Bones & Bone Tissue: Structure and Function

Bones & Bone Tissue

Functions of the Skeletal System

The skeletal system is essential for multiple physiological and structural roles in the human body. Its functions extend beyond mere support, encompassing protection, storage, and movement.

• Protection: Bones protect vital organs such as the brain (skull), heart and lungs (rib cage), and spinal cord (vertebral column).

• Mineral Storage & Acid-Base Homeostasis: Bones store minerals, primarily calcium and phosphorus, and help regulate acid-base balance by releasing or absorbing these minerals as needed.

• Blood Cell Production (Hematopoiesis): Red bone marrow within spongy bone is the site of blood cell formation.

• Fat Storage: Yellow bone marrow stores triglycerides, serving as an energy reserve.

• Movement: Bones act as levers for muscles, facilitating movement.

• Support: The skeleton provides a rigid framework that supports the body and maintains its shape.

Bone Structure

Bones are classified by their shapes and internal structures, each adapted for specific functions.

• Long Bones: Longer than they are wide (e.g., femur, humerus). Consist of a shaft (diaphysis) and two ends (epiphyses).

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

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

• Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae, facial bones).

• Sesamoid Bones: Small, round bones embedded within tendons (e.g., patella).

Structure of a Long Bone

Long bones have a specialized structure to support their functions in movement and weight-bearing.

• Periosteum: Dense connective tissue membrane covering the outer surface of bone; contains nerves and blood vessels.

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

• Epiphyses: The ends of the bone, containing spongy bone and red marrow.

• Medullary Cavity: Central cavity within the diaphysis, containing yellow marrow in adults.

• Endosteum: Thin membrane lining the medullary cavity.

• Articular Cartilage: Hyaline cartilage covering the epiphyses, reducing friction at joints.

• Compact (Cortical) Bone: Dense outer layer providing strength.

• Spongy (Trabecular/Cancellous) Bone: Porous inner bone containing red marrow.

• Epiphyseal Plates/Lines: Growth plates in children; become epiphyseal lines after growth ceases.

Microscopic Structure of Bone Tissue

Bone tissue is a dynamic, living tissue composed of cells and an extracellular matrix (ECM).

• Extracellular Matrix (ECM):

  • Inorganic Matrix: Mainly hydroxyapatite crystals (calcium phosphate), providing hardness and strength.

  • Organic Matrix (Osteoid): Collagen fibers and proteoglycans, providing flexibility and tensile strength.

• Bone Cells:

  • Osteoblasts: Bone-forming cells responsible for bone deposition.

  • Osteocytes: Mature bone cells derived from osteoblasts; maintain bone matrix.

  • Osteoclasts: Large, multinucleated cells responsible for bone resorption (breakdown).

Types of Bone Tissue

• Compact (Cortical) Bone: Dense, organized into osteons (Haversian systems) for strength.

• Spongy (Cancellous/Trabecular) Bone: Lattice-like network of trabeculae; spaces filled with red bone marrow.

Example Table: Bone Types and Features

Bone Type	Location	Main Function
Long Bone	Femur, Humerus	Movement, support
Short Bone	Carpals, Tarsals	Stability, some movement
Flat Bone	Sternum, Skull	Protection, muscle attachment
Irregular Bone	Vertebrae	Protection, support
Sesamoid Bone	Patella	Reduces friction, modifies pressure

Key Terms

• Hematopoiesis: The process of blood cell formation, occurring in red bone marrow.

• Hydroxyapatite: The mineral form of calcium phosphate in bone matrix.

• Osteon: The structural unit of compact bone.

• Trabeculae: The lattice-like structures in spongy bone.

Additional info:

• Bone tissue is highly vascularized and innervated, allowing for rapid repair and adaptation to stress.

• Bone remodeling is a continuous process involving osteoblasts and osteoclasts, essential for growth, repair, and calcium homeostasis.