L-2 Integ Skin Pathology: Structure and Function

• Skin pathology lecture 2 objectives include understanding skin structure and function, discussing its vulnerabilities to pathology, and identifying macroscopic and microscopic features of skin pathology.

Skin Structure and Function

• The skin is the largest organ in the body and is part of the integumentary system.
• Key functions include protection, sensation, thermoregulation, communication, and self-repairing after injury.
• Containment prevents loss of body fluid.
• Protection is provided by keratinocytes, melanocytes, and collagen in the dermis.
• Langerhans cells and mast cells trigger an inflammatory response for immunologic features.
• Wound healing involves the basal layer of the epidermis responding to damage by proliferating to cover the site of injury, and fibroblasts producing scar formation.
• Merkel cells are responsible for sensation
• Thermoregulation is controlled by the skin transfer of body heat across body's surface

Skin Structures: Epidermis

• The epidermis is the outermost layer of the skin with five sublayers.
• Stratum corneum is the outermost layer of dead, protective cells.
• Stratum lucidum is present in thick skin.
• Stratum granulosum accumulates keratohyaline granules.
• Stratum spinosum is the "prickle cell layer" attached by desmosomes and tonofilaments.
• Stratum basale is mitotically active and attached to the basement membrane by hemidesmosomes.
• There are no blood vessels in the epidermis.

Epidermal Cells

• Keratinocytes are the most numerous cells in the epidermis.
• Melanocytes produce melanin and transfer melanosomes to keratinocytes.
• Melanocytes are derived from neural crest cells.
• Langerhans cells are antigen-presenting dendritic cells found in the stratum spinosum/basale.
• Merkel cells are associated with nerve endings and are responsible for tactile sensation (light touch).

Skin Structures: Dermis

• The dermis consists of dense fibroconnective collagen.
• The papillary dermis is adjacent to the epidermis and contains fine collagen while the reticular dermis is under the papillary dermis and contains course collagen.
• Elastic fibers provide mechanical strength.
• Ground substance serves as a substrate for diffusion.
• Mast cells, lymphocytes, and macrophages provide immune system surveillance.

Nerve Endings in the Skin

• Nerve endings provide sensory perception.
• Merkel's touch corpuscles are located at the base of the epidermis on the soles of feet and palms of hand.
• Meissner's corpuscles are light touch receptors in the dermal papillae of soles of feet and fingertips.
• Pacinian corpuscles are located deeper in the dermis and detect deep pressure.

Specialized Skin Structures

• Sweat glands (merocrine) are tubular glands for thermoregulation.
• Eccrine sweat glands are found over most of the body.
• Apocrine sweat glands are in the axilla, pubic, and perianal regions.
• Sebaceous glands (holocrine) are associated with hair follicles, nerve endings, and smooth muscle.
• Sebaceous glands secrete sebum (oil) for lubricating hair and skin.

Subcutaneous Layer

• The subcutaneous layer (hypodermis) is the deepest layer of the skin between the dermis and skeletal muscle.
• It's mostly composed of adipose tissue, fibroblasts, macrophages, and blood vessels.
• It functions as an energy reserve and provides attachment to bone and muscle, protection (padding).

Vulnerability of Skin

• Skin is a dynamic organ that is continuously renewing and altering itself in response to endogenous and exogenous stimuli.
• Vulnerability factors include aging, UV radiation damage, and genetic predisposition.
• Preventable factors include irritants, adhesive dressings, occlusion, and exposure to infection or allergens as well as aging and UV light causing loss of elasticity, hydration, and decreased cell turnover.
• Health care professionals need to be aware of problems with vulnerable skin in patients with wounds.

Disturbed Skin Barriers

• Disturbances can be caused by environmental hazards, physical trauma, and micro-organisms.
• Genetic disorders can cause aberrant skin barriers (atopic dermatitis).
• Bullous diseases and lowered mechanical strength/elasticity in aged skin can contribute or be caused by disturbed skin barriers.
• Disruptions can lead to increased transepidermal water loss, loss of calcium gradient, and cutaneous immune reactions, as well as skin failure resulting in loss of temperature control and loss of protein/electrolytes.

Skin Diseases

• Skin barriers are compromised by skin disease.
• Ichthyosis involves defects in lipid metabolism or protein components in the stratum corneum.
• Tuberous sclerosis involves growth of noncancerous tumors of the skin, brain, and kidneys.
• Inflammatory skin diseases disturb normal cell turnover, maturation, and function.
• Eczema, in both acute and chronic forms, can cause vesicular lesions that lead to ulcerations and erosions.
• Desquamative inflammatory dermatoses, such as seborrheic dermatitis and psoriasis, reduce the effectiveness of the skin barrier.
• Accumulations of debris, sebum, and topical medication remnants induce inflammation.
• Moisture retention leads to colonization by fungi, yeasts, and bacteria, causing inflammatory skin infections.

Common Skin Pathologies

• Acute inflammation includes urticaria, acute eczema, and erythema multiforme.
• Chronic inflammation includes psoriasis, chronic eczema, and lichen planus.
• Infections include bacterial (impetigo), viral (warts), and fungal (tinea) infections.
• Blistering diseases include pemphigus, pemphigoid, and dermatitis herpetiformis.
• Benign neoplasms include nevi, actinic keratosis, and seborrheic keratosis.
• Malignant neoplasms include basal cell carcinoma, squamous carcinoma, and melanoma.

Specific Skin Conditions

• Urticaria is characterized by lymphocytic infiltrate with vasculitis.
• Eczema's histopathology shows spongiosis, vasculitis, and lymphocytic and eosinophilic dermal infiltrate.
• Psoriasis histopathology includes parakeratosis, hyperkeratosis, acanthosis, and Munroe's micro abscesses.
• Impetigo, a bacterial infection, causes ulceration and acute inflammation.
• Seborrheic keratosis is characterized by horn cysts, acanthosis, and hyperkeratosis.