Skeletal System: Cartilage, Bone Structure, and Ossification

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Skeletal Cartilages

Types of Cartilage

Cartilage is a resilient and flexible connective tissue found in various parts of the skeletal system. It provides support, flexibility, and cushioning in joints and other structures.

Hyaline cartilage: Most abundant type; provides support, flexibility, and resilience. Found in articular surfaces, costal cartilages, respiratory tract, and nasal cartilages.
Elastic cartilage: Contains more elastic fibers; maintains shape and flexibility. Found in the external ear and epiglottis.
Fibrocartilage: Contains thick collagen fibers; provides great tensile strength. Found in intervertebral discs, pubic symphysis, and menisci of knee.

Growth of Cartilage

Mechanisms of Cartilage Growth

Cartilage grows by two main processes, allowing for development and repair.

Appositional growth: Cells secrete matrix against the external face of existing cartilage, increasing width.
Interstitial growth: Chondrocytes divide and secrete new matrix from within, expanding cartilage from the inside.
Calcification: Occurs during normal bone growth and old age, where cartilage is replaced by bone tissue.

Bones of the Skeleton

Main Groups of Bones

The human skeleton is divided into two main groups, each with specific functions and locations.

Axial skeleton: Includes the skull, vertebral column, and rib cage.
Appendicular skeleton: Includes the limbs and girdles (shoulder and pelvic).

Classification of Bones by Shape

Bones are classified based on their shapes, which relate to their functions.

Long bones: Longer than they are wide (e.g., femur, humerus).
Short bones: Cube-shaped (e.g., wrist and ankle bones).
Flat bones: Thin, flat, and usually curved (e.g., sternum, scapula).
Irregular bones: Complicated shapes (e.g., vertebrae, hip bones).
Sesamoid bones: Form within tendons (e.g., patella).

Functions of Bones

Main Functions

Bones serve several vital functions in the body:

Support: Framework for the body and soft organs.
Protection: Protects vital organs (e.g., skull protects brain).
Movement: Levers for muscles to act upon.
Mineral storage: Reservoir for minerals, especially calcium and phosphate.
Blood cell formation: Occurs in red marrow cavities (hematopoiesis).
Triglyceride storage: Energy storage in bone cavities.

Bone Markings

Projections

Projections are sites of muscle and ligament attachment or help form joints.

Projection	Description
Process	Any bony prominence
Crest	Narrow ridge of bone
Trochanter	Large, blunt, irregular surface
Line	Narrow ridge, less prominent than crest
Tubercle	Small rounded projection
Epicondyle	Raised area above a condyle

Depressions and Openings

These allow passage of nerves and blood vessels.

Depression/Opening	Description
Foramen	Round or oval opening through a bone
Fossa	Shallow, basin-like depression
Meatus	Canal-like passageway
Sinus	Cavity within a bone, filled with air and lined with mucous membrane

Bone Textures

Types of Bone Tissue

Bones consist of two main types of tissue:

Compact bone: Dense outer layer; appears smooth and solid.
Spongy bone: Internal layer; honeycomb of small needle-like or flat pieces called trabeculae.

Bone Tissue Structure

Structure of a Long Bone

Long bones have a characteristic structure:

Diaphysis: Shaft; contains medullary cavity filled with yellow marrow.
Epiphyses: Ends of the bone; contain spongy bone and red marrow.
Membranes: Periosteum (outer) and endosteum (inner) cover and line bone surfaces.

Membranes of Bone

Periosteum: Double-layered membrane covering external bone surface; contains osteoblasts and osteoclasts.
Endosteum: Delicate membrane lining internal bone surfaces; also contains bone cells.

Location of Hematopoietic Tissue (Red Marrow) & Yellow Marrow

Distribution in Adults and Infants

Red marrow: Found in trabecular cavities of spongy bone in long bones and flat bones (e.g., sternum, pelvis).
Yellow marrow: Fat storage; found in medullary cavities of long bones.

Structure of Short, Flat, Irregular, and Sesamoid Bones

General Structure

Thin plates of spongy bone covered by compact bone.
No shaft or epiphyses.
Bone marrow between trabeculae, but no marrow cavity.

Bone Cells

Types and Functions

Cell Type	Function
Osteogenic cells	Stem cells that differentiate into osteoblasts
Osteoblasts	Bone-forming cells
Osteocytes	Mature bone cells; maintain bone matrix
Osteoclasts	Bone-resorbing cells

Anatomy of Bone: Compact Bone

Haversian System (Osteon)

The structural unit of compact bone is the osteon.

Lamellae: Weight-bearing, column-like matrix tubes.
Central (Haversian) canal: Contains blood vessels and nerves.
Perforating (Volkmann's) canals: Right angles to the central canal; connect blood vessels and nerves of periosteum and central canal.
Lacunae: Small cavities that contain osteocytes.
Canaliculi: Hairlike canals that connect lacunae to each other and the central canal.

Anatomy of Bone: Spongy Bone

Structure and Function

Trabeculae align along lines of stress to help resist stress.
No osteons present; nutrients reach osteocytes by diffusing through canaliculi from capillaries in the endosteum.

Chemical Composition of Bone

Organic and Inorganic Components

Organic: Osteogenic cells, osteoblasts, osteocytes, osteoclasts, and osteoid (matrix produced by osteoblasts).
Inorganic: Hydroxyapatites (mineral salts), mainly calcium phosphate crystals.

Organic components provide flexibility and tensile strength; inorganic components provide hardness and resistance to compression.

Bone Formation (Ossification)

Types of Ossification

Ossification is the process of bone tissue formation, occurring in two main ways:

Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones (e.g., clavicles, cranial bones).
Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most bones of the skeleton.

Intramembranous Ossification

Larger bones have more than one primary ossification center.
Begins within fibrous connective tissue membranes formed by mesenchymal cells.
Forms flat bones of the skull and clavicles.

Endochondral Ossification

Bone development of bones below head (except clavicles).
Uses hyaline cartilage models for most bones.
Begins in the second month of development.
Many bones complete ossification by age 7.

Key Equations

Hydroxyapatite formation:

Example

During endochondral ossification, the femur develops from a cartilage model, gradually being replaced by bone tissue as the child grows.

Additional info: Academic context and definitions have been expanded for clarity and completeness.