The distribution and ultrastructure of sensory elements in the baroreceptor region of the truncus arteriosus of the lizard Trachydosaurus rugosus.

The proximal truncus arteriosus of the lizard Trachydosaurus rugosus was studied with light-, fluorescence- and electron-microscopical techniques. Three vessels comprised the truncus; the pulmonary, left aortic, and carotico-aortic arteries. Right and left truncal nerves, each derived from the ipsilateral vagus nerve, innervated the truncus, particularly its proximal 3 mm. Ultrastructurally, the nerves had a variety of appearances: some were clearly adrenergic, c-type or p-type. A number of profiles contained large numbers of mitochondria and were classified as sensory. Some profiles defied exact classification, having characteristics common to two different types of profile. Within the outer medial layers, profiles up to 7 micrometers in diameter were found. These contained large numbers of mitochondria, myelin bodies and structures intermediate between the two. In addition, the profiles contained large amounts of glycogen and small numbers of vesicles. These nerve fibres were classified as baroreceptors, since they closely resemble carotid sinus and aortic arch baroreceptors in mammals. Large numbers of chromaffin cells were found, particularly in the common wall of the pulmonary and left aortic arteries. Many of these cells emitted a long tapering process, which sometimes entered a nearby nerve bundle. Sensory, p-type and c-type profiles, but not adrenergic profiles, made extensive close contacts with chromaffin cells.