Human Skeletal System

Structure of Bone

• Articular cartilage provides a smooth gliding surface for tendons and ligaments to move.
• Epiphyseal line is a remnant of the epiphyseal plate where the bone grows in length.
• Epiphysis is the rounded end of the long bone, formed by a secondary ossification center.
• Diaphysis is the shaft of the long bone.
• Medullary cavity contains yellow marrow in adults and red marrow in infants.
• Periosteum is a tough connective tissue sheath that surrounds the bone and anchors tendons and ligaments.
• Compact bone is made up of tightly packed osteons (cylindrical structures containing Haversian canals).
• Cancellous (spongy) bone is made up of a network of thin, plate-like trabeculae that form a honeycomb-like structure.

Histology of Bone (Osseous Tissue)

• Osteocytes are mature bone cells that maintain the bone matrix.
• Osteoblasts are bone-forming cells that secrete the organic matrix of bone.
• Osteogenic cells are stem cells that develop into osteoblasts.
• Lacunae are small spaces in the bone matrix that house the osteocytes.
• Canalliculi are tiny channels that connect the lacunae and allow nutrients and waste products to pass through.
• Haversian canals are central canals in the osteons that contain blood vessels and nerves.
• Volkmann's canals are channels that connect the Haversian canals and allow blood vessels and nerves to pass through the bone.

Factors Affecting Bone Growth

• Minerals: calcium and phosphorus are needed for bone growth.
• Vitamins:
  • Vitamin A stimulates osteoblast activity.
  • Vitamin C is required for collagen synthesis.
  • Vitamin D is required for calcium absorption from foods.
  • Vitamin K and B12 are needed for bone protein synthesis.
• Hormones:
  • Growth hormone (GH) is produced by the pituitary gland.
  • Insulin-like growth factors are produced by the liver and bone.
  • Sex hormones: estrogen and testosterone.

Calcium Homeostasis

• Parathyroid hormone (PTH) is secreted by parathyroid glands and increases blood calcium levels by:
  • Increasing bone resorption (release of calcium from bone into the blood).
  • Decreasing calcium excretion by the kidneys.
• Calcitonin (CT) is secreted by parafollicular cells of the thyroid gland and decreases blood calcium levels by:
  • Decreasing bone resorption.
  • Increasing calcium excretion by the kidneys.

Divisions of the Skeletal System

• The Axial skeleton consists of bones arranged along the longitudinal axis of the body and consists of 80 bones.
• The Appendicular skeleton consists of 126 bones in the upper and lower extremities (limbs) and pectoral (shoulder) and pelvic (hip) girdles.

Types of Bones

• Long bones: mostly compact bone tissue in their diaphyses, but have considerable amounts of spongy bone tissue in their epiphyses.
• Short bones: cube-shaped, consists of spongy bone tissue except at the surface, which has a thin layer of compact bone.
• Flat bones: generally thin, composed of two parallel plates of compact bone tissue enclosing a layer of spongy bone tissue, provide extensive areas for muscle attachment.
• Irregular bones: complex shapes.
• Sesamoid bones: develop in certain tendons, not always completely ossified, protect tendons from excessive wear and tear.