LAMELLAR BODY FORMATION AND DARK MULTIVESICULAR BODY POCKETS IN FETAL AND POSTNATAL NORMAL RAT ALVEOLAR TYPE II CELLS: ULTRASTRUCTURAL STUDY

Abstract

To investigate the osmiophilic lamellar body (LB) formation in the alveolar type II cells, the authors examined lung tissues from fetal (day 19 of gestation) and postnatal normal rat by electron microscopy. Lamellar body formation was identical in fetal and postnatal rat lungs. Immature dark multivesicular body (MVB) showed incomplete formation of the limiting membrane and contained a cluster of vesicles and an electron-dense granule probably originated from Golgi complex. Dark and light MVBs were seen in alveolar type II cells. However, only dark MVB was involved in the production of LB. First, a drumstick-shaped projection arising from the surface of MVB appeared, which partly trapped cytoplasmic materials such as glycogen granules and ribosomes. The projection was ultimately fused with the MVB surface to form an MVB pocket described here for the first time. In prelamellar body, lamellar structures appeared to provide a communicative MVB pocket and even found in a drumstick-shape. Mature LBs showed disappearance of multivesicles, dense matrix, and MVB pocket. Thus, lamellar body formation might not only need MVB materials, but also glycogen granules and ribosomes in the MVB pocket.