Module 5: The Musculoskeletal System: Bones (PDF)

Module 5 The Musculoskeletal System: Bones  Functions of Bone Protecting and supporting soft tissues Attachment site for muscles making movement possible Storage of the minerals, calcium & phosphate -- mineral homeostasis Blood cell production occur...

Module 5 The Musculoskeletal System: Bones  Functions of Bone Protecting and supporting soft tissues Attachment site for muscles making movement possible Storage of the minerals, calcium & phosphate -- mineral homeostasis Blood cell production occurs in red bone marrow (hemopoiesis) Energy storage in yellow bone marrow  Classification of Bone Shape Bones are classified based of their shape: Long bones - longer than they are wide; cylindrical Femur and phalanges are examples Short bones – nearly equal in width and length; cube shape Carpals and tarsals are examples Flat bones – thin and flat Many bones in the skull, sternum, ribs and shoulder blades Classification of Bone Shape Irregular bones – don’t fit in any of the previous categories; complex in shape Vertebrae, bones in the hip Sesamoid bones – develop in the tendons under friction and stress Patella is an example Sutural Bones – classifed by location rather than shape; tiny bones in sutures between cranial bones Anatomy of a Long Bone Diaphysis = shaft Epiphysis = one end of a long bone Metaphyses are the areas between the epiphysis and diaphysis and include the epiphyseal plate in growing bones. Articular cartilage over joint surfaces acts as friction reducer & shock absorber Medullary cavity = marrow cavity Endosteum = lining of marrow cavity Periosteum = tough membrane covering bone but not the cartilage Histology of Bone Tissue A type of connective tissue as seen by widely spaced cells separated by matrix Matrix of 25% water, 25% collagen fibers & 50% crystalized mineral salts Minerals: Hydroxyapatite (85%) Calcium carbonate (10%) Others: calcium fluoride, magnesium fluoride, etc 4 types of cells in bone tissue Bone Cells 1. Osteogenic cells undergo cell division and develop into osteoblasts. 2. Osteoblasts are bone-building cells. 3. Osteocytes are mature bone cells and the principal cells of bone tissue. They maintain the bone tissue 4. Osteoclasts are derived from monocytes and serve to break down bone tissue.  Matrix of Bone Inorganic mineral salts provide bone’s hardness hydroxyapatite (calcium phosphate) & calcium carbonate Organic collagen fibers provide bone’s flexibility their tensile strength resists being stretched or torn remove minerals with acid & rubbery structure results Bone is not completely solid since it has small spaces for vessels and red bone marrow spongy bone has many such spaces compact bone has very few such spaces Compact Bone Compact bone is arranged in units called osteons. Osteons contain blood vessels, lymphatic vessels, nerves, and osteocytes along with the calcified matrix. Osteons are aligned in the same direction along lines of stress. These lines can slowly change as the stresses on the bone changes.  Histology of Compact Bone Osteon is concentric rings (lamellae) of calcified matrix surrounding a vertically oriented blood vessel Osteocytes are found in spaces called lacunae Osteocytes communicate through canaliculi filled with extracellular fluid that connect one cell to the next cell Interstitial lamellae represent older osteons that have been partially removed during tissue remodeling Spongy Bone Spongy bone does not contain osteons. It consists of trabeculae surrounding many red marrow filled spaces. It forms most of the structure of short, flat, and irregular bones, and the epiphyses of long bones. Spongy bone tissue is light and supports and protects the red bone marrow. The Trabeculae of Spongy Bone Latticework of thin plates of bone called trabeculae oriented along lines of stress Spaces in between these struts are filled with red marrow where blood cells develop Found in ends of long bones and inside flat bones such as the hipbones, sternum, sides of skull, and ribs. BONE FORMATION Bone formation is termed osteogenesis or ossification and begins when mesenchymal cells provide the template for subsequent ossification. Two types of ossification occur. Intramembranous ossification is the formation of bone directly from or within fibrous connective tissue membranes. Endochondral ossification is the formation of bone from hyaline cartilage models. Intramembranous Intramembranous ossification forms the flat bones of the skull and the mandible. Also forms the clavicles. An ossification center forms from mesenchymal cells as they convert to osteoblasts and lay down osteoid matrix. The matrix surrounds the cell and then calcifies as the osteoblast becomes an osteocyte. The calcifying matrix centers join to form bridges of trabeculae that constitute spongy bone with red marrow between. On the periphery the mesenchyme condenses and develops into the periosteum. Intramembranous ossification Intramembranous ossification forms the flat bones of the skull and the mandible. Also forms the clavicles. Osteoblasts Osteocytes Red bone marrow Endochondral Endochondral ossification involves replacement of cartilage by bone and forms most of the bones of the body. The first step in endochondrial ossification is the development of the cartilage model.  Endochondral Ossification Development of Cartilage model Mesenchymal cells form a cartilage model of the bone during development Growth of Cartilage model in length by chondrocyte cell division and matrix formation ( interstitial growth) in width by formation of new matrix on the periphery by new chondroblasts from the perichondrium (appositional growth) cells in midregion burst and change pH triggering calcification and chondrocyte death  Endochondral Ossification Development of Primary Ossification Center perichondrium lays down periosteal bone collar nutrient artery penetrates center of cartilage model periosteal bud brings osteoblasts and osteoclasts to center of cartilage model osteoblasts deposit bone matrix over calcified cartilage forming spongy bone trabeculae osteoclasts form medullary cavity Endochondral Ossification Development of Secondary Ossification Center blood vessels enter the epiphyses around time of birth spongy bone is formed but no medullary cavity Formation of Articular Cartilage cartilage on ends of bone remains as articular cartilage.  Growth in Length To understand how a bone grows in length, one needs to know details of the epiphyseal or growth plate. The epiphyseal plate consists of four zones: zone of resting cartilage, zone of proliferation cartilage, zone of hypertrophic cartilage, and zone of calcified cartilage The activity of the epiphyseal plate is the only means by which the diaphysis can increase in length. When the epiphyseal plate closes, is replaced by bone, the epiphyseal line appears and indicates the bone has completed its growth in length.  Bone Growth in Length Epiphyseal plate or cartilage growth plate cartilage cells are produced by mitosis on epiphyseal side of plate cartilage cells are destroyed and replaced by bone on diaphyseal side of plate Between ages 18 to 25, epiphyseal plates close. cartilage cells stop dividing and bone replaces the cartilage (epiphyseal line) Growth in length stops at age 25 Zones of Growth in Epiphyseal Plate Zone of resting cartilage anchors growth plate to bone Zone of proliferating cartilage rapid cell division (stacked coins) Zone of hypertrophic cartilage cells enlarged & remain in columns Zone of calcified cartilage thin zone, cells mostly dead since matrix calcified osteoclasts removing matrix osteoblasts & capillaries move in to create bone over calcified cartilage Figure 6.10 Which ankle is older? Growth in Thickness Bone can grow in thickness or diameter only by appositional growth. The steps in these process are: Periosteal cells differentiate into osteoblasts which secrete collagen fibers and organic molecules to form the matrix. Ridges fuse and the periosteum becomes the endosteum. New concentric lamellae are formed. Osetoblasts under the peritsteum form new circumferential lamellae.  Bone Growth in Width Only by appositional growth at the bone’s surface Periosteal cells differentiate into osteoblasts and form bony ridges and then a tunnel around periosteal blood vessel. Concentric lamellae fill in the tunnel to form an osteon. Fractures A fracture is a broken bone Will typically heal on its own, but resetting it is necessary to insure that it heals in the proper position. Some fractures may be reset without surgery (closed reduction) or may require surgery (open reduction) Types of fractures  Repair of a Fracture Formation of fracture hematoma damaged blood vessels produce clot in 6-8 hours, bone cells die inflammation brings in phagocytic cells for clean-up duty new capillaries grow into damaged area Formation of fibrocartilagenous callus formation fibroblasts invade the procallus & lay down collagen fibers chondroblasts produce fibrocartilage to span the broken ends of the bone Repair of a Fracture Formation of bony callus osteoblasts secrete spongy bone that joins 2 broken ends of bone lasts 3-4 months Bone remodeling compact bone replaces the spongy in the bony callus surface is remodeled back to normal shape