Bones and the Skeletal System: Structure, Function, and Development

Chapter 6: Bones and the Skeletal System

Skeletal Cartilage

The skeleton begins as cartilage in the embryo, which is gradually replaced by bone. Some cartilage remains in adults, serving important structural and functional roles.

• Composition: Skeletal cartilage is made of cartilage connective tissue, consisting primarily of water for resilience.

• Avascular: Contains no nerves or blood vessels.

• Perichondrium: Dense irregular connective tissue surrounding cartilage, resists outward expansion, and contains blood vessels to nourish cartilage cells.

• Cells: Chondrocytes (cartilage cells) reside in lacunae (small cavities within the extracellular matrix).

Types of Cartilage

• Hyaline Cartilage: Provides support, flexibility, and resilience. Contains fine collagen fibers (not visible under light microscope). Found in articular (joints), costal (ribs), respiratory, and nasal cartilages.

• Elastic Cartilage: Similar to hyaline but with more elastic fibers. Found in the external ear and epiglottis.

• Fibrocartilage: Highly compressible with great tensile strength. Contains parallel rows of chondrocytes alternating with thick collagen fibers. Found in areas subject to pressure and stretch (e.g., menisci of the knee, intervertebral discs).

Cartilage Growth

• Appositional Growth: Cells in the perichondrium secrete new matrix on the external surface, expanding cartilage from the outside.

• Interstitial Growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

• Growth typically stops during adolescence. Cartilage can calcify (harden) but calcified cartilage is not bone.

Bone: Functions and Classification

Bones are dynamic organs with multiple functions and diverse shapes.

• Functions of Bone:

  1. Support

  2. Protection

  3. Movement

  4. Mineral and growth factor storage

  5. Blood cell formation (hematopoiesis)

  6. Triglyceride (fat) storage

  7. Hormone production

• Skeleton Divisions:

  • Axial Skeleton: Skull, vertebral column, rib cage. Functions: protection, support, carrying body parts.

  • Appendicular Skeleton: Limbs and girdles. Functions: locomotion, manipulation of environment.

• Bone Shapes:

  1. Long Bones: Longer than wide, shaft with two ends (e.g., femur, phalanges).

  2. Short Bones: Cube-shaped (e.g., carpals, tarsals). Includes sesamoid bones (form in tendons, e.g., patella).

  3. Flat Bones: Thin, flattened, slightly curved (e.g., sternum, scapulae, ribs, cranial bones).

  4. Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Bone Structure Overview

Bones are organs containing several tissue types. The main tissue is osseous (bone) tissue, but bones also contain:

• Nervous tissue

• Cartilage (in articular surfaces)

• Fibrous connective tissue (lining surfaces)

• Muscle and epithelial tissue (in blood vessels)

Compact and Spongy Bone

• Compact Bone: Dense, smooth outer layer.

• Spongy (Cancellous) Bone: Internal layer with trabeculae (needle-like structures) and marrow-filled spaces.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone (diploë) covered by compact bone.

• Covered by periosteum (outer) and endosteum (inner) membranes.

• No shaft or expanded ends.

Structure of Long Bones

• Diaphysis: Tubular shaft forming the long axis. Thick collar of compact bone surrounds the medullary (marrow) cavity, which contains yellow marrow in adults.

• Epiphyses: Bone ends. Outer shell of compact bone, inner spongy bone. Joint surfaces covered with articular cartilage. Epiphyseal line (remnant of growth plate) separates diaphysis and epiphysis.

Bone Membranes

• Periosteum: Covers external bone surface (except joint surfaces). Has a fibrous outer layer (dense irregular connective tissue) and an osteogenic inner layer (contains osteoprogenitor cells, osteoclasts, osteoblasts). Rich in nerves and blood vessels. Sharpey's fibers anchor periosteum to bone. Provides attachment for tendons and ligaments.

• Endosteum: Covers internal bone surfaces, trabeculae of spongy bone, and canals. Contains same cell types as periosteum's osteogenic layer.

Blood Vessels and Nerves

• Bones are highly vascularized, unlike cartilage. Nerves accompany blood vessels.

Hematopoietic Tissue

• Red marrow: Blood-forming tissue found in trabecular cavities of spongy bone and diploë of flat bones.

Bone Markings

• Projections: Sites of muscle and ligament attachment.

• Surfaces: Form joints.

• Depressions/Openings: Allow passage of blood vessels and nerves.

Bone Cells

Five major types of bone cells maintain bone structure and function:

• Osteoprogenitor Cells: Mitotically active stem cells in periosteum and endosteum. Differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells. Secrete unmineralized bone matrix (osteoid), including collagen and calcium-binding proteins. Become osteocytes when surrounded by matrix.

• Osteocytes: Mature bone cells in lacunae. Maintain and monitor bone matrix. Communicate with osteoblasts and osteoclasts for bone remodeling.

• Bone Lining Cells: Flat cells on bone surfaces where remodeling is not occurring. Help maintain matrix.

• Osteoclasts: Large, multinucleate cells that resorb (break down) bone. Derived from hematopoietic stem cells (like macrophages).

Microscopic Anatomy of Bone

Compact Bone

• Osteon (Haversian System): Structural unit of compact bone. Consists of concentric lamellae (layers) of bone matrix around a central canal.

• Lamellae: Collagen fibers run in alternating directions in adjacent lamellae, resisting torsion.

• Canals:

  • Central (Haversian) Canal: Contains blood vessels and nerves.

  • Perforating (Volkmann's) Canals: Connect blood and nerve supply of periosteum to central canals.

  • Lacunae: Small cavities at lamellae junctions containing osteocytes.

  • Canaliculi: Tiny canals connecting lacunae to each other and to the central canal.

• Interstitial and Circumferential Lamellae: Fill gaps between osteons or encircle the bone.

Spongy Bone

• Composed of trabeculae aligned along lines of stress. No osteons present. Trabeculae contain irregular lamellae and osteocytes connected by canaliculi.

Chemical Composition of Bone

• Organic Components: Cells and osteoid (ground substance and collagen fibers).

• Inorganic Components: Mineral salts, mainly hydroxyapatites (calcium phosphates).

Bone Development (Ossification)

Bone tissue formation (ossification or osteogenesis) occurs in the embryo, continues through growth, and is ongoing in remodeling and repair.

• Begins just before week 8 of embryonic development. Prior to this, the skeleton is made of hyaline cartilage and fibrous membranes.

• Flexible embryonic skeleton allows for growth and birth.

Types of Ossification

• Endochondral Ossification: Bone forms by replacing hyaline cartilage. Forms most bones below the skull (except clavicles). Steps:

  1. Bone collar forms around diaphysis of cartilage model. Perichondrium becomes periosteum. Osteoid is secreted against cartilage, forming collar. Chondrocytes in shaft hypertrophy, creating primary ossification center.

  2. Cartilage in center calcifies and develops cavities.

  3. Periosteal bud invades cavities, bringing blood vessels, nerves, red marrow, osteogenic cells, and osteoclasts. Osteoclasts erode calcified cartilage; osteoblasts form spongy bone.

  4. Diaphysis elongates; medullary cavity forms. Secondary ossification centers appear in epiphyses near birth.

  5. Epiphyses ossify. Hyaline cartilage remains at articular surfaces and epiphyseal plates.

• Intramembranous Ossification: Bone develops from fibrous connective tissue membrane. Forms flat bones (cranial bones, clavicles). Steps:

  1. Ossification centers develop in fibrous membrane.

  2. Osteoid is secreted and calcifies.

  3. Trabeculae and immature spongy bone form.

  4. Compact bone replaces immature spongy bone; red marrow develops.

Bone Growth After Birth

• Longitudinal Growth: Occurs at epiphyseal plates via four zones:

  1. Proliferation Zone: Rapid cell division lengthens bone.

  2. Hypertrophic Zone: Chondrocytes enlarge, lacunae erode and enlarge.

  3. Calcification Zone: Matrix calcifies, chondrocytes die, matrix deteriorates, blood vessels invade.

  4. Ossification Zone: Calcified spicules invaded by marrow elements; osteoblasts cover spicules with new bone, forming spongy bone.

• Appositional Growth: Bone thickens by addition of new bone at the periosteum (osteoblasts) and resorption at the endosteum (osteoclasts).

Bone Remodeling

• Bone is continuously resorbed and deposited. About 5-10% of the skeleton is replaced annually; the entire skeleton is replaced every 10 years.

• Prevents bones from becoming brittle or too heavy. Occurs mainly at the endosteal surface.

• Involves osteoclasts (resorption) and osteoblasts (deposition).

Bone Resorption

• Osteoclasts attach to bone via a ruffled border, secrete H+ (acid) and lysosomal enzymes to dissolve minerals and digest matrix. Waste is transcytosed and released into blood. Osteoclasts undergo apoptosis after resorption.

Bone Deposition

• Osteoblasts deposit new osteoid, which then calcifies. Osteoid proteins bind calcium, raising local concentration. Osteoblasts release matrix vesicles with alkaline phosphatase, which increases phosphate concentration. When calcium and phosphate are high enough, hydroxyapatite crystals form, mineralizing the bone.

Control of Bone Remodeling

• Hormonal Control: Maintains calcium homeostasis. Parathyroid hormone (PTH) is released when blood calcium is low, stimulating osteoclasts to resorb bone and release calcium. Negative feedback turns off PTH when calcium is restored.

• Mechanical Stress: Bone remodels in response to mechanical demands (Wolff's law). Bones are thickest where stress is greatest. Spongy bone trabeculae align along lines of stress. Large projections form where muscles attach. Bones atrophy if not used.

Bone Fractures and Repair

• Classification:

  1. Position of bone ends: Nondisplaced (normal position) vs. displaced (misaligned).

  2. Completeness: Complete (all the way through) vs. incomplete.

  3. Skin penetration: Open (compound, bone pierces skin) vs. closed (simple, no skin break).

• Other classifications: location, appearance, nature of break.

Fracture Treatment and Repair

• Reduction: Realignment of bone ends. Closed (manual) or open (surgical with hardware).

• Immobilization: Bone is immobilized to allow healing.

• Repair Stages:

  1. Hematoma Formation: Blood vessels break, forming a clot. Bone cells die, tissue swells.

  2. Fibrocartilaginous Callus Formation: New blood vessels grow. Fibroblasts produce collagen fibers; chondroblasts secrete cartilage matrix, forming a soft callus (splint).

  3. Bony Callus Formation: Osteoblasts form spongy bone, replacing soft callus with hard (bony) callus.

  4. Bone Remodeling: Excess material is removed, compact bone reconstructs shaft walls.

Bone Diseases

• Osteomalacia: Poorly mineralized, soft bones. In children, called rickets (bowed legs, deformities). Caused by insufficient calcium or vitamin D.

• Osteoporosis: Bone resorption exceeds deposition; bone mass declines, bones become fragile. Most common in postmenopausal women due to decreased estrogen. Other risk factors: low calcium/vitamin D/protein, smoking, genetics, alcohol, certain medications.

• Paget's Disease: Excessive, disorganized bone remodeling. High ratio of spongy to compact bone, reduced mineralization, localized weakening (often spine, pelvis, femur, skull).

Summary Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Support, flexibility, resilience; fine collagen fibers	Articular, costal, respiratory, nasal cartilages
Elastic	More elastic fibers; flexible, returns to shape	External ear, epiglottis
Fibrocartilage	Thick collagen fibers; compressible, strong	Menisci, intervertebral discs

Summary Table: Major Bone Cells

Cell Type	Function	Location
Osteoprogenitor	Stem cells; differentiate into osteoblasts	Periosteum, endosteum
Osteoblast	Bone formation; secrete osteoid	Bone surfaces
Osteocyte	Maintain bone matrix	Lacunae in bone
Bone lining cell	Maintain matrix (inactive areas)	Bone surfaces
Osteoclast	Bone resorption	Bone surfaces (resorption sites)

Key Equations

• Calcium Homeostasis (Negative Feedback): $\text{Low blood Ca}^{2+} \rightarrow \text{PTH release} \rightarrow \text{Osteoclast activation} \rightarrow \text{Bone resorption} \rightarrow \text{Increased blood Ca}^{2+}$

• Hydroxyapatite Formation: $\text{Ca}^{2+} + \text{PO}_4^{3-} \rightarrow \text{Ca}_5(\text{PO}_4)_3\text{OH}$

Additional info: Some details (e.g., specific cell names, steps of ossification, and disease risk factors) were inferred from standard anatomy and physiology knowledge to ensure completeness and clarity.