The vasa vasorum (i.e. ‘vessels of the vessels’, VV) is a network of nourishing small vessels in the walls of large blood vessels giving rise to a capillary network within the tunica adventitia, which may extend into the adjacent tunica media (Wheater et al. 1987). Veins are supplied much more abundantly with VV than arteries. Retrograde filling of VV in saphenous veins (SV) used in coronary artery bypass surgery has been described, suggesting that in certain cases VV may empty into the lumen of this vessel (Souza, 2002). We have used a combination of immunohistochemistry and light microscopy (LM) and transmission (TEM) and scanning (SEM) electron microscopy to identify VV in SV used as bypass conduits.

Following local ethics committee approval and patients’ informed consent, segments of human SV (n = 10) were collected from patients undergoing coronary artery bypass surgery. For the LM study, frozen transverse sections were used for the immunohistochemical identification of endothelial cells (CD31) and vascular smooth muscle cells (anti-alpha smooth muscle actin) using the ABC peroxidase method. Microscopic examination revealed the distribution of endothelial and smooth muscle cells. Standard EM procedures (JEOL-1010 TEM and JEOL-4010LV SEM) were also used on the same tissue.

Endothelial cells lining the lumen were observed in all vessels as well as clusters that were scattered throughout the vessel wall. Although the majority of these were located in the adventitia and media, in five of the vessels, endothelial cells were located within 20 µm of the intima, and in two cases the VV terminated in the vessel lumen (Fig. 1). Endothelial cells often co-localised with vascular smooth muscle cells in microvessels measuring approximately 25 µm in diameter. TEM and SEM of SV confirmed the presence of VV-like profiles at the adventitia-media border as well as in the intimal layer.

The VV are situated in close proximity to nerves in the SV. Furthermore, these microvessels are embedded within cells that are potential sources of numerous factors possessing both vasoactive and proliferative properties. The observation that VV empty into the lumen of the SV suggests that this microvessel network may represent a transport system for the local delivery of tissue- or nerve-derived factors within this vessel.

M.R.D. and R.A. were supported by the British Heart Foundation.