Study Guide: Bone Structure, Function, and Calcium Homeostasis

Bones of the Skeleton

Functions of Bones

Bones serve multiple essential functions in the human body, contributing to structure, movement, protection, and metabolic processes.

• Support: Provide a rigid framework for the body and maintain its shape.

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

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

• Mineral Storage: Store minerals such as calcium and phosphorus.

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

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

Types of Cells in Bone Tissue

Bone tissue contains several specialized cell types, each with distinct roles in bone maintenance and remodeling.

• Osteoblasts: Cells responsible for bone formation and deposition.

• Osteoclasts: Cells that resorb (break down) bone tissue.

• Osteocytes: Mature bone cells that maintain bone matrix.

• Bone lining cells: Regulate passage of calcium into and out of bone.

Types of Bones (Based on Shape)

Bones are classified by their shapes, which relate to their functions.

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

• Short bones: Cube-shaped (e.g., carpals, tarsals).

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

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

• Sesamoid bones: Embedded in tendons (e.g., patella).

Bone Matrix and ECM Components

The extracellular matrix (ECM) of bone provides strength and flexibility.

• Collagen fibers: Provide tensile strength.

• Hydroxyapatite: Mineral component (calcium phosphate) that gives hardness.

• Proteoglycans: Contribute to compressive strength.

Bone Marrow

Bone marrow is a soft tissue found within bone cavities.

• Red marrow: Site of hematopoiesis (blood cell formation); found in flat bones and ends of long bones in children and adults.

• Yellow marrow: Stores fat; found in the medullary cavity of long bones, especially in adults.

Types of Bone Tissue

Bones are composed of two main types of tissue:

• Compact bone: Dense, forms the outer layer of bones.

• Spongy (cancellous) bone: Porous, found at the ends of long bones and inside flat bones.

Epiphysis and Diaphysis

Long bones have distinct regions:

• Diaphysis: Shaft of the bone, composed mainly of compact bone.

• Epiphysis: Ends of the bone, composed mainly of spongy bone and covered by articular cartilage.

Bone Formation and Growth

Ossification

Ossification is the process of bone tissue formation. There are two main types:

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

• Endochondral ossification: Bone develops by replacing hyaline cartilage (e.g., long bones).

During ossification, osteoblasts deposit bone matrix, while osteoclasts resorb bone to shape and remodel it.

Bone Remodeling

Bone remodeling is a continuous process involving bone resorption and deposition, essential for growth, repair, and calcium homeostasis.

• Resorption: Osteoclasts break down bone tissue, releasing minerals into the blood.

• Deposition: Osteoblasts build new bone tissue.

This process occurs throughout life and is important for adapting bone structure to stress and repairing microdamage.

Growth in Length and Width

Bones grow in length at the epiphyseal plate (growth plate) through endochondral ossification. Growth in width occurs by appositional growth, where new bone is added to the surface.

• Epiphyseal plate: Region of cartilage where bone lengthening occurs until adulthood.

• Appositional growth: Osteoblasts add bone to the outer surface, increasing diameter.

Abnormal Growth Conditions

• Achondroplastic dwarfism: Genetic disorder affecting cartilage formation, resulting in short stature.

• Gigantism: Excessive growth due to overproduction of growth hormone in childhood.

• Acromegaly: Excessive growth hormone in adulthood, causing enlargement of bones.

Calcium Homeostasis and Hormonal Regulation

Key Hormones in Calcium Regulation

Calcium levels in the blood are tightly regulated by several hormones, which act on bones, kidneys, and intestines.

Hormone	What stimulates its release?	Effects on targets	Overall effect on blood calcium
PTH (Parathyroid Hormone)	Low blood calcium	Bone: Stimulates osteoclasts to resorb bone, releasing calcium. Kidney: Promotes reabsorption of calcium, reduces loss in urine. Effect on calcitriol: Stimulates formation of calcitriol (active vitamin D).	Increases blood calcium
Calcitonin	High blood calcium	Bone: Inhibits osteoclasts, stimulates osteoblasts to deposit bone. Kidney: Promotes loss of calcium in urine.	Decreases blood calcium
Calcitriol (Vitamin D)	Stimulated by PTH and low blood calcium	Intestine: Increases absorption of calcium from the diet.	Increases blood calcium

Effects of Hormonal Imbalance

• Hypercalcemia: Excess calcium in blood; can decrease neuromuscular excitability.

• Hypocalcemia: Low calcium in blood; increases neuromuscular excitability, risk of tetany.

Bone Diseases

• Rickets: Defective mineralization in children due to vitamin D deficiency.

• Osteoporosis: Loss of bone mass and density, increasing fracture risk.

• Osteogenesis imperfecta: Genetic disorder causing brittle bones.

Bone Repair

Steps in Fracture Repair

Bone repair is a multi-step process that restores bone integrity after injury.

1. Hematoma formation: Blood clot forms at the fracture site.

2. Fibrocartilaginous callus formation: Soft callus bridges the fracture.

3. Bony callus formation: Osteoblasts produce new bone, replacing the soft callus.

4. Bone remodeling: Bone is reshaped to its original form.

Additional Info

• Wolff's Law: Bone adapts to the mechanical stresses placed upon it; increased stress leads to increased bone density.

• Primary vs. Secondary Ossification Centers: Primary centers form in the diaphysis during fetal development; secondary centers form in the epiphyses after birth.