Skin Biology Quiz

Skin Structure and Function

• The skin is the largest organ of the body, accounting for 15-20% of total body weight.
• It consists of three layers: epidermis, dermis, and hypodermis.
• Dermal papillae are projections located at the junction between the dermis and epidermis. They interdigitate with epidermal ridges to strengthen adhesion between dermis and epidermis

Skin Properties

• Dermatoglyphs (fingerprints) are dermal-epidermal interdigitations forming a unique pattern for each individual.
• Skin is elastic and expands rapidly to accommodate swollen areas.
• Skin continually self-renews throughout life.

Skin Functions

• Protection: Acting as a physical barrier against thermal and mechanical damage (e.g., friction). Protects against potential pathogens and foreign materials through macrophages and antigen-presenting cells. Skin protects against UV rays with melanin, and forms a permeability barrier preventing excessive water loss or uptake. Allows for administration of lipophilic drugs, steroids, hormones, and medications.

• Sensory: Constantly monitors the surroundings and mechanoreceptors regulate body's interaction with physical objects

• Thermoregulation: Maintains constant body temperature through skin's insulating components (fatty layer and hair) and mechanisms that accelerate heat loss (sweat production & dense superficial microvasculature).

• Metabolic: Synthesis of Vitamin D3 through the local action of UV light on vitamin D precursor. Vitamin D3 is needed for calcium metabolism and proper bone formation. Excess electrolytes are removed through sweat. Fat cells store fat.

• Sexual Signaling: Pheromones produced by apocrine sweat glands.

Epidermis Layers

• Basal Layer/ Stratum Basale: A single layer of basophilic cuboidal or columnar cells. Hemidesmosomes bind these cells to the basal lamina, and desmosomes bind the cells laterally and apically. High mitotic activity and contains progenitor cells for the epidermal layers. Keratinocytes contain cytoskeletal keratins, with increasing amount and type as the cells move upward.

• Spinous Layer/ Stratum Spinosum: Thickest layer of cells, especially in epidermal ridges. Consist of polyhedral cells with central nuclei and cytoplasm actively synthesizing keratins. Cells are still dividing here, in stratum germinativum. Intercellular bridges form between cells. "Spines" or prickles are increased in regions of high friction, such as the soles of your feet

• Granular Layer/ Stratum Granulosum: Three to five layers of flattened cells undergoing keratinization. Cytoplasm filled with keratohyaline granules and filaggrin which forms structures. Lamellar granules undergo exocytosis at the terminal stage of keratinization, producing a lipid-rich, impermeable layer around the cells.

• Stratum Lucidum: Found only in thick skin (palms, soles of feet), thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes. Nuclei and organelles are lost. Cytoplasm consists almost exclusively of packed keratin.

• Stratum Corneum: Consists of 15-20 layers of keratinized, squamous cells filled with keratin. Fully keratinized cells, called squames, are continuously shed.

Cells of the Epidermis

• Melanocytes: Specialized cells in the epidermis found among the cells of the basal layer and in hair follicles. Produce eumelanin (brown/black pigments) and pheomelanin (red pigment in red hair). Melanocytes migrate to the stratum basale in a 1:6 ratio. Pale-staining round cell bodies attached to basal lamina by hemidesmosomes. Melanocytes synthesize melanin-containing granules.

• Langerhans Cells: Antigen-presenting cells derived from monocytes. Make up 2-8% of epidermal cells and are primarily in the spinous layer. Cytoplasmic processes extend between keratinocytes to bind, process, and present antigens to T-lymphocytes.

• Merkel Cells: Epithelial tactile cells (mechanoreceptors) specialized in gentle touch. Abundant in areas with high sensitivity. Located in the stratum basale, attached to keratinocytes by desmosomes. Have few melanocytes but abundant Golgi-derived dense-core granules and make synaptic contacts with nerves located at the basal lamina.

Dermis

• A layer of connective tissue supporting the epidermis and binding it to hypodermis.

• Dermal papillae extend into epidermis to form dermal-epidermal junction.

• Dermis provides nutrients through its rich vasculature to epidermis through basement membrane.

• Consists of two layers: papillary and reticular.

• Papillary Dermis: Includes the dermal papillae. Composed of loose connective tissue with type I and III collagen fibers, fibroblasts, mast cells, dendritic cells, and leukocytes. Anchoring fibrils of type IV collagen attach the dermis to the epidermis.

• Reticular Dermis: Composed of dense irregular connective tissue. Denser fibers than papillary dermis. Contains network of elastic fibers, providing elasticity, and rich in dermatan sulfate (a proteoglycan of connective tissue)

Skin Nutrition and Innervation

• Subpapillary Plexus: Network of blood vessels and nerves located between the papillary and reticular dermis.
• Larger Plexus: Location between the reticular dermis and subcutaneous tissue. Thermoregulatory function and arterivorenous anastomoses/shunts
• Sensory Afferent Nerve Fibers: Forms a network in the papillary dermis and around hair follicles (responding to stimuli)
• Autonomic Efferent Nerve Fibers: Regulate sweat glands and smooth muscles in the skin.

Hypodermis

• Sometimes called subcutaneous layer or superficial fascia, loose adipose connective tissue. Binds skin loosely to underlying muscle tissue. Contains adipocytes and thin connective tissue fibers. Extensive blood vessels supply support rapid uptake injected drugs.

Cutaneous Sensory Receptors

• Skin functions as an extensive receiver of stimuli from the environment. Diverse encapsulated and unencapsulated receptors are present.

• Unencapsulated Receptors: Include Merkel cells (light touch and texture), free nerve endings (temperature, pain, itch), and root hair plexuses (hair movement).

• Encapsulated Receptors: Include Meissner's corpuscles (light touch, low-frequency stimuli), Pacinian corpuscles (pressure, vibrations), Ruffini corpuscles (stretch, tension, twisting), and Krause end bulbs (low-frequency vibrations).

Epidermal Appendages

• Hair: Keratinized structures within epidermal invaginations (hair follicles). Grows discontinuously with phases of growth and rest.

• Parts: Hair bulb, hair root, and hair shaft

• Hair Papilla: Consists of dermal hair papilla penetrating the base of the hair bulb, containing a vascular supply for cells.

• Hair Follicle: Connective tissue root sheath, glassy membrane, epithelial tissue root sheath, internal and external root sheaths.

• Medulla, Cortex, Cuticle: Parts of hair follicle structure

• Arrector pili muscle: A smooth muscle bundle extending from the midpoint of the hair sheath to dermal papillary layer. Contractions of arrector pili muscle causes hair to raise, providing insulation.

• Nails: Keratinized hard plates formed on dorsal surface of distal phalanx. Nail root, cuticle, nail bed and nail matrix parts

• Sebaceous glands: Branched acinar glands in the dermis, except palms/soles, empty into hair follicles. Contain sebocytes producing sebum and a mixture of lipids and components. Maintain stratum corneum, hair shaft, function as a weak antibacterial / antifungal agent.

• Sweat glands: Long epidermal invaginations in the dermis. Eccrine sweat glands: widely distributed across the body, produce sweat as a physiological response to temperature change. Coiled secretory and duct structures. Secretion of watery sweat and composed of clear (basal lamina), dark (eosinophilic granules) and myoepithelial cells (move sweat to duct). Duct contains two layers of acidophilic cells and has cell membranes rich in Na-K-ATPase to absorb Na+ from secreted water.

• Apocrine sweat glands: Confined to axillary and perineal regions, develop functionally after puberty. More viscous secretion which often gains odor due to bacterial activity. May contain pheromones.