Distribution and quantity of contractile tissue in postnatal development of rat alveolar interstitium

Alpha-smooth muscle actin (alpha-SMA) -expressing cells are important participants in lung remodeling, during both normal postnatal ontogeny and after injury. Developmental dysregulation of these contractile cells contributes to bronchopulmonary dysplasia in newborns, and aberrant recapitulation in adults of the normal ontogeny of these cells has been speculated to underlie disease and repair in mature lungs. The significance of airway smooth muscle has been widely investigated, but contractile elements within the pulmonary parenchyma, although also of structural and functional consequence in developing and mature lungs, are relatively unstudied and little quantitative information exists. Here, we quantify the areal density of alpha-SMA expression in lung parenchyma and assess changes in its spatiotemporal distribution through postnatal ontogeny. Using an antibody against alpha-SMA, we immunofluorescently labeled contractile elements in lung sections from a postnatal growth series of rats. Images were segmented using thresholded pixel intensity. Alpha-SMA areal density in the alveolar interstitium was calculated by dividing the area of alpha-SMA-positive staining by the tissue area. The areal density of alpha-SMA in 2-day neonates was 3.7%, almost doubled, to 7.2% by 21 days, and decreased to 3% in adults. Neonates had large, elongate concentrations of alpha-SMA, and alpha-SMA localized both at septal tips and within the interstitium. In adults, individual areas of alpha-SMA expression were smaller and more round, and located predominately in alveolar ducts, at alveolar ends and bends. The results are consistent with increasing alpha-SMA expression during the period of peak myofibroblast activity, corresponding to the phase of rapid alveolarization in the developing lung.