The pulmonary lipofibroblast is located in the alveolar interstitium and is recognizable by its characteristic lipid droplets. During alveolar development it participates in the synthesis of extracellular matrix structural proteins, such as collagen and elastin, and as an accessory cell to the type II pneumocyte, in the synthesis of surfactant. The lipofibroblast contains cortical contractile filaments and is thereby related to the contractile interstitial cells that are normally found at the alveolar septal tips and after lung injury. The morphologic, immunologic, and biochemical characteristics of the lipofibroblast and its probable physiologic functions are reviewed. The retinoid and lipid metabolism of the lipofibroblast is compared with that of the hepatic lipocyte and the adipocyte. Although the functions of the lipofibroblast remain incompletely characterized, this cell type is emerging as an important contributor to pulmonary alveolar septal development.

Platelet‐derived growth factor‐A and its receptor, platelet‐derived growth factor receptor‐alpha (PDGF‐Rα), are required for formation of the secondary pulmonary alveolar septa in mice. However, it remains unclear how these molecules direct the secondary septation process. We have examined the abundance, location, and the accumulation of alpha‐smooth muscle actin (αSMA), neutral lipid droplets, and elastin in the proximity of PDGF‐Rα‐expressing alveolar cells during postnatal days 4 through 12 in the mouse. PDGF‐Rα‐expressing cells preferentially have characteristics of myofibroblasts and were more likely to contain αSMA than are alveolar cells that do not express PDGF‐Rα. PDGF‐Rα expressing cells were preferentially located in the alveolar entry ring (AER) where αSMA and elastic fibers accumulate. In contrast, PDGF‐Rα expression inversely correlated with neutral lipid accumulation, which was more prominent at the alveolar base, distant from the AER. PDGF‐Rα‐expressing alveolar cells accumulate in the AER where they may promote mechanical stability during respiration. In addition to defining how alveolar septa form, these findings may have implications for the treatment of diseases which involve alveolar effacement such as emphysema and pulmonary fibrosis. Anat Rec, 2008. © 2008 Wiley‐Liss, Inc.

In mammals, including rats and mice, the development of pulmonary alveolar septa is primarily limited to late gestation and the early periods of postnatal life. Before this time, the rat lung contains a relatively large supply of endogenous retinyl ester that, together with its metabolite retinoic acid, has been shown to increase elastin gene expression and the number of alveoli. We have hypothesized that mice bearing a deletion of one or more genes encoding for retinoic acid receptors (which are DNA binding proteins that alter transcription of retinoic acid-responsive genes) may demonstrate abnormalities in retinoid-mediated alveolar formation. Our studies demonstrate that the absence of the retinoic acid receptor-gamma (RARgamma) is associated with a decrease in the steady-state level of tropoelastin messenger RNA in a subpopulation of lung fibroblasts at Postnatal Day 12. RARgamma gene deletion also resulted in a decrease in whole lung elastic tissue and alveolar number, and an increase in mean cord length of alveoli (L(m)) at Postnatal Day 28. The additional deletion of one retinoid X receptor (RXR)alpha allele resulted in a decrease in alveolar surface area and alveolar number, and an increase in L (m). These data indicate that RARgamma is required for the formation of normal alveoli and alveolar elastic fibers in the mouse, and that RAR/RXR heterodimers are involved in alveolar morphogenesis.

During lung development the extracellular matrix regulates cellular growth, migration, and differentiation. Pulmonary cells reciprocate and regulate extracellular matrix formation by elaborating a variety of peptides that affect gene transcription, RNA processing, translation, and posttranslational modifications of proteins. This regulation involves prenatal events such as the branching of airways and postnatal events such as alveolar septal formation. Normal airway branching requires multiple extracellular matrix proteins, proteoglycans, and the expression of cellular receptors for these molecules. Alveolar septal formation exemplifies how cells regulate the production, export, and deposition of an important structural protein, elastin, which is essential for the development of normal gas exchange units. Repair after injury to the adult lung engages some of the same factors that regulate lung development. However, in this case, inflammatory cells that enter the lung after injury elaborate many of these regulatory peptides. The effects of these peptides on the resident cells, which produce extracellular matrix, are a major determinant of whether the repair restores normal pulmonary architecture or progresses to fibrosis and additional impairment of gas exchange.