Porth's Essentials of Pathophysiology, 4e

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Musculoskeletal Function

U N I T 1 2

Running through the core of each osteon is a central canal, called a haversian canal , which contains the blood ves- sels and nerves that supply the osteon (Fig. 42-4A). Canals of a second type called perforating , or Volkmann canals , lie at right angles to the long axis of the bone, connect- ing the vascular and nerve supplies of the periosteum and medullary cavity. Spider-shaped osteocytes (mature bone cells) occupy small cavities, or lacunae , at the junctions of the lamellae (Fig. 42-4B). In compact bone (e.g., diaph- ysis of long bones), the lamellae exhibit a characteristic organization of outer circumferential lamellae, inner cir- cumferential lamellae, and interstitial lamellae. The outer circumferential lamellae are located just beneath the peri- osteum and the inner circumferential lamellae are arranged concentrically around a central haversian canal. Between the two circumferential lamellae are the irregularly shaped interstitial lamellae. These are the remnants of haversian systems that have been destroyed during bone growth or remodeling. Trabecular lamellar bone forms the coarse spongy bone of the medullary cavity. It exhibits plates of lamellar bone perforated by marrow spaces. In contrast to compact bone, immature or woven bone , consisting of trabeculae, looks like poorly orga- nized bone. It is deposited more rapidly than lamellar bone, has low tensile strength, and serves as temporary scaffolding for support. It is found in the developing fetus, in areas surrounding tumors and infections, and as part of a healing fracture. Blood Supply Bones are richly supplied with blood from nutrient and perforating arteries (see Fig. 42-2C). The nutrient arter- ies enter the bone through a nutrient foramen and sup- ply the marrow space and the internal half of the cortex. The perforating arteries are small arteries that extend inward from the periosteal arteries on the external sur- face of the periosteum and anastomose in the cortex with branches of the nutrient arteries coming from the bone marrow. The distribution of blood in the cortex occurs through the haversian and Volkmann canals (see Fig. 42-4A). Veins accompany arteries through the nutrient formania. Many large veins also leave through openings near the articulating ends of bones. Bones con- taining red bone marrow have numerous large veins. Bone Cells Five types of cells participate in the formation and main- tenance of bone tissue: osteoprogenitor cells, osteo- blasts, osteocytes, osteoclasts, and bone-lining cells (Table 42-1). With the exception of the osteoclast, each type of bone cell originates from the same basic cell type, undergoing transformation as it matures. Osteoprogenitor Cells. Osteoprogenitor cells are undif- ferentiated or resting cells that are found in the perios- teum, endosteum, and epiphyseal plate of growing bone. Derived from stem cells in the bone marrow, they have the potential to differentiate into many different cell types, including adipocytes, fibroblasts, and osteoblasts.

and an inner, more cellular layer of osteoprogenitor cells which can differentiate into osteoblasts (bone-forming cells, to be discussed). The periosteum contains blood vessels and acts as an anchorage point for blood vessels as they enter and leave the bone. The endosteum is the membrane that lines the spaces of spongy bone, the mar- row cavities, and the haversian canals of compact bone. It is considerably thinner than the periosteum and is composed of a single layer of flattened osteoprogenitor cells and small amounts of connective tissue. The prin- cipal functions of the periosteum and endosteum are the nutrition of bone tissue and continuous supply of new osteoblasts for repair and growth of bone. Bone marrow occupies the medullary cavities of the long bones (see Fig. 42-2C) throughout the skeleton and the cavities of spongy bone in the vertebrae, ribs, sternum, and flat bones of the pelvis. The cellular composition of the bone marrow varies with age and skeletal location. Red bone marrow contains developing red blood cells and is the site of blood cell formation. Yellow bone mar- row is composed largely of adipose cells. At birth, nearly all of the marrow is red and hematopoietically active. As the need for red blood cell production decreases during postnatal life, red marrow is gradually replaced with yel- low bone marrow in most of the bones. In the adult, red marrow persists in the vertebrae, ribs, sternum, and ilia. BoneTissue Bone, or osseous , tissue is perhaps one of the most dis- tinctive tissues in the body. It is typical of other con- nective tissue in that it consists of cells, fibers, and an extracellular matrix. However, its extracellular compo- nents are hard and calcified, a feature that enables it to serve both supportive and protective functions. The extracellular bone matrix can be divided into two parts: the organic matrix and inorganic mineral matrix. The organic matrix of bone and other connective tis- sues is a composite of collagenous fibers and an amor- phous mixture of protein and polysaccharides called ground substance . The ground substance provides support and adhesion between cellular and fibrous ele- ments and serves an active role in many metabolic func- tions necessary for bone growth, repair, and remodeling. The inorganic mineral matrix of bone is composed largely of calcium phosphate in the form of hydroxyapa- tite crystals. It comprises about 25% of the bones’ vol- ume, but because of its high density, it is responsible for about half its weight. In addition to calcium and phos- phate, bone contains considerable amounts of the body’s content of carbonate, magnesium, and sodium. Bone may also take up lead and other heavy metals, thereby removing these toxic substances from the circulation. Laminar and Woven Bone. There are two distinct types of bone tissue based on their stage of develop- ment: laminar (mature) and woven (immature). Laminar or mature bone is composed largely of cylinder- shaped units of calcified matrix, called osteons , that are oriented parallel to the long axis of the bone. Functionally, osteons can be thought of as tiny weight-bearing pillars.