Bone Structure, Function, and Calcium Homeostasis: Study Notes for Anatomy & Physiology

Bone Structure and Function

Functions of the Bones of the Skeleton

• Support: Bones provide a rigid framework that supports the body and cradles soft organs.

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

• Movement: Bones act as levers for muscles to pull on, enabling movement.

• Mineral Storage: Bones store minerals, especially calcium and phosphorus, and release them into the bloodstream as needed.

• Blood Cell Formation: Hematopoiesis occurs in the red marrow of certain bones, producing red and white blood cells and platelets.

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

Types of Cells Found in Bone and Their Functions

• Osteoblasts: Bone-forming cells that secrete bone matrix and are responsible for bone deposition.

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

• Osteoclasts: Large, multinucleated cells that break down bone matrix (bone resorption).

• Osteoprogenitor Cells: Stem cells that differentiate into osteoblasts.

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, flattened, and usually curved (e.g., sternum, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

• Sesamoid Bones: Bones that form within tendons (e.g., patella).

Bone Matrix and Extracellular Matrix (ECM) Components

• Organic Components: Mainly collagen fibers, providing tensile strength and flexibility.

• Inorganic Components: Primarily hydroxyapatite (calcium phosphate crystals), providing hardness and resistance to compression.

Types of Bone Marrow

• Red Marrow: Site of hematopoiesis (blood cell formation); found in flat bones and the epiphyses of long bones in adults.

• Yellow Marrow: Primarily fat storage; found in the medullary cavity of long bones in adults.

Types of Bone Tissue

• Compact Bone: Dense outer layer; provides strength and protection.

• Spongy (Cancellous) Bone: Porous, inner layer; contains trabeculae and often houses red marrow.

Bone Anatomy: Diaphysis, Epiphysis, and Epiphyseal Plate

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

• Epiphysis: The ends of a long bone; composed mostly of spongy bone.

• Epiphyseal Plate (Growth Plate): Hyaline cartilage plate where bone growth occurs in children and adolescents.

Bone Formation and Growth

Ossification (Osteogenesis)

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

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue. This process forms most flat bones, such as those of the skull and clavicle.

• Endochondral Ossification: Bone forms by replacing hyaline cartilage. This process forms most bones of the skeleton, including long bones.

Primary and Secondary Ossification Centers

• Primary Ossification Center: The first area of a bone to start ossifying, usually in the diaphysis.

• Secondary Ossification Centers: Appear later in the epiphyses of long bones.

Bone Growth in Length and Width

• Length: Occurs at the epiphyseal plate through interstitial growth of cartilage, which is then replaced by bone.

• Width (Appositional Growth): Osteoblasts in the periosteum add new bone tissue to the surface, increasing diameter.

Bone Remodeling

• Continuous process of bone resorption (by osteoclasts) and deposition (by osteoblasts).

• Important for bone strength, repair, and calcium homeostasis.

Bone Resorption vs. Bone Deposition

• Bone Resorption: Breakdown of bone matrix by osteoclasts, releasing minerals into the blood.

• Bone Deposition: Formation of new bone matrix by osteoblasts.

Wolff's Law

• Bones grow and remodel in response to the forces placed upon them.

• Increased stress leads to stronger bones; lack of stress leads to bone loss.

Calcium Homeostasis and Hormonal Regulation

Key Hormones in Calcium Regulation

Hormone	What stimulates its release?	Effects on targets	Overall effect on blood calcium
PTH (Parathyroid Hormone)	Low blood calcium levels	Bone: Stimulates osteoclasts to resorb bone, releasing calcium. Kidney: Increases reabsorption of calcium, decreases loss in urine. Effect on Calcitriol: Stimulates formation of calcitriol (active vitamin D).	Increases blood calcium
Calcitonin	High blood calcium levels	Bone: Inhibits osteoclast activity, stimulates osteoblasts. Kidney: Increases excretion of calcium in urine.	Lowers blood calcium
Calcitriol (Vitamin D)	PTH stimulates its synthesis; low blood calcium	Intestine: Increases absorption of calcium from the gut. Bone: Promotes bone resorption when needed. Kidney: Reduces excretion of calcium.	Increases blood calcium

Effects of Hormone Dysregulation

• Excess PTH: Can lead to hypercalcemia, bone loss, and kidney stones.

• Excess Calcitonin: Rarely causes problems, but could lead to hypocalcemia if unchecked.

Normal Blood Calcium Levels and Effects of Imbalance

• Normal Range: Approximately 8.5–10.5 mg/dL.

• Hypercalcemia: Can cause muscle weakness, kidney stones, and decreased neuromuscular excitability.

• Hypocalcemia: Can cause muscle spasms, tetany, and increased neuromuscular excitability.

Bone Disorders and Diseases

Rickets

• Caused by vitamin D deficiency in children, leading to soft, weak bones and skeletal deformities.

Osteoporosis

• Characterized by decreased bone mass and increased fracture risk, often due to imbalance between bone resorption and deposition.

Osteogenesis Imperfecta (Brittle Bone Disease)

• Genetic disorder resulting in defective collagen production, leading to fragile bones.

Achondroplastic Dwarfism, Gigantism, Acromegaly

• Achondroplastic Dwarfism: Caused by abnormal cartilage formation, leading to short stature.

• Gigantism: Excess growth hormone in children, causing excessive bone growth.

• Acromegaly: Excess growth hormone in adults, causing thickening of bones.

Bone Fracture and Repair

Steps in Fracture Repair

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

2. Fibrocartilaginous Callus Formation: Soft callus of collagen and cartilage forms to bridge the gap.

3. Bony Callus Formation: Osteoblasts replace the soft callus with spongy bone.

4. Bone Remodeling: Compact bone replaces spongy bone, restoring the bone's original shape.

Additional info:

• Some questions in the original file prompt students to draw and label diagrams of bone growth and structure. For study purposes, students should be familiar with the anatomy of a long bone, including the periosteum, endosteum, medullary cavity, and the arrangement of compact and spongy bone.

• Students should also be able to compare and contrast the processes of intramembranous and endochondral ossification, and understand the role of hormones in bone growth and calcium regulation.