ULTRASTRUCTURE OF SMOOTH MUSCLE CELL OF CAT NICTITATING MEMBRANE

Abstract

The smooth muscle layer of cat nictitating membrane was fixed in permanganate, embedded in epon and observed with electronmicroscope. The smooth muscle cells are enclosed with basement membrane, except where the specialized cell membrane interconnection (intrusion body) is present. This intrusion body is frequently observed in the areas where nerve elements are sparse. Fused membrane or quintuple layered system can not be observed.The unmyelinated nerve endings are distributed among the smooth muscle cells. In the wide extracellular spaces where the nervous elements are rather scant, naked axons are found with basement membrane. But when the nerve terminals are abundant, naked axons keep often intimate relationship with the muscle cells. They are either contact directly with the muscle cells without basement membrane or embedded deep in muscle cells.Occasionally, the smooth muscle cell membrane at the point of contact with the nerve ending shows the presence of endoplasmic reticulum and pinocytotic vesicles. The closest apposition of the axon and the muscle cell is about 80 A. At these contact points, the specialized structure such as secondary synaptic cleft seen in skeletal muscle is not clear. The synaptio vesicles in the axon are divided into two types following their average diameters of 500 A and 800 A. The smaller vesicles with 500 A diameter could further be classified into the granular and agranular types, and they are often found in the same axon. The functional differentiation of them is impossible at present.