Distribution of antigen epitopes shared by nerves and the myocardium of the embryonic chick heart using different neuronal markers.

We examined which neuronal elements and nonneuronal tissues in the embryonic myocardium are stained with antibodies traditionally used for staining nerve tissue. Furthermore, we studied whether nonneuronal myocardial staining was confined to regions determining initial nerve entry points and development of cardiac ganglia. The third focus was whether nerves preferentially distribute in regions of the conduction system. Different neuronal markers were used such as the HNK-1 antibody against neural crest and nerve tissue, Tyrosine Hydroxylase antibody (TH) against putative sympathetic nerve tissue, anti-GFAP against glia cells, antibodies against phosphorylated neurofilaments DO170, RMO270, 3A10, and RT97, and finally the antibody Snap25 against a synaptic protein. Chick embryonic hearts between stage HH25-44 where immunohistochemically evaluated. Transient HNK-1 staining in the basal region of the heart coincided with ingrowing vagal branches and crest-derived neuronal precursor cells seeding the region of the atrioventricular sulcus, suggesting a role for HNK-1 in the homing of the parasympathetic plexus. Transient TH staining was confined to regions of the atrial myocardium coincident with the localization of the few early TH-positive nerve fibers before stage HH40, whereas the second wave of TH-positive nerve fibers at HH42 was mainly localized around myocardial coronary arteries. This transient myocardial TH staining might be involved in early emergence of the catecholaminergic phenotype, while coronary arteries or blood borne factors might be involved in later differentiation. Some myocardial expression, not related with initial nerve ingrowth, using Snap25, TH, HNK-1, DO170, and RMO270 was confined to regions of the ventricular conduction system. HNK-1 is the only marker staining the region of the putative sinoatrial node. Just before hatching nerve fibers, including TH-positive nerve fibers, are uniformly distributed throughout the myocardium, without being specifically confined to regions containing the conduction system or coronary arteries.