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Expression of cathepsin K in the human embryo and fetus

Cathepsin K is a protease with high collagenolytic and elastinolytic activity. Its cellular expression was previously thought to be restricted to osteoclasts and osteoclast‐mediated bone resorption. In this study, the expression of cathepsin K in the human embryo and fetus was demonstrated by immunohistochemistry, in situ hybridization, and by Northern blotting of fetal tissue extracts. Besides osteoclasts and chondroclasts and their precursors, epithelial cells of various organ systems expressed significant amounts of this enzyme. Respiratory and gastrointestinal mucosa, including bile duct epithelia and urothelia, showed high levels of cathepsin K expression. With the exception of the urothelium, showing a more homogenous expression pattern, the protease was usually accentuated in the surface cell layers of pithelia. In summary, these findings in the human embryo and early fetus demonstrated a significant expression of cathepsin K in different epithelial cell types besides osteoclasts. The functional aspects of cathepsin K expression in nonosteoclastic cells and potential conclusions on physiological and pathological conditions in the embryo‐fetal or adult organism remain to be investigated. Dev Dyn 1999;216:89–95. © 1999 Wiley‐Liss, Inc.