EMBRYONIC EXPRESSION OF TENASCIN-X SUGGESTS A ROLE IN LIMB, MUSCLE, AND HEART DEVELOPMENT

Tenascin-X (TN-X) is the newest member of the tenascin family of extra cellular matrix proteins and it is highly expressed in muscular tissue s during development. To gain insight into the possible functions of T N-X during development, we evaluated its expression in the rat embryo. Using an 800 bp cDNA encoding the fibrinogen-like domain of TN-X, we show that TN-X expression begins in migrating cells of the epicardium in the E12 heart, The epicardium provides progenitors of fibrous and v ascular tissue to the developing heart. After the epicardium is comple te, TN-X is expressed in the sub-epicardial space in association with developing blood vessels, and later by non-myocytes dispersed through the myocardial wall, A similar pattern of TN-X expression, first in co nnective tissue surrounding muscle, and then by a subset of cells with in muscle, was seen in para-axial, body wall, craniofacial, and append icular muscle, This pattern suggests a role in connective tissue cell migration and late muscle morphogenesis. TN-X is also highly expressed in the interdigital space at E15 and surrounding developing tendons, suggesting an additional role in cell fate determination. Although the pattern of TN-X expression is distinct from that of tenascin C, they are frequently expressed in close proximity, Indirect genetic evidence in humans suggests an essential function for TN-X, and the pattern of TN-X expression in heart, skeletal muscle, and limb is consistent wit h this hypothesis. (C) 1995 Wiley-Liss, Inc.