Cartilage and Bone: Structure, Function, and Classification

Cartilage: Structure, Types, and Growth

Basic Structure and Characteristics

Cartilage is a specialized connective tissue that provides support and flexibility to various parts of the body. It is avascular (lacks blood vessels) and receives nutrients via diffusion.

• Perichondrium: Dense irregular connective tissue surrounding cartilage, providing nutrients and resisting outward expansion.

• Chondrocytes: Mature cartilage cells located in spaces called lacunae within the extracellular matrix.

• Matrix: Composed of ground substance and fibers, giving cartilage its strength and resilience.

Types of Cartilage

• Hyaline Cartilage: Most abundant; provides support with flexibility and resilience. Found in articular surfaces, costal cartilages, respiratory structures, and embryonic skeleton.

• Elastic Cartilage: Contains more elastic fibers, making it more flexible. Located in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers, providing great tensile strength. Found in intervertebral discs, pubic symphysis, and menisci of the knee.

Growth of Cartilage

• Appositional Growth: Cartilage-forming cells in the perichondrium secrete new matrix on the external surface of existing cartilage.

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

Additional info: Cartilage growth is most active during childhood and adolescence.

Comparison of Cartilage and Bone

The following table summarizes the main differences between cartilage and bone:

	Cartilage	Bone
Covering	Perichondrium	Periosteum
Blood Vessels	No blood vessels or nerves (except in perichondrium)	Richly vascularized and innervated
Matrix	Flexible extracellular matrix	Hard extracellular matrix (mineralized)
Growth	Appositional and interstitial	Appositional only

Bone: Structure, Function, and Classification

Functions of Bone

Bones serve multiple essential functions in the body:

• Support: Framework for the body and cradles soft organs.

• Protection: Protects vital organs (e.g., skull protects brain, rib cage protects thoracic organs).

• Movement: Muscles attach to bones via tendons, using bones as levers.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphate.

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

• Triglyceride Storage: Fat storage in yellow marrow.

• Hormone Production: Osteocalcin production helps regulate bone formation and metabolism.

Classification of Bones by Shape

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

• Short Bones: Cube-shaped (e.g., carpals, tarsals); sesamoid bones (e.g., patella) are a special type.

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

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

Gross Anatomy of Bones

Compact and Spongy Bone

• Compact Bone: Dense outer layer; provides strength and protection.

• Spongy Bone (Cancellous Bone): Internal network of trabeculae; spaces filled with red or yellow marrow.

Structure of a Typical Long Bone

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

• Epiphyses: Ends of the bone; spongy bone interior, covered by articular cartilage.

• Metaphysis: Region between diaphysis and epiphysis; includes the epiphyseal plate (growth plate) in children.

• Periosteum: Double-layered membrane covering external bone surface (except joints); contains osteogenic cells.

• Endosteum: Delicate membrane lining internal bone surfaces.

Bone Marrow

• Red Marrow: Site of hematopoiesis; found in trabecular cavities of spongy bone and diploë of flat bones.

• Yellow Marrow: Fat storage; can convert to red marrow if necessary.

Bone Markings

Projections, Surfaces, and Depressions/Openings

Bones have various markings that serve as sites for muscle, ligament, and tendon attachment, as well as passages for blood vessels and nerves.

Name of Bone Marking	Description	Example
Tuberosity	Large rounded projection; may be roughened	Tibia
Crest	Narrow ridge of bone; usually prominent	Ilium
Trochanter	Very large, blunt, irregularly shaped process	Femur
Line	Narrow ridge of bone; less prominent than a crest	Femur
Tubercle	Small rounded projection or process	Humerus
Epicondyle	Raised area on or above a condyle	Femur
Spine	Sharp, slender, often pointed projection	Scapula
Process	Any bony prominence	Mastoid process

Additional info: Other markings include facets, condyles, foramina, and fossae, which serve as articular surfaces or passageways.

Bone Cells and Microscopic Anatomy

Types of Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Stem cells found in periosteum and endosteum; differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete bone matrix (osteoid), including collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells that maintain bone matrix and act as stress sensors.

• Osteoclasts: Multinucleated cells that resorb bone, important for bone remodeling and calcium homeostasis.

Microscopic Anatomy of Compact Bone

• Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae (layers) of bone matrix surrounding a central canal containing blood vessels and nerves.

• Lamellae: Collagen fibers in adjacent lamellae run in different directions, providing strength and resistance to torsion.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae, allowing for nutrient and waste exchange.

Key Equations and Concepts

• Bone Growth: Occurs via appositional growth (increasing thickness) and endochondral ossification (lengthening).

• Mineralization: Deposition of hydroxyapatite crystals ($Ca_{10}(PO_4)_6(OH)_2$) in the bone matrix.