Skeletal System

1. Humans have endoskeletons, composed of bone and cartilage that grows with us
• does not limit space available for internal organs
• supports greater weight
• have a vertebral column (backbone) 
2. Cartilage
• type of dense connective tissue made of tough protein fibers
• function is to provide smooth surfaces for the movement of bones at a joint
• babies and children have more cartilage, and as they grow, the cartilage becomes bone tissue
3. Functions of bones
• gives shape and form to body
• structural support to body against the flow of gravity, and lower bones support trunk when standing
• protection of internal organs, especially soft ones
• fused bones of cranium make it less vulnerable to injury
• spinal cord and bones of rib cage help protect hear and lungs
• provides attachment surfaces for muscles and tendons, allowing body movement
• bones work together with muscles to produce body movement
• blood cell production takes place in certain bone marrow
• stores minerals, such as calcium, to support homeostasis
4. Structures of bones
• bones are organs
• compact bone
• dense outer layer of bones
• spongy bone
• lighter and less dense, and found toward center of bone
• periosteum
• tough, shiny, white layer that covers all surface of bones (except joints) 
• composed of layer of fibrous connective tissue and layer of bone forming cells
5. Compact bone (in detail) 
• accounts for 80% of total bone mass
• extremely hard, made of osteons (haversian systems) 
• act like strong pillars in bone to give bones strength and let it bear the weight of attached muscles
• made up of rings of calcium salts and collagen fibers called bone matrix
• calcium salts give bone strength, but shatter when stressed
• collagen fibers are tough and flexible
• together, they give the bones ability to bend and twist without breaking easily
• center of osteon is haversian canals
1. passageway for blood vessels and nerves
• within osteon, there are osteocytes
1. found in pockets called lacunae that are between layers of bone matrix
2. responsible for monitoring protein and mineral content of bone
• osteoblasts are responsible for growth of new bone
1. found near surface of bones
• osteoclasts remove calcium salts from bone matrix
6. Spongy bone (in detail) 
• occurs at ends of long bones
• less dense than compact bone
• "spongy" refers to appearance only, because spongy bone is quite strong
• forms porous network of bony branches (trabiculae), and gives bone strength and makes bone lighter
• allows room for blood vessels and bone marrow
• do not have osteons
• nutrients reach spongy bone by diffusion through tiny openings
• makes up bulk of interior of bones
7. Bone marrow
• connective tissue
• two types
• red bone marrow
1. produces red blood cells, platelets, and most white blood cells
2. newborns have red bone marrow only
3. as child ages, yellow bone marrow replaces red bone marrow
4. found mostly in flat bones of skull, ribs, vertebrae, and pelvic bone
• yellow bone marrow
1. produces white blood cells
2. color due to high number of fat cells
8. Periosteum
• hast tough, external fibrous layer
• internal layer with osteoblasts
• richly supplied with blood, lymph, and nociceptors
• provides nourishment to bone through rich blood supply
• connected to bone by strong collagen fibers (Sharpey's fibres) 
9. Bone shapes
• long bones
• longer than they are wide
• long shaft with two bulky ends
• mostly made of compact bone, but may have large amount of spongy bone at the ends
• classification refers to shape rather than size
• short bones
• roughly cube shaped
• have thin layer of compact bone surrounding spongy interior
• wrists and ankles
• sesamoid bones
• embedded in tendons
• act to hold tendon away from joint
• force of muscle is increased
• flat bones
• thin and generally curved
• two parallel layers of compact bones sandwiching spongy bone
• skull (cranium) bones, and sternum
• irregular bones
• bones that do not fit into above categories
• consists of thin layers of compact bone surrounding spongy interior
• pelvis and vertebrae
10. Cellular structure of bone
• osteoblasts
• bone-forming cells on inner and outer surfaces of bones
• make protein mixture that becomes bone matrix
• immature bone cells
• osteoblasts that get trapped in bone matrix become osteocytes, and direct the release of calcium from bones
• osteocytes
• originate from osteoblasts
• star-shaped
• occupy spaces called lacunae
• matrix maintenance and calcium homeostasis
• mature bone cells
• osteoclasts
• responsible for bone resorption
• remodeling of bone to reduce volume
• large cells with many nuclei
• located on bone surface
• secrete acids that dissolve calcium salts, releasing into blood stream
• causes calcium and phosphate concentration to increase
• constantly remove minerals from bone
11. Bone cells and calcium homeostasis
• bone resorption by osteoclasts releases stored calcium into bloodstream
• important in regulating calcium balance
12. Development of bones
• skeleton begins forming in early fetal development
• ossification begins after eight weeks
• at first, skeleton made of cartilage
• nutrients diffuse through matrix to chondrocytes
• bones of the body gradually harden in process called endochrondrial ossification
• cartilage still remains in your ear, joints, rib cage, the tip of your nose, and little discs between the vertebrae
13. Endochondral ossification
• process of replacing cartilage with bony tissue
• third month after birth, blood vessels transport osteoblasts and stem cells into interior to change cartilage into bone tissue
• osteoblasts form bone collar of compact bone around diaphysis of the bone
• osteoclasts remove material from center of bone to form central cavity
• cartilage at ends of long bones keep growing
• secondary ossification
• similar to ossification at center of bone
• however, spongy bone is kept instead of broken down to form cavity
• cartilage totally replace, except 2 areas
• region of cartilage over the surface of epiphysis
• another inside long bones at either end 
• called growth region
14. Intramembranous ossification
• happens in flat bones (cranial and clavicles) 
• in developing fetus
• future bones formed as connective tissue membranes
• osteoblasts migrate to membranes and secrete osteoid
• osteoid forms bony matrix