Chapter 6: Notes on Bones and Bone Structure

6-1: Describe the major functions of the skeletal system.
6-2: Classify bones according to shape and structure, giving examples of each type, and explain the functional significance of each of the major types of bone markings.
6-3: Identify the cell types in bone, and list their major functions.
6-4: Compare the structures and functions of compact bone and spongy bone.
6-5: Compare the mechanisms of endochondral ossification and intramembranous ossification.
6-6: Describe the remodeling and homeostatic mechanisms of the skeletal system.
6-7: Discuss the effects of exercise, nutrition, and hormones on bone development and on the skeletal system.
6-8: Explain the role of calcium as it relates to the skeletal system.
6-9: Describe the types of fractures, and explain how fractures heal.
6-10: Summarize the effects of the aging process on the skeletal system.

Components: The skeletal system includes:
- Bones of the skeleton
- Cartilages, ligaments, and other connective tissues
Primary Functions:
- Support: Framework for the body.
- Storage of minerals and lipids: Stores calcium, phosphate, and lipids in adipocytes within the bone.
- Blood Cell Production: Hematopoiesis occurs in red bone marrow.
- Protection: Protects vital organs (e.g., skull protects the brain).
- Leverage: Acts as levers to facilitate movement.

Basis of Classification: Bones can be classified by their shape and structure.
Bone Shapes:
- Sutural Bones: Small, flat, irregularly shaped bones found between flat bones of the skull (e.g., Wormian bones).
- Irregular Bones: Complex shapes (e.g., vertebrae, pelvis).
- Short Bones: Boxlike appearance (e.g., carpal and tarsal bones).
- Flat Bones: Thin and parallel surfaces (e.g., skull roof, sternum, ribs).
- Long Bones: Long and slender (e.g., femur, humerus).
- Sesamoid Bones: Small, round, flat bones found near joints (e.g., patella).
Bone Markings (Surface Features):
- Projections: Where muscles, tendons, and ligaments attach or at articulations with other bones.
- Openings and Depressions: For passage of blood vessels and nerves.

Projections:
- Process: Projection or bump.
- Ramus: Part of a bone that forms an angle with the rest of the structure.
- Trochanter: Large, rough projection.
- Tubercle: Small, rounded projection.
Openings:
- Foramen: Rounded passageway for blood vessels and/or nerves.
- Fissure: Deep furrow, cleft, or slit.
- Sinus: Chamber within a bone, normally filled with air.

Bone Tissue Definition: Dense, supportive connective tissue containing specialized cells and a solid extracellular matrix.
Characteristics:
- Dense matrix due to deposits of calcium salts.
- Osteocytes within lacunae organized around blood vessels.
- Canaliculi: Narrow passageways that allow for nutrient and waste exchange.
- Periosteum: Covers outer surfaces of bones except at joints, consisting of outer fibrous and inner cellular layers.

Types of Bone Cells:
- Osteogenic Cells: Stem cells in periosteum and endosteum that produce osteoblasts.
- Osteoblasts: Immature cells that secrete the organic components of the matrix.
- Osteocytes: Mature bone cells that maintain the bone matrix.
- Osteoclasts: Multinucleate cells that dissolve bone matrix by secreting acids and enzymes for osteolysis.

Compact Bone:
- Osteon: Functional unit containing a central canal.
- Lamellae: Layers of bone matrix around the central canal, with concentric, interstitial, and circumferential lamellae.
Spongy Bone:
- Lacks osteons; forms an open network of trabeculae.
- Red bone marrow fills spaces and contains blood vessels; provides nutrients to osteocytes.
- Yellow bone marrow stores fat.

Bone Development: Includes ossification (osteogenesis) and calcification (deposition of calcium salts).
Types of Ossification:
- Endochondral Ossification: Bone replaces hyaline cartilage.
- Seven main steps, including enlargement of cartilage, development of the primary ossification center, and formation of secondary ossification centers.
- Epiphyseal Closure: Occurs at puberty when the rate of cartilage production decreases.
- Intramembranous Ossification: Occurs in dermis and forms dermal bones.

Interstitial Growth: Growth in length; involves secondary ossification centers and epiphyseal plates.
Appositional Growth: Growth in width; new layers of bone are added at the outer surface.

Bone Remodeling: Continuous process throughout life involving recycling and renewing bone matrix.
Main Cells Involved: Osteocytes, osteoblasts, and osteoclasts.
Balance: If removal of bone is faster than deposition, the bone weakens.

Exercise Effects: Heavily stressed bones become thicker and stronger; rapid degeneration occurs without activity.
Nutritional Requirements: Calcium and phosphorus are crucial, along with vitamins C, A, K, D, and B12.
Hormonal Influences: Growth hormone and sex hormones promote bone growth; parathyroid hormone and calcitonin manage calcium levels.

Calcium Reserve: Bones store 99% of body calcium; calcium is vital for physiological processes.
Hormonal Regulation: PTH increases blood calcium by stimulating osteoclasts, while calcitonin decreases it by inhibiting osteoclasts and enhancing calcium excretion.

Definition: Cracks or breaks in bones under stress; types include:
- Open (compound) and closed (simple)
- Specific types: transverse, displaced, compression, spiral, epiphyseal, comminuted, greenstick, Colles, Pott’s.

Fracture Hematoma Formation: A blood clot establishes a fibrous network.
Callus Formation: Cells migrate into fracture zone, stabilizing the break.
Spongy Bone Formation: Osteoblasts replace cartilage with spongy bone.
Compact Bone Formation: Repaired bone may be thicker and stronger than the original.

Bone Thinning: Decreases density; osteopenia begins around ages 30-40, with women losing 8% and men 3% bone mass per decade.
Osteoporosis: Severe loss affecting 29% of women and 18% of men over 45; accelerates post-menopause.
Cancer Effects: Can release factors that stimulate osteoclasts, leading to osteoporosis.