Bone Tissue

Overview of Bone Tissue

Bone tissue, also known as osseous tissue, is a specialized connective tissue that forms the rigid part of bones. It plays a crucial role in support, protection, movement, mineral storage, and blood cell formation.

• 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: Serves as levers for muscles to act upon.

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

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

Types of Bone Tissue

• Compact Bone: Dense and solid, forms the outer layer of bones. Provides strength for weight bearing.

• Spongy (Cancellous) Bone: Porous and lighter, found at the ends of long bones and inside flat bones. Contains red bone marrow.

Bone Composition

• Cells: Osteoblasts (bone-forming), osteocytes (mature bone cells), osteoclasts (bone-resorbing).

• Matrix: Composed of organic (collagen fibers) and inorganic (hydroxyapatite crystals) components.

Hydroxyapatite: The main mineral component, providing hardness and strength. Its chemical formula is:

Classification of Bones

Types of Bones by Shape

Bones are classified according to their shapes and structures, which relate to their functions.

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

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., sternum, skull bones).

• Irregular Bones: Complex shapes (e.g., vertebrae, some facial bones).

• Sesamoid Bones: Develop within tendons (e.g., patella).

Structure of a Long Bone

Gross Anatomy

• Diaphysis: The shaft, composed mainly of compact bone.

• Epiphyses: The ends of the bone, consisting mostly of spongy bone covered by a thin layer of compact bone.

• Metaphysis: The region between diaphysis and epiphysis; contains the epiphyseal (growth) plate in growing bones.

• Medullary Cavity: Central cavity containing yellow bone marrow (fat storage) in adults.

• Periosteum: Dense connective tissue membrane covering the outer surface of bone (except at joints).

• Endosteum: Thin membrane lining the medullary cavity.

Microscopic Structure of Bone

Compact Bone (Haversian System)

• Osteon (Haversian System): The structural unit of compact bone, consisting of concentric lamellae (layers) around a central canal.

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

• Lacunae: Small spaces containing osteocytes.

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

Spongy Bone

• Trabeculae: Lattice-like network of bone tissue; spaces between trabeculae are filled with red bone marrow.

Bone Development (Ossification)

Overview of Ossification

Ossification is the process by which bone forms. There are two main types:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue. Forms most flat bones (e.g., skull, clavicle).

• Endochondral Ossification: Bone develops by replacing hyaline cartilage. Forms most bones of the body, especially long bones.

Endochondral Ossification: Steps

1. Development of cartilage model

2. Growth of cartilage model

3. Development of primary ossification center (in diaphysis)

4. Development of medullary cavity

5. Development of secondary ossification centers (in epiphyses)

6. Formation of articular cartilage and epiphyseal plate

Epiphyseal Plate: Also known as the growth plate, it is responsible for longitudinal bone growth during childhood and adolescence.

Table: Classification of Bones by Shape

Additional info:

• Bone remodeling is a continuous process involving bone resorption by osteoclasts and bone formation by osteoblasts.

• Disorders of bone tissue include osteoporosis (loss of bone mass) and osteomalacia (softening of bones).