Bones and Skeletal Tissues

Introduction

The skeletal system provides the framework for the human body, supporting movement, protecting organs, and serving as a reservoir for minerals. This chapter explores the structure, function, development, and disorders of bones and skeletal tissues.

Skeletal Cartilages

Basic Structure, Types, and Locations

Cartilage is a resilient, semi-rigid connective tissue found in various parts of the skeleton. It is avascular and receives nutrients by diffusion. There are three main types of cartilage:

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

• Elastic cartilage: Contains more elastic fibers, allowing flexibility. Found in the external ear and epiglottis.

• Fibrocartilage: Highly compressible with great tensile strength. Found in intervertebral discs, pubic symphysis, and menisci of the knee.

Locations: Cartilage is present in joints, rib attachments, respiratory passages, ear, nose, and intervertebral discs.

Growth of Cartilage

• Appositional growth: Cells in the perichondrium secrete new matrix on the external surface of cartilage.

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

Functions of Bones

Major Functions

• Support: Provides a framework for the body and supports soft tissues.

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

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

• Mineral storage: Reservoir for calcium and phosphate.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities.

• Triglyceride storage: Fat stored in yellow marrow.

• Hormone production: Osteocalcin helps regulate bone formation and protects against obesity and diabetes.

Classification of Bones

Bone Shapes

Bones are classified by shape:

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

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

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

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

Bone Structure

Gross Anatomy

• Compact bone: Dense outer layer; smooth and solid.

• Spongy bone (trabecular bone): Internal honeycomb of trabeculae filled with red or yellow marrow.

Structure of a Long Bone

• Diaphysis: Shaft; compact bone surrounding medullary cavity.

• Epiphyses: Ends of bone; spongy bone interior, articular cartilage covers joint surfaces.

• Membranes: Periosteum (outer) and endosteum (inner) cover bone surfaces.

Bone Markings

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

Microscopic Anatomy of Bone

Cells of Bone Tissue

Bone contains five major cell types:

• Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete bone matrix.

• Osteocytes: Mature bone cells that maintain the bone matrix.

• Bone-lining cells: Flat cells on bone surfaces believed to help maintain matrix.

• Osteoclasts: Large cells that resorb or break down bone matrix.

Compact Bone Structure

• Osteon (Haversian system): Structural unit of compact bone; elongated cylinder parallel to long axis of bone.

• Lamellae: Concentric rings of bone matrix.

• Central (Haversian) canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) canals: Connect blood vessels and nerves of periosteum to those in central canal.

• Lacunae: Small cavities containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae to each other and the central canal.

Spongy Bone Structure

• Trabeculae: Align along lines of stress; no osteons present.

• Spaces between trabeculae are filled with red or yellow marrow.

Chemical Composition of Bone

Organic and Inorganic Components

• Organic: Includes cells and osteoid (ground substance and collagen fibers), which contribute to flexibility and tensile strength.

• Inorganic: Mainly hydroxyapatites (mineral salts, primarily calcium phosphates), which provide hardness and resistance to compression.

Bone Development (Ossification)

Formation of the Bony Skeleton

Ossification (osteogenesis) is the process of bone tissue formation. Two main types:

• Endochondral ossification: Bone forms by replacing hyaline cartilage; forms most of the skeleton below the skull.

• Intramembranous ossification: Bone develops from fibrous membrane; forms flat bones of the skull and clavicles.

Postnatal Bone Growth

• Longitudinal growth: Occurs at the epiphyseal plate through zones of proliferation, hypertrophy, calcification, and ossification.

• Appositional growth: Bone increases in thickness.

Bone Remodeling

Bone Deposit and Resorption

• Bone deposit: Occurs where bone is injured or added strength is needed; requires a diet rich in protein, vitamins, and minerals.

• Bone resorption: Osteoclasts break down bone matrix, releasing calcium into the blood.

Control of Remodeling

• Hormonal control: Parathyroid hormone (PTH) increases blood calcium by stimulating osteoclasts; calcitonin may lower blood calcium.

• Mechanical stress: Bone adapts to the loads under which it is placed (Wolff's law).

Bone Repair

Fracture Classification

Fractures are classified by their location, completeness, orientation, and whether the bone ends penetrate the skin.

Fracture Treatment and Repair

1. Hematoma formation: Blood clot forms at fracture site.

2. Fibrocartilaginous callus formation: Soft callus of collagen and cartilage forms.

3. Bony callus formation: New bone trabeculae appear in callus.

4. Bone remodeling: Compact bone is laid down to reconstruct shaft walls.

Bone Disorders

Osteomalacia and Rickets

• Osteomalacia: Bones are inadequately mineralized, causing them to soften; often due to vitamin D deficiency in adults.

• Rickets: Analogous disease in children; results in bowed legs and deformities.

Osteoporosis

• Bone resorption outpaces deposit; bones become fragile and porous.

• Common in postmenopausal women due to decreased estrogen.

Paget’s Disease

• Excessive and haphazard bone deposit and resorption; bone has a high ratio of spongy to compact bone and reduced mineralization.

• Unknown cause; may be viral.

Additional info: Bone health is influenced by nutrition, hormonal balance, and mechanical stress. Disorders often reflect imbalances in these factors.