5 The Skeletal System

The Skeletal System Parts of the skeletal system Bones (skeleton) Joints Cartilages Ligaments Two subdivisions of the skeleton Axial skeleton Appendicular skeleton

Functions of Bones Support the body Protect soft organs Skull and vertebrae for brain and spinal cord Rib cage for thoracic cavity organs Allow movement due to attached skeletal muscles Store minerals and fats Calcium and phosphorus Fat in the internal marrow cavity Blood cell formation (hematopoiesis)

Bones of the Human Body The adult skeleton has 206 bones Two basic types of bone tissue Compact bone Homogeneous Spongy bone Small needle-like pieces of bone Many open spaces

Spongy bone Compact bone Figure 5.1

Classification of Bones on the Basis of Shape Bones are classified as: Long Short Flat Irregular

Figure 5.2

Classification of Bones Long bones Typically longer than they are wide Shaft with heads situated at both ends Contain mostly compact bone All of the bones of the limbs (except wrist, ankle, and kneecap bones) Example: Femur Humerus

Figure 5.2a

Classification of Bones Short bones Generally cube-shaped Contain mostly spongy bone Includes bones of the wrist and ankle Sesamoid bones are a type of short bone which form within tendons (patella) Example: Carpals Tarsals

Figure 5.2d

Classification of Bones Flat bones Thin, flattened, and usually curved Two thin layers of compact bone surround a layer of spongy bone Example: Skull Ribs Sternum

Spongy bone Compact bone Figure 5.1

Figure 5.2c

Classification of Bones Irregular bones Irregular shape Do not fit into other bone classification categories Example: Vertebrae Hip bones

Figure 5.2b

Anatomy of a Long Bone Diaphysis Epiphysis Shaft Composed of compact bone Epiphysis Ends of the bone Composed mostly of spongy bone

Articular cartilage Proximal epiphysis Spongy bone Epiphyseal line Periosteum Compact bone Medullary cavity (lined by endosteum) Diaphysis Distal epiphysis (a) Figure 5.3a

Anatomy of a Long Bone Periosteum Perforating (Sharpey’s) fibers Outside covering of the diaphysis Fibrous connective tissue membrane Perforating (Sharpey’s) fibers Secure periosteum to underlying bone Arteries Supply bone cells with nutrients

Endosteum Yellow bone marrow Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries (c) Figure 5.3c

Anatomy of a Long Bone Articular cartilage Covers the external surface of the epiphyses Made of hyaline cartilage Decreases friction at joint surfaces

Articular cartilage Compact bone Spongy bone (b) Figure 5.3b

Anatomy of a Long Bone Epiphyseal plate Epiphyseal line Flat plate of hyaline cartilage seen in young, growing bone Epiphyseal line Remnant of the epiphyseal plate Seen in adult bones

Articular cartilage Proximal epiphysis Spongy bone Epiphyseal line Periosteum Compact bone Medullary cavity (lined by endosteum) Diaphysis Distal epiphysis (a) Figure 5.3a

Anatomy of a Long Bone Marrow (medullary) cavity Cavity inside of the shaft Contains yellow marrow (mostly fat) in adults Contains red marrow for blood cell formation in infants In adults, red marrow is situated in cavities of spongy bone and epiphyses of some long bones

Articular cartilage Proximal epiphysis Spongy bone Epiphyseal line Periosteum Compact bone Medullary cavity (lined by endosteum) Diaphysis Distal epiphysis (a) Figure 5.3a

Bone Markings Surface features of bones Sites of attachments for muscles, tendons, and ligaments Passages for nerves and blood vessels Categories of bone markings Projections or processes—grow out from the bone surface Terms often begin with “T” Depressions or cavities—indentations Terms often begin with “F”

Microscopic Anatomy of Compact Bone Osteon (Haversian system) A unit of bone containing central canal and matrix rings Central (Haversian) canal Opening in the center of an osteon Carries blood vessels and nerves Perforating (Volkmann’s) canal Canal perpendicular to the central canal

Osteon (Haversian system) Lamellae Blood vessel continues into medullary cavity containing marrow Spongy bone Perforating fibers Compact bone Periosteal blood vessel Central (Haversian) canal Periosteum Perforating (Volkmann’s) canal (a) Blood vessel Figure 5.4a

Microscopic Anatomy of Bone Lacunae Cavities containing bone cells (osteocytes) Arranged in concentric rings called lamellae Lamellae Rings around the central canal Sites of lacunae

Canaliculus Lacuna (b) Lamella Osteocyte Canaliculus Lacuna (b) Central (Haversian) canal Figure 5.4b

Osteon Lacuna (c) Central canal Interstitial lamellae Figure 5.4c

Microscopic Anatomy of Bone Canaliculi Tiny canals Radiate from the central canal to lacunae Form a transport system connecting all bone cells to a nutrient supply

Canaliculus Lacuna (b) Lamella Osteocyte Canaliculus Lacuna (b) Central (Haversian) canal Figure 5.4b

Formation of the Human Skeleton In embryos, the skeleton is primarily hyaline cartilage During development, much of this cartilage is replaced by bone Cartilage remains in isolated areas Bridge of the nose Parts of ribs Joints

Bone Growth (Ossification) Epiphyseal plates allow for lengthwise growth of long bones during childhood New cartilage is continuously formed Older cartilage becomes ossified Cartilage is broken down Enclosed cartilage is digested away, opening up a medullary cavity Bone replaces cartilage through the action of osteoblasts

Bone Growth (Ossification) Bones are remodeled and lengthened until growth stops Bones are remodeled in response to two factors Blood calcium levels Pull of gravity and muscles on the skeleton Bones grow in width (called appositional growth)

Articular cartilage Hyaline cartilage Spongy bone New center of bone growth New bone forming Epiphyseal plate cartilage Growth in bone width Medullary cavity Bone starting to replace cartilage Invading blood vessels Growth in bone length New bone forming Bone collar Hyaline cartilage model Epiphyseal plate cartilage In an embryo In a fetus In a child Figure 5.5

Bone starting to replace cartilage Bone collar Hyaline cartilage model In an embryo Figure 5.5, step 1

Hyaline cartilage New center of bone growth Medullary cavity Invading blood vessels Growth in bone length In a fetus Figure 5.5, step 2

Articular cartilage Spongy bone New bone forming Epiphyseal plate Growth in bone width Invading blood vessels New bone forming Epiphyseal plate cartilage In a child Figure 5.5, step 3

is replaced by bone here. 4 Bone is resorbed here. 3 Bone growth Bone remodeling Bone grows in length because: Growing shaft is remodeled as: Articular cartilage Cartilage grows here. 1 Epiphyseal plate Cartilage is replaced by bone here. 2 1 Bone is resorbed here. Cartilage grows here. 3 2 Bone is added by appositional growth here. Cartilage is replaced by bone here. 4 Bone is resorbed here. 3 Figure 5.6

Types of Bone Cells Osteocytes—mature bone cells Osteoblasts—bone-forming cells Osteoclasts—giant bone-destroying cells Break down bone matrix for remodeling and release of calcium in response to parathyroid hormone Bone remodeling is performed by both osteoblasts and osteoclasts

Bone Fractures Fracture—break in a bone Types of bone fractures Closed (simple) fracture—break that does not penetrate the skin Open (compound) fracture—broken bone penetrates through the skin Bone fractures are treated by reduction and immobilization

Common Types of Fractures Comminuted—bone breaks into many fragments Compression—bone is crushed Depressed—broken bone portion is pressed inward Impacted—broken bone ends are forced into each other Spiral—ragged break occurs when excessive twisting forces are applied to a bone Greenstick—bone breaks incompletely

Repair of Bone Fractures Hematoma (blood-filled swelling) is formed Break is splinted by fibrocartilage to form a callus Fibrocartilage callus is replaced by a bony callus Bony callus is remodeled to form a permanent patch

Figure 5.7 Hematoma External callus Bony callus of spongy bone New blood vessels Internal callus (fibrous tissue and cartilage) Healed fracture Spongy bone trabecula Hematoma forms. 1 Fibrocartilage callus forms. 2 Bony callus forms. 3 Bone remodeling occurs. 4 Figure 5.7

Hematoma Hematoma forms. 1 Figure 5.7, step 1

Figure 5.7, step 2 Hematoma External callus New blood vessels Internal (fibrous tissue and cartilage) Spongy bone trabecula Hematoma forms. 1 Fibrocartilage callus forms. 2 Figure 5.7, step 2

Figure 5.7, step 3 Hematoma External callus Bony callus of spongy bone New blood vessels Internal callus (fibrous tissue and cartilage) Spongy bone trabecula Hematoma forms. 1 Fibrocartilage callus forms. 2 Bony callus forms. 3 Figure 5.7, step 3

Figure 5.7, step 4 Hematoma External callus Bony callus of spongy bone New blood vessels Internal callus (fibrous tissue and cartilage) Healed fracture Spongy bone trabecula Hematoma forms. 1 Fibrocartilage callus forms. 2 Bony callus forms. 3 Bone remodeling occurs. 4 Figure 5.7, step 4