Bone Structure, Physiology, and Development: Study Notes for Anatomy & Physiology

Bone Structure, Physiology, and Development

Functions of the Skeletal System

The skeletal system provides the framework for the body and serves several essential functions:

• Support: Provides structural support for the entire body.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as levers for muscles to produce movement.

• Mineral Storage: Stores minerals, especially calcium and phosphorus.

• Blood Cell Production: Houses bone marrow, which produces blood cells (hematopoiesis).

• Energy Storage: Yellow bone marrow stores lipids as an energy reserve.

Bone Shapes

Bones are classified by shape, which relates to their function:

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., sternum, skull bones).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvic bones).

• Sesamoid Bones: Small, round bones embedded in tendons (e.g., patella).

Bone Markings

Bones have surface features called markings that serve as attachment points for muscles, tendons, and ligaments, or as passageways for nerves and blood vessels.

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, trochanter).

• Depressions and Openings: Allow passage of blood vessels and nerves (e.g., foramen, fossa, groove).

Long Bone Structure

Long bones have a characteristic structure that supports their function:

• Diaphysis: The shaft, composed mainly of compact bone.

• Epiphyses: The ends of the bone, consisting mostly of spongy bone covered by compact bone.

• Metaphysis: The region between diaphysis and epiphysis, containing the growth plate in children.

• Medullary Cavity: Central cavity containing bone marrow.

• Periosteum: Dense connective tissue covering the outer surface of bone.

• Endosteum: Thin membrane lining the medullary cavity.

Bones and Bone Structure

Bones are dynamic organs composed of several types of tissue:

• Bone (Osseous) Tissue: The primary tissue, providing strength and support.

• Cartilage: Covers joint surfaces and supports bone growth.

• Connective Tissue: Includes periosteum and endosteum.

• Blood Vessels and Nerves: Supply nutrients and sensation.

Bone Matrix

The bone matrix is the intercellular substance of bone tissue, providing strength and flexibility:

• Organic Component: Mainly collagen fibers, providing tensile strength.

• Inorganic Component: Primarily hydroxyapatite (calcium phosphate crystals), providing hardness.

Cells of Bones

Bone tissue contains several specialized cell types:

• Osteoblasts: Bone-forming cells that secrete bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Large cells that resorb (break down) bone matrix.

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

Compact Bone

Compact bone is dense and forms the outer layer of bones, providing strength for weight bearing:

• Osteons (Haversian Systems): Structural units composed of concentric lamellae around a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Connect central canals transversely.

Spongy Bone

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

• Trabeculae: Lattice-like network of bone tissue.

• Spaces: Filled with red bone marrow, site of hematopoiesis.

Bone Marrow

Bone marrow is a soft tissue found in the medullary cavities of bones:

• Red Bone Marrow: Produces red blood cells, white blood cells, and platelets.

• Yellow Bone Marrow: Stores fat and serves as an energy reserve.

Blood and Nerve Supplies to Bone

Bones are richly supplied with blood vessels and nerves:

• Nutrient Arteries: Enter through nutrient foramina to supply the bone interior.

• Periosteal Vessels: Supply the outer compact bone.

• Nerves: Accompany blood vessels and are important for pain sensation and regulation of blood flow.

Appositional Growth

Appositional growth refers to the increase in bone diameter:

• Process: Osteoblasts in the periosteum add new bone tissue to the outer surface, while osteoclasts resorb bone on the inner surface, enlarging the medullary cavity.

• Result: Bones become thicker and stronger without becoming excessively heavy.

Bone Formation and Growth

Bone formation (ossification) occurs through two main processes:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones of the skull).

• Endochondral Ossification: Bone replaces a cartilage model (e.g., long bones).

Process of Endochondral Ossification

Endochondral ossification is the process by which most bones form:

1. Cartilage model develops.

2. Cartilage calcifies and a periosteal bone collar forms.

3. Primary ossification center forms in the diaphysis.

4. Blood vessels invade, bringing osteoblasts.

5. Secondary ossification centers form in the epiphyses.

6. Cartilage remains at the epiphyseal (growth) plates and articular surfaces.

Bone Remodeling

Bone remodeling is a continuous process of bone resorption and formation:

• Purpose: Maintains bone strength, repairs microdamage, and regulates calcium levels.

• Cells Involved: Osteoclasts (resorption) and osteoblasts (formation).

Fractures

A fracture is a break in the bone, classified by its characteristics:

• Simple (Closed) Fracture: Bone breaks but does not penetrate the skin.

• Compound (Open) Fracture: Bone breaks and pierces the skin.

• Other Types: Comminuted, greenstick, spiral, etc.

Factors Impacting Bone Development

Several factors influence bone growth and health:

• Genetics: Determines potential size and shape.

• Hormones: Regulate growth and remodeling.

• Nutrition and Diet: Adequate intake of calcium, vitamin D, and protein is essential.

• Physical Activity: Mechanical stress stimulates bone formation.

Hormones Affecting Bone

Hormones play a crucial role in bone development and maintenance:

• Growth Hormone: Stimulates bone growth during childhood.

• Thyroid Hormones: Regulate bone growth and maturation.

• Sex Hormones (Estrogen and Testosterone): Promote growth spurt and closure of growth plates.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption.

• Calcitonin: Lowers blood calcium by inhibiting bone resorption.

Calcium and Bone Physiology

Calcium is vital for bone strength and many physiological processes:

• Bone Reservoir: Bones store 99% of the body's calcium.

• Calcium Homeostasis: Maintained by hormonal regulation.

Calcium Balance

Calcium balance is regulated by the interplay of dietary intake, bone storage, and excretion:

• Absorption: Occurs in the intestines, enhanced by vitamin D.

• Excretion: Mainly through the kidneys.

• Bone Remodeling: Releases or stores calcium as needed.

Parathyroid Hormone (PTH)

PTH is secreted by the parathyroid glands in response to low blood calcium:

• Stimulates osteoclast activity to release calcium from bone.

• Increases calcium reabsorption in the kidneys.

• Stimulates activation of vitamin D, increasing intestinal absorption of calcium.

Calcitonin

Calcitonin is produced by the thyroid gland and acts to lower blood calcium:

• Inhibits osteoclast activity, reducing bone resorption.

• Promotes calcium deposition in bones.

Nutrition and Diet

Proper nutrition is essential for bone health:

• Calcium: Required for bone mineralization.

• Vitamin D: Necessary for calcium absorption.

• Protein: Needed for collagen synthesis.

• Other Nutrients: Magnesium, phosphorus, and vitamins A, C, and K also contribute to bone health.

Comparison of Compact and Spongy Bone

Feature	Compact Bone	Spongy Bone
Location	Outer layer of bones	Ends of long bones, inside flat bones
Structure	Osteons (Haversian systems)	Trabeculae
Function	Strength, weight bearing	Lightweight, supports marrow
Marrow	Medullary cavity (yellow marrow)	Spaces filled with red marrow

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.