THE FINE STRUCTURE OF THE MESENTERIC ARTERIES OF THE RAT.

An account is given of the ultrastructural details of normal rat mesenteric arteries. Material was obtained from 150-gm rats and processed for electron microscopy. In general terms, the arteries were found to be similar in morphologic detail to previous descriptions of other normal arteries. They were composed of three cellular layers, intima, media and adventitia, separated by two elastic laminae. The intima included a lining of endothelial cells lying on an internal elastic lamina (IEL). Collagen fibers were often found lying between the IEL and endothelium. The IEL comprised randomly orientated fibers lying in a homogenous matrix. Spindle-shaped smooth muscle cells arranged in a helical fashion and separated by collagen and small elastic strips formed the media. Depending on the size of the artery, an external elastic lamina (EEL) was present separating the media from the adventitia; the latter contained nerves, fibroblasts and collagen. Bundles of collagen were arranged longitudinally, transversely and radially. The fibroblasts were long, spindle-shaped cells with much ergastoplasm. Schwann cells containing many unmyelinated axons were located, in a few cases, extremely close to the outermost muscle cell layer of the media. Occasionally, projections from these muscle cells containing mitochondria and vesicles protruded through pores in the EEL into Schwann cells. [ABSTRACT FROM AUTHOR]