Thickening of the airway smooth muscle layer from late gestation to first year of life is accompanied by a reduction in smooth muscle cell density

Background: The volume of the airway smooth muscle (ASM) layer increases from birth until adulthood. However it is unknown if the increased volume is due to ASM cell hypertrophy and/or hyperplasia, or an increase in the proportion of extracellular matrix (ECM). It is important to investigate the mechanisms that produce the normal increase in the volume of the ASM layer as these may have different effects on airway function and/or be susceptible to change resulting in abnormal function. Aim: Characterise the structural mechanisms producing growth of the ASM layer in early life. Methods: Airways from post-mortem cases were available from 4 different age groups: Results: Airway size and muscle thickness approximately doubled from the prenatal period to the first year of life. The increase in ASM layer thickness was accompanied by a reduction in ASM cell density. There was a non-significant trend for an increase in NL. Conclusion: From late gestation to the first year of life, the ASM layer thickens with a concomitant reduction in ASM cell density. This may due to cell hypertrophy or a disproportionate expansion of the ECM between cells. There is a suggestion of increased cell number between 6–12 months of age.