Bones and Skeletal Tissue: Structure, Function, and Composition

Bones and Skeletal Tissue

I. Skeletal Cartilages

Skeletal cartilages are specialized connective tissues that provide support, flexibility, and resilience in various parts of the body. They are classified based on their structure and function.

• Basic Structure, Types, and Location:

  • Hyaline cartilage: Most abundant type; provides support with flexibility. Found in articular surfaces of joints, costal cartilages, respiratory tract, and nasal cartilages.

  • Elastic cartilage: Contains more elastic fibers; maintains shape and flexibility. Located in the external ear and epiglottis.

  • Fibrocartilage: Highly compressible and strong; found in intervertebral discs, pubic symphysis, and menisci of knee.

• Growth of Cartilage:

  • Appositional growth: Cartilage grows by addition of new layers on the surface.

  • Interstitial growth: Cartilage expands from within by division of chondrocytes and secretion of new matrix.

II. Classification of Bones

Bones are classified by their shape and structure, which relate to their function in the body.

• Shape:

  • Long bones: Longer than they are wide (e.g., humerus, femur).

  • Short bones: Cube-shaped (e.g., talus, carpals).

  • Flat bones: Thin, flattened, and usually curved (e.g., sternum, skull bones).

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

III. Functions of Bones

Bones perform several vital functions necessary for movement, protection, and homeostasis.

• Support: Framework for the body and attachment for soft tissues.

• Protection: Shields vital organs (e.g., skull protects brain, rib cage protects heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Storage: Reservoir for minerals, especially calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow.

IV. Bone Structure

A. Gross Anatomy

• Types of Tissue:

  • Compact bone: Dense, smooth, and solid; forms the outer layer of bones.

  • Spongy bone (cancellous): Honeycomb-like structure; contains trabeculae and spaces filled with bone marrow.

B. Typical Long Bone Structure

• Diaphysis: Shaft of the bone; composed of compact bone surrounding the medullary cavity.

• Epiphysis: Ends of the bone; mostly spongy bone covered by compact bone.

• Epiphyseal Line: Remnant of the growth plate; separates diaphysis from epiphysis in adults.

• Periosteum: Fibrous outer layer covering the bone; contains nerves and blood vessels.

• Endosteum: Delicate inner layer lining the medullary cavity and trabeculae of spongy bone.

• Articular Cartilage: Hyaline cartilage covering joint surfaces; reduces friction and absorbs shock.

C. Short, Irregular, and Flat Bones

• These bones have thin layers of compact bone surrounding spongy bone and lack a shaft or epiphyses.

D. Compact and Spongy Bone

• Compact bone: Organized into osteons (Haversian systems) with concentric lamellae around a central canal.

• Spongy bone: Consists of trabeculae and lamellae; spaces are filled with bone marrow.

V. Location of Hematopoietic Tissue in Bones

• Red Marrow: Site of blood cell production; found in trabecular cavities of spongy bone in long bones and flat bones.

• Yellow Marrow: Stores fat; found in medullary cavities of long bones in adults.

VI. Microscopic Structure of Bone

A. Compact Bone

• Haversian System (Osteon): Structural unit of compact bone; consists of concentric lamellae around a central canal.

• Osteocytes: Mature bone cells located in lacunae.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Tiny canals connecting lacunae; allow for nutrient and waste exchange.

• Interstitial Lamellae: Fill spaces between osteons.

• Circumferential Lamellae: Extend around the entire circumference of the bone.

B. Spongy Bone

• Trabeculae: Irregularly arranged lamellae; form the framework of spongy bone.

• Lamellae: Layers of bone matrix within trabeculae.

VII. Chemical Composition of Bone

• Organic Components:

  • Cells: Osteogenic cells (stem cells), osteoblasts (bone-forming), osteocytes (mature bone cells), and osteoclasts (bone-resorbing).

  • Osteoid: Unmineralized bone matrix composed of collagen and ground substance; provides tensile strength and flexibility.

• Inorganic Components:

  • Hydroxyapatite: Mineral salts (mainly calcium phosphate) that give bone its hardness and strength. $Ca_{10}(PO_4)_6(OH)_2$

VIII. Bone Cell Types

Cell Type	Function
Osteogenic cell	Stem cell that differentiates into osteoblasts
Osteoblast	Matrix-synthesizing cell responsible for bone growth
Osteocyte	Mature bone cell that maintains bone matrix
Osteoclast	Bone-resorbing cell; breaks down bone matrix

IX. Summary Table: Types of Cartilage

Type	Main Locations	Key Properties
Hyaline	Articular surfaces, costal cartilages, nose, trachea, larynx	Support, flexibility, resilience
Elastic	External ear, epiglottis	Maintains shape, flexibility
Fibrocartilage	Intervertebral discs, pubic symphysis, menisci	Strength, compressibility

X. Summary Table: Bone Types

Bone Type	Example	Main Function
Long	Femur, humerus	Movement, support
Short	Carpals, tarsals	Stability, support
Flat	Sternum, skull	Protection, muscle attachment
Irregular	Vertebrae, hip bones	Protection, support

XI. Example: Bone Growth

• During childhood and adolescence, bones grow in length by interstitial growth at the epiphyseal plate and in thickness by appositional growth at the periosteum.

Additional info: The notes have been expanded to include definitions, examples, and tables for clarity and completeness.