Integumentary System

Functions of Skin

The skin is the largest organ of the body and serves as a critical barrier between internal tissues and the external environment. Its functions are essential for maintaining homeostasis and protecting the body.

• Protection: Acts as a physical, chemical, and biological barrier against pathogens, mechanical injury, and harmful substances.

• Body Temperature Regulation: Regulates heat loss through sweat production and blood vessel dilation or constriction.

• Cutaneous Sensations: Contains sensory receptors for touch, pain, pressure, and temperature.

• Metabolic Functions: Synthesizes vitamin D when exposed to sunlight; participates in other metabolic processes.

• Blood Reservoir: Stores up to 5% of the body's blood volume, which can be redirected as needed.

• Excretion: Eliminates waste products such as urea, ammonia, and salts through sweat.

Skin Cancer

Skin cancer arises from uncontrolled cell growth in the skin. Most skin tumors are benign and do not metastasize, but malignant forms can spread and become life-threatening. Risk factors include overexposure to ultraviolet (UV) radiation and frequent skin irritation.

• Basal Cell Carcinoma:

  • Least malignant and most common (about 80% of cases).

  • Originates from stratum basale cells, which invade the dermis and subcutaneous tissue.

  • Usually cured by surgical excision (99% success rate).

• Squamous Cell Carcinoma:

  • Second most common; can metastasize.

  • Involves keratinocytes of the stratum spinosum.

  • Presents as a scaly, reddened papule, often on scalp, ears, lower lip, or hands.

  • Good prognosis if treated early by radiation or surgery.

• Melanoma:

  • Cancer of melanocytes; most dangerous due to high metastatic potential and resistance to chemotherapy.

  • Accounts for about 1% of skin cancer cases.

  • Treated by wide surgical excision and radiation therapy.

Burns

Burns are injuries to skin tissue caused by heat, electricity, radiation, or chemicals. The primary damage is due to protein denaturation, which destroys cells. The most immediate threat is dehydration and electrolyte imbalance.

• Rule of Nines:

  • Used to estimate the percentage of body surface area affected by burns.

  • The body is divided into 11 sections, each representing 9% of the total surface area (genitals account for 1%).

  • Helps estimate fluid loss and guide treatment.

• Classification by Severity:

  • First-degree: Affects only the epidermis; symptoms include localized redness, swelling (edema), and pain.

  • Second-degree: Damages the epidermis and upper dermis; blisters appear. Both first- and second-degree burns are called partial-thickness burns.

  • Third-degree: Involves the entire thickness of the skin (full-thickness burns); skin may appear gray-white, cherry red, or blackened. Little or no edema, and the area may not be painful due to destroyed nerve endings. Skin grafting is usually necessary.

Developmental Aspects of the Integumentary System

Embryo to Fetus

The skin develops from distinct embryonic layers and undergoes several changes before birth.

• Epidermis: Develops from the embryonic ectoderm.

• Dermis and Subcutaneous Tissue: Develop from the mesoderm.

• By the 4th month, the skin is well developed.

• During the 5th and 6th months, the fetus is covered with a lanugo coat (delicate, colorless hairs), which is shed by the 7th month as vellus hairs grow.

Infancy to Adulthood

• Skin thickens and accumulates more subcutaneous fat.

• Sweat and sebaceous gland activity increases, often leading to acne.

• Optimal skin appearance occurs during the 20s and 30s.

• After age 30, cumulative environmental damage becomes apparent, with increased scaling and dermatitis (skin inflammation).

Bones and Skeletal Tissue

Cartilage Structure, Types, and Locations

The human skeleton initially consists of cartilage, which is gradually replaced by bone except in areas requiring flexibility. Cartilage is made of chondrocytes within lacunae, surrounded by a jelly-like extracellular matrix.

• Hyaline Cartilage:

  • Most abundant; provides support, flexibility, and resilience.

  • Contains only collagen fibers.

  • Locations: articular (joints), costal (ribs), respiratory (larynx and passageways), nasal (external nose).

• Elastic Cartilage:

  • Similar to hyaline but contains elastic fibers.

  • Locations: external ear, epiglottis.

• Fibrocartilage:

  • Thick collagen fibers provide tensile strength.

  • Locations: menisci of knee, pubic symphysis, intervertebral discs.

Growth of Cartilage

Cartilage grows by two mechanisms, both important during development and healing.

• Appositional Growth: New matrix is laid down on the surface by cells in the perichondrium.

• Interstitial Growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

• Cartilage growth ends during adolescence.

Functions of Bones

Bones are dynamic organs with several vital functions:

• Support: Framework for the body and soft organs.

• Protection: Shields the brain, spinal cord, and vital organs.

• Anchorage: Acts as levers for muscle action.

• Mineral Storage: Stores calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis occurs in red bone marrow.

• Triglyceride Storage: Fat stored in bone cavities serves as an energy source.

• Hormone Production: Osteocalcin regulates insulin secretion, glucose homeostasis, and energy expenditure.

Classification of Bones

By Location

• Axial Skeleton: Long axis of the body; includes skull, vertebral column, and rib cage.

• Appendicular Skeleton: Bones of upper and lower limbs, and girdles attaching limbs to axial skeleton.

By Shape

• Long Bones: Longer than wide, with a medullary cavity; e.g., most limb bones.

• Short Bones: Cube-shaped; e.g., wrist and ankle bones.

• Flat Bones: Thin, flat, slightly curved; e.g., sternum, ribs, cranial bones.

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

Bone Structure

Bones are organs composed of several tissue types. Their structure can be described at three levels: gross, microscopic, and chemical.

• Compact Bone: Dense outer layer that appears smooth and solid.

• Spongy Bone: Honeycomb of small, needle-like or flat pieces called trabeculae; spaces filled with red or yellow bone marrow.

Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone covered by compact bone.

• Compact bone is sandwiched between connective tissue membranes: periosteum (outside) and endosteum (inside).

• Bone marrow is scattered throughout spongy bone; no defined marrow cavity.

Structure of a Typical Long Bone

• Diaphysis: Tubular shaft forming the long axis.

• Epiphyses: Bone ends, consisting of compact bone externally and spongy bone internally.

• Epiphyseal Line: Remnant of the childhood epiphyseal plate where bone growth occurred.

• Membranes:

  • Periosteum: Double-layered membrane covering external surfaces except joints.

    • Fibrous layer: Dense irregular connective tissue with Sharpey's fibers anchoring it to bone matrix.

    • Osteogenic layer: Contains stem cells that give rise to most bone cells.

  • Endosteum: Delicate connective tissue membrane covering internal bone surfaces and trabeculae of spongy bone; contains osteogenic cells.

Hematopoietic Tissue in Bones

• Red bone marrow is found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum).

• In newborns, medullary cavities and all spongy bone contain red marrow.

• In adults, red marrow is located in the heads of femur and humerus, and most active in flat bone diploë and some irregular bones (e.g., hip bone).

Bone Markings

Bones have distinctive markings that serve as sites for muscle, ligament, and tendon attachment, joint formation, or conduits for blood vessels and nerves. Bone markings are classified into three categories:

• Projections: Sites of muscle and ligament attachment.

• Surfaces: Form joints.

• Depressions and Openings: Allow passage for blood vessels and nerves.

Table: Main Categories of Bone Markings

Additional info: Bone markings are essential for anatomical identification and clinical reference, especially in radiology and surgery.