Cardiac fibronectin: Developmental distribution and quantitative comparison of possible sites of synthesis

Evidence accumulating from other cell systems suggests that fibronectin, if present in the heart, might modulate patterns of cell organization during myocardial development. Accordingly, we examined hearts of fetal, neonatal and adult rats for the presence and distribution of fibronectin, and quantitatively measured fibronectin synthesis by cultured cardiac myocytes and mesenchymal cells to assess the relative importance of these potential intracardiac sources of this glycoprotein. Fibronectin was present in the hearts from all age groups and was distributed in a reticular pattern suggesting a pericellular or interstitial location. In fetal hearts, specific immuno-fluorescence was particularly prominent at the crest of the developing interventricular septum. In post-confluent cardiac mesenchymal cultures, a dense fibrillar network of fibronectin formed over the surface of the cells. The content of fibronectin in the culture medium assessed by enzyme-linked immunosorbent assay increased substantially at confluence from 2 to 17 mg/dish, stabilizing at 11 mg in post-confluent mesenchymal cultures. In myocytic cultures, fibronectin formed a fine reticular pattern over cell surfaces and the fibronectin content of the medium ranged from 0.8 to 2.1 mg. Thus, fibronectin is present in the heart, may be synthesized in the neonate predominantly by cardiac mesenchymal cells, and may contribute by mechanisms still to be characterized in vivo to normal development of the critical septal region during embryogenesis.