The ultrastructure and connections of blood vessels supplying the rat carotid body and carotid sinus.

Abstract

The morphology of blood vessels supplying the carotid body and carotid sinus was analysed in 41 rats by using a combination of light microscopic, transmission electron microscopic and scanning electron microscopic methods. We found that a large sphincter-like intimai cushion was located at the orifice of thecarotid body artery, where the vessel arose from the external carotid or occipital artery. The sphincter contained circumferential smooth muscle and constricted the diameter of the orifice to less than half. After reaching the carotid body, the carotid body artery typically divided into three or four first-order and five or more second-order branches. Usually three or foursecond-order branches supplied the carotid body, but all other branches continued on to such structures as the superior cervical ganglion, nodose ganglion, vagus nerve and carotid sinus.Third andfourth-order branches gave rise toterminal arterioles that supplied the glomus tissue. Vessels resemblingprecapillary sphincters were located at the junction of terminal arterioles and capillaries. Precapillary sphincters had a wall comprised of protruding endothelial cells surrounded by smooth muscle cells or pericytes. Most terminal arterioles gave rise to two types of capillaries.Type I capillaries penetrated a glomus cell cluster and had an intimate association with glomus cells of that cluster had a luminal diameter ranging from about 8 to over 20 μm, but varied in calibre along their length. These vessels followed a winding course, made one or more U-shaped turns, and usually had multiple connections with venules. Type I capillaries had a thin fenestrated endothelium, an incomplete covering of pericytes, and a thin basal lamina. By contrast,type II capillaries did not penetrate glomus cell clusters, had a uniform diameter of about 7 μm, and had both straight and curved regions. Both types of capillaries were bypassed byarteriovenous anastomoses formed by terminal arterioles that joined small venules directly.Venules of the carotid body were interconnected with one another and joined major veins of the neck via several routes. Arterioles derived from the carotid body artery also supplied an extensive network of vasa vasorum in the adventitia of the carotid sinus. Short capillaries and larger shunt vessels connected arterioles with the numerous venules in the sinus wall; and the venules in turn were connected to the venous plexus at the surface of the carotid body. We conclude that the arterial branching pattern, intimai cushions and precapillary sphincters participate in the control of carotid body blood flow and also may influence plasma skimming. However, the existence of arteriovenous anastomoses in addition to at least two types of capillaries indicates that blood flow to the chemoreceptive tissue can be regulated independently of total blood flow. Furthermore, the redundancy of venous connections may be related to the sensitivity of carotid body chemoreceptors to changes in venous pressure.