The Skeletal System Chapter 6

The Skeletal System The skeletal system includes:  Bones  Cartilages  Joints (articulations)  Ligaments  Other connective tissues

The Skeletal System Functions of the Skeletal System  Support against gravity  Protection of soft internal organs  Movement (Leverage)  Storage Minerals (calcium, phosphorous) – within the matrix of bone tissue Energy reserve (adipose) – within the yellow marrow of long bones  Blood cell production – within red marrow of spongy bone tissue

The Structure of a Typical Bone All bones have a combination of spongy & compact bone tissue Figure 6-3(a)

The Histological Features of Spongy Bone  Lamellae (layers) of matrix laid down in trabeculae - branching network of bony tissue  Osteocytes located within lacunae  Canaliculi branch out from lacunae  Many marrow cavities which contain red marrow & small blood vessels

The Histological Features of Compact Bone Osteon/Haversian system - basic functional unit of compact bone Concentric lamellae (layers) of matrix surrounding central (Haversian) canal Osteocytes located within lacunae Canaliculi branch out radially from lacunae Central canals (containing BVs) run vertically down the length of the bone Perforating (Volkmann’s) canals (containing BVs) run horizontally across the width

Bone Cells In addition to Osteocytes, the mature bone cells found within the matrix of bone tissue, there are three other cells that are associated with bones  Osteoprogenitor cells  Osteoblasts  Osteoclasts

Bone Cells Osteoprogenitor cells  Derived from mesenchymal cells  Can undergo mitosis  Mature into osteoblasts

Bone Cells Osteoblasts  Responsible for osteogenesis – create bone tissue  Mature into osteocytes

Bone Cells Osteoclasts  Derived from embryological WBCs  Secrete enzymes for osteolysis – resorb/break down bone tissue  Necessary for calcium homeostasis

Anatomical Classification of Bones Bones are classified by their basic shape  Long bones (e.g. humerus, femur)

Anatomical Classification of Bones Short bones (e.g. carpals, tarsals)

Anatomical Classification of Bones  Flat bones (e.g. parietal bone)

Anatomical Classification of Bones  Irregular bones (e.g. vertebra)

Anatomical Classification of Bones Sesamoid bones (e.g. patella)

The Structure of a Typical Long Bone diaphysis (compact bone) Figure 6-2

The Structure of a Long Bone diaphysis epiphysis (spongy bone) epiphysis

The Structure of a Long Bone diaphysis epiphysis articular cartilage

The Structure of a Long Bone epiphysis metaphysis – location of epiphyseal plate – in children epiphyseal line – in adults diaphysis metaphysis articular cartilage

The Structure of a Long Bone medullary cavity filled with yellow marrow in adults lined with endosteum

The Structure of a Long Bone periosteum  Double layered membrane surrounding bone (except at articular cartilage)  Inner cellular layer contains osteoprogenitor cells, osteoblasts, osteoclasts  Outer fibrous layer of dense irregular CT for attachment of tendons, ligaments

Bone Formation and Growth Ossification - Process of converting other tissues to bone Two types of ossification processes occur during embryological formation:  Intramembranous  Endochondral

Intramembranous Ossification  Forms flat bones of skull, mandible, clavicle  Replacement of mesenchymal membrane with osseous tissue  Mesenchymal cells differentiate to osteoprogenitor cells, which then become osteoblasts  Osteoblasts create spongy bone tissue, which then remodels into compact bone where necessary  Other mesenchymal cells around the developing bone form the periosteum Mesenchymal tissue forms Osteoblasts begin to secrete osteoid forming spongy bone tissue Blood vessels infiltrate tissue. Calcium salts deposit in osteoid. Periosteum develops

Endochondral Ossification  Most bones formed this way  Mesenchyme creates Cartilage model, which gets replaced by bone  Replacement begins in middle (diaphysis) & follows in ends (epiphyses) Enlarging chondrocytes within calcifying matrix Chondrocytes at the center of the growing cartilage model enlarge and then die as the matrix calcifies. Newly derived osteoblasts cover the shaft of the cartilage in a thin layer of bone. Blood vessels penetrate the cartilage. New osteoblasts form a primary ossification center. The bone of the shaft thickens, and the cartilage near each epiphysis is replaced by shafts of bone. Cartilage model Bone formation Epiphysis Diaphysis Marrow cavity Primary ossification center Blood vessel Marrow cavity Blood vessel Secondary ossification center Epiphyseal cartilage Articular cartilage perichondrium periosteum Perichondrium now called periosteum

Bone Growth Interstitial growth  Occurs throughout childhood as long as epiphyseal plate cartilage remains  Replacement of cartilage by osseous tissue (ossification) at diaphyseal end of plate; replacement of cartilage cells at epiphyseal end of plate  eventually no more cartilagenous growth resulting in epiphyseal line Appositional growth  Occurs throughout life through activity of cells in periosteum and endosteum  important in repair and remodeling

Fracture - A crack or break in a bone  Steps in fracture repair Fracture hematoma - bleeding in bone tissue causes a clot; occurs within hours Callus formation - Mitoses in cells of periosteum, endosteum forms internal & external callus of spongy bone tissue; occurs within few days Bone remodeling - replacement of spongy bone with compact bone; occurs over months to years Injury and Repair

Figure of 5 Steps in the Repair of a Fracture PLAY Bone fragments Immediately after the fracture, extensive bleeding occurs. Over a period of several hours, a large blood clot, or fracture hematoma, develops. Fracture hematoma Dead bone New bone Periosteum Spongy bone of internal callus Cartilage of external callus An internal callus forms as a network of spongy bone unites the inner edges, and an external callus of cartilage and bone stabilizes the outer edges. The cartilage of the external callus has been replaced by bone, and struts of spongy bone now unite the broken ends. Fragments of dead bone and the areas of bone closest to the break have been removed and replaced. A swelling initially marks the location of the fracture. Over time, this region will be remodeled, and little evidence of the fracture will remain. Internal callus External callus External callus Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

The Skeleton Figure 6-8(a)

An Overview of the Skeleton Skeletal Divisions Axial skeleton (80 bones)  Skull  Thoracic cage and sternum  Vertebral column Appendicular skeleton (126 bones)  Upper, lower limbs  Pectoral girdle  Pelvic girdle There are 206 bones in the adult human body

Bone Markings (Surface Features) Surface Features of Bones Table 6-1 (in textbook)

Bone Markings (Surface Features) Spinous process Transverse process Depressions/openings:  fossa  foramen (canal)  sulcus(groove)  meatus

Bone Markings (Surface Features) Processes for articulations: head condyle facet

Lateral epicondyle Bone Markings (Surface Features) Processes for attachment:  Trochanter (only on femur)  tubercle  tuberosity  epicondyle  crest  spine  spinous process (SP) (only on vertebrae)  transverse process (TP) (only on vertebrae) Spinous process Transverse process