Fibronectin distribution during early chick embryo heart development.

The distribution of fibronectin (FN) during early stages of chick embryo heart development has been studied by indirect immunofluorescence methods. The cardiac extracellular matrix (cardiac jelly) was almost devoid of FN-positive material throughout the period studied (stages 8-18). Intensely extracellular fluorescent material was only demonstrated at the heart midline and in the dorsal mesocardium. Fluorescence associated with the basal surface of the myocardium was demonstrated first at the time of fusion of the two heart tubes. While the heart remains attached to the embryonic trunk by the dorsal mesocardium, two different myocardial basal zones can be distinguished according to the intensity of fluorescence: an intensely stained dorsal zone and a much less fluorescent ventral zone. The endocardium did not present a strongly fluorescent basement membrane until stage 13. The intensity of fluorescence of the endocardial basal surface varied according to the rostrocaudal levels of the heart and also to the development stage of the embryo. The levels of fluorescence increased in myocardium and endocardium at the onset of trabeculation but decreased as trabeculation was completed. The quantitative and qualitative variations of FN distribution have been associated with a number of developmental events.