An electron microscopic study of the cardiac innervation in larval lamprey

Larval lampreys (Lampetra japonica) 13 and 21 mm in body length were examined by serial section electron microscopy and it was found that the young 13-mm larvae which was 26 days old had no nerves to, and in, the heart. However, the heart of 21-mm larval lampreys had two sets of nerve fibers entering the heart. One of the nerve fibers entered the heart via the porta venosa, ran along the vena jugularis impar, and ended in the sinus venosus. The other nerve entered with the porta arteriosa and terminated in the proximal region of the bulbus cordis. Two characteristic types of nerve endings were observed. One type of nerve ending contained numerous, small, clear vesicles about 40 nm in diameter. These endings were found only in the walls of the vena jugularis impar and the sinus venosus. The second type of ending characteristically contained distinctive large-cored vesicles 60-130 nm in diameter mixed with numerous small, clear vesicles. These endings were present in the walls of the vena jugularis impar, the sinus venosus, and the bulbus cordis. It should be emphasized that the bulbus contained only the second type of nerve ending. The nerves in the heart were confined to specific regions and those from the two sources remained separate. Furthermore, the atrium, ventricle, ducts of Cuvier, and hepatic veins were completely devoid of nerves. There were no ganglion cells in any region of the heart.