Figure 1.

Active recruitment of neutral lipid from lipofibroblasts by alveolar Type II cells. Neutral lipids stored in lipofibroblasts are actively “trafficked” to alveolar Type II cells by means of Adipocyte Differentiation Related Protein (ADRP), regulated by Parathyroid Hormone-related Protein (PTHrP) produced by Type II cells. The Type II cells secrete Prostaglandin E₂, stimulating the secretion of the neutral lipids, and the uptake of the neutral lipid by the Type II cells is regulated by leptin produced by the lipofibroblasts. Each of these steps is coordinately stretch-regulated to increase surfactant phospholipid synthesis by the Type II cell. The net result is surfactant phospholipid production integrated with the distension of the alveolar wall during breathing.