Ultrastructural analysis of lamellar bodies in type II alveolar epithelial cells in the human lung

Pulmonary surfactant is produced by type II alveolar epithelial cells (AEC2) and stored in lamellar bodies (LBs) before secretion. Here, we characterize AEC2 and their LBs in the human lung ultrastructurally and quantitatively. Five human lungs were analyzed by transmission electron microscopy, serial section electron tomography, and stereology. A human lung contained about 24 billion AEC2 with a mean size of about 650 µm³. The number of AEC2 as well as the total volume of LBs per lung, about 1.9 mL, strongly correlated with total lung volume. A single AEC2 contained an LB volume of about 74 µm³. This amount was packed in about 324 LBs with a mean size of 0.24 µm³. Three morphologically distinct subpopulations of LBs were identified: 1) isolated LBs which make up the majority (average 300 per AEC2), 2) LBs connected to each other via pores (average 23 per AEC2), and 3) LBs connected to the plasma membrane via a fusion pore (average 1 per AEC2). Along this sequence of subpopulations, the mean size of LBs increased. LBs that were connected either with each other or to the plasma membrane contained about 14% of an AEC2's LB volume. This is in line with the concept of an intermediate surfactant pool, stored in LBs either directly or indirectly connected to the plasma membrane. In summary, this study provides quantitative reference data on surfactant-storing LBs in AEC2 as well as morphological evidence for an intermediate surfactant pool in the human lung.NEW & NOTEWORTHY Human lung type II alveolar epithelial cells (AEC2) and their surfactant-storing lamellar bodies (LBs) are characterized quantitatively and ultrastructurally by transmission electron microscopy, serial section electron tomography, and stereology. On average, the 24 billion AEC2 in a human lung contain 324 LBs each. An intermediate surfactant pool in the human lung, comprising LBs in AEC2 not only directly but also indirectly connected to the plasma membrane via inter-LB connections, is demonstrated morphologically and characterized quantitatively.