Ch 7: Bone Tissue

Skeletal System

The sturdy internal frame composed of bone, cartilage, ligaments

Skeletal System Functions

1. Support

2. Protection

3. Movement

4. Electrolyte balance

5. Acid-base balance

6. Blood formation

Support

body weight, teeth, posture

Protection

brain, lungs, heart, spinal cord

Movement

limbs, breathing

Electrolyte balance

stores Ca2+ & phosphate

Acid-base balance

buffers blood against large pH changes

Blood formation

red bone marrow

Bone can refer to:

• Osseous tissue (the hard connective tissues)

OR

• the bone organ (osseous tissue + blood + cartilage + adipose + nerve + connective tissue)

Flat bones

Thin, curved; protect organs

Example: Cranial bones

Long bones

Important for movement; longer than wide

Example: Femur

Short bones

Equal length and width; glide in many directions

Example: Carpals (wrist bones)

Irregular bones

Elaborate shapes

Example: Vertebrae

Compact bone

Dense outer shell that encloses marrow cavity

Spongy bone

Found at ends of long bones

Marrow cavity

Space that contains bone marrow

Nutrient foramina

Tiny holes for blood vessels

Periosteum

Outer covering

• Fibrous outer layer

• Inner osteogenic layer

Endosteum

Lines inner surfaces; contains osteogenic cells

Diaphysis

Shaft of long bone that provides leverage

Epiphysis

Enlarged end of long bone; strengthen joint; anchors tendons/ligments

Articular Cartilage

Layer on hyaline cartilage, allows joint move freely

Epiphyseal Plate

Hyaline cartilage that separates epiphyses and diaphysis of children bones

Epiphyseal Line

Adult remnant of plate

Four Types of Bone Cells

1. Osteogenic Cells

2. Osteoblasts

3. Osteocytes

4. Osteoclasts

Osteogenic Cells

Stem cells that will become osteroblasts

Osteoblasts

From bone; build matrix

Osteocytes

Former osteoblasts; maintain bone; help resorb/deposit

Osteoclasts

Dissolve bone; large cells formed by fusion of stem cells

Bone Matrix

The ground substance of osseous tissue (hard dense substance)

1. Organic matter (1/3 of bone matrix)

2. Inorganic matter (2/3 of bone matrix)

Organic matter

• Collagen fibers

• Glycosaminoglycans (GAGs)

• Proteoglycans

• Glycoproteins

Purpose:

-Adds flexibility

-Prevents bone from becoming brittle

Inorganic matter

Made of:

• 85% hydroxyapatite (calcium phosphate crystals)

• 10% calcium carbonate

• Other minerals (magnesium, sodium, potassium)

Purpose:

-Provides strength, hardness, and shape

-Resists compression

Osteon

• Central Canal

• Lamellae

Central Canal

Holds blood vessels + nerves

Lamellae

Rings of matrix around the central canal

Collagen Arrangement

• Collagen spirals around each lamella

• Each layer spirals in the opposite direction

→ This increases strength and resistance to twisting or compression

Perforating (Volkmanns) Canals

• Run perpendicular/diagonal to central canals

Connect:

• One osteon to another

• Central canals to nutrient foramina on the surface

Lacunae

• Small cavities between lamellae

• Contains osteocytes

Canaliculi

• Tiny channels linking lacunae

• Allow osteocytes to exchange nutrients/waste through gap junctions

Spongy Bone Structure

• Network of spicules (slivers) and trabeculae (thin plates/beams)

• Spaces between trabeculae are filled with red bone marrow

• Trabeculae align along lines of mechanical stress → strengthens bones without adding weight

Bone Marrow

Soft tissue inside bones

Red Marrow

Function:

Contains hematopoietic (produces all blood cells)

Location:

• Children: in almost every bone

• Adults: only in specific bones

-skull

vertebrae

-ribs

-sternum

-pelvic girdle

-proximal ends of humerus + femur

Yellow Marrow

• Mostly fat (adipose tissue)

• Replaces red marrow in adult limb bones

• Does NOT produce blood cells

Bone Formation (Ossification)

1. Intramembranous

2. Endochondrial

Steps of Endochondrial Ossification Steps 1 & 2

1. Starts with a body of hyaline cartilage

2. A primary ossification center forms in the middle

-osteoblasts deposit a bony collar around the cartilage

Steps of Endochondrial Ossification Step 3

• Blood vessels enter the primary ossification center

They bring in:

• Osteoclasts → digest calcified cartilage and create the primary marrow cavity

• Osteoblasts → begin laying bone tissue, lengthening the bone

• Blood vessels also invade the epiphysis, forming a secondary ossification center

Steps of Endochondrial Ossification Step 4 (By Birth)

• The metaphysis forms at each end

→ This is the transition zone between cartilage and bone

• A secondary marrow cavity appears in the epiphysis

Steps of Endochondrial Ossification Step 5 (Childhood)

• Epiphyses fill with spongy bone

• Cartilage remains in two places:

1. Covers ends of bones (Articular cartilage)

2. Epiphysial plate

-Between primary and secondary marrow cavities

Steps of Endochondrial Ossification Step 6 (Early 20s)

Epiphyseal plate converts to bone, ceasing growth

How Bones Grow in Length

Occurs at:

• The epiphyseal plate (hyaline cartilage band)

• Each side of the plate is bordered by the metaphysis, where cartilage turns into bone

Process Bones Grow in Length

1. Chondrocytes near marrow cavity stop dividing

2. Minerals are deposited → cartilage calcifies

3. Osteoclasts dissolve calcified cartilage

4. Osteoblasts lay down new bone on top

5. The plate shifts upward from both ends → bone lengthens

What Controls Lengthening

Hormones:

• Growth hormones= lengthening

• Sex hormones= start & stop of lengthening of long bones

Appositional Growth

Continual bone growth in diameter and thickness

How Bones Grow in Width/Thickness Process

• Osteoblasts under the periosteum deposit new osteoid tissue

→ This calcifies, adding layers to the outer surface

• These layers from circumferential lamellae

• Meanwhile, osteoclasts in the endosteum enlarge the marrow cavity

→ Prevents the bone from becoming too heavy

Why Does Remodeling Happen

Allow the skeleton to:

• Repair microfractures

• Balance minerals; release/absorb minerals from bloodstream

• Change shape and width

• Adjust bone strength depending on stress placed on it

• Form bony features (ridges, bumps, epicondyles)

How Does Remodeling Work

• Osteoclasts break down (resorb) bone

• Osteoblasts build bone

• These two must work together to keep bone density healthy

When Remodeling Goes Wrong

• If osteoclasts > osteoblasts, bone density decreases

• This can lead to osteoporosis, where bones become weak and brittle

Factors that Affect Remodeling

• Nutrition (calcium, vitamin D)

• Body weight

• Exercise level (especially weight-bearing exercise)

• Hormones

• Medical history

Wolff’s Law

• Bone grows stronger in response to mechanical stress

• Bone becomes weaker when stress is removed

How Wolff’s Law Works

When a bone bends slightly:

1. The inner curvature experiences compressive force

2. This force generates small electrical currents

3. These currents stimulate osteoblasts

4. Bone is added where compressive force is greatest

5. Bone is removed where force is low

Results;

A bone reshapaes itself to better withstand the stress placed on it

Physiology of Mature Osseous Tissue

Even fully grown bone is:

• Metabolically active

• Constantly undergoing growth, maintenance, and remodeling

• A major regulator of mineral balance in the body

Why Mineral Balance Matters

Calcium Levels Affect:

• Nervous system function

• Muscle contraction

• Heart rhythm

Mineral Deposition

Osteoblasts build bone by depositing calcium and phosphate from the blood

How Mineral Deposition Happens

1. Osteoblasts produce collagen fibers

2. Minerals (Ca2+ and PO43-) stick to the collagen

3. Crystals of hydroxyapatite form

4. More calcium/phosphate is attracted

→ Blood Ca2+ and phosphate levels go Down

Mineral Resorption

Osteoclasts break down bone to release minerals into the blood

How Mineral Resorption Happens

1. Osteoclasts pump out HCl (acid) → dissolves bone minerals

2. They secrete an enzyme that digests collagen

3. Calcium and phosphate enter the bloodstream

→ Blood Ca2+ and phosphate levels go up

Resorption= breaking down bone to release minerals