SPECIALIZED SYMPATHETIC NEUROEFFECTOR ASSOCIATIONS IN RAT IRIS ARTERIOLES

Vascular sympathetic neuroeffector associations have been examined in rat iris arterioles using serial section electron microscopy and recon struction techniques. Examination of random sections showed that, of a ll profiles of varicosities (199) seen to lie closer than 4 mu m to va scular smooth muscle cells, only a small proportion (29/199) were foun d in close association with vascular smooth muscle cells, where adjace nt membranes were separated by less than 100 nm. However, serial secti on examination, from intervaricose region to intervaricose region, of 79 varicosities similarly observed lying within 4 mu m of vascular smo oth muscle cells showed that 54 formed close associations with vascula r smooth muscle cells. In serial sections, all these varicosities were also closely associated with melanocytes and of the 25 remaining vari cosities, 22 formed close associations with melanocytes alone, whilst 3 did not come into close association with any effector cell. The incr eased observation of close associations with vascular smooth muscle ce lls in serial sections, compared with random sections, is consistent w ith the demonstration that the area of contact only occupies, on avera ge, a small percentage (5%) of the total surface area of the varicosit y as seen in the 3-dimensional reconstructions. In both random and ser ial sections, close associations were observed between varicosities an d vascular smooth muscle cells or melanocytes irrespective of whether fibres were present singly or in small nerve bundles. Three-dimensiona l reconstruction of associations of varicosities and vascular smooth m uscle cells demonstrated several common features, such as accumulation s of synaptic vesicles and loss of Schwann cell covering at the region of membrane facing the effector cell. The similarity in the appearanc e of the neuroeffector association seen in this study and those descri bed in previous studies provides evidence for the existence of a commo n sympathetic neuroeffector association, irrespective of the receptor subtype involved in neurotransmission.