Bones and Skeletal Tissues

Introduction to the Human Skeleton

The human skeleton is a dynamic framework that provides structure, protection, and support for the body. Initially, the skeleton is primarily composed of cartilage and fibrous membranes, which are gradually replaced by bone tissue during development, except in regions where flexibility is essential.

• Cartilage and Fibrous Membranes: Early embryonic skeleton is made of these tissues.

• Ossification: The process by which cartilage is replaced by bone.

• Flexibility: Some areas, such as joints, retain cartilage for movement and shock absorption.

Types of Cartilage

Hyaline Cartilage

Hyaline cartilage is the most prevalent type in the body, providing support, flexibility, and resilience. It contains only collagen fibers and forms the template for most bones during development.

• Key Features: Smooth, glassy appearance; resists compression.

• Locations:

  • Articular cartilage: Covers ends of bones at joints.

  • Costal cartilage: Connects ribs to sternum.

  • Respiratory cartilage: Forms larynx and reinforces air passages.

  • Nasal cartilage: Supports external nose.

Elastic Cartilage

Elastic cartilage is similar to hyaline cartilage but contains more elastic fibers, allowing it to withstand repeated bending.

• Key Features: Flexible and resilient.

• Locations:

  • External ear (auricle)

  • Epiglottis (flap covering the larynx during swallowing)

Fibrocartilage

Fibrocartilage contains thick collagen fibers, providing great tensile strength and the ability to absorb compressive shock.

• Key Features: Highly durable, resists compression and tension.

• Locations:

  • Menisci of the knee

  • Intervertebral discs

  • Pubic symphysis

Growth of Cartilage

Mechanisms of Cartilage Growth

• Appositional Growth: Chondroblasts in the perichondrium secrete new matrix on the external surface of cartilage, increasing its size from the outside.

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

Functions of Bones

Bones serve multiple essential functions in the human body:

• Support: Framework for the body and soft organs.

• Protection: Shields vital organs (e.g., skull protects the brain).

• Movement: Attachment sites for muscles; act as levers.

• Mineral Storage: Reservoir for calcium and phosphate.

• Blood Cell Formation: Hematopoiesis occurs in red marrow.

• Triglyceride Storage: Fat stored in yellow marrow.

Classification of Bones

By Location

• Axial Skeleton: Forms the long axis of the body (skull, vertebral column, rib cage).

• Appendicular Skeleton: Includes upper and lower limbs, and girdles attaching limbs to the axial skeleton.

By Shape

• Long Bones: Longer than they are wide; shaft with two ends (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., wrist and ankle bones); sesamoid bones (e.g., patella) form within tendons.

• Flat Bones: Thin, flat, and slightly curved (e.g., sternum, scapulae, ribs, most skull bones).

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

Bone Structure

Gross Anatomy

• Compact Bone: Dense outer layer; appears smooth and solid.

• Spongy Bone: Honeycomb structure (trabeculae); spaces filled with red or yellow marrow.

Structure of Short, Irregular, and Flat Bones

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

• Compact bone sandwiched between connective tissue membranes (periosteum and endosteum).

• Bone marrow scattered throughout spongy bone.

• Hyaline cartilage covers areas of bone that form movable joints.

Structure of Long Bones

• Diaphysis: Shaft; compact bone surrounds medullary cavity (contains yellow marrow in adults).

• Epiphyses: Ends of bone; compact bone externally, spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of the epiphyseal plate (growth plate) separating diaphysis and epiphysis.

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces (except joints); outer fibrous layer (dense irregular CT), inner osteogenic layer (contains stem cells).

• Endosteum: Connective tissue membrane covering internal bone surfaces; contains osteogenic cells.

Hematopoietic Tissue in Bones

• Red Marrow: Found in trabecular cavities of spongy bone and diploë of flat bones; in infants, also in medullary cavities; in adults, mainly in heads of femur and humerus.

• Yellow Marrow: Can convert to red marrow if increased blood cell production is needed (e.g., anemia).

Bone Markings

• Projections: Outward bulges for muscle, ligament, and tendon attachment.

• Surfaces: Form joints.

• Depressions/Openings: Passageways for blood vessels and nerves.

Microscopic Anatomy of Bone

Bone Cells

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

• Osteoblasts: Bone-forming cells; secrete unmineralized bone matrix (osteoid); become osteocytes when surrounded by matrix.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix; act as stress/strain sensors.

• Bone-Lining Cells: Flat cells on bone surfaces; help maintain matrix; called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Multinucleate cells from white blood cell lineage; responsible for bone resorption; have ruffled borders to increase surface area for enzyme activity.

Compact Bone Structure

• Osteon (Haversian System): Structural unit; elongated cylinder parallel to long axis; consists of concentric lamellae (rings of bone matrix with collagen fibers).

• Central (Haversian) Canal: Runs through core of osteon; contains blood vessels and nerves.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Tiny canals connecting lacunae and central canal; allow communication and nutrient/waste exchange between osteocytes.

Spongy Bone Structure

• No osteons present.

• Trabeculae (irregularly arranged lamellae and osteocytes) act as struts, providing strength with minimal weight.

• Canaliculi connect osteocytes; nutrients supplied by capillaries in endosteum.

Chemical Composition of Bone

Organic Components

• Cells: Osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts.

• Osteoid: Unmineralized bone matrix secreted by osteoblasts; consists of ground substance and collagen fibers, providing strength and flexibility.

Inorganic Components

• Mineral Salts: Make up 65% of bone mass; mainly tiny calcium phosphate crystals in and around collagen fibers.

• Function: Provide hardness and resistance to compression.

Summary Table: Types of Cartilage and Their Locations

Key Equations

• Bone Mass Composition:

• Mineral Content:

of bone mass