Introduction to the Skeletal System

The skeletal system is a complex organ system composed of bones, cartilages, ligaments, and other connective tissues that stabilize and support the body. The primary tissue of the skeletal system is bone tissue (also called osseous tissue), which is dynamic and made up of several tissue types.

• Support: Bones provide the structural framework for the body.

• Storage of minerals: Bones store calcium salts; approximately 98% of the body's calcium is found in bones.

• Blood cell production: Bone marrow produces new blood cells (hematopoiesis).

• Protection: Bones protect delicate internal organs (e.g., skull protects the brain).

• Leverage: Muscles pull on bones to produce movement.

Structure of Bone (Osseous Tissue)

Components of Bone Tissue

Bone is a specialized form of supporting connective tissue with a solid matrix containing calcium salts and specialized cells. It is covered externally by the periosteum and lined internally by the endosteum.

• Periosteum: Outer covering of bone, continuous with the deep fascia.

• Endosteum: Inner cellular lining of bone.

Histological Organization of Mature Bone

The matrix of bone tissue is composed of inorganic and organic components, providing both strength and flexibility.

• Matrix:

  • 2/3 calcium phosphate (in the form of hydroxyapatite crystals), which are very resistant to compression.

  • 1/3 collagen fibers, which are very resistant to stretch.

  • The combination of collagen and hydroxyapatite makes bone tissue extremely strong.

  • Cells make up about 2-3% of bone tissue by weight.

Cells of Mature Bone

Bone tissue contains several types of cells, each with specialized functions:

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoblasts: Immature, active cells found on bone surfaces; they produce osteoid (the organic component of the matrix) and are responsible for osteogenesis (new bone formation).

• Osteoprogenitor cells: Stem cells found on bone surfaces that divide and differentiate into new osteoblasts.

• Osteoclasts: Large, multinucleated cells that perform osteolysis (breakdown of bone matrix).

Histological Structure of a Typical Bone

The following table summarizes the main bone cell types and their functions:

Example: During bone remodeling, osteoclasts resorb bone tissue, while osteoblasts lay down new bone, maintaining bone strength and mineral homeostasis.

Additional info: The balance between osteoblast and osteoclast activity is essential for healthy bone maintenance and is regulated by hormones such as parathyroid hormone (PTH) and calcitonin.