1. Skeletal System: Bones and Bone Tissue
Chapter 6
Skeletal System: Bones and Bone Tissue

2. Functions of the Skeletal System
Support. Bone is hard and rigid; cartilage is flexible yet strong. Cartilage in nose, external ear, thoracic cage and trachea. Ligaments- bone to bone
Protection. Skull around brain; ribs, sternum, vertebrae protect organs of thoracic cavity
Movement. Produced by muscles on bones, via tendons. Ligaments allow some movement between bones but prevent excessive movement (joint: formed where two or more bones come together------allows movement between bones)
Storage. Ca and P. Stored then released as needed. Fat stored in marrow cavities
Blood cell production. Bone marrow that gives rise to blood cells and platelets

3. Components of Skeletal System
Bone
Cartilage: three types
Hyaline
Fibrocartilage
Elastic
Tendons and ligaments (Tendons: muscle to bone
Ligament: bone to bone)

4. Hyaline Cartilage Consists of specialized cells that produce matrix
Chondroblasts: form matrix
Chondrocytes: surrounded by matrix; are lacunae
Matrix. Collagen fibers for strength, proteoglycans for resiliency
Perichondrium. Double-layered C.T. sheath. Covers cartilage except at articulations
Inner. More delicate, has fewer fibers, contains chondroblasts
Outer. Blood vessels and nerves penetrate. No blood vessels in cartilage itself
Articular cartilage. Covers bones at joints; has no perichondrium
Growth
Appositional. New chondrocytes and new matrix at the periphery
(chondroblasts in perichondrium lay down new matrix & new chondrocytes)
Interstitial. Chondrocytes within the tissue divide and add more matrix between the cells.

5. Bone Histology
Bone matrix. Like reinforced concrete. Steel bars is collagen fibers, cement is hydroxyapatite
Organic: collagen and proteoglycans (35%)
Inorganic: hydroxyapatite. CaPO4 crystals (65%)
If mineral removed, bone is too bendable
If collagen removed, bone is too brittle

6. Bone Histology Bone cells
Osteoblasts
Osteocytes
Osteoclasts
Stem cells or osteochondral progenitor cells
Woven bone: collagen fibers randomly oriented
Lamellar bone: mature bone in sheets
Cancellous bone (spongy): contain trabeculae
-less bone matrix & more space
Compact bone: dense
-more bone matrix & less space

7. Bone Cells Osteoblasts
Formation of bone through ossification or osteogenesis. Collagen produced by E.R. and golgi. Released by exocytosis. Precursors of hydroxyapetite stored in vesicles, then released by exocytosis.
Ossification: formation of bone by osteoblasts. It occurs by appositional growth on surface of previously existing bone. Osteoblasts communicate through gap junctions. Bone matrix produced by osteoblasts covers the older bone & surrounds osteoblasts results in new layer of bone.

8. Bone Cells
Osteocytes. Mature bone cells once surrounded by matrix. They are relatively inactive, but can make small amounts of matrix to maintain it.
Lacunae: spaces occupied by osteocyte cell body
Canaliculi: canals occupied by osteocyte cell processes
Nutrients diffuse through tiny amount of liquid surrounding cell and filling lacunae and canaliculi. Then can transfer nutrients from one cell to the next through gap junctions.

9. Bone Cells
Osteoclasts. Responsible for resorption of bone (breakdown of bone)
Ruffled border (projections): where cell membrane borders bone and resorption is taking place.
H ions pumped across membrane, acid forms, eats away bone.
Release enzymes that digest the bone.
Derived from monocytes (which are formed from stem cells in red bone marrow)
Multinucleated
Stem Cells. Mesenchyme (Osteochondral Progenitor Cells) become chondroblasts or osteoblasts. (Remember: osteocytes derived from osteoblasts)

10. Woven and Lamellar Bone
Woven bone. Collagen fibers randomly oriented.
- first formed during fetal development or repair of fracture
- after formation, osteoclasts break down woven bone & osteoblasts build new matrix this process called remodeling
Remodeling
Removing old bone and adding new
Woven bone is remodeled into lamellar bone
Lamellar bone
Mature bone in sheets called lamellae. Fibers are oriented in one direction in each layer, but in different directions in different layers for strength.

11. Cancellous (Spongy) Bone
Trabeculae: interconnecting rods or plates of bone. Like scaffolding.
Spaces filled with marrow & blood vessels.
Covered with endosteum.
Oriented along stress lines
Trabeculae consist of several lamellae (thin sheets) w/ osteocytes located in lacunae between them

12. Compact Bone Central or Haversian canals: parallel to long axis
Lamellae: thin sheets/layers
concentric (circles), circumferential (perimeter, periphery), interstitial (between osteon)
Osteon or Haversian system: central canal (also called haversian canals------contains blood vessels that run parallel to long axis of bone), associated concentric lamellae and osteocytes
Perforating or Volkmann’s canal: perpendicular to long axis. Both perforating and central canals contain blood vessels. Central canals receive blood vessels from perforating canals & nutrients in blood enter central canals, pass into canaliculi, move through osteocytes to osteocytes (by gap junctions). Waste removed in reverse direction.

13. Bone Shapes Long Ex. Upper and lower limbs Short
Ex. Carpals and tarsals
Flat
Ex. Ribs, sternum, skull, scapulae
Irregular
Ex. Vertebrae, facial

14. Structure of a Long Bone
Diaphysis
Shaft
Compact bone
Epiphysis
End of the bone
Cancellous bone
Epiphyseal plate: growth plate (separates epiphysis from diaphysis)
Hyaline cartilage; present until growth stops
Epiphyseal line: bone stops growing in length (epiphyseal plate becomes epiphyseal line)
Medullary cavity (space): In children medullary cavity is red marrow, gradually changes to yellow in limb bones and skull (except for epiphyses of long bones). Rest of skeleton is red.

15. Structure of a Long Bone, cont.
Periosteum
Outer is fibrous (contains blood vessels & nerves)
Inner is single layer of bone cells including osteoblasts, osteoclasts and osteochondral progenitor cells
Tendons & ligaments attach to bone & become continuous with fibers of periosteum.
Sharpey’s fibers: some collagen fibers of tendons or ligaments penetrate the periosteum and into the bone. Strengthen attachment of tendon or ligaments to bone.
Endosteum. Similar to periosteum, but more cellular. Lines all internal surfaces of all cavities including spaces in cancellous bone.

16. Structure of Flat, Short, and Irregular Bones
Flat Bones
No diaphyses, epiphyses
Sandwich of cancellous between compact bone
Short and Irregular Bone
Compact bone that surrounds cancellous bone center; similar to structure of epiphyses of long bones (bec. have processes that possess epiphyseal growth plates
No diaphyses and not elongated
Some flat and irregular bones of skull have sinuses (air filled spaces) lined by mucous membranes.

17. Bone Development Intramembranous ossification
Takes place in connective tissue membrane
Endochondral ossification
Takes place in cartilage
Both methods of ossification
Produce woven bone that is then remodeled
After remodeling, formation cannot be distinguished as one or other