Morphological innervation pattern of the developing rabbit heart

The morphological innervation pattern of developing fetal and neonatal rabbit hearts was delineated histochemically by a cholinesterase/silver procedure and immunohistochemically with the monoclonal antibody HNK1, an antibody which recognizes some cells derived from neuroectoderm. Cholinesterase-containing nerves appeared distally on the outflow tract by gestational day 15 (G15). Isolated cells with cholinesterase-stained fine processes were present near the base of the pulmonary trunk. HNK1 antibody stained the same nerves and ganglia revealed by the cholinesterase reaction and other nerves in the rabbit heart. It was used to confirm that cells with fine neuron-like processes were present before nerve ingrowth. The G14 heart contained many HNK1 staining cells in the right atrium, outflow, and inflow tracts; cells with fine processes were few but increased at G16. By G17, a plexus of interweaving nerves and associated cells began to form at the base of the pulmonary trunk. Fine nerves encircled the base of the aorta, and others crossed the intercaval region dorsally. At G19, nerves 1) extended downward from a rich "bulbar" plexus along the front ventricular surface, 2) grew near the epicardial surface at the base of the heart along the atrial floor and ventricular roof, 3) traversed the vena cavae and intercaval region to enter the atrial roof, and 4) crossed the coronary sinus to reach the back ventricular walls. By G23, cholinesterase-staining nerves and ganglia in the atria and, epicardially, in the ventricles formed the general innervation pattern of the newborn and adult rabbit heart.