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 numerous binding sites to the other elements of the matrix and to the cells. The aim of this study was to determine the relative importance of fibronectin isoform expression (FN-EIIIA, FN-EIIIB) during fetal and postnatal development of the rat heart. Methods: In situ hybridisation and immunolabelling approaches were used to describe the cellular synthesis of fibronectin and its distribution throughout the rat heart from 11 d postconception until adulthood. The distribution of fibronectin was compared to that of laminin and of alpha type III procollagen. Results: The accumulation and pattern of distribution of the major fibronectin mRNA isoforms were identical, that is, there was a progressive decrease in their accumulation as a function of time after 11 d postconception, resulting in a complete absence in the adult. The distribution of fibronectin and procollagen type III mRNAs were, however, quite distinct. At the protein level the time course of synthesis and secretion of the locally synthesised fibronectin (c-FN) did not follow fibronectin mRNA expression, the accumulation of the protein being rather poor, except just before birth. where it was found mainly in the coronary vessels. Conclusions: During the development of the fetal rat heart fibronectin gene transcription is active and progressively decreases with age, whereas the translation of the mRNAs into their corresponding proteins is always relatively poor. If fibronectin is involved in fetal and postnatal morphogenesis of the rat myocardium, it is the plasma form (p-FN) that is most probably involved in the process of growth and differentiation of the rat heart.