover Medical School, 30625 Hannover, Germany
4
Hanover Adventure Zoo, 30175 Hannover, Germany
organelles, the so-called lamellar bodies (Weaver et al. 2002). AE2 cells are present in all terrestrial mammalian lungs ranging from the 2 g weighing Etruscan sh
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