Smooth muscle cells in the body are connected to each other through adherens junctions, which are thickened regions of cell membranes with a small gap containing dense granular material. These cells do not fuse during embryonic development and each cell has a single centrally located nucleus. Compared to skeletal muscle cells, smooth muscle cells are smaller in size but still quite large, typically measuring 40-600 µm in length. They have a diameter of 2 to 10 µm near the nucleus and taper towards their ends. When these cells contract, they become distorted due to the force exerted on them by attachments to other cells or the extracellular matrix. As a result, cross sections of these cells often appear irregular. Unlike skeletal muscle cells, smooth muscle cells do not have T tubules. However, they do have longitudinal rows of small sac-like structures called caveolae on the sarcolemma, which is the cell membrane. These caveolae serve as inpocketings and provide a unique structural feature of smooth muscle cells.
B Surface couplings Sarcoplasmic reticulum 0.5 mµ0.2 mµ 0.5 mµ •Fig. 14.3 Junctions,membranes,andmyofilaments,insmoothmuscle.A,TransmissionelectronmicrographofGapjunctionbetweenintestinalsmoothmusclecells.B,Longitudinalviewofapulmonaryarterysmoothmusclecell.Thesarcoplasmicreticulumisstainedwithosmiumferricyanideandappearstoformacontinuousnetworkthroughoutthecellconsistingoftubules,fenestratedsheets(long arrows), andsurfacecouplingsatthecellmembrane(short arrows). C,Transversesectionofabundleofvenoussmoothmusclecellsillustratingtheregularspacingofthickfilaments(long line) andtherelativelylargenumberofsurroundingthin(actin)filaments(inset). Densebodies(arrowheads) aresitesofattachmentforthethinactinfilamentsandequivalenttotheZlinesofstriatedmuscles.Elementsofsarcoplasmicreticulum(short line) occurattheperipheryofthesecells.D,Scanningelectronmicrographoftheinnersurfaceofthesarcolemmaofanintestinalsmoothmusclecell.Longitudinalrowsofcaveolaeprojectintothemyoplasm(small, light-colored spheres), surroundedbydarkerelementsofthetubularsarcoplasmicreticulum.Theattachmentsofthinfilamentstothesarcolemmabetweentherowsofmembraneelementswereremovedduringpreparationofthespecimen.(AandD,FromMottaPM[ed].Ultrastructure of Smooth Muscle. Norwell,MA:KluwerAcademic;1990.BandC,FromSomlyoAP,SomlyoAV.Smoothmusclestructureandfunction.In:FozzardHAetal.[eds].The Heart and Cardiovascular System.2nded.NewYork:RavenPress;1992.) muscle cells. The adherens junction appears as thickened regions of opposing cell membranes that are separated by a small gap (≈60 nm) containing dense granular material. Thin filaments extend into the adherens junction to allow the contractile force generated in one smooth muscle cell to be transmitted to adjacent smooth muscle cells.
Embryonic smooth muscle cells do not fuse, and each differentiated cell has a single centrally located nucleus. Though dwarfed by skeletal muscle cells, smooth muscle cells are nevertheless quite large (typically 40–600 µm long). These cells are 2 to 10 µm in diameter in the region of the nucleus, and most taper toward their ends. Contracting cells become quite distorted as a result of the force exerted on the cell by attachments to other cells or to the extracellular matrix, and cross sections of these cells are often very irregular.
Smooth muscle cells lack T tubules, the invaginations of the skeletal muscle sarcolemma that provide electrical links to the sarcoplasmic reticulum (SR). However, the sarcolemma of smooth muscle has longitudinal rows of tiny sac-like inpocketings called caveolae (see