Bones and Skeletal System

Skeletal Cartilage

The skeletal cartilage forms the initial framework of the skeleton in embryonic development and is gradually replaced by bone. Cartilage remains in adults in limited locations, serving important functions.

• Composition: Made of cartilage tissue, primarily water, with no nerves or blood vessels.

• Perichondrium: Dense irregular connective tissue surrounding cartilage, resists expansion, and contains blood vessels for nourishment.

• Basic Components:

  • Cells called chondrocytes

  • Cavities in ECM called lacunae that enclose cells

• Types of Cartilage:

  • Hyaline cartilage: Provides support with flexibility and resilience; contains fine collagen fibers.

  • Elastic cartilage: Contains more elastic fibers, allowing it to "bounce back" after bending; found in ear and epiglottis.

  • Fibrocartilage: Highly compressible, great tensile strength; found in menisci of knee and intervertebral discs.

Cartilage Growth

Cartilage grows by two mechanisms, accommodating mitosis and expansion.

• Appositional growth: Cells in perichondrium secrete matrix, expanding cartilage externally.

• Interstitial growth: Chondrocytes divide within lacunae, secreting new matrix and expanding cartilage internally.

• Growth stops during adolescence as skeleton matures.

• Cartilage may calcify but calcified cartilage is NOT bone.

Functions of Bones

Bones serve multiple vital functions in the body.

• Support

• Protection

• Anchorage

• Mineral storage (especially calcium and phosphate)

• Blood cell formation (hematopoiesis)

• Triglyceride storage

• Hormone production

Bone Classification

Bones are classified by location and shape.

• Axial skeleton: Long axis of body (skull, vertebral column, rib cage); supports and protects body parts.

• Appendicular skeleton: Limbs and girdles; enables movement and manipulation of environment.

Bone Shapes

• Long bones: Longer than wide, with shaft and two ends (e.g., femur, phalanges).

• Short bones: Cube-shaped (e.g., carpals, tarsals); includes sesamoid bones (e.g., patella).

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

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

Bone Structure Overview

Bones are organs containing several tissue types, with osseous tissue dominating.

• Osseous tissue: Main bone tissue

• Cartilage: Found in articular surfaces

• Dense connective tissue: External surface

• Muscle and epithelial tissue: Found in blood vessels

Bone structure can be studied at three levels:

• Gross

• Microscopic

• Chemical

Compact and Spongy Bone

• Compact bone: Dense, solid, smooth outer layer

• Spongy bone: Internal layer with small needle-like trabeculae; air spaces filled with bone marrow

Short, Irregular, and Flat Bones

• Thin plates of spongy bone covered by compact bone

• No shaft or expanded ends

• Compact bone covered by periosteum and endosteum

Long Bones

• Diaphysis: Tubular shaft, thick collar of compact bone, central medullary cavity (contains yellow marrow)

• Epiphyses: Bone ends, outer shell of compact bone, inner spongy bone, joint surfaces with hyaline cartilage

• Epiphyseal line: Remnant of growth plate (disc of hyaline cartilage)

Membranes

• Periosteum: Covers bone except joint surfaces; outer fibrous layer (dense irregular connective tissue), inner osteogenic layer (contains osteoprogenitor cells)

• Endosteum: Covers internal bone surfaces, trabeculae of spongy bone, and canals in compact bone; contains similar cells as periosteum

Blood Vessels and Nerves

• Bones are highly vascularized and heal quickly

• Nerves accompany blood vessels

Hematopoietic Tissue

• Blood-forming tissue, also called red bone marrow

Bone Markings

Bone markings explain how bones and ligaments work together.

• Sites of muscle and ligament attachment

• Surfaces: Form joints

• Depressions/openings: For blood vessels and nerves

Bone Cells

Five major types of bone cells are involved in bone formation, maintenance, and remodeling.

• Osteoprogenitor cells: Mitotically active stem cells; found in periosteum and endosteum; differentiate into osteoblasts

• Osteoblasts: Bone-forming cells; secrete unmineralized bone matrix (osteoid); actively mitotic

• Osteocytes: Mature bone cells; occupy lacunae; monitor and maintain bone matrix

• Bone lining cells: Flat cells where bone remodeling is not occurring; help maintain matrix

• Osteoclasts: Multinucleate, large cells; bone resorption; derived from macrophage lineage

Compact Bone Microscopic Anatomy

• Osteon: Structural unit; weight-bearing pillars

• Lamellae: Matrix tubes; collagen fibers run in opposite directions in adjacent lamellae

• Canals and Canaliculi:

  • Central canal: Vascularized and innervated

  • Perforating canals: Connect blood and nerve supply of medullary cavity to central canals

  • Lacunae: Located at lamella junctions; contain osteocytes

  • Canaliculi: Connect lacunae to each other and to central canal

• Interstitial and Circumferential Lamellae: Not part of complete osteons; fill gaps or surround osteons

Spongy Bone Microscopic Anatomy

• Contains trabeculae; resists stress

• No osteons present

Chemical Bone Anatomy

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

• Inorganic components: Mineral salts (mostly calcium phosphates)

Formation of the Bony Skeleton

Bone tissue formation (ossification or osteogenesis) begins in embryos and continues through adulthood.

• Embryonic skeleton: hyaline cartilage and fibrous membranes

• Two main processes:

  • Endochondral ossification: Bone develops by replacing hyaline cartilage

  • Intramembranous ossification: Bone develops from fibrous membrane

Endochondral Ossification

• Forms most bones below the skull (except clavicles)

• Uses hyaline cartilage models as patterns

Step 1: Formation of Bone Collar

• Perichondrium converts to periosteum

• Osteoid secretion forms bone collar around diaphysis

Step 2: Cavitation

• Cartilage calcifies and develops cavities

Step 3: Formation of Spongy Bone

• Periosteal bud invades cavities, bringing artery, vein, nerve fibers, red marrow, osteoprogenitor cells, and osteoclasts

• Osteoclasts erode calcified cartilage; osteoblasts form spongy bone

Step 4: Elongation and Medullary Cavity Formation

• Diaphysis elongates; spongy bone broken down by osteoclasts to form medullary cavity

• Secondary ossification centers form in epiphyses

Step 5: Ossification of Epiphyses

• Spongy bone retained; no medullary cavity formed

• Epiphyseal plate remains between diaphysis and epiphysis

Intramembranous Ossification

• Forms cranial bones and clavicle (mostly flat bones)

• Begins around week 8-9 of embryonic development

• Four main steps:

  1. Development of ossification center in fibrous connective tissue

  2. Secretion and calcification of osteoid

  3. Formation of periosteum and immature spongy bone

  4. Replacement of immature spongy bone with compact bone and development of red bone marrow

Bone Growth

• Long bones grow by interstitial growth of epiphyseal plate cartilage

• Zones:

  • Proliferation zone: Chondrocytes divide, lengthening bone

  • Hypertrophic zone: Chondrocytes enlarge, lacunae erode

  • Calcification zone: Cartilage matrix calcifies, chondrocytes die, blood vessels invade

  • Ossification zone: Calcified spicules invaded by marrow elements, replaced by bone

• Bone shape maintained by both resorption and deposition

• Bone thickness grows by appositional growth (osteoblasts in periosteum, osteoclasts in endosteum)

Bone Remodeling

• Bone cells continuously break down (resorb) and deposit bone

• Occurs every year, replacing about 5-10% of skeleton

• Occurs primarily at endosteal surface; consists of both resorption and deposition

• Key players: osteoblasts and osteoclasts

Bone Resorption

• Osteoclasts attach to bone, secrete acids and enzymes to dissolve bone matrix

• Phagocytose dead osteocytes and undigested matrix

• Osteoclasts undergo apoptosis after resorption

Bone Deposition

• Osteoblasts deposit new bone matrix (osteoid)

• Osteoid calcification: proteins bind calcium ions, raising local concentration

Key Equations and Terms

• Ossification: Process of bone tissue formation

• Osteogenesis: Synonym for ossification

• Osteoid: Unmineralized bone matrix

• Bone resorption: Breakdown of bone tissue by osteoclasts

• Bone deposition: Formation of new bone tissue by osteoblasts

Table: Types of Cartilage

Table: Bone Cell Types

Example: Endochondral Ossification in the Femur

The femur develops from a hyaline cartilage model. The process begins with the formation of a bone collar, followed by cavitation, invasion of the periosteal bud, elongation, and finally ossification of the epiphyses, resulting in a mature long bone.

Key Formula: Bone Matrix Mineralization

The mineralization of bone matrix involves the deposition of calcium phosphate:

This reaction forms hydroxyapatite crystals, which give bone its hardness.