Skin: Pathology and Clinical Assessment

Major Types of Skin Cancer

Skin cancer is the uncontrolled growth of abnormal skin cells. The three major types are classified based on the cell of origin and their clinical behavior.

• Basal Cell Carcinoma (BCC): Originates from basal cells in the epidermis. It is the most common and least aggressive type, rarely metastasizing.

• Squamous Cell Carcinoma (SCC): Arises from squamous cells. It is more likely to invade deeper tissues and can metastasize if untreated.

• Melanoma: Develops from melanocytes, the pigment-producing cells. It is the most dangerous type due to its high potential for metastasis.

Example: Melanoma often appears as a new or changing mole with irregular borders and coloration.

Burns: Severity and Clinical Implications

Burns are injuries to the skin and underlying tissues caused by heat, chemicals, electricity, or radiation. Serious burns are life threatening due to fluid loss, infection risk, and impaired temperature regulation.

• First-degree burns: Affect only the epidermis; symptoms include redness and mild pain. Healing occurs without scarring.

• Second-degree burns: Involve the epidermis and part of the dermis; symptoms include blistering, severe pain, and possible scarring.

• Third-degree burns: Extend through the dermis into deeper tissues; skin may appear white, charred, or leathery. Nerve endings are destroyed, so pain may be less than expected. Require medical intervention and often result in significant scarring.

Why Serious Burns Are Life Threatening:

• Loss of protective skin barrier leads to increased risk of infection.

• Severe fluid loss can cause hypovolemic shock.

• Impaired temperature regulation and metabolic disturbances.

Estimating Burn Extent: The Rule of Nines

The "rule of nines" is a method used to estimate the percentage of body surface area affected by burns, aiding in clinical assessment and treatment planning.

Example: A burn involving the entire left leg and anterior trunk would cover 36% of the body surface area.

Bones: Structure, Function, and Growth

Bone Classes: Structure and Examples

Bones are classified based on their shapes and functions. The four major classes are:

Functions of Bones

Bones perform several vital functions in the human body:

• Support: Provide structural framework for the body.

• Protection: Shield vital organs (e.g., skull protects brain).

• Movement: Serve as levers for muscles.

• Mineral Storage: Store calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis occurs in red marrow.

• Triglyceride Storage: Fat stored in yellow marrow.

• Hormone Production: Osteocalcin regulates bone formation and energy metabolism.

Anatomy of Long and Flat Bones

Bone anatomy can be described at both gross and microscopic levels.

• Long Bone Gross Anatomy: Consists of a diaphysis (shaft), epiphyses (ends), metaphyses, articular cartilage, periosteum (outer membrane), and medullary cavity (contains marrow).

• Flat Bone Gross Anatomy: Composed of two thin layers of compact bone enclosing a layer of spongy bone (diploë).

• Microanatomy: Includes osteons (Haversian systems) in compact bone, trabeculae in spongy bone, and various bone cells (osteocytes, osteoblasts, osteoclasts).

Example: The femur is a long bone with a central medullary cavity and a thick periosteum.

Bone Formation: Intramembranous vs. Endochondral Ossification

Bone development occurs via two main processes:

• Intramembranous Ossification: Bone forms directly from mesenchymal tissue. Occurs mainly in flat bones (e.g., skull, clavicle).

• Endochondral Ossification: Bone replaces a hyaline cartilage model. Responsible for formation of most bones, especially long bones.

Comparison Table:

Bone Growth: Longitudinal and Appositional

Bones grow in length and thickness through distinct processes:

• Longitudinal Growth: Occurs at the epiphyseal plate via endochondral ossification. Chondrocytes proliferate, hypertrophy, and are replaced by bone tissue.

• Appositional Growth: Bone increases in diameter as osteoblasts add new bone to the periosteum while osteoclasts resorb bone from the endosteum.

Equation:

Regulation of Bone Remodeling

Bone remodeling is a continuous process regulated by hormones and mechanical stress.

• Hormonal Regulation: Parathyroid hormone (PTH) increases blood calcium by stimulating osteoclasts; calcitonin lowers blood calcium by inhibiting osteoclasts; growth hormone and sex hormones stimulate bone growth.

• Physical Stress: Mechanical loading (exercise, weight-bearing) stimulates bone formation via osteoblast activity (Wolff's Law).

Example: Athletes have denser bones due to increased mechanical stress.

Additional info: Vitamin D is essential for calcium absorption and bone health.