Histology and Physiology of Bones Chapter 6 Histology and Physiology of Bones Osteon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Functions of the Skeletal System Skeletal system has four components Bones Cartilage Tendons Ligaments Bones are organs composed of Nerve tissue Vascular tissue

Function of Bones Support: form the framework that supports the body and cradles soft organs Protection: provide a protective case for the brain, spinal cord, and vital organs Movement: provide levers for muscles Storage: reservoir for minerals, especially calcium and phosphorus Blood cell production: hematopoiesis occurs within the marrow cavities of bones

Cartilage Chondroblasts produce cartilage and become chondrocytes Chondrocytes are located in lacunae surrounded by matrix The matrix of cartilage contains collagen fibers (for strength) and proteoglycans (trap water) The perichondrium surrounds cartilage The outer layer contains fibroblasts The inner layer contains chondroblasts Cartilage grows by appositional and interstitial growth

Fig. 6.1

Bone Histology Bone Matrix Approximately 35% organic and 65% inorganic material Organic Collagen provides flexible strength Proteoglycans Inorganic Hydroxyapatite (calcium phosphate crystal) provides weight-bearing strength

Effects of Changing the Bone Matrix Fig. 6.2

Bone Histology Bone Cells Osteoblasts produce bone matrix and become osteocytes Osteoblasts connect to one another through cell processes and surround themselves with bone matrix to become osteocytes Osteocytes are located in lacunae and are connected to one another through canaliculi Osteoclasts break down bone Osteoblasts originate from osteochondral progenitor cells Osteoclasts originate from stem cells in red bone marrow

Bone Histology Ossification (Osteogenesis) Osteoblasts on a preexisting surface, such as cartilage or bone. The cell processes of different osteoblasts join together Osteoblasts have produced bone matrix. The osteoblasts are now osteocytes Fig. 6.3

Bone Histology Bone tissue is classified as either woven or lamellar bone, according to the organization of collagen fibers Woven bone Has collagen fibers oriented in many different directions It is remodeled to form lamellar bone Lamellar bone Mature bone Arranged in thin layers called lamellae Has collagen fibers oriented parallel to one another

Bone Histology Bone can be classified according to the amount of bone matrix relative to the amount of space present within the bone Cancellous bone has many spaces Internal layer which is a honeycomb of trabeculae filled with red or yellow bone marrow Compact bone is dense with few spaces External layer Fig. 6.4

Bone Histology Cancellous Lamellae combine to form trabeculae Trabeculae are oriented along lines of stress and provide structural strength Fig. 6.4

Bone Histology Compact Bone Consists of organized lamellae Circumferential lamellae form the outer surface of compact bones Concentric lamellae surround central canals, forming osteons Interstitial lamellae are remnants of lamellae left after bone remodeling Canals within compact bone provide a means for the exchange of gases, nutrients, and waste products From the periosteum (endosteum) perforating canals carry blood vessels to central canals Canaliculi connect central canals to osteocytes

Fig. 6.5

Bone Anatomy Individual bones are classified according to their shape Long bones Longer than they are wide Most bones of the upper and lower limbs Short bones About as wide as they are long Bones of the wrist (carpals) and ankle (tarsals) Flat bones Relatively thin, flattened shape and are usually curved Certain bones of the skull, all the ribs, the breastbone (sternum), and the shoulder blades (scapulae) Irregular bones Do not fit into the other three categories Vertebrae, pelvic girdle and facial bones

Bone Anatomy Structure of Long Bone Long bones consist of a diaphysis and an epiphysis Diaphysis Tubular shaft that forms the axis of long bones Composed of compact bone that surrounds the medullary cavity Yellow bone marrow (fat) is contained in the medullary cavity Not to the same extent, but certain bones also contain red marrow

Bone Anatomy Structure of Long Bone Epiphyses Expanded ends of long bones Exterior is compact bone, and the interior is spongy bone Joint surface is covered with articular (hyaline) cartilage Epiphyseal line separates the diaphysis from the epiphyses Epiphyseal plate is the site of bone growth in length

Bone Anatomy Bone Membranes Periosteum: double layer of protective membrane covering the outer surface of bone Outer fibrous layer is dense regular connective tissue, which contains blood vessels and nerves Inner osteogenic layer contains osteoblasts, osteoclasts, and osteochondral progenitor cells Endosteum: delicate membrane covering internal surfaces of bone Contains osteoblasts, osteoclasts, and osteochondral progenitor cells

Fig. 6.6

Bone Anatomy Structure of Flat, Short, and Irregular Bones Flat bones contain an interior framework of cancellous bone sandwiched between two layers of compact bone Short and Irregular bones have a composition similar to the ends of long bones

Bone Development Begins at week 8 of embryo development Intramembranous ossification: bone develops from a fibrous membrane Some skull bones, part of the mandible, and the diaphyses of the clavicles Endochondral ossification: bone forms by replacing hyaline cartilage Bones of the base of the skull, part of the mandible, the epiphyses of the clavicles, and most of the remaining skeletal system

Bone Development Intramembranous Ossification Within the membrane at centers of ossification, osteoblasts produce bone along the membrane fibers to form cancellous bone Beneath the periosteum, osteoblasts lay down compact bone to form the outer surface of the bone Fontanels are areas of membrane that are not ossified at birth Bones formed by intramembranous ossification are yellow and bones formed by endochondral ossification are blue Fig. 6.7

Fig. 6.7

Bone Development Endochondral Ossification Uses hyaline cartilage “bones” as models for bone construction Requires breakdown of hyaline cartilage prior to ossification The perichondrium covering the hyaline cartilage “bone” is infiltrated with blood vessels, converting it to a vascularized periosteum The change in nutrition transforms the underlying osteochondral progenitor cells into osteoblasts Formation of bone collar around the diaphysis of the hyaline cartilage model

Bone Development Endochondral Ossification cont. Blood vessels grow into the calcified cartilage, bringing osteoblasts and osteoclasts from the periosteum A primary ossification center forms as osteoblasts lay down bone matrix Medullary cavity forms Appearance of secondary ossification centers in the epiphyses Ossification of the epiphyses, with hyaline cartilage remaining only in the epiphyseal plates

Endochondral Ossification Fig. 6.9a

Endochondral Ossification Continued Fig. 6.9b

Bone Growth Bones increase in size only by appositional growth Adding of new bone on the surface of older bone or cartilage Trabeculae grow by appositional growth

Bone Growth Growth in Bone Length Bone length increases because of growth at the epiphyseal plate Epiphyseal plate growth involves Interstitial growth of cartilage Followed by appositional bone growth on the cartilage Epiphyseal plate growth results in an increase in the length of the diaphysis and bony processes Bone growth in length ceases when the epiphyseal plate becomes ossified and forms the epiphyseal line

Postnatal Bone Growth Growth in Long Bones Ossified bone The epiphyseal plate is organized into four zones Zone of resting cartilage Cartilage attaches to the epiphysis Zone of proliferation New cartilage is produced on the epiphyseal side of the plate as the chondrocytes divide and form stacks of cells Zone of hypertrophy Chondrocytes mature and enlarge Zone of calcification Matrix is calcified, and chondrocytes die Ossified bone The calcified cartilage on the diaphyseal side of the plate is replaced by bone

Fig. 6.10

Fig. 6.11

Bone Growth Growth at Articular Cartilage Involves the interstitial growth of cartilage followed by appositional bone growth on the cartilage Results in larger epiphyses and an increase in the size of bones that do not have epiphyseal plates

Bone Growth Growth in Bone Width Appositional bone growth beneath the periosteum increases the diameter of long bones and the size of other bones Osteoblasts from the periosteum form ridges with grooves between them The ridges grow together, converting the grooves into tunnels filled with concentric lamellae to form osteons Osteoblasts from the periosteum lay down circumferential lamellae, which can be remodeled Fig. 6.12

Bone Growth Factors Affecting Bone Growth Genetic factors determine bone shape and size The expression of genetic factors can be modified Factors that alter the mineralization process or the production of organic matrix Deficiencies in vitamin D Hormones Growth hormone, estrogen, and testosterone stimulate bone growth

Bone Remodeling Remodeling converts woven bone to lamellar bone and allows bone to Change shape Adjust to stress Repair itself Regulate body calcium levels Basic multicellular units (BMUs) make tunnels in bone, which are filled with concentric lamellae to form osteons BMUs are temporary assemblies of osteoclasts and osteoblasts BMU activity renews the entire skeleton every 10 years Interstitial lamellae are remnants of bone not removed by BMUs

Remodeling of a Long Bone Fig. 6.13

Bone Fractures (Breaks) Bone fractures are classified by: The position of the bone ends after fracture The completeness of the break The orientation of the bone to the long axis Whether or not the bone ends penetrate the skin

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Bone Repair Fig. 6.14

Bone Repair Hematoma formation Torn blood vessels hemorrhage A mass of clotted blood (hematoma) forms at the fracture site Site becomes swollen, painful, and inflamed Fig. 6.14

Bone Repair Callus formation Granulation tissue (soft callus) forms a few days after the fracture Capillaries grow into the tissue and phagocytic cells begin cleaning debris Fig. 6.14

Bone Repair Callus formation cont. The external callus forms when: Osteoblasts and fibroblasts migrate to the fracture and begin reconstructing the bone Fibroblasts secrete collagen fibers that connect broken bone ends Osteoblasts begin forming woven bone Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies

Bone Repair Callus ossification The fibers and cartilage of the internal and external calluses are ossified to produce woven, cancellous bone Cancellous bone formation in the callus is usually complete 4-6 weeks after the injury Fig. 6.14

Bone Repair 4. Bone remodeling Excess material on the bone shaft exterior and in the medullary canal is removed Compact bone is laid down to reconstruct shaft walls The remodeling process may take more than a year to complete Fig. 6.14

Bone Repair Fig. 6.14

Calcium Homeostasis Bone is the major storage site for calcium (Ca2+) Two hormones regulate Ca2+ levels in the blood: parathyroid hormone (PTH) and calcitonin PTH is the major regulator of blood Ca2+ Falling blood Ca2+ levels signal the parathyroid glands to release PTH PTH signals Osteoclasts to degrade bone matrix and release Ca2+ into the blood Ca2+ absorption from the small intestines Reabsorption of Ca2+ from the urine Calcitonin Rising blood Ca2+ levels trigger the thyroid to release calcitonin Calcitonin stimulates calcium salt deposition in bone by decreasing osteoclast activity

Effects of Aging on the Skeletal System With aging, bone matrix is lost and the matrix becomes more brittle Cancellous bone loss results from a thinning and loss of trabeculae Compact bone loss mainly occurs from the inner surface of bones and involves less osteon formation Loss of bone Increases the risk for fractures Causes deformity Loss of height Pain Stiffness Loss of teeth

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