In Depth Notes on Bone Functions and Anatomy

Support: Provides structural support for body and soft organs.
Protection: Shields vital organs (e.g., brain, spinal cord).
Movement: Serves as levers for muscle actions.
Mineral and Growth Factor Storage: Acts as a reservoir for minerals like calcium and phosphorus.
Blood Cell Formation: Hematopoiesis occurs in red marrow cavities in certain bones.
Triglyceride Storage: Stores fat used as an energy source in bone cavities.
Hormone Production: Osteocalcin secreted by bones regulates insulin, glucose levels, and metabolism.

Total Number of Bones: 206 named bones in the human skeleton divided into two main groups.
- Axial Skeleton: Includes skull, vertebral column, rib cage; forms the long axis of the body.
- Appendicular Skeleton: Composed of bones of the limbs and girdles attaching them to the axial skeleton.
Shapes of Bones:
- Long Bones: Longer than they are wide (e.g., limb bones).
- Short Bones: Cube-shaped (e.g., wrist, ankle); includes sesamoid bones (e.g., patella).
- Flat Bones: Thin, flat, slightly curved (e.g., sternum, scapulae, ribs).
- Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Compact and Spongy Bone:
- Compact Bone: Dense outer layer of every bone.
- Spongy Bone: Honeycomb-like trabecular structure with open spaces filled with red or yellow bone marrow.
Structure of Short, Irregular, and Flat Bones:
- Composed of thin plates of spongy bone (diploë) covered by compact bone.
- Bone marrow is scattered in spongy bone, no defined marrow cavity.
- Hyaline cartilage covers areas of bone at movable joints.
Structure of Long Bones:
- Diaphysis: Tubular shaft forming the long axis, composed of compact bone and a central medullary cavity filled with yellow marrow.
- Epiphyses: Ends of long bones made of compact bone externally and spongy bone internally.
- Articular cartilage covers joint surfaces; epiphyseal line indicates your growth.

Red Marrow: Found in trabecular cavities of spongy bone. In adults, concentrated in heads of femur and humerus, among others.
Yellow Marrow: Fat storage can convert back to red if the individual becomes anemic.

Bone Cells: Five major types:
- Osteogenic Cells: Stem cells differentiating into osteoblasts or bone-lining cells.
- Osteoblasts: Bone-forming cells secreting unmineralized matrix (osteoid), primarily collagen.
- Osteocytes: Mature cells maintaining the bone matrix; respond to mechanical stress.
- Bone-Lining Cells: Maintain the matrix on bone surfaces.
- Osteoclasts: Multinucleated cells breaking down bone matrix, aiding in resorption.

Osteon (Haversian System): Structural unit consisting of an elongated cylinder parallel to the bone's long axis.
- Lamellae: Rings of bone matrix around the osteon containing collagen fibers and bone salts.
Canals and Canaliculi:
- Central Canal: Contains blood vessels and nerve fibers.
- Perforating Canals: Connect blood vessels and nerves to central canals; occur at right angles.
- Canaliculi: Small canals connecting osteocytes to each other and to the central canal.

Organic Components: Include bone cells and osteoid (33% of organic bone matrix).
Inorganic Components: Hydroxyapatites (65% by mass), mainly calcium phosphate crystals, provide hardness.

Ossification: Process of bone tissue formation, starting in the second month of development.
Long Bone Growth: Occurs until 18-21 years of age in females and males, respectively.
Growth in Length: Involves zones of cartilage (resting, proliferation, hypertrophic, calcification, ossification) at the epiphyseal plate.

Hematoma Formation: After a fracture, blood vessels break and form a hematoma.
Fibrocartilaginous Callus Formation: Fibroblasts and cartilage cells create a callus bridging the fracture.
Bony Callus Formation: Osteoblasts lay down new spongy bone within the callus.
Bone Remodeling: Excess material from the fracture is removed and compact bone is laid down.

Definition: A disease where bone resorption exceeds deposition, leading to a decrease in bone mass.
Risk Factors: Include aging, postmenopausal women, genetics, lack of physical activity, poor nutrition, and hormonal imbalances.
Treatment: Consists of calcium and vitamin D supplements, exercise, and medications like bisphosphonates and selective estrogen receptor modulators.

Bone density changes over a lifetime, influenced by genetic factors and hormonal changes.
Aging tends to favor bone resorption over formation, leading to decreased bone mass and density, particularly in women post-menopause.