Porth's Essentials of Pathophysiology, 4e

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Musculoskeletal Function

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where flexibility is important, such as in the auricle of the ear, the auditory canal, the epiglottis, and the larynx. Fibrocartilage is characterized by a matrix containing a combination of chrondrocytes and dense fibrous tissue. It is found in areas subjected to pulling forces such as attachments of ligaments to the cartilaginous surface of bones, the symphysis pubis, and intervertebral disks. Hyaline cartilage is the most abundant and the best studied of the three types. It forms the articulating sur- faces of the moveable joints in the body, costal carti- lages that join the ribs to the sternum and vertebrae, and many of the cartilages of the respiratory tract. Hyaline cartilage is also essential for growth before and after birth. In the embryo, most of the axial and appendicular skeleton is formed first as a cartilage model and then is replaced by bone. In postnatal life, hyaline cartilage continues to play an essential role in the growth of long bones and persists as articular cartilage in the adult. The matrix of hyaline cartilage is composed of col- lagen fibers embedded in a firm, hydrated gel of struc- tural glycoproteins and proteoglycans, which have a high affinity for water. This high degree of hydration in the cartilage matrix contributes to the weight-bearing properties of cartilage and allows the cartilage matrix to respond to varying pressure loads. The multiadhesive glycoproteins influence interactions between the chon- drocytes and matrix molecules and have clinical value as markers of cartilage turnover and degeneration. Repair of Hyaline Cartilage Hyaline cartilage can tolerate considerable amounts of intense and repetitive stress, but manifests a striking inability to heal from even the most minor injuries. This lack of response to injury is attributable to the avascular nature of cartilage, the immobility of chondrocytes, and the limited ability of chondrocytes to proliferate. Some repair can occur, but only if the defect involves the peri- chondrium, in which case chondrogenic cells from the perichondrium enter the defect and form new cartilage. If the defect is large, the cells form dense connective tis- sue to repair the injury. Hyaline cartilage is also subject to calcification, a process in which calcium phosphate crystals become embedded in the cartilage matrix. In most situations, given sufficient time, cartilage that cal- cifies will be replaced by bone. Chondrocytes normally derive all of their nutrients and dispose of waste materi- als through the extracellular matrix. When the matrix becomes heavily calcified, diffusion is impeded and the chondrocytes swell and die. The ultimate consequence of this event is removal of the calcified matrix and its replacement by bone. Bones and Bone Structure Bones are the major component of the skeletal system. Besides contributing to body shape and form, bones perform several other important functions. They pro- vide the hard framework that supports the body and provides protection for its delicate soft tissues and organs. For example, the bones of lower limbs act as pillars to support the body trunk when we stand, and

Axial skeleton

Appendicular skeleton

FIGURE 42-1. The axial (blue) and appendicular (uncolored) skeleton.

to cartilage, the extracellular matrix of bone is mineral- ized, producing a hard tissue capable of providing sup- port for the body and protection for its vital structures. Cartilage Cartilage is an avascular tissue that consists of cells, called chondrocytes , and an extensive extracellular matrix composed of fibers and ground substance. The chondrocytes are sparse, but are essential to the produc- tion and maintenance of the matrix. A connective tissue sheath, called the perichondrium , overlies the free sur- faces of most cartilage. The perichondrium is vascular and its blood vessels supply nutrients to the chrondro- cytes. In areas where the cartilage has no perichon- drium, such as the articular surfaces of moving joints, the chondrocytes receive their nourishment from the synovial fluid that bathes the joint surfaces. Types of Cartilage There are three types of cartilage, each exhibiting varia- tions in matrix composition: elastic, fibrocartilage, and hyaline. Elastic cartilage has a dense network of elastic fibers scattered throughout its matrix. It is found in areas