Similar encapsulated receptors, Golgi tendon organs, are found in the tendons of muscle and respond to increased tension on the muscle. These receptors contain only sensory (afferent, Ib) nerve fibers, and they monitor muscle tension (or the force of contraction) within an optimal range.
Development, Repair, Healing, and Renewal
Development of myogenic stem cell linage depends on expression of various myogenic regulatory factors.
Myoblasts are derived from a self-renewing population of multipotential myogenic stem cells that originate in the embryo from unsegmented paraxial mesoderm (cranial muscle progenitors) or segmented mesoderm of somites (epaxial and hypaxial muscle progenitors). Early in  embryonic development, these cells express MyoD transcription factor, which, along with other myogenic regulatory factors (MRFs), plays a key role in activation of muscle-specific gene expressions and differentiation of all skeletal muscle lineages. A balancing effect on skeletal-muscle development is achieved by the expression of  negative regulatory myostatin gene, which leads to synthesis of myostatin, a 26-kilodalton protein belonging to the bone morphogenetic protein/transforming growth factor(BMP/TGF-) protein superfamily. Myostatin exerts an inhibitory effect on muscle growth and differentiation. It is thought that MyoD preferentially upregulates myostatin gene expression and controls myogenesis during not only the embryonic and fetal periods but also postnatal stages of the development. The hypermuscular phenotypes observed on inactivation of the myostatin gene in animals and humans have confirmed the role of myostatin as a negative regulator of skeletal-muscle development. Experimental studies have demonstrated that muscle mass increases through myostatin inhibition, and the myostatin signaling
FIGURE 11.12 • Muscle spindle. a. Schematic diagram of a muscle spindle. The diameter of the spindle is expanded to illustrate structural details. Each spindle contains approximately two to four nuclear bag fibers and six to eight nuclear chain fibers. In the nuclear bag fibers, the muscle fiber nuclei are clumped in the expanded central portion of the fiber, hence the name bag. In contrast, the nuclei concentrated in the central portion of the nuclear chain fibers are arranged in a chain. Both afferent Ia (sensory) and  efferent (motor) nerve fibers supply muscle spindle cells. The afferent nerve fibers respond to excessive stretching of the muscle, which in turn inhibits the somatic motor stimulation of the muscle. The efferent nerve fibers regulate the sensitivity of the afferent endings in the muscle spindle. b. Photomicrograph of a cross section of a muscle spindle, showing two bundles of spindle cells in the encapsulated, fluid-filled receptor. In one bundle, several of the spindle cells are cut at the level that reveals their nuclei. An internal capsule surrounds the spindle cells. The external capsule of the muscle spindle and the adjacent perimysium can be seen as a faint double-layer boundary of the receptor. Immediately above and outside of the muscle spindle is a nerve that may be supplying the spindle. The several types of nerves associated with the spindle cells as well as the type of spindle cells cannot be distinguished in this H&E–stained section. Near one of the bundles of spindle cells is a small blood vessel. The flocculent material within the capsule consists of precipitated proteoglycans and glycoproteins from the fluid that filled the spindle before fixation. 550.
pathway may be a potent therapeutic intervention point in the treatment of muscle-wasting diseases, such as muscular dystrophy, amyotrophic lateral sclerosis (ALS), AIDS, and cancer. Pharmacologic manipulation of myostatin expression could also lead to the development of new therapeutic approaches in a variety of musculoskeletal pathologies.
Skeletal muscle progenitors differentiate into early and late myoblasts.