4. Bone Tissue

Repair of Bone Fractures

(a) Blood vessels torn within the fracture release blood that clots to produce a large fracture hematoma. (b) This isgradually removed by macrophages and replaced by a soft fibrocartilage-like mass of procallus tissue rich in collagen and fibroblasts. If broken, the periosteum reestablishes continuity over this tissue. (c) This soft procallus is invaded by regrowing blood vessels and osteoblasts. In the next few weeks the fibrocartilage is gradually replaced by trabeculae of woven bone, forming a hard callus throughout the original area of fracture. (d) The woven bone is then remodeled as compact and cancellous bone in continuity with the adjacent uninjured areas and fully functional vasculature is reestablished.

Epiphyseal Zones

1. The resting zone consists of hyaline cartilage with typical chondrocytes. 2. In the proliferative zone, chondrocytes begin to divide rapidly and form columns of stacked cells parallel to the long axis of the bone. 3. The hypertrophic cartilage zone contains swollen, degenerative chondrocytes whose cytoplasm has accumulated glycogen. This hypertrophy compresses the matrix into thin septa between the chondrocytes. 4. In the calcified cartilage zone, loss of the chondrocytes by apoptosis is accompanied by calcification of the septa of cartilage matrix by the formation of hydroxyapatite crystals. 5. In the ossification zone, bone tissue first appears. Capillaries and osteoprogenitor cells originally from the periosteum invade the cavities left by the chondrocytes. Many of these cavities will be merged and become the marrow cavity. Osteoblasts settle in a layer over the septa of calcified cartilage matrix and secrete osteoid over these structures, forming woven bone.

Lacuna

A small space containing an osteocyte in bone or chondrocyte in cartilage. The lacunae are situated between the lamellae, and consist of a number of oblong spaces. In an ordinary microscopic section, viewed by transmitted light, they appear as fusiform opaque spots. Each lacuna is occupied during life by a branched cell, termed an osteocyte, bone-cell or bone-corpuscle. A lacuna never contains more than one osteocyte.

Osteocytes

Account for 90-95% of bone cells. Many osteoblasts are gradually surrounded by the material they secreted and differentiate further as osteocytes enclosed singly within the lacunae that are regularly spaced throughout the mineralized matrix. In the transition from osteoblasts to osteocytes, the cells extend many long dendritic processes, which also become surrounded by calcifying matrix. When compared with osteoblasts, the flat, almond shaped osteocytes exhibit significantly less RER, smaller Golgi complexes, and more condensed nuclear chromatin. These cells maintain the bony matrix, and their death is followed by rapid matrix resorption. Osteocytes express a different array of genes compared to osteoblasts, and osteocyte products such as the protein sclerostin and certain cytokines help regulate bone remodeling. The extensive lacunar-canalicular network of osteocytes and their communication with all other bone cells suggest additional roles for osteocytes in calcium homeostasis and as sensors for detection of mechanical stresses on bone, which is also important in directing bone remodeling.

Osteoblasts

Although representing only 4-6% of total resident cells, osteoblasts attend the crucial functiono f building the bone. Osteoblasts synthesize and secrete the organic components of bone matrix, which include type I collagen fibers, proteoglycans, and several glycoproteins such as osteonectin. Deposition of the inorganic components of bone also depends on viable osteoblasts. Mature osteoblasts are located exclusively at the surfaces of bone matrix, usually side by side in a layer somewhat resembling a simple epithelium. When actively engaged in matrix synthesis, osteoblasts have a cuboidal to columnar shape and basophilic cytoplasm. When their synthesizing activity declines, they flatten and basophilia is reduced. Inactive osteoblasts represent most of the flattened bone lining cells in both the endosteum and periosteum.

Macrophage-like synovial cells (A cells)

Are derived from blood monocytes and remove wear-and-tear debris from the synovial fluid. These modified macrophages, which represent approximately 25% of the cells lining the synovium, are important in regulating inflammatory events within diarthrotic joints.

Inner Circumferential Lamellae

Are located around the marrow cavity.

External Circumferential Lamellae

Are located immediately beneath the periosteum.

Canaliculi

Are microscopic canals between the lacunae of ossified bone. The processes of the osteocytes project into these canals. These cytoplasmic processes are joined together by gap junctions. Osteocytes do not entirely fill up the canaliculi. The remaining space is known as the periosteocytic space, which is filled with periosteocytic fluid. This fluid contains substances too large to be transported through the gap junctions that connect the osteocytes.

Articular Cartilage

Articular cartilage is hyaline cartilage on the articular surfaces of bones inside the joint cavity of synovial joints. Such cartilage does not contribute to bone growth. It does not have a perichondrium. Cartilage calcifies and merges with bone tissue

Flat bones

As they sound, strong, thin, flat and usually curved. They consist of trabecular bone ("diploe") sandwiched between 2 layers of cortical bone. In adults, the highest number of red blood cells are formed in the bone marrow of flat bones. Examples: scapula (shoulder blade), Sternum (breast bone), cranium (skull), Coxae (hip bone), pelvis and ribs

Endochondral Ossification

Begins at 5 weeks of development and forms all the bones below the base of the skull. Takes place within a piece of hyaline cartilage whose shape resembles a small version, or model, of the bone to be formed. This type of ossification is principally responsible for initiating most bones of the body and is especially well studied in developing long bones. The first bone tissue appears as a collar surrounding the diaphysis of the cartilage model. This bone collar is produced by activity of osteoblasts that form within the surrounding perichondrium. The collar impedes diffusion of oxygen and nutrients into the underlying cartilage, promoting degenerative changes there. The chondrocytes begin to produce alkaline phosphatase and swell up (hypertrophy), enlarging their lacunae. These changes both compress the matrix into narrow trabeculae and lead to calcification in these structures. Death of the chondrocytes creates a porous structure consisting of calcified cartilage remnants which become covered by a layer of osteoblasts. Blood vessels from the perichondrium (now the periosteum) penetrate through the bone collar, bringing osteoprogenitor cells to the porous central region. Next, osteoblasts adhere to the remnants of calcified cartilage matrix and produce woven bone. The calcified cartilage at this stage appears basophilic, and the new bone is more acidophilic. This process in the diaphysis forms the primary ossification center, beginning in many bones as early as the first trimester. Secondary ossification centers appear later at the epiphyses of the cartilage model and develop in a similar manner. During their expansion and remodeling, the primary and secondary ossification centers produce cavities that are gradually filled with bone marrow and trabeculae of cancellous bone. With the primary and secondary ossification centers, two regions of cartilage remain: articular cartilage and epiphyseal cartilage.

Intramembranous Ossification

Begins at 8 weeks of development and forms the bones of the skull and clavicle (flat bones). The starting points for bone formation are called ossification centers. In these areas mesenchymal cells differentiate into osteoprogenitor cells which proliferate and form incomplete layers of osteoblasts around a network of developing capillaries. From their surfaces facing away from these blood vessels, the polarized osteoblasts secrete the osteoid components that calcify and form trabeculae of woven bone. Differentiating osteocytes now enclosed within matrix lacunae retain intercellular contacts via their thin cytoplasmic processes within matrix canaliculi. Continued matrix secretion, calcification, and trabecular growth lead slowly to the fusion of neighboring ossification centers and gradually produce layers of compact bone that broadly enclose a region of cancellous bone with marrow and larger blood vessels.

Long Bone

Body is ~2-time longer than it is wide They have growth plates (epiphysis) at either/both ends, a hard outer surface of cortical (compact) bone and a spongy inner known an cancellous/trabecular bone containing bone marrow. Both ends of the bone are covered in hyaline cartilage to help protect the bone and provide friction-less joint movements. Examples: humerus, ulna, radius, femur, tibia, fibula, metatarsals, metacarpals and phalanges.

Short Bone

Body is ~as long as it is wide They provide support and stability with little movement. They consist of only a thin layer of cortex with trabecular bone on the inside along with relatively large amounts of bone marrow. Examples: carpals (wrist) and tarsals (ankle)

Osteogenesis

Bone development or osteogenesis occurs by one of two processes: ■ Intramembranous ossification, in which osteoblasts differentiate directly from mesenchyme and begin secreting osteoid ■ Endochondral ossification, in which a preexisting matrix of hyaline cartilage is eroded and invaded by osteoblasts, which then begin osteoid production. In both processes, the bone tissue that appears first is temporary woven bone, which is soon replaced by stronger lamellar bone.

Bone Remodeling

Bone remodeling is continuous throughout life and involves a process of bone resorption and bone formation. In compact bone, remodeling resorbs parts of old osteons and produces new ones. Resorption involves the actions of osteoclasts, often working in groups to remove old bone in tunnel-like cavities having the approximate diameter of new osteons. Such tunnels are quickly invaded by many osteoprogenitor cells from the endosteum or periosteum and sprouting loops of capillaries. Osteoblasts develop, line the wall of the tunnels, and begin to secrete osteoid in a cyclic manner, forming the concentric lamellae of bone with trapped osteocytes. In healthy adults 5%-10% of the bone turns over annually.

Calcitonin

Calcitonin, synthesized within the thyroid gland, reduces elevated blood calcium levels by opposing the effects of PTH in bone. This hormone directly targets osteoclasts to slow matrix resorption and bone turnover.

Compact bone

Compact bone, also called cortical bone, dense bone in which the bony matrix is solidly filled with organic ground substance and inorganic salts, leaving only tiny spaces (lacunae) that contain the osteocytes, or bone cells. Compact bone makes up 80 percent of the human skeleton. Compact bone forms a shell around cancellous bone and is the primary component of the long bones of the arm and leg and other bones, where its greater strength and rigidity are needed.

Epiphyseal cartilage (epiphyseal plate or growth plate)

Connects each epiphysis to the diaphysis. The epiphyseal cartilage is responsible for the growth in length of the bone and disappears at adulthood, causing bone growth to cease. Elimination of these epiphyseal plates ("epiphyseal closure") occurs at different times with different bones and is complete in all bones by about age 20. Once the epiphyses have closed, additional growth in length of bones is no longer possible although bone widening may still occur.

Irregular bones

Do not fall into any other category, due to their non-uniform shape. They primarily consist of trabecular bone, with a thin outer layer of compact bone. Examples: vertebrae, sacrum, hyoid, and mandible.

Periosteum

External surfaces of bone are covered by tissue layers with bone-forming cells, called periosteum. The periosteum is organized much like the perichondrium. The outer layer is dense connective tissue, with small blood vessels, collagen bundles, and fibroblasts. Bundles of periosteal collagen fibers, called perforating (or Sharpey) fibers, penetrate the bone matrix, binding the periosteum to bone. The inner region of periosteum is a more cellular layer containing bone lining cells, osteoblasts, and mesenchymal stem cells called osteoprogenitor cells. With the potential to proliferate and differentiate into osteoblasts, osteoprogenitor cells play a prominent role in bone growth and in bone repair. The principal functions of periosteum are to nourish the osseous tissue and provide a continuous supply of new osteoblasts for appositional bone growth or repair.

Symphyses

Immobile joints with a pad of fibrocartilage between the articular cartilage covering the ends of the bones. All symphyses, such as the pubic symphysis, occur in the midline of the body.

Osteoclast Function

In areas of bone undergoing resorption, osteoclasts lie within enzymatically etched depressions or cavities in the matrix known as resorption cavities (also called Howship lacunae). In active osteoclasts, the surface against the bone matrix is folded into irregular projections, forming a ruffled border surrounded by a cytoplasmic zone rich in actin filaments, which is the site of adhesion to the matrix. This circumferential adhesion zone creates a microenvironment between the osteoclast and the matrix in which bone resorption occurs. Into this subcellular pocket the osteoclast secretes collagenase, cathepsin K, and other enzymes and pumps protons to produce an acidic environment locally for dissolving hydroxyapatite and promoting the localized digestion of matrix proteins. Osteoclast activity is controlled by local signaling factors and hormones. Osteoclasts have receptors for calcitonin, a thyroid hormone. Osteoblasts activated by parathyroid hormone (PTH) produce M-CSF, RANKL, and other factors that regulate the formation and activity of osteoclasts.

Bone Matrix

Inorganic material represents about 50% of the dry weight of bone matrix. Calcium hydroxyapatite is most abundant, but bicarbonate, citrate, magnesium, potassium, and sodium ions are also found. Significant quantities of amorphous (noncrystalline) calcium phosphate are also present. The surface ions of hydroxyapatite crystals are hydrated The organic matter embedded in the calcified matrix includes type I collagen, proteoglycan aggregates, and bonespecific multiadhesive glycoproteins such as osteonectin. Calcium-binding glycoproteins, notably osteocalcin, and the phosphatases released in matrix vesicles by osteoblasts promote calcification of the matrix. O Because of its high collagen content, decalcified bone matrix is usually acidophilic. The association of minerals with collagen fibers during calcification is responsible for the hardness and resistance of bone tissue. After a bone is decalcified, its shape is preserved, but it becomes as flexible as a tendon.

Bone Tissue

Is a specialized connective tissue composed of calcified extracellular material, the bone matrix, and three major cell types: osteocytes, osteoblasts and osteoclasts.

Woven Bone

Is nonlamellar and characterized by random disposition of type I collagen fibers and is the first bone tissue to appear in embryonic development and in fracture repair. Woven bone tissue is usually temporary and is replaced in adults by lamellar bone, except in a very few places in the body, for example, near the sutures of the calvaria and in the insertions of some tendons. In addition to the irregular, interwoven array of collagen fibers, this type of bone has a lower mineral content and often a higher proportion of osteocytes than mature lamellar bone. These features reflect the fact that woven bone forms more quickly but has less strength than lamellar bone.

Nucleus Pulposus

Is situated in the center of the annulus fibrosus and allows each disc to function as a shock absorber within the vertebral column. It typically contains scattered, vacuolated cells (the only cells derived from the embryonic notochord), but it is largely composed of water in a gel-like matrix rich in hyaluronan and fibers of type II collagen. The nucleus pulposus is large in children, but these structures gradually become smaller with age and are partially replaced by fibrocartilage.

Synarthroses Joints

Joints classified as synarthroses allow very limited or no movement. Synarthroses can be subdivided into fibrous and cartilaginous joints, depending on the type of tissue that joins the bones. Major subtypes of synarthroses include: Synostoses, Syndesmoses and Symphyses

Synostoses

Joints in which bones are united only by bone tissue and no movement takes place. In older adults. Synostoses unite the skull bones, which in children and young adults are held together by sutures, or thin layers of connective tissue with osteogenic cells.

Types of Bones

Long bones Short bones Flat bones Irregular bones Sesamoid bones

Lamellar Bone

Most bone in adults, compact or cancellous, is organized as lamellar bone, characterized by multiple layers or lamellae of calcified matrix, each 3-7 μm thick. The lamellae are organized either parallel to each other or concentrically around a central canal. In each lamella, type I collagen fibers are aligned in parallel, with the pitch of the fibers' orientation shifted orthogonally (by about 90 degrees) in successive lamellae. This organization of collagen fibers in lamellae adds greatly to the strength of lamellar bone.

Interstitial Lamellae

Numerous irregularly shaped groups of parallel lamellae which are scattered among the intact osteon. These structures are lamellae remaining from osteons partially destroyed by osteoclasts during growth and remodeling of bone. In compact bone (eg, the diaphysis of long bones) besides forming osteons, the lamellae also exhibit a typical organization consisting of multiple external circumferential lamellae and often some inner circumferential lamellae.

Calcification of Matrix

Osteocalcin and certain glycoproteins which are secreted into the matrix, bind Ca2+ with high affinity. Osteoblasts also release very small membrane-enclosed matrix vesicles with which alkaline phosphatase and other enzymes are associated. These enzymes hydrolyze PO4- ions from various macromolecules, creating a high concentration of these ions locally. The high ion concentrations cause calcified nanocrystals to form in and around the matrix vesicles. The crystals grow and mineralize further with formation of small growing masses of calcium hydroxyapatite, which surround the collagen fibers and all other macromolecules. Eventually the masses of hydroxyapatite merge as a confluent solid bony matrix as calcification of the matrix is completed.

Osteoclast Diffrentiation

Osteoclast development from monocytes requires two polypeptides produced by osteoblasts: macrophage-colony-stimulating factor (M-CSF) and the receptor activator of nuclear factor-κB ligand (RANKL).

Osteoclasts

Osteoclasts are very large, motile cells with multiple nuclei that play a major role in matrix resorption during bone growth and remodeling. The large size and multinucleated condition of osteoclasts are due to their origin from monocytes. They reprsent "specialized macrophage" in the bone tissue.

Bone Growth and Remodeling

Osteogenesis and bone growth involves the partial resorption of bone tissue formed earlier, while simultaneously laying down new bone at a rate exceeding that of bone removal. The rate of bone turnover is very active in young children, where it can be 200 times faster than that of adults. The constant remodeling of bone ensures that, despite its hardness, this tissue remains plastic and capable of adapting its internal structural in the face of changing stresses.

Osteogenic Cells

Osteogenic cells are undifferentiated cells with high mitotic activity. They are the only bone cells that divide. Immature osteogenic cells are found in the deep layers of the periosteum and the marrow. When they differentiate, they develop into osteoblasts.

PTH

PTH from the parathyroid glands acts in bone to raise low blood calcium levels by stimulating osteoclasts and osteocytes to resorb matrix and release Ca2+. The PTH effect on osteoclasts is indirect; PTH receptors occur on osteoblasts, which respond by secreting paracrine factors that stimulate osteoclast activity.

Diarthroses Joints

Permit free bone movement. Diarthroses such as the elbow and knee generally unite long bones and allow great mobility. In a diarthrosis, ligaments and a capsule of dense connective tissue maintain proper alignment of the bones. The capsule encloses a sealed joint cavity that contains synovial fluid, a clear, viscous liquid. The joint cavity is lined, not by epithelium, but by a specialized connective tissue called the synovial membrane that extends folds and villi into the cavity and secretes the lubricant synovial fluid. Synovial fluid is derived from blood plasma, but with a high concentration of hyaluronan produced by cells of the synovial membrane. In different diarthrotic joints the synovial membrane may have prominent regions with dense connective tissue or fat.

Central Canal

Processes of adjacent cells are in contact via gap junctions, and all cells of an osteon receive nutrients and oxygen from the microvasculature in the central canal.

Fibroblastic synovial cells (B cells)

Produce abundant hyaluronan and other extracellular components. Much of this material is transported by water from the capillaries into the synovial fluid, which lubricates the joint, reducing friction on all internal surfaces, and supplies nutrients and oxygen to the articular cartilage.

Osteon (Haversian System)

Refers to the complex of concentric lamellae surrounding a small central canal that contains blood vessels, nerves, loose connective tissue, and endosteum. Between successive lamellae are lacunae, each with one osteocyte, interconnected by canaliculi containing the cells' dendritic processes. The outer boundary of each osteon is a more collagen- rich layer called the cement line.

Sesamoid bones

Short or irregular bones, embedded in a tendon that passes over a joint, such as the hand, knee, and foot. Functionally, they act to protect the tendon and to increase its mechanical effect. Like short and irregular bones, they consist of trabecular bone, with a thin outer layer of compact bone. Examples: patella (knee cap), pisiform (smallest carpal bone) and the two small bones at the base of the 1st metatarsal.

Endochondral Ossification

Stage 1-3: during fetal week 9 to 9th month. Stage 4: just before birth. Stage 5: skeletal growth until end of adolescence.

Syndesmoses

Syndesmoses join bones by dense connective tissue only. Examples include the interosseous ligament of the inferior tibiofibular joint and the posterior region of the sacroiliac joints.

Annulus Fibrosus

The annulus fibrosus of each disc has an external layer of dense connective tissue but is mainly composed of overlapping laminae of fibrocartilage in which collagen bundles are orthogonally arranged in adjacent layers. The multiple lamellae provide the disc with unusual resilience and enable it to withstand pressures generated by the vertebrae.

Spongy (Cancellous or Trabecular) Bone

The bone matrix, or framework, is organized into a three-dimensional latticework of bony processes, called trabeculae, arranged along lines of stress. The spaces between are often filled with red marrow and blood vessels. Cancellous bone makes up about 20 percent of the human skeleton, providing structural support and flexibility without the weight of compact bone. It is found in most areas of bone that are not subject to great mechanical stress. It makes up much of the enlarged ends (epiphyses) of the long bones and is the major component of the ribs, the shoulder blades, the flat bones of the skull, and a variety of short, flat bones elsewhere in the skeleton. Cancellous bone is usually surrounded by a shell of compact bone, which provides greater strength and rigidity. The open structure of cancellous bone enables it to dampen sudden stresses, as in load transmission through the joints. Do not have true ostons.

Perforating Canals (Volkmann canals)

The central canals communicate with the marrow cavity and the periosteum and with one another through transverse perforating canals (or Volkmann canals).

Hyaline Cartilage in Diarthroses Joints

The collagen fibers of the hyaline articular cartilage are disposed as arches with their tops near the exposed surface, which, unlike most cartilage, is not covered by perichondrium (Figure 8-21). This arrangement of collagen helps distribute more evenly the forces generated by pressure on joints. The resilient articular cartilage is also an efficient absorber of the intermittent mechanical pressures to which many joints are subjected.

Matrix Synthesis by Osteoblasts

The matrix is organic and consists of type 1 collagen and ground substanc (GAG chondroitin sulfate and glycoproteins). During matrix synthesis, osteoblasts have the ultrastructure of cells actively synthesizing proteins for secretion. Matrix components are secreted at the cell surface in contact with existing bone matrix, producing a layer of new (but not yet calcified) material called osteoid between the osteoblast layer and the preexisting bone surface.

Diaphysis

The shaft or central part of a long bone. Is almost totally composed of compact bone, with a thin region of spongy bone on the inner surface around the central marrow cavity.

Metabolic Role of Bones

The skeleton serves as the calcium reservoir, containing 99% of the body's total calcium in crystals of hydroxyapatite. The concentration of calcium in the blood and tissues is generally quite stable because of a continuous interchange between blood calcium and bone calcium. The principal mechanism for raising blood calcium levels is the mobilization of ions from hydroxyapatite crystals to interstitial fluid, which occurs primarily in cancellous bone. Ca2+ mobilization from hydroxyapatite is regulated mainly by paracrine interactions between bone cells, and two polypeptide harmones: PTH and Calcitonin.

Epiphyses

The the end part of a long bone. Are composed of spongy bone covered by a thin layer of compact bone

Endosteum

The very thin endosteum covers small trabeculae of bony matrix that project into the marrow cavities. Although considerably thinner than the periosteum, endosteum also contains osteoprogenitor cells,osteoblasts, and bone lining cells.

Intervertebral Discs

Thick discs of fibrocartilage between the articular surfaces of successive bony vertebral bodies. These discoid components of the intervertebral joints facilitate movements of the vertebral column. Has 2 parts: annulus fibrosus and nucleus pulposus.

Osteoid

Unmineralized (or poorly mineralized) matrix secreted by osteoblasts . The osteoid components are: 90% collagen type I and ground substance (non-collagenous proteins) . The ground substance consists of: GAG chondroitin -4 and -6 sulfate; keratan sulfate. Glycoproteins: bone sialoprotein (OSP, BSP2), osteonectin osteocalcin...