Fetal Lung Development

ABSTRACT Cathepsins are involved in lyso-somal protein degradation, proenzyme activa-tion, antigen processing, and hormone matura-tion. They are secreted by tumor cells and macrophages and catalyze the remodeling of ex-tracellular matrix proteins. To gain insight into the expression pattern of cathepsins during fetal lung development, the expression of cathepsins B, H, K, L, and S at protein and mRNA levels were evaluated by using immunohistochemistry and in situ hybridization. Early expression of ca-thepsins B, H, and K was found in epithelial cells of the branching presumptive bronchi (<12th week of gestation). The most intense cathepsin K–specific immunoreactivity was found in devel-oping airways with a lumen. Cathepsin K was found in epithelial cells only, whereas in con-trast, cathepsins B and H were detected both in epithelial and interstitial cells. During fetal mat-uration, interstitial cells displayed cathepsin L immunoreactivity and, in the saccular phase (>26th week of gestation), both cathepsin L and S immunoreactivities. A continuous decline in the proportion of cathepsin H-positive interstitial CD68-positive cells was observed. These discrete temporal and spatial variations in cathepsin ex-pression during organogenesis of the human lung indicate different physiological roles for the individual enzymes in different cell types and developmental stages. © 2002 Wiley-Liss, Inc. Key words: cathepsin; fetal development; hu-man; immunohistochemistr