Bones (Ch 6)

The skeletal system

• Bones of the skeleton

• Cartilages, ligaments, and other connective tissues

Functions:

• movement and locomotion

• Structural support and attachment

• Protection: thoracic cage, vertebrae, skull

• Storage of minerals and lipids (calcium and yellow bone marrow)

• Blood cell production

Bone Structure: Long Bone

Diaphysis:

• Wall of compact bone

• Inside: medullary cavity with yellow bone marrow

Metaphysis: narrow zone between diaphysis and epiphysis

Epiphysis:

• Spongy (trabecular) bone filled with red bone marrow

• Cortex: thin covering of compact bone

Bone Structure (Flat bone)

• Core of spongy bone with red bone marrow

• Covered by compact bone

Features of bone

• Osteocytes are the main cell type

• Depend on diffusion through canaliculi for nutrient and waste removal

• Little ground substance with dense mineralized matrix

• Surrounded by periosteum (dense irregular connective tissue)

• Good blood supply: high regeneration and repair

Bone matrix (identical in compact and spongy bone)

Composed of:

• Hydroxyapatite crystals: Ca3(PO4)2 and Ca(OH)2

• Calcium carbonate: CaCO3

• Minerals for compressive strength

• Collagen fibers: flexible and tensile strength

• Composite materia

Osteoprogenitor cells

• Mesenchymal stem cells produce osteoblasts

• Located in inner cellular layer of periosteum/endosteum

• Assist in fracture repair

Osteoblast cell

• immature bone cells

• Secrete matrix compounds to produce new bone (osteogenesis)

• Osteoid: matrix produced by osteoblasts, not yet calcified

• Secrete calcium phosphate

• Surrounded by bone become osteocytes

Osteocytes

• Mature bone cells: maintain bone matrix

• Live in lacunae (1 cell each)

• Between lamellae (layers) of matrix

• Connect by cytoplasmic extensions through canaliculi

• Do not divide but may produce osteoblasts that repair the matrix

Osteoclast

• Derived from same stem cells as monocytes

• Giant, multinucleate cells

• Secrete acids and proteases (protein digesting enzymes) to dissolve matrix (osteolysis)

• Release stored minerals, making bones weaker

Osteoblast=add matrix vs Osteoclast=destroy matrix

Structure of Compound Bone

Structure of Spongy Bone

• Matrix formed by a network of trabeculae (no osteons)

• Produces red blood cells

• No blood vessels in trabeculae

• Some yellow bone marrow stores fat

Periosteum

• Covers the exterior of all compact bones except at joint capsules

• Contiguous with articular connective tissue and synovial membranes

• Made of an outer, fibrous layer and an inner cellular layer

• Perforating fibers: collagen fibers of periosteum entering bones

• Also ligaments and tendons

Endosteum

• Lines the medullary cavity

• lines central canals

• Covers trabeculae of spongy bone

• Contains osteoblasts, osteoprogenitor, and osteoclasts

• Active in bone growth and repair

Endochondral Ossification

1. Hyaline cartilage grows (interstitial)

2. Chondrocytes get bigger (so do the lacunae)

3. Matrix gets calcified and chondrocytes die

4. Blood vessels grow

5. Cells from perichondrium become osteoblast

6. Compact bone is produced through appositional growth

7. Capillaries and fibroblast invade cartilage

8. Fibroblast take space left by chondrocytes

9. Fibroblasts become osteoblasts

10. Primary ossification center forms (spongy bone)

11. Osteoclast form medullary cavity

12. Shaft becomes thicker

13. Cartilage at the edge lifts

14. Capillaries and osteoblast create secondary ossification center

15. Epiphyses fill with spongy bone

16. Some stays as articular cartilage

17. Epiphyseal cartilage separates

18. New bone grows

Dermal Ossification (intramembranous)

1. Mesenchymal cells and differentiate as osteoblasts

2. Bone expands as spicules

3. Spicule interconnect, trapping blood vessels

4. Bone remodels into spongy bone

Repair of a Bone Fracture

1. Hematoma production

2. An external callus (cartilage and bone) and internal callus (bone) stabilizes the broken bone

Effects of Parathyroid Hormone

• Decreases Ca2+ storage in bones; stimulates osteoclast activity and Ca2+ release from bone into plasma

• Decreases Ca+ excretion in kidneys and induces production of calcitriol Vit D

• Indirectly, increases intestinal absorption of Ca+