Ch. 6 Anatomy- Bone and Bone Tissues (Part 1)

Skeletal System Consists of

1. Bones

2. Joints

3. Supporting Tissues

Bones

Main organ of skeletal system

Types of Tissue in Bone

1. Dense Regular

2. Irregular Collagenous CT

3. Bone Marrow

Functions of Skeletal System

1. Protection of Underlying Organs

2. Minerals Storage and Acid-Base Homeostasis

3. Blood Cell Formation

4. Fat Storage

5. Movement

6. Support

Mineral Storage and Acid-Base Homeostasis (2.)

• Bones store calcium, phosphorus, magnesium salts

• Minerals are also present in blood as electrolytes, acids, bases

Blood Cell Formation (3.)

• Bones house red bone marrow- CT involved in formation of blood cells (hematopoiesis)

Fat Storage (4.)

• Bones also contain yellow bone marrow- made up of fat cells (adipocytes), store triglycerides

Movement (5.)

• Bones serve as sites for attachment of most skeletal muscles

Support (6.)

• Skeleton supports weight of body

• Provides structural framework

Bone Structure

Can be organized into 5 classes of bones for all 206 bones

5 Classes of Bones

1. Long Bones

2. Short Bones

3. Flat Bones

4. Irregular Bones

5. Sesamoid Bones

Long Bones

• longer than they are wide

• include most bones in arms and legs

Short Bones

• roughly cube-shaped or about as long as they are wide

• include bones of wrist or carpals and ankle or tarsals

Flat Bones

• thin and broad bones

• include ribs, pelvis, sternum, and most skull bones

Irregular Bones

• irregular shapes

• include vertebrae and certain skull bones

Sesamoid Bones

• specialized bones located within tendons

• usually small, flat, oval-shaped

• gives tendons mechanical advantage

• patella (kneecap)

Structure of a long bone (Parts)

1. Periosteum

2. Perforating Fibers (Sharpey’s Fibers)

3. Diaphysis

4. Medullary Cavity (Marrow Cavity)

5. Compact Bone

6. Spongy Bone (Cancellous Bone)

7. Bony Struts

8. Epiphyseal Lines

Periosteum

• Composed of dense irregular collagenous CT (rich with blood vessels and nerves)

• surround outer surface of long bones

Perforating Fibers (Sharpey’s Fibers)

• Anchors periosteum firmly to underlying bone surface by penetrating deep into bone matrix

Diaphysis

• Shaft of long bone

• Each end is Epiphysis

• Covered with thin layer of hyaline cartilage (articular cartilage) found within joints (articulations) between bones

Medullary Cavity (marrow cavity)

• Within diaphysis

• Contains either red or yellow bone marrow (depends on bone and age)

Compact Bone

• hard, dense outer region

• allows bone to resist linear compression and twisting forces

Spongy Bone (cancellous bone)

• inside compact bone

• honey-comb like framework of bony struts

• allows long bones to resist forces from different directions

• provides cavity for bone marrow

Endosteum (Thin membrane)

Bony struts in spongy bone and all inner surfaces are covered by this thin membrane

Epiphyseal Lines

• separates both proximal and distal epiphyses from diaphysis

• remnant of epiphyseal plates (growth plates); line of hyaline cartilage found in developing bones of children

Structure of short, flat, irregular, and sesamoid bones

• They do not have diaphyses, epiphyses, medullary cavities, epiphyseal lines or epiphyseal plates

• Covered by periosteum

• Internal structure of two outer layers of thin compact bone with middle layer of spongy bone (diploë)

• Some flat and irregular bones of skull contain hollow, air-filled spaces (sinuses), which reduce bone weight

Blood Supply of Long Bones

• 1/3 comes from periosteum to supply compact bone

• 2/3 comes from one or two nutrient arteries, enter bone through small hole in diaphysis (nutrient foramen)

Blood Supply of Other Bones

• Provided mostly by vessels in periosteum

Blood and nerve supply to bone

Bones are well supplied with blood vessels and sensory nerve fibers

Red Marrow Presence in Adults

• Decreases with age

• Present in pelvis, proximal femur, humerus, vertebrae, ribs, sternum, clavicles, scapulae, and some skull bones

Extracellular matrix (ECM) of bone (unique)

• Inorganic matrix- minerals make up 65% of bone’s total weight

• Organic matrix- makes up remaining 35%, consists of collagen fibers and usual ECM components

Inorganic Matrix

• predominantly calcium salts

• bone stores around 85% of total calcium ions and large amount of phosphorus

• bicarbonate, potassium, magnesium, and sodium are also present

Hydroxyapatite Crystal

• Large molecules of calcium and phosphorus salts

• This crystalline structure makes bone one of the hardest substances in the body

Organic Matrix (known as Osteoid)

• Consists of protein fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins

• Collagen is the predominant protein fiber

• Collagen fibers align themselves with hydroxyapatite crystals

• Glycosaminoglycans and proteoglycans create an osmotic gradient; draws water into osteoid to help tissue resist compression

• Glycoproteins in osteoid bind osteoid and inorganic matrix together

Torsion and Tensile Forces

Torsion- twisting

Tensile- pulling or stretching

Note: Collagen helps bone resist these forces

Bone is a DYNAMIC tissue

Bone is continually changing as older bone is broken down for raw materials to build new bones

3 Types of Bone Cells

1. Osteoblasts

2. Osteocytes

3. Osteoclasts

Osteoblasts

• Metabolically active in periosteum and endosteum

• Perform bone deposition (bone-building)

• Osteogenic cells differentiate into osteoblasts when stimulated by specific chemical signals

Osteocytes

Osteocytes are formed when osteoblasts surround themselves with matrix in small cavities (lacunae)

• no longer actively synthesize bone matrix, but maintain it

Osteoclasts

• Responsible for bone resorption (cell secretes hydrogen ions (inorganic matrix) and enzymes (organic matrix) to break down bone matrix)

• Large multinucleated cells derived from fusion of cells from bone marrow

• Eventually located in shallow depressions of internal and external surfaces of bone

Compact Bone Cross Section

• Resembles forest of tightly packed trees

• Each tree is a unit called a osteon

• Rings of each tree are made up of thin layers of bone called lamellae

• Each osteon contains 4-20 lamellae (arranged concentric)

Osteon Structure

• Central Canal- endosteum-lined hole in center of each osteon (blood vessels and nerves supply bone

• Osteons are not permanent structures

• Interstitial lamellae- fill spaces between circular osteons, are remnants of old osteons

Trabeculae (Spongy Bone)

• Struts or ribs of bone, covered with endosteum

• Composed of concentric lamellae with osteocytes in lacunae, communicate through canaliculi

• Obtain blood from vessels in bone marrow