Basic Res Cardiol 97: 189 – 197 (2002) © Steinkopff Verlag 2002 ORIGINAL CONTRIBUTION Anna Ratajska Jack P. M. Cleutjens Embryogenesis of the rat heart: the expression of collagenases  Abstract Little is known about extracellular matrix (ECM) remodeling dur- ing heart development. Matrix degrading metalloproteinases are possible candidates contributing to degradation of ECM during these complex bio- logical events. We described here different forms of MMPs, based on their substrate specificity, molecular weight, immunolocalization and in situ zymography within embryonic rat myocardium at different stages of heart development (from embryonic day – ED12 until ED 21). Murine collagenase- 3 (MMP-13), stromelysin (MMP-3) and gelatinases A&B (MMP-2 & -9) were expressed in prenatal hearts, as demonstrated by quantitative zymography and immunohistochemistry. MMP-2, -3 and -9 were found within myo- cardium of avascular (ED12) and vascularized heart (ED14-21). An extensive immunolabeling over the heart trabeculae, epicardial tissue and a weaker labeling in the endocardial and truncoconal cushion tissue was observed at all stages of the heart development. Utilizing quantitative zymography we found that MMP-13 activity gradually increased from ED14-ED16 reaching a plateau from ED16-ED21, while MMP-2 activity demonstrated a transient increase starting at ED13, peaked at ED16 and declined thereafter. As to MMP-9 activity, it was seen only between ED16 and ED 18. In situ zymo- graphy with gelatin as a substrate represented activity of MMPs within the myocardium of the atria and the ventricles and a very strong activity in the interstitial tissue of the endocardial and the conotruncal cushion tissue. Con- clusion MMPs expressed in embryonic heart correspond to all major classes of these enzymes. They may contribute to embryonic remodeling of the heart.  Key words Embryonic heart – rat – coronary neovascularization – matrix metalloproteinases – extracellular matrix remodeling BRC 343 A. Ratajska () The Medical University of Warsaw Department of Pathological Anatomy Chalubinskiego 5 02-004 Warsaw, Poland Tel./Fax: +48-22/629-98-92 E-Mail: aratajska@ib.amwaw.edu.pl J. P. M. Cleutjens CARIM Cardiovascular Research Institute Maastricht Department of Pathology University Maastricht P.O. Box 616 6200 MD Maastricht, The Netherlands Tel.: +31-43/3876631 Fax: +31-43/3876613 Received: 5 July 2001 Returned for revision: 26 July 2001 Revision received: 27 November 2001 Accepted: 12 December 2001 Introduction The extracellular matrix (ECM) plays a pivotal role in embryogenesis of the heart. It forms a supportive scaf- fold for cellular elements of developing myocardium and mediates various cellular activities, like cell migration, formation of cellular extensions, proliferation, epithe- lial-mesenchymal transformation during formation of heart valves, and coronary vessel vasculogenesis/angio- genesis (27, 29, 33, 35, 48, 50). ECM can be also consid- ered as a compartment for the storage (accumulation) and release of growth factors and other signaling mole- cules which are required for intercellular communica- tions (36). Furthermore the ECM itself undergoes remod- eling during heart morphogenesis which is manifested by deposition of different molecules like fibrillar types I & III collagen, types IV, V, VI, VIII, XV collagen,