TYPE-II PNEUMOCYTES REVISITED - INTRACELLULAR MEMBRANOUS SYSTEMS, SURFACE CHARACTERISTICS, AND LAMELLAR BODY SECRETION

BACKGROUND: Type II pneumocytes, the producers of pulmonary surfactant , have been extensively studied during the last 20 years because of th e importance of their metabolism in lung function and integrity. The u ltrastructural studies of the 1970s and 1980s have shown that these ce lls present unique elements. EXPERIMENTAL DESIGN: In this work, we use d thin-section, freeze-fracture, and fracture-flip electron microscopy techniques to obtain new information on the ultrastructural peculiari ties of isolated rat type II pneumocytes, focusing our study on the in tracellular membranous systems and their interrelationships and the mi croanatomy of their plasma membrane during secretory process. RESULTS: In thin-sections of pneumocytes postfixed with osmium tetroxide and p otassium ferricyanide, we observed that lamellar bodies (LBs) are usua lly connected to membranes of the endoplasmic reticulum, and seem to e merge and grow from them. Unusual connections between the endoplasmic reticulum and mitochondria were detected, as well as numerous ''bar-li ke structures'' (BLSs), most of them in the early stages of developmen t and often generating from the nuclear membrane. Membranes of the smo oth endoplasmic reticulum that closely follow the outlines of mitochon dria also appear to be the origin of some BLSs. Possible transition fo rms, BLS --> LB, were also detected, although they were rare. New imag es of the surface of the pneumocytes and its changes during LB secreti on showed a segregation and clearing of membrane particles at the area s of LB extrusion. CONCLUSIONS: We propose that LBs can originate dire ctly from membranes of the endoplasmic reticulum or from BLSs. An indi rect participation of mitochondria appears possible. The plasma membra ne of pneumocytes displays structural changes associated with the secr etion of LBs as visualized by a redistribution of intramembrane and su rface particles.