HumanA&P 7: Bones

Bone Tissue Composition

o Bone tissue (osseous tissue)
o Dense regular & irregular connective tissue
o Bone marrow (mix of tissue types)
o Epithelium
o Adipose tissue
o Nervous tissue

7 Bone Functions

1. Protection

2. Mineral Storage & Acid-Base Homeostasis

3. Blood Cell Formation

4. Fat Storage

5. Movement

6. Support

7. Hormone Production: Osteacalcin

2 Divisions of Skeletal System

1. Axial Skeleton

2. Appendicular Skeleton

Axial Skeleton

80 bones that lie around longitudinal
axis of human body:
the bones of the skull, auditory ossicles (ear bones), hyoid
bone, ribs, sternum, and vertebrae.

Appendicular Skeleton

126 bones of upper and lower
limbs or appendages plus the bone groups called girdles
that connect the limbs to the axial skeleton.

6 Bone Classifications

1. Sutural Bones

2. Irregular Bones

3. Short Bones

4. Flat Bones

5. Long Bones

6. Sesamoid Bones

Long Bones

• Longer than they are wide

• Most bones in arms & legs

Short Bones

• Roughly cube-shaped

• Includes carpals & tarsals

Flat Bones

• Thin & broad bones

• Include:

  • ribs

  • pelvis

  • sternum

  • skull bones

Irregular Bones

• Irregular shape

• Include vertebrae & skull bones

Sesamoid Bones

• Specialized bones located within tendons

• Usually small, flat, oval shaped

• Gives tendons mechanical advantage

Appositional Cartilage Growth

• Cartilage-forming cells in perichondrium secrete matrix against external face of existing cartilage

  • New matrix laid down on surface of cartilage

Interstitial Cartilage Growth

• Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within

• New matrix made within cartilage

Anatomy of Long Bones

• Diaphysis

• Epiphysis

• Articular Cartilage

• Periosteum

• Medullary Cavity

• Endosteum

Diaphysis

• Long bones

• Middle shaft of long bones

Epiphysis

• Long bones

• 2 rounded ends of long bones

Articular Cartilage

• Long bones

• Thin layer of hyaline cartilage covering regions of the epiphysis where bone articulates with other bones

Periostium

• Long bones

• Sheath of dense irregular connective tissue and blood vessels surrounding parts of the bone outside of the articular cartilage

Endosteum

• Long bones

• Thin membrane that lines medullary cavity in

Anatomy of Short, Flat, Irregular & Sesamoid Bones

• No diaphysis, epiphysis, or medullary cavities

• Covered by periosteum, perforating fibers

• Internal structure:

  • 2 outer layers of thin, compact bone

  • Middle layer of spongey bone

  • Red bone marrow

Blood Supply to Long Bones

• 1/3 from periosteum

• 2/3 from nutrient arteries through nutrient foramen

Blood Supply to Short, Flat, Irregular & Sesamoid Bones

• Provided mostly from the periosteum

Red Bone Marrow

• Loose connective tissue

• Blood-forming hematopoietic cells

• Amount decreases with age

Yellow Bone Marrow

• Triglyceride storage

• Blood vessels

• Adipocytes

Microscopic Bone Structure

• Osseous tissue composed mostly ECM with small population of cells scattered throughout

Extracellular Matrix of Bone

• Inorganic materials: minerals 60% of bone weight

• Organic materials: Collagen fibers & usual ECM components make up the rest

Inorganic Bone Matrix

• Predominantly calcium salts

• Stores around 85% of calcium ions, lots of phosphorus

• Ca + P = hydroxyapatite crystal

  • Makes bone hardest substance in body

• Bicarbonate, potassium, magnesium, sodium

• Bone shatters easily without it

Organic Bone Matrix

• Osteiod

• Consists of:

  • protein fibers

  • proteoglycans

  • glycosaminoglycans

  • bone-specific proteins

  • collagen

• Bone cannot resist compression without it

4 Bone Cells

1. Osteoprogenitor Cells

2. Osteoblasts

3. Osteocytes

4. Osteoclasts

Osteoprogenitor Cells

• Stem cells that develop into osteoblasts

Osteoblasts

• Bone deposition

• Synthesize & secrete ECM that calcifies into bone

• Eventually surround themselves with matrix in lacunae and become osteocytes

Osteocytes

• Mature bone cells

• Most numerous cells in bone tissue

• Maintain bone tissue

• Monitor mechanical stress

• Help repair damaged bone

Osteoclasts

• Bone reabsorption

  • Secretes H+ and enzymes to break down materials

• Large multinucleated cells

• Break down ECM to release nutrients

• Help bones grow & heal

Osteogenic Cells

Immature cells that differentiate into osteoblasts when stimulated by specific chemical signals

3 Steps of Osteoblasts —> Osteocytes

Bone Histology

• Hard outer compact bone

  • Organized by osteons

  • Inner layers called lamellae

• Porous inner spongy bone

  • Trabeculae

Lacunae

Small cavities or spaces

Osteon Structure

• Osteon contains 4-20 lamellae ring structures that make the bone strong

• Central canal contain blood vessels & nerves

• Osteocytes reside in lacunae

• Canaliculi are extensions of osteocytes that connect lacunae together

Spongy Bone

• Resists forces from multiple directions

• Made of trabeculae: projections of bone making “web”

  • Made of lamellae with osteocytes localized to lacunae that communicate through canaliculi

• No osteons

Bone Density

• Compact Bone:

  • Denser, arranged into osteons with osteonic canal

  • Concentric lamellae, lacunae within

• Spongy Bone:

  • No osteons

  • Arranged in trabeculae

  • Cavities are filled with red bone

Factors Affecting Bone Homeostasis

• Require multiple factors to be successful:

  • Adequate minerals

  • Vitamins A, C, D

  • hGH & insulin-like growth factors

  • Weight-bearing exercise that stresses bones

• Growth driven by hormone increases, repair/remodeling occur throughout life

Parathyroid Hormone

• Higher levels stimulate osteoclasts to break down more bone matrix, lower levels slow activity

Bone Resorption

Removal of minerals and collagen by osteoclasts

Bone Deposition

Addition of minerals & collagen by osteoblasts

Bone Remodeling

• Continuous process, old tissue replaced

  • Absorption & deposition

• Blood Ca levels determine when matrix is broken or formed

• Gravity & muscles determine where matrix is broken or formed

Bone Remodeling & Ca2+ Homeostasis

Bone Spurs

• Growth of bone from excessive rubbing on the bone

• Can cause problems if they end up rubbing on other structures

4 Types of Bone Fractures

1. Partial: incomplete break

2. Complete: bone is broken in 2 or more pieces

3. Closed (simple): bone does not break through skin

4. Open (compound): bone protrudes through skin

4 Steps of Bone Formation After Fracture

Exercise & Bone Tissue

• Bone becomes stronger when placed under stress

Osteoporosis

• Long term lack of calcium

• Age-related osteoporosis linked to reduced estrogen during menopause

• Related to overactive thyroid, parathyroid, or adrenal glands