Bones and Skeletal Tissues

Introduction to the Human Skeleton

The human skeleton forms the structural framework of the body, providing support, protection, and facilitating movement. Initially, the skeleton is composed mainly of cartilage and fibrous membranes, which are gradually replaced by bone during development, except in regions where flexibility is essential.

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

• Bone Replacement: Most cartilage is replaced by bone as development proceeds.

• Flexibility: Cartilage remains in areas such as joints, rib tips, and respiratory structures to allow movement and flexibility.

Types of Cartilage

Hyaline Cartilage

Hyaline cartilage is the most prevalent type in the human body, providing support, flexibility, and resilience. It contains only collagen fibers and is found in several key locations.

• Support and Flexibility: Maintains shape and allows movement.

• Locations:

  • Articular cartilage: Covers joint surfaces.

  • Costal cartilage: Connects ribs to sternum.

  • Respiratory cartilage: Forms larynx and reinforces airways.

  • Nasal cartilage: Supports the nose.

Elastic Cartilage

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

• Locations:

  • External ear

  • Epiglottis

Fibrocartilage

Fibrocartilage contains thick collagen fibers, providing great tensile strength and the ability to tolerate compression.

• Locations:

  • Menisci of knee

  • Intervertebral discs

  • Pubic symphysis

Growth of Cartilage

Cartilage grows in two ways:

• Appositional Growth: Chondroblasts in the perichondrium produce new matrix on the surface of existing cartilage.

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

Functions of Bones

Major Functions

Bones serve several critical functions in the body:

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

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

• Mineral Storage: Reservoir for calcium and phosphate.

• Triglyceride Storage: Yellow bone marrow stores fat.

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 wide, with a shaft and 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 called trabeculae; spaces filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone covered by compact bone.

• Compact bone is sandwiched between connective tissue membranes (periosteum externally, endosteum internally).

• Bone marrow is scattered throughout spongy bone.

• Hyaline cartilage covers areas forming movable joints.

Structure of Long Bones

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

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

• Epiphyseal Line: Separates diaphysis and epiphysis; remnant of childhood epiphyseal plate.

Membranes Covering Bone

• Periosteum: White, double-layered membrane covering external surfaces (except joints).

  • Fibrous layer: Dense irregular connective tissue.

  • Osteogenic layer: Contains osteogenic stem cells that give rise to bone cells.

  • Rich in nerve fibers and blood vessels; anchoring points for tendons and ligaments.

• 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: Located in medullary cavities and all spongy bone.

• In adults: Located in heads of femur and humerus; yellow marrow can convert to red marrow if anemic.

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

Major Cell Types

• Osteogenic (Osteoprogenitor) Cells: Mitotically active 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 and act as stress/strain sensors.

• Bone-Lining Cells: Maintain bone matrix; called periosteal cells (external surface) and endosteal cells (internal surface).

• Osteoclasts: Multinucleate cells derived from white blood cell lineage; responsible for bone resorption.

Microscopic Anatomy - Compact Bone

• Osteon (Haversian System): Structural unit; elongated cylinder acting as weight-bearing pillar; consists of concentric rings (lamellae) of bone matrix with collagen fibers.

• Canals and Canaliculi:

  • 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.

Microscopic Anatomy - Spongy Bone

• Trabeculae: Network of struts and plates; contain irregularly arranged lamellae and osteocytes interconnected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Cells: All five types (osteogenic, osteoblasts, osteocytes, bone-lining cells, osteoclasts).

• Osteoid: Secreted by osteoblasts; consists of ground substance and collagen fibers; provides strength and flexibility.

Inorganic Components

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

• Provide hardness and resistance to compression.

Summary Table: Types of Cartilage and Locations

Key Formula: Bone Composition

Additional info: The notes above expand on brief points and fill in missing context for a comprehensive overview of bone and cartilage anatomy, suitable for college-level Anatomy & Physiology students.