Pneumocytes multivesicular bodies

At the distal respiratory site, the alveolar epithelial cell layer is much flatter (0.1 -0.5 pm) and composed of two major cell types, squamous type I and agranular type II pneumocytes. Type I pneumocytes are non-phagocytic and highly flattened cells with broad and thin extensions. They occupy -95 % of the alveolar luminal surface, although they are less numerous than type II cells. The remaining surface is occupied by type II pneumocytes, which have blunt microvilli and contain multivesicular bodies [3, 11]. 

Hackett and Sunderman 1968). Great enlargement of the nuclei and nucleoli was observed two to six days. The cytoplasm contained prominent arrays of rough endoplasmic reticulum, as well as increased numbers of free ribosomes and mitochondria. In some of the type 1 pneumocytes, the endoplasmic reticulum was distended with flocculent osmiophiUc material. Abundant Golgi zones were present. Multivesicular bodies were found two and four days after Ni(CO)4 appUcation. They were usually surrounded by a single membrane. 

Acid phosphatase (EC 3.1.3.2) was chiefly found in the characteristic laminated vacuoles of the type II pneumocytes of the rat (Corrin et al. 1969). Not all such vacuoles nor indeed all such cells contained the enzyme but in many, acid phosphatase reaction product formed a thin rim about the periphery of the vacuoles. The enzyme was not limited to the smaller of these cytosomes and was often present in the larger and presumably older vacuoles. Occasionally reaction product was aligned alongside or within channels of the endoplasmic reticulum. The multivesicular bodies described by Sorokin (1966) and Goldenberg et al. (1967) generally contained acid phosphatase at their periphery and between the individual vesicles. 

To follow the retrieval of lamellar body membrane from the cell surface in rat type II pneumocytes, Schaller-Bals et al. (2000) instilled the monoclonal antibody 3C9 into rat lungs. In vivo it was endocytosed by type II cells but not by other lung cells. In type II cells that were isolated from rat lungs by elastase digestion and cultured on plastic for 24 h, Mab 3C9 first bound to the cell surface, then was found in endosomes, vesicular structures, and multivesicular bodies and, finally, clustered on the luminal face of lamellar body membranes. 

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