An ultrastructural and fluorescence histochemical investigation of the innervation of retial arteries in Monodon monoceros

Abstract

In this study, the innervation of cerebrally related retial arteries in the narwhal Monodon monoceros was examined. Vessels were processed for the demonstration of adrenergic nerve endings by fluorescence histochemistry, and the results were confirmed by electron microscopy. Innervation of cerebrally related retial arteries was compared to that of a system situated in the haemal canal and supplying the tail. The retial arteries were poorly innervated. Adrenergic nerve endings, as indicated by fluorescence, occurred only in caudal portions of the spinal rete. Ultrastructurally, nerves were found in most retial vessels examined. However, except for arteries from caudal portions of the spinal rete, nerve numbers were few and because they occurred in outer layers of the adventitia were probably not functionally significant. In contrast, vessels in the haemal canal were well innervated. Nerve endings possessing neurotransmitter vesicles were adjacent to the smooth muscle cells. The cetacean rete mirabile, a system which supplies blood to the entire central nervous system, is apparently not under extensive nervous control, even though most reports suggest there is a relationship, possibly based on the presence of adjacent nerve trunks. Any vasomotor activity that does occur, possibly does so in response to catecholamines or other vasoactive agents circulating in the blood.

Dynamics of blood circulation during diving in the bottlenose dolphin (Tursiops truncatus). The role of the retia mirabilia

“…The rete mirabilis has been considered to receive considerable adrenergic control (Diéguez et al, 1983) and exert a dampening effect on the brain circle of ruminants (Lluch et al, 1985). But a specific study on the innervation of the retia mirabilia (in the narwhal) identified only a few nerve fibers (Vogl et al, 1981). Although we have shown that nerve fibers are present within the retia (Fig.…”

Cardiovascular design in fin whales: high-stiffness arteries protect against adverse pressure gradients at depth

“…Direct neural control of retial arteries would allow rapid blood redistribution (Nagel et al, 1968), but although ample innervation runs in close proximity to dolphin and fin whale rete (Nagel et al, 1968;Ommanney, 1932), direct adrenergic connections have been shown to be poor in the rete of the narwhal (Vogl et al, 1981). Pfeiffer et al (Pfeiffer and Kinkead, 1990), studying nonthoracic retial arteries from the bowhead whale, Balaena mysticetus, came to a similar conclusion based on a number of morphological features including the observation of ganglia-like complexes in the media, but no diffuse neurons.…”

On how whales avoid decompression sickness and why they sometimes strand

“…This also implies that the velocity of the blood in traversing the rete will be reduced almost to nil, which will greatly increase the chances for trans-vascular exchange between the blood and the surrounding adipose tissue. Both Vogl and associates (Vogl et al, 1981;Vogl and Fisher, 1982) and Melnikov (Melnikov, 1997) mention that the retia consist of thick-walled arteries surrounded by fat, but this statement of fact was not taken any further. It has been known, however, for more than 100years (Vernon, 1907) that nitrogen is at least six times more soluble in fat than in water.…”

“…Vogl and associates (Vogl et al, 1981) reported that no adrenergic endings could be demonstrated in vessels of any of the retia and suggested that any vasomotor activity that may occur probably does so in response to catecholamines or other vasoactive agents circulating in the blood. The lack of innervation of the retial arteries was confirmed in the present study, while Nagel and associates reported that they 'noted what appears to be an elaborate innervation to these retial vessels'.…”