Skeletal System Overview

The skeletal system provides the structural framework for the human body, supports movement, protects internal organs, and plays a vital role in mineral storage and blood cell production.

• Mineral storage and release: Bones store calcium and phosphate, releasing them as needed.

• Blood cell production: Red bone marrow produces red blood cells (RBCs), white blood cells (WBCs), and platelets.

• Triglyceride (lipid) storage: Yellow bone marrow stores fat.

• Movement: Bones attach to muscles and each other, acting as levers.

• Support: Bones form the structural framework of the body.

Bone Membranes

Periosteum

The periosteum is a membrane covering bones except at joints, consisting of a fibrous outer layer and a cellular inner layer.

• Functions:

  • Isolates bone from surrounding tissues

  • Provides a route for blood vessels and nerves

  • Assists in bone repair and growth

Endosteum

The endosteum is an incomplete single layer of osteoprogenitor cells lining the marrow cavity and covering the trabeculae of spongy bone and inner canal walls in compact bone.

• Involved in bone growth, repair, and remodeling

• Contains osteoblasts, osteoclasts, and osteoprogenitor cells

Bone Structure

Structure of Compact Bone

Compact bone is found in the external layers of bones and in the diaphysis (shaft) of long bones.

• Osteon (Haversian system): The basic structural unit of compact bone.

  • Osteocytes in lacunae are arranged in concentric circles around a central canal (Haversian canal).

  • Central canal contains capillaries and nerves.

  • Perforating canals (Volkmann's canals) run perpendicular to the long axis, connecting blood and nerve supply of the periosteum to the central canals and medullary cavity.

  • Lamellae: Rings of bone matrix; different types include:

    • Interstitial lamellae: Fill spaces between osteons.

    • Circumferential lamellae: Form the inner and outer surfaces of the shaft.

Structure of Spongy Bone

Spongy bone (cancellous bone) is found mainly at the ends of long bones and inside flat bones.

• No osteon arrangement.

• Lamellae are arranged in an irregular honeycomb called trabeculae.

• Spaces between trabeculae are filled with red bone marrow, which produces blood cells.

• Spongy bone reduces the weight of the skeleton and supports/protects bone marrow.

Bone Extracellular Matrix/Histology

• Matrix: Dense, containing calcium salts and collagen fibers.

• Calcium salts: Provide hardness and resistance to compression.

• Collagen fibers: Provide flexibility and resistance to tension and twisting.

Bone Cells

• Osteogenic/Osteoprogenitor cells: Stem cells in the inner periosteum and endosteum; divide and differentiate into osteoblasts.

• Osteoblasts: Bone-building cells; produce new bone matrix (osteoid) and promote mineralization.

• Osteocytes: Mature bone cells trapped in lacunae; maintain bone matrix and communicate via canaliculi.

• Osteoclasts: Large, multinucleated cells; break down bone matrix, releasing calcium and phosphate into the blood.

Bone Physiology

Ossification

Ossification is the process of replacing other tissues with bone.

• Calcification: The deposition of calcium salts within a tissue.

Intramembranous Ossification

Bone develops from mesenchymal (stem) cells or fibrous connective tissue, forming flat bones (e.g., skull, mandible, clavicle).

1. Mesenchymal cells cluster and differentiate into osteoblasts, forming ossification centers.

2. Osteoblasts secrete matrix, becoming trapped and differentiating into osteocytes.

3. Bone forms spicules, which fuse to form spongy bone; outer layers may become compact bone.

Endochondral Ossification

Bone replaces hyaline cartilage, forming most bones of the skeleton.

1. Chondrocytes enlarge, matrix calcifies, and chondrocytes die, leaving cavities.

2. Osteoprogenitor cells differentiate into osteoblasts, forming a bone collar.

3. Blood vessels invade, osteoblasts form spongy bone at the primary ossification center.

4. Remodeling occurs, marrow cavity forms, and bone grows in length and diameter.

5. Secondary ossification centers form in the epiphyses.

Appositional Growth

Appositional growth increases the diameter of a developing bone by adding new bone tissue at the surface.

• Bone deposition by osteoblasts in the periosteum

• Bone resorption by osteoclasts in the endosteum

Bone Remodeling

Bone remodeling is a lifelong process where mature bone tissue is removed and new bone tissue is formed.

• Maintains bone strength and mineral homeostasis

• Involves osteoblasts (formation), osteoclasts (resorption), and osteocytes (regulation)

• Exercise and mechanical stress stimulate bone remodeling, increasing bone density and strength

Physiology of Calcium Homeostasis

Calcium and phosphate levels in the blood are tightly regulated by hormones through negative feedback mechanisms.

• Too Low:

  • Calcitriol (Vitamin D): Increases intestinal absorption of calcium and phosphate, reduces urinary excretion, and promotes osteoclast activity.

  • Parathyroid hormone (PTH): Increases osteoclast activity, inhibits osteoblast activity, and decreases excretion of calcium by the kidneys.

• Too High:

  • Calcitonin: Inhibits osteoclast activity, stimulates osteoblast activity, and increases excretion of calcium by the kidneys.

Summary Table: Bone Cells and Their Functions

Key Equations

• Calcium Homeostasis:

Additional info:

• Bone remodeling is influenced by hormones, diet, mechanical stress, and age.

• Disorders of bone metabolism include osteoporosis (increased bone resorption) and osteomalacia (defective bone mineralization).