The musculoskeletal system consists of different connective tissues arranged in two systems: skeletal and muscle. A considerable part of the extracellular matrix (ECM) of connective tissue is filled by a network of proteins and polysaccharides produced by local cells. In cartilage and bone, these cells are chondroblasts and osteoblasts, which belong to the family of less specialized but most versatile fibroblasts. The major components of the ECM are proteoglycans (PGs), which regulate the activity and stability of other matrix proteins, and signaling molecules.
Skeletal system The skeletal system provides support and locomotion to the body structures by means of three specialized tissues: cartilage, bone, and skeletal muscle. Cartilage consists of chondrocytes embedded in a matrix rich in collagen fibers, PGs, and elastin fibers whose relative amounts and organization define three main types of cartilage: hyaline, elastic, and fibrocartilage. Cartilage tissue is usually found in close association with bone, acting as the framework for bony skeleton growth. Unlike other connective tissues, cartilage is avascular and has a limited regenerative capacity; however, it can grow by expanding as the chondrocytes divide and secrete matrix.
Hyaline cartilage, the most common, is durable, yet flexible and elastic. It is rich in collagen and PGs uniformly distributed, whereby it performs as a good shock absorber. It is the main cartilage in embryonic skeleton, persisting in adults at the end of bones to form the articular surface of synovial joints. Cartilage participates in bone growth, acting as a template during endochondral ossification. Elastic cartilage not only has a similar composition, but also contains elastin fibers dispersed throughout the matrix. It helps to maintain the shape and flexibility of organs, while also supporting these structures. Fibrocartilage is found in areas subjected to frequent stress, like intervertebral disks, the pubic symphysis, and the attachments of tendons and ligaments.
Besides cartilage, bone tissue constitutes the main component of the skeleton. It consists of osteoblasts and osteoclasts, surrounded by a rigid matrix of collagenous fibers, calcium salts, and water. It is a highly dynamic tissue that grows by apposition and is subjected to a continuous remodeling greatly affected by mechanical, metabolic, and endocrine stimuli. In addition to its structural function, bone fulfils two key metabolic functions: maintenance of mineral homeostasis and hematopoiesis.
Muscle system
Supportive and locomotor features of the musculoskeletal system are only possible with the aid of muscles. Muscle tissue is characterized by its ability to contract and relax under nervous stimuli. There are three types of muscle: the main type, skeletal muscle or “voluntary,” is responsible for the skeletal movement through anchoring to the bone by tendons and ligaments. Smooth muscle or “involuntary” is mainly found in internal organs, blood vessels, intestines, etc. The third type, only present in the heart, is also an involuntary muscle, the cardiac muscle.
The joints
Joints are sites where bones meet. According to their ability to allow or disallow different movements, and the type and function of the cartilage present, joints are classified into three groups: fibrous joints, like skull sutures, which connect bones without allowing any movement (synarthroses); cartilaginous joints, attached by cartilage and with limited movement, like the spine or ribs (amphiarthroses); and diarthroses, the most versatile and majoritary synovial joints, which are freely movable and are present in shoulders, elbows, wrists, hips, fingers, knees, and ankles.
The basic structure of a synovial joint consists of bone ends coated with articular cartilage specialized in shock absorption which provide a smooth, frictionless surface during movement. A capsule made of two layers encloses the joint. The outer rigid membrane has structural functions, connecting bones to keep them in place. The vascularized inner membrane is a metabolically active connective tissue called synovium whose main cells are synoviocytes. These can be type A or macrophagic, derived from blood mononuclear cells, and type B or fibroblast-like synoviocyte (FLS) cells, involved in the production of matrix and synovial fluid (SF) constituents.
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Conceptualmente una célula madre (stem cell) es aquella a partir de la que se originan las demás células. Biológicamente la diferencia fundamental entre una célula madre y otro tipo de células reside en dos características particulares: una, la capacidad de dividirse de forma perpetua (auto renovación) y otra, el potencial intrínseco para diferenciarse en otros tipos célulares (multipotencialidad).