Study Guide: Bone Structure, Function, and Calcium Homeostasis

Bone Structure and Function

Functions of the Bones of the Skeleton

The bones of the skeleton serve several essential functions in the human body:

• Support: Provides a rigid framework that supports the body and maintains its shape.

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

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

• Mineral Storage: Stores minerals such as calcium and phosphorus, which can be released into the bloodstream as needed.

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

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

Types of Cells Found in Bone and Their Functions

• Osteoblasts: Cells responsible for bone formation by producing new bone matrix.

• Osteocytes: Mature bone cells that maintain bone tissue and communicate with other bone cells.

• Osteoclasts: Cells that break down bone tissue (bone resorption), helping in bone remodeling and calcium release.

• Bone lining cells: Flat cells on bone surfaces involved in maintenance and nutritional support.

Types of Bones Based on Shape

• 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, ribs, sternum).

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

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

Bone Matrix and Extracellular Components

The extracellular matrix (ECM) of bone consists of:

• Collagen fibers: Provide tensile strength and flexibility.

• Hydroxyapatite crystals: Calcium phosphate-based minerals that give bone its hardness.

• Proteoglycans and glycoproteins: Contribute to the structure and function of the matrix.

The ECM is crucial for bone strength, flexibility, and mineral storage.

Bone Marrow: Location and Function

• Red bone marrow: Found in the spongy bone of flat bones and the ends of long bones; responsible for hematopoiesis (blood cell production).

• Yellow bone marrow: Found in the medullary cavity of long bones; stores fat.

Types of Bone Tissue

• Compact bone: Dense, strong outer layer of bone; provides strength and protection.

• Spongy bone (cancellous bone): Porous, inner layer; contains red bone marrow and supports hematopoiesis.

Bone Anatomy: Diaphysis, Epiphysis, and Arrangement

• Diaphysis: Shaft of a long bone; composed mainly of compact bone.

• Epiphysis: Ends of a long bone; composed mainly of spongy bone and contains red marrow.

• Metaphysis: Region between diaphysis and epiphysis; contains the growth plate in children.

Bone Formation and Growth

Ossification: Types and Processes

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

• Intramembranous ossification: Bone develops directly from sheets of mesenchymal connective tissue. Occurs mainly in flat bones (e.g., skull, clavicle).

• Endochondral ossification: Bone develops by replacing hyaline cartilage. Occurs in most bones, especially long bones.

During ossification, osteoblasts deposit bone matrix, while osteoclasts resorb bone tissue as needed for remodeling.

Bone Growth and Remodeling

• Longitudinal growth: Occurs at the epiphyseal plate (growth plate) in children and adolescents.

• Appositional growth: Increase in bone thickness; occurs throughout life.

• Remodeling: Continuous process of bone resorption and formation to maintain bone strength and mineral homeostasis.

Primary and Secondary Ossification Centers

• Primary ossification center: First area of bone formation, usually in the diaphysis.

• Secondary ossification center: Develops later in the epiphyses.

Cartilage Transformation in Bone Growth

During endochondral ossification, cartilage is gradually replaced by bone tissue as the skeleton matures.

Bone Disorders Related to Growth

• Achondroplastic dwarfism: Genetic disorder affecting cartilage formation, leading to short stature.

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

• Acromegaly: Excessive growth hormone in adulthood, causing enlarged bones of hands, feet, and face.

Calcium Homeostasis and Hormonal Regulation

Key Hormones in Calcium Regulation

Calcium levels in the blood are tightly regulated by three main hormones:

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: Increases loss of calcium in urine.	Decreases blood calcium
Calcitriol (Vitamin D)	PTH stimulates its formation	Intestine: Increases absorption of calcium from the diet.	Increases blood calcium

Effects of Hormonal Imbalance

• If PTH release does not stop after calcium homeostasis is restored, blood calcium may become excessively high (hypercalcemia), leading to muscle weakness, kidney stones, and neurological symptoms.

• If calcitonin release does not stop, blood calcium may become abnormally low (hypocalcemia), causing muscle spasms and cardiac arrhythmias.

Normal Blood Calcium Levels and Effects of Imbalance

• Normal range: 8.5–10.5 mg/dL

• Hypercalcemia: Excess calcium; can depress neuromuscular activity.

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

Bone Remodeling and Repair

Importance of Bone Remodeling

Bone remodeling is essential for:

• Maintaining bone strength and integrity

• Adapting to mechanical stress

• Regulating calcium and phosphate levels

Steps in the Repair of a Fracture

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

2. Fibrocartilaginous callus formation: Soft callus of collagen and cartilage forms.

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

4. Bone remodeling: Bone is reshaped to restore its original structure.

Bone Diseases

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

• Osteoporosis: Decreased bone mass and increased fracture risk, common in older adults.

• Osteogenesis imperfecta: Genetic disorder causing brittle bones due to defective collagen.

Wolff's Law of Bone

Wolff's Law states that bone grows and remodels in response to the mechanical stresses placed upon it. Increased stress leads to increased bone density and strength.

Additional Info

• Epiphyseal plate: Site of bone growth in length during childhood and adolescence.

• Bone resorption: Process by which osteoclasts break down bone tissue, releasing minerals into the blood.

• Bone deposition: Process by which osteoblasts build new bone tissue.