Unit 12B Skin SS

Structure and Function of the Skin

Integumentary Function

  • The skin, known as the integumentum, is among the body's most versatile organs, contributing approximately 16% of total body weight.

  • It serves as the primary interface between internal organs and the external environment.

  • Skin thickness can range from less than 1 mm to more than 5 mm.

  • It acts as the body's first line of defense, constantly exposed to potentially harmful environmental agents such as solid matter, gases, sunlight, and microorganisms.

  • The skin has remarkable properties, including a continuous cycle of shedding, healing, and cell regeneration.

  • Systemic diseases may manifest through skin disorders, such as:
        - Jaundice associated with liver disease.
        - Bronze skin in Addison’s disease.
        - Malar rash seen in systemic lupus erythematosus.

Skin Structures

  • The skin consists of three major layers:
       1. Epidermis: The outer layer.
       2. Dermis: The inner layer.
       3. Subcutaneous fat layer: A deeper layer of loose connective and fatty tissue.

  • The basement membrane separates the epidermis and dermis.

  • The subcutaneous tissue binds the dermis to underlying body tissues.

  • Variations in skin structure can occur in different body parts, making it challenging to define 'normal' skin. Variations include:
       - Thickness of skin layers.
       - Distribution and density of sweat glands.
       - Number and size of hair follicles.

Epidermis

  • Epidermis is the outermost layer that covers the body and is specialized in various areas to form skin appendages such as hair, nails, and glands.

  • Keratinocytes, the primary cells of the epidermis, produce keratin, a fibrous protein crucial for skin protection.

  • The epidermis contains three other cell types:
       - Melanocytes: Produce melanin, responsible for skin color and protection against ultraviolet radiation.
       - Merkel cells: Provide sensory information.
       - Langerhans cells: Link the epidermis to the immune system.

Keratinocytes

  • Keratinocytes constitute about 85% of the epidermis cells and consist of stratified squamous keratinized epithelium with five distinct layers:
       1. Stratum germinativum (stratum basale): A single layer of basal cells attached to the basal lamina, consisting of columnar cells undergoing mitosis to produce new keratinocytes that migrate towards the skin surface. These basal cells provide stability for the epidermis.
       2. Stratum spinosum: Formed by the descendants of basal cells, with 2-4 layers of cells that differentiate as they migrate outward. These cells often display a spiny appearance and are referred to as prickle cells.
       3. Stratum granulosum: Only a few cells thick (1-3 layers) and consists of granular cells that are the most differentiated skin cells. Some cells lose cytoplasm while others synthesize keratin.
       4. Stratum lucidum: A thin, transparent layer found only in thick skin areas, such as palms and soles, containing transitional cells retaining some functions of living cells below.
       5. Stratum corneum: The topmost layer consists of dead or keratinized cells ranging from 15-25 layers, sometimes exceeding 100 layers in specialized areas like the palms and soles.

  • Keratinization refers to the transformation of viable cells into dead cells of the stratum corneum, occurring during the keratinocyte migration to the skin's surface over about 20-30 days.

Morphological Changes of Keratinocytes

  • As keratinocytes move towards the outer layer, they undergo morphological changes:
       - Basal keratinocytes become flatter in the granular layer and elongated in the stratum corneum.

  • Keratinocytes connected by desmosomes maintain integrity during continual skin motion.

  • Other cellular junctions include:
       - Adherens junctions: Provide strong mechanical connections between cells.
       - Gap junctions: Allow ion and small molecule passage.
       - Tight junctions: Further strengthen connections between cells.

Melanocytes

  • Melanocytes are pigment-synthesizing cells scattered in the basal layer, responsible for skin color by producing pigment granules (melanin).

  • There are two major forms of melanin:
       1. Eumelanin: Brown and black; the most abundant form that protects against UV rays and is responsible for tanning and freckle formation.
       2. Pheomelanin: Yellow and red; concentrated in lips, nipples, glans penis, and vagina, prevalent in red-haired individuals and contributing to skin cancer susceptibility.

  • Melanin production is regulated by hormones and proteins stimulating melanocytes.

  • The enzyme tyrosinase is critical for melanin synthesis; its genetic deficiency leads to albinism.

  • Dark-skinned and light-skinned individuals have the same number of melanocytes, but melanin production and packaging differ significantly.

Merkel Cells

  • Merkel cells are specialized cells within the stratum basale of the epidermis, associated with sensory function and connected to afferent nerve terminals known as Merkel discs.

  • They serve as touch receptors and are more abundant in fingers, toes, lips, and the outer hair follicle shaft.

Langerhans Cells

  • Langerhans cells are a type of dendritic cell found in the suprabasal epidermis.

  • They originate from bone marrow precursors and continuously repopulate the epidermis.

  • Function as immunologic cells, recognizing foreign antigens and presenting them to the immune system.

  • Langerhans cells are innervated by sympathetic fibers, indicating a possible link to stress-related skin conditions.

Basement Membrane

  • The basement membrane is a matrix layer at the interface between the dermis and epidermis that separates the two layers and provides adhesion.

  • It plays a role in skin diseases involving blister formation and acts as a site for immunoglobulin and complement deposition.

  • Comprises three zones:
       1. Lamina lucida: An electron-lucent layer facilitating adherence to proteins.
       2. Lamina densa: Contains type IV collagen and specific adhesion materials.
       3. Lamina fibroreticularis: Contains anchoring microfibrils.

Dermis

  • The dermis is a connective tissue layer that separates the epidermis from the subcutaneous fat layer and supports epidermal nutrition.

  • Composed of two layers:
       1. Papillary dermis: The thin, vascularized layer rich in capillaries and nerve tissue, containing dermal papillae that reinforce epidermal-Dermal attachment.
       2. Reticular dermis: Thicker layer forming the bulk of the dermis, characterized by collagen bundles and elastic fibers in a rich ground substance.

  • Darker skin indicates a more compact dermis, correlating with reduced wrinkling.

Immune Cells

  • Immune cells in the reticular dermis include:
       - Dermal dendrocytes: Cells with phagocytic properties and antigen-presenting functions.
       - Macrophages: Present antigens to T cells.
       - T cells: Most are activated or memory T cells responsive to antigens.
       - Mast cells: Involved in immediate hypersensitivity reactions.

Blood Vessels

  • Skin is nourished by blood vessels forming two plexuses:
       - One between the dermis and subcutaneous tissue.
       - The second between the papillary and reticular layers.

  • Arteriovenous anastomoses facilitate temperature regulation through direct blood flow control.

Innervation

  • The skin has a complex innervation providing sensory information.

  • The dermis is well supplied with sensory neurons for detecting temperature, touch, and pain.

  • Specific receptors include:
       - Pacinian corpuscles: Detect pressure and vibrations.
       - Meissner corpuscles: Touch receptors, mainly in hairless skin areas.
       - Ruffini corpuscles: Respond to heavy pressure and joint movement.

Subcutaneous Tissue

  • The subcutaneous tissue layer consists primarily of fat cells and connective tissues, providing support for vascular and neural structures.

  • Thickness varies by location and contributes to immunological functions.

Skin Appendages

  • Various skin appendages include:
       - Sweat glands: Eccrine and apocrine types.
       - Sebaceous glands: Produce sebum for skin lubrication.
       - Hair: Produced by hair follicles in the dermis.
       - Nails: Hardened keratinized plates protecting fingers and toes.

Functions of the Skin

  • Skin serves multiple functions including:
       - Protection against environmental elements.
       - Regulation of moisture loss.
       - Immune regulation through various skin-associated lymphoid tissues.
       - Sensory functions and temperature regulation.
       - Endocrine function via vitamin D synthesis from UV rays.

Manifestations of Skin Disorders

  • Skin disorders are diverse and may vary in appearance and cause.

  • Characteristics may change due to factors like itching or infection.

Lesions and Rashes

  • Lesions: Traumatic or pathologic losses of normal skin structure.

  • Rashes: Temporary skin eruptions associated with various diseases or reactions.

  • Can vary in appearance, size, and color.

Pruritus

  • Pruritus: Sensation of itch as a common symptom across skin disorders, often linked to internal diseases.

  • Can induce scratching, leading to secondary skin issues.

Dry Skin

  • Dry skin (xerosis) can result from aging, environmental causes, or systemic diseases.

  • Often leads to discomfort, particularly in extremities and poses risks of fissuring or other skin maladies.

Skin Variations in Dark-Skinned Individuals

  • Melanin's production influences skin color, providing better UV protection in darker-skinned individuals, although skin cancer is still a risk.

  • Unique disorders among different racial and ethnic groups highlight the need for careful assessment to avoid misdiagnosis.