title: Type II pneumocytes revisited: intracellular membranous systems, surface characteristics, and lamellar body secretion.
abstract: BACKGROUND
  Type II pneumocytes, the producers of pulmonary surfactant, have been extensively studied during the last 20 years because of the importance of their metabolism in lung function and integrity. The ultrastructural studies of the 1970s and 1980s have shown that these cells present unique elements.
  
  
  METHODS
  In this work, we used thin-section, freeze-fracture, and fracture-flip electron microscopy techniques to obtain new information on the ultrastructural peculiarities of isolated rat type II pneumocytes, focusing our study on the intracellular membranous systems and their interrelationships and the microanatomy of their plasma membrane during secretory process.
  
  
  RESULTS
  In thin-sections of pneumocytes postfixed with osmium tetroxide and potassium ferricyanide, we observed that lamellar bodies (LBs) are usually connected to membranes of the endoplasmic reticulum, and seem to emerge and grow from them. Unusual connections between the endoplasmic reticulum and mitochondria were detected, as well as numerous "bar-like structures" (BLSs), most of them in the early stages of development and often generating from the nuclear membrane. Membranes of the smooth endoplasmic reticulum that closely follow the outlines of mitochondria also appear to be the origin of some BLSs. Possible transition forms, BLS--LB, were also detected, although they were rare. New images of the surface of the pneumocytes and its changes during LB secretion showed a segregation and clearing of membrane particles at the areas of LB extrusion.
  
  
  CONCLUSIONS
  We propose that LBs can originate directly from membranes of the endoplasmic reticulum or from BLSs. An indirect participation of mitochondria appears possible. The plasma membrane of pneumocytes displays structural changes associated with the secretion of LBs as visualized by a redistribution of intramembrane and surface particles.
authors.affiliation: Biological Carcinogenesis and Development Program, NCI-Frederick Cancer Research and Development Center, Maryland.
authors.forename: C
authors.initials: C
authors.lastname: Risco
authors.forename: C
authors.initials: C
authors.lastname: Romero
authors.forename: M A
authors.initials: MA
authors.lastname: Bosch
authors.forename: P
authors.initials: P
authors.lastname: Pinto da Silva
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000648:ultrastructure; D004721:Endoplasmic Reticulum / Q000502:physiology / Q000648:ultrastructure; D005614:Freeze Fracturing; D002479:Inclusion Bodies / Q000502:physiology*; D007425:Intracellular Membranes / Q000502:physiology / Q000648:ultrastructure*; D008168:Lung / Q000166:cytology* / Q000378:metabolism / Q000648:ultrastructure; D008297:Male; D008928:Mitochondria / Q000648:ultrastructure; D009685:Nuclear Envelope / Q000648:ultrastructure; D011663:Pulmonary Surfactants / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
chemical_list: D011663:Pulmonary Surfactants