EXPRESSION OF FIBRONECTIN DURING RAT FETAL AND POSTNATAL-DEVELOPMENT - AN IN-SITU HYBRIDIZATION AND IMMUNOHISTOCHEMICAL STUDY

Objective: Fibronectin is a protein of the extracellular matrix with n umerous binding sites to the other elements of the matrix and to the c ells. The aim of this study was to determine the relative importance o f fibronectin isoform expression (FN-EIIIA, FN-EIIIB) during fetal and postnatal development of the rat heart. Methods: In situ hybridisatio n and immunolabelling approaches were used to describe the cellular sy nthesis of fibronectin and its distribution throughout the rat heart f rom 11 d postconception until adulthood. The distribution of fibronect in was compared to that of laminin and of alpha type III procollagen. Results: The accumulation and pattern of distribution of the major fib ronectin mRNA isoforms were identical, that is, there was a progressiv e decrease in their accumulation as a function of time after 11 d post conception, resulting in a complete absence in the adult. The distribu tion of fibronectin and procollagen type III mRNAs were, however, quit e distinct. At the protein level the time course of synthesis and secr etion of the locally synthesised fibronectin (c-FN) did not follow fib ronectin mRNA expression, the accumulation of the protein being rather poor, except just before birth. where it was found mainly in the coro nary vessels. Conclusions: During the development of the fetal rat hea rt fibronectin gene transcription is active and progressively decrease s with age, whereas the translation of the mRNAs into their correspond ing proteins is always relatively poor. If fibronectin is involved in fetal and postnatal morphogenesis of the rat myocardium, it is the pla sma form (p-FN) that is most probably involved in the process of growt h and differentiation of the rat heart.