Fig. 4. (A) Light micrograph of lateral collecting vessel, showing a thick endothelial layer, but no distinct medial layer. The vessel sits within a tube of dense connective tissue, formed by two opposed crescent-shaped structures. Numerous blood vessels are readily observed within the `adventitial' layer (arrowheads). Scale bar, 100 µm. Lissamine Fast Red, 5 µm section. (B) Transmission electron micrograph of the lateral collecting vessel shows that the endothelial cell layer (EC) sits on an elastic lamina (EL), and there is no presence of smooth muscle cells in the vessel. Leukocytes can be seen migrating through the endothelial layer (arrowheads). Blood vessels are frequently seen within the adventitial layer of the vessel wall. Scale bar, 20 µm.