Each muscle fiber contains multiple nuclei that lie immediately beneath the sarcolemma, which is its plasma membrane. A small proportion of the nuclei at the periphery of the myofiber are stem cells, also called satellite cells, which can repopulate damaged fibers after injury.5 The cytoplasm, called sarcoplasm, is similar to that of other cells. It contains a cellular matrix, organelles, and a variety of other molecules. Among the organelles, the Golgi apparatus and mitochondria are abundant and lie close to the nuclei. The sarcoplasmic reticulum is a continuous branching network of membrane, which is a specialized form of endoplasmic reticulum that is unique to muscle. Glycogen, lipid droplets, and myoglobin are among other cytoplasmic components.
Other cell types within muscle include fibroblasts, endothelial and smooth muscle cells constituting blood vessels, and Schwann cells around the sheath of nerve axons.
The cells in tendons are called tenocytes. These cells are typical fibroblasts with long cytoplasmic extensions. The cell density of tendons is similar to that of ligaments but lower than that of other tissues such as bone marrow or liver, conferring mechanical strength to these structures.
External to the sarcolemma is a basement membrane that merges with the extracellular matrix (ECM) to form the endomysium. The basement membrane is rich in protein and carbohydrate components, including collagen, laminin, fibronectin, and a variety of glycoproteins.
The ECM of tendons is composed of dense, parallel bundles of collagen fibers. These bundles are oriented along the line of tension between muscle and bone insertion for maximal transmission of load, and they have less crimp than the collagen bundles in ligaments. The collagen is almost 95% type I, with the remainder primarily type III col- lagen and proteoglycans. Tendons generally attach to bone via a specialized direct inser- tion site that has four zones: tendon, fibrocartilage, mineralized fibrocartilage, and bone. Sharpey’s fibers are collagen bundles that extend from the tendon or periosteum into the bone.
Blood vessels run parallel to the axis of myofiber in the connective tissue, often with several capillaries around each myofiber. They are arranged with enough redundancy to permit changes in length during the contraction-extension cycle of a muscle.