In this higher-magnification view of a portion of the wall of the vein in the figure above, the endothelial cells (EN) are more easily recognized and are seen to be plumper than those of the arterial endothelium. The margin between the tunica intima (TI) and the thin tunica media (TM) is difficult to discern, but the smooth muscle cells (SM) in the thin media are more easily recognized than in the figure above because of the shape of their nuclei and the slight basophilia of their cytoplasm. The tunica adventitia (TA) is about twice as thick as the tunica media and appears to contain only bundles of collagen fibers and fibroblasts, with the latter recognizable by their nuclei (N). The collagen bundles of the loose connective tissue beneath the tunica adventitia are larger than those of the adventitia, and there are fewer cells in this portion of the specimen.
PLATE 35 Arterioles, Venules, and Lymphatic Vessels
The terminal components of the arterial tree just before a capillary bed or an arteriovenous shunt are the arterioles. Arterioles have an en-dothelial lining and smooth muscle in the wall but the smooth muscle is limited in thickness to one or two cells. There may or may not be an internal elastic membrane, according to the size of the vessel. Arterioles control blood flow into capillary networks. In the normal relationship between an arteriole and a capillary network, contraction of the smooth muscle of the arteriole wall reduces or shuts off the blood going to the capillaries. A precapillary sphincter is formed by a slight thickening of the smooth muscle at the origin of a capillary bed from an arteriole. Nerve impulses and hormonal stimulation can cause the muscle cells to contract, directing blood into capillary beds where it is most needed.
Arteriole, venule, and small nerve, fngertip, human, H&E ×600.
This micrograph shows two cross-sectioned arterioles (A) and a venule (V). The arteriole on the left is identified as a large arteriole, based on the presence of two discrete layers of smooth muscle cells that form the tunica media of the vessel. The nu clei of the muscle cells appear in longitudinal profile as a result of the cir 438 cumferential arrangement of the cells. The endothelial cell nuclei of the vessel appear as small round profiles surrounding the lumen. These cells are elongate and oriented with their long axis in the direction of flow. Thus,
Arteriole, fngertip, human, H&E ×350.
This micrograph shows a longitudinal section of an arteriole. Because of its twisting path through the section, its wall has been cut such that the single layer of muscle cells of the tunica media is seen in different planes along its length. In the segment numbered 1, at the left, the vessel wall has been cut tan gentially. Thus, the vessel lumen is not included in the plane of section, but the smooth muscle cell nuclei of the tunica media are seen in longitudinal
Lymphatic vessel, fngertip, human, H&E ×175.