Bone Structure, Growth, and Disorders: Study Notes for Anatomy & Physiology

Bones: Classification, Structure, and Growth

Classification of Bones

Bones are classified based on their shapes and functions. The six main classifications are:

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

• Short bones: Cube-shaped, nearly equal in length and width (e.g., carpals, tarsals).

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

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

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

• Sutural (Wormian) bones: Small bones within cranial sutures.

Additional info: Classification helps in understanding bone function and location.

Main Parts of a Long Bone

• Diaphysis: Shaft of the bone.

• Epiphyses: Ends of the bone.

• Metaphysis: Region between diaphysis and epiphysis, contains growth plate.

• Medullary cavity: Central cavity containing bone marrow.

• Articular cartilage: Covers joint surfaces.

• Periosteum: Outer membrane covering the bone.

• Endosteum: Membrane lining the medullary cavity.

Location of Spongy and Compact Bone

• Spongy bone is found primarily in the epiphyses (ends) of long bones and inside flat bones.

• Compact bone forms the outer layer of all bones and the shaft (diaphysis) of long bones.

Distinguishing Features of Spongy Bone

• Contains a network of trabeculae (thin columns and plates).

• Spaces between trabeculae are filled with red bone marrow.

• Lighter and less dense than compact bone.

Osteocyte Location and Function

• Osteocytes reside in small spaces called lacunae within the bone matrix.

• They maintain bone tissue and communicate via canaliculi.

Bone Cells and Their Functions

• Osteogenic (osteoprogenitor) cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Build new bone matrix by secreting osteoid.

• Osteocytes: Mature bone cells that maintain bone tissue.

• Osteoclasts: Remove bone matrix by resorption.

Bone Membranes

• Periosteum: Tough outer membrane covering the bone except at joint surfaces.

• Endosteum: Thin membrane lining the medullary cavity.

Medullary Cavity

• The medullary cavity is the central hollow region in the diaphysis of long bones, containing yellow bone marrow (fat storage).

Bone Growth: Thickness and Length

• Appositional growth: Bone increases in thickness by addition of new bone tissue at the periosteum.

• Longitudinal growth: Bone lengthens at the epiphyseal plate (growth plate) through endochondral ossification.

Bone Formation Types

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

• Endochondral ossification: Bone forms by replacing a cartilage model (e.g., long bones).

Primary Steps in Endochondral Ossification

1. Development of cartilage model.

2. Growth of cartilage model.

3. Development of primary ossification center.

4. Development of medullary cavity.

5. Development of secondary ossification centers.

6. Formation of articular cartilage and epiphyseal plate.

Abnormalities of Bone: Discs and Disorders

• Intervertebral discs can degenerate, herniate, or become inflamed, leading to back pain and nerve compression.

• Osteomalacia: Softening of bones due to vitamin D deficiency in adults.

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

• Paget's disease: Abnormal bone remodeling, resulting in enlarged and misshapen bones.

Hormonal Regulation of Bone and Calcium

Growth Hormone (GH) Effects

• Hypersecretion of GH in childhood leads to gigantism.

• Hyposecretion of GH in childhood leads to dwarfism.

Primary Hormones Controlling Blood Calcium Levels

• Parathyroid hormone (PTH): Increases blood calcium by stimulating osteoclasts and increasing calcium reabsorption in kidneys.

• Calcitonin: Decreases blood calcium by inhibiting osteoclast activity.

• Vitamin D (calcitriol): Increases calcium absorption from the intestine.

Sites of Hormone Secretion

• PTH is secreted by the parathyroid glands.

• Calcitonin is secreted by the thyroid gland.

• Vitamin D is activated in the kidneys.

Bone Fractures: Types and Descriptions

Open vs. Closed Fractures

• Open (compound) fracture: Bone breaks through the skin.

• Closed (simple) fracture: Bone does not penetrate the skin.

Displaced vs. Non-Displaced Fractures

• Displaced fracture: Bone fragments are out of alignment.

• Non-displaced fracture: Bone fragments remain in correct alignment.

Complete vs. Incomplete Fractures

• Complete fracture: Bone is broken all the way through.

• Incomplete fracture: Bone is only partially broken.

Common Types of Fractures

Type of Fracture	Description
Transverse	Break is at a right angle to the bone's axis.
Oblique	Break has a curved or sloped pattern.
Spiral	Bone is twisted apart.
Comminuted	Bone is broken into several pieces.
Greenstick	Bone bends and cracks, common in children.
Compression	Bone is crushed, often in vertebrae.

Additional info: Fracture types are important for diagnosis and treatment planning.

Bone Disorders: Osteomalacia, Osteoporosis, and Paget's Disease

Osteomalacia

• Softening of bones due to defective bone mineralization.

• Commonly caused by vitamin D deficiency.

• Symptoms include bone pain and muscle weakness.

Osteoporosis

• Loss of bone density and mass, leading to fragile bones.

• Risk factors: aging, hormonal changes, lack of calcium/vitamin D.

• Common in postmenopausal women.

Paget's Disease

• Chronic disorder causing abnormal bone remodeling.

• Results in enlarged, deformed bones.

• May cause bone pain, arthritis, and fractures.