Chapter_5 The Skin

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Title: Skin.

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Integumentary System

  • Components:

    • Skin

    • Hair

    • Nails

    • Sweat glands

    • Sebaceous (oil) glands

  • Functions:

    • Primarily protective roles against pathogens and dehydration.

    • Maintains boundaries of the body.

  • Weight: Approximately 9-11 lbs.

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Structure of Skin

  • Two distinct regions:

    • Epidermis:

      • Superficial, avascular (lacks blood vessels)

    • Dermis:

      • Deep, vascular (rich in blood vessels)

  • Components:

    • Hair shaft

    • Dermal papillae

    • Sweat pore

    • Appendages of skin:

      • Eccrine sweat gland

      • Arrector pili muscle

      • Sebaceous gland

      • Hair follicle

      • Hair root

  • Layers of skin:

    • Hypodermis: not part of skin, contains adipose tissue.

    • Nervous structures include sensory nerve fiber, lamellar corpuscle, hair follicle receptor.

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Hypodermis

  • Location: Deep to the skin.

  • Function:

    • Vascular and primarily composed of adipose tissue.

    • Absorbs shock and insulates.

    • Anchors the skin to underlying structures, primarily muscles.

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Cells of the Epidermis

  • Keratinized tissue type: keratinized epithelium.

  • Four cell types:

    • Keratinocytes:

      • Produce fibrous keratin, the main component that protects skin.

      • Millions slough off daily; new epidermis every month.

    • Melanocytes:

      • Located in the deepest epidermis; produce melanin which provides pigmentation.

    • Langerhans cells:

      • Function as immune cells in skin.

    • Nerve Receptors:

      • Act as sensory receptors responding to stimuli.

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Layers of the Epidermis

  • Total Layers: Four or five distinct layers depending on skin type:

    • Thick skin: Found in high-abrasion areas like palms and soles.

    • Thin skin: Present on most of the body.

  • Strata:

    • Stratum corneum: Most superficial, composed of flattened dead cells filled with keratin.

    • Stratum granulosum: Flattened cells with organelles deteriorating, containing granules.

    • Stratum spinosum: Several layers with keratin bundles, begins keratin synthesis.

    • Stratum basale: Deepest layer, high mitotic activity; contains stem cells and melanocytes.

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Stratum Basale

  • Characteristics:

    • Deepest layer, attached to the dermis by a basement membrane.

    • One row of mitotic stem cells; some cells progress towards surface, taking 25-45 days.

    • Cells undergo differentiation as they ascend.

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Stratum Spinosum

  • Structure: Several layers thick.

  • Composition: Cells contain a network of intermediate prekeratin filaments.

  • Function: Synthesis of pre-keratin begins here.

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Stratum Granulosum

  • Thickness: Four to six cells thick.

  • Change in Cells: Cells begin to flatten and loss of organelles occurs.

  • Coating: Cells are coated with keratin-like granules.

  • Nutritional Access: Cells in this layer and above are too far from dermal blood supply.

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Stratum Lucidum

  • Location: Only found in thick skin.

  • Description: Layer consists of thin, translucent band of dead keratinocytes.

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Stratum Corneum

  • Description: Known as the "horny layer".

  • Structure: 20-30 layers of flat, keratinized, anucleated cells;

    • Comprises three-quarters of the epidermal thickness.

  • Functions:

    • Protect deeper cells, prevent water loss, protect from abrasion, and act as barrier against various assaults.

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Dermis

  • Function: Supports the epidermis.

  • Composition: Strong and flexible connective tissue containing nerves, blood vessels, lymphatic vessels, sweat glands, and hair follicles.

  • Layers:

    • Papillary layer: Contains dermal papillae.

    • Reticular layer: Provides strength and elasticity.

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Papillary Layer

  • Description: Superficial layer of areolar connective tissue; accounts for 20% of the dermis.

  • Features:

    • Dermal papillae contain capillary loops (function to supply blood).

    • Contains free nerve endings (function in sensation).

    • Contains tactile corpuscles (touch receptors).

  • Functionality: Increases surface area for exchange and sensation.

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Reticular Layer

  • Description: Makes up about 80% of dermal thickness.

  • Structure: Comprised of dense irregular connective tissue.

  • Features: Contains the cutaneous plexus (network of blood vessels) and hair follicles.

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Hair

  • Structure: Composed of dead keratinized cells.

  • Locations: Absent on palms, soles, lips, nipples, and certain areas of external genitalia.

  • Functions:

    • Warn of insects on the skin.

    • Protect against physical trauma, heat loss, and shield skin from sunlight.

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Regions of Hair

  • Shaft: Area extending above the scalp, complete keratinization.

  • Root: Area located within the scalp, where keratinization is ongoing.

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Anatomy of Hair

  1. Origin: Strands of hair originate in a hair follicle, which penetrates the dermis.

  2. Shaft: The visible part of hair not anchored to the follicle.

  3. Root: Anchored part of hair within the scalp; ends deep in the dermis at the hair bulb.

  4. Hair bulb: Surrounds the papilla, composed of connective tissue with blood vessels and nerve endings.

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Structure of Hair

  • Core: The central core of hair is called the medulla.

  • Surrounding Layer: The cortex is a layer of compressed keratinized cells.

  • Outer Layer: The outer layer is known as the cuticle, made of hard keratinized cells.

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Sweat Glands

  • Distribution: Present on all skin surfaces except for nipples and parts of external genitalia; about 3 million per person.

  • Types:

    • Eccrine (merocrine): Most numerous; tubules in dermis connecting to pores.

    • Apocrine: Found in axillary and anogenital areas.

  • Function: Contract upon nervous stimulation to release sweat into ducts.

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Eccrine Sweat Glands

  • Type: Merocrine glands; most numerous.

  • Location: Abundant on palms, soles, and forehead.

  • Secretion: Composed of 99% water, salts, and antibodies; maintains acidic pH known as acid mantle.

  • Functions: Play a role in body temperature regulation and waste elimination.

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Apocrine Sweat Glands

  • Location: Confined to axillary and anogenital areas.

  • Secretion: Viscous, yellowish sweat containing fatty substances/proteins.

  • Function: Bacteria break down sweat, leading to body odor.

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Ceruminous Gland

  • Type: Specialized sweat gland.

  • Location: Found in the external ear canal (outer 1/3).

  • Secretion: Produces cerumen (earwax), which protects the ear canal.

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Sebaceous (Oil) Glands

  • Distribution: Widely distributed except in thick skin.

  • Function: Secrete oil into hair follicles, which softens and lubricates hair and skin; exhibits anti-bacterial properties.

  • Hormonal Influence: Inactive until puberty when stimulated by hormones; associated with acne development.

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Functions of Skin

  • Primary Role: Acts as a barrier against environmental factors.

  • Main Functions:

    • Protection

    • Regulation of body temperature

    • Sensation

    • Metabolic functions (e.g., vitamin D synthesis)

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Protection

  • Environmental Threats: Skin protects against microorganisms, mechanical abrasions, temperature extremes, and harmful chemicals.

  • Barrier Types:

    • Chemical barrier: Involves secretion of sweat and other chemicals.

    • Physical barrier: Physical integrity of skin structure.

    • Biological barrier: Immune functioning through various cells.

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Chemical Barrier

  • Secretions: Includes sweat (contains antimicrobial proteins) and sebum.

  • Acid Mantle: Formed by acidic secretions, providing protection.

  • Melanin: Offers protection against UV radiation damage.

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Barrier Functions

  • Physical Barrier: Stratum corneum's dead keratinized cells block most water and soluble substances.

  • Biological Barrier: Contains phagocytic cells; Langerhans cells act in immune response, while macrophages are present in the dermis.

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Body Temperature Regulation

  • Controlled by: Hypothalamus in the brain.

  • When temperature rises: Sweating occurs, and dermal blood vessels undergo vasodilation; skin becomes more flushed.

  • When temperature drops: Blood vessels constrict; vasoconstriction reduces heat loss, causing skin temperature to drop.

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Cutaneous Sensations

  • Sensory Receptors: Part of the nervous system, respond to various stimuli.

    • Types of Stimuli: Temperature, touch, and pain (via free nerve endings).

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Metabolic Functions

  • Vitamin D Synthesis: Skin synthesizes vitamin D, crucial for calcium absorption and bone health.

  • Conversion Process: Cholesterol in skin is converted to pre-vitamin D, then transported through the body.

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Rickets Disease

  • Cause: Insufficient vitamin D levels.

    • Vitamin D is vital for calcium absorption and bone health.

  • Notably, only skin can produce vitamin D independently; fortified milk provides additional sources of calcium.

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Skin Cancer

  • Overview: Most skin tumors are benign and do not metastasize.

  • Risk Factors: Overexposure to UV radiation and frequent skin irritation.

  • Types of Skin Cancer:

    • Basal cell carcinoma

    • Squamous cell carcinoma

    • Melanoma

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Cancer Mechanisms

  • Uncontrolled Cell Growth: Due to genetic mutations or external factors.

  • Nomenclature: Cancers are typically named based on origin type:

    • Carcinoma: Develops from epithelial tissue

    • Sarcoma: Develops from connective tissue

    • Lymphoma: Cancer of lymphatic tissue.

  • Example: Osteosarcoma (bone cancer).

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Benign vs. Malignant Tumors

  • Benign Tumors: Non-cancerous; local growth, do not invade other tissues.

  • Malignant Tumors: Cancerous; invade nearby tissues and can metastasize via blood and lymphatic vessels.

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Metastasis

  • Routes: Can occur via blood vessels or lymphatic system.

  • Primary Cancer: Starts at a specific site, invades nearby tissues.

  • Angiogenesis: Tumors develop their own blood supply.

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Basal Cell Carcinoma

  • Characteristics: Least aggressive and most common form of skin cancer.

  • Pathology: Proliferation of stratum basale cells leads to invasion of dermis and hypodermis.

  • Risk Factor: Commonly occurs in sun-exposed areas.

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Squamous Cell Carcinoma

  • Type: Second most common skin cancer; can metastasize.

  • Origin: Squamous cells of stratum spinosum.

  • Appearance: Typically appears as scaly, reddened lesions on the scalp, ears, lower lip, or hands.

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Melanoma

  • Description: Cancer of melanocytes; highly metastatic and resistant to chemotherapy.

  • Identification: Abnormal growth of moles (clusters of melanocytes).

  • Treatment: Requires wide surgical excision and immunotherapy; monitored by the ABCD rule (Asymmetry, Border, Color, Diameter).

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Burns

  • Types:

    • First-degree burn: Affects the epidermis.

    • Second-degree burn: Affects the epidermis and dermis.

    • Third-degree burn: Extends through the full thickness of skin to subcutaneous tissue.

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Dermatitis

  • Definition: Also known as eczema; refers to inflammation of the skin.

  • Causes: Can result from various irritants or allergens.

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Psoriasis

  • Nature: An autoimmune disease, believed to have genetic components.

  • Characterization: Results in the formation of abnormal skin patches.

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Clinical Indicators of Skin Health

  • Cyanosis: Bluish skin color due to low oxygenation of hemoglobin.

  • Erythema: Redness of skin indicating fever, inflammation, or allergic reaction.

  • Pallor: Pale skin due to anemia or low blood pressure.

  • Jaundice: Yellowing of skin related to high bilirubin levels.

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Key Clinical Detailing

  • Indicators of skin color changes can often signal underlying health issues such as circulatory or hepatic problems.

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Skin Structure Illustration

  • Components:

    • Hair shaft

    • Dermal papillae

    • Epidermis

    • Papillary layer

    • Subpapillary plexus

    • Sweat pore

    • Appendages such as eccrine sweat gland, arrector pili muscle, sebaceous gland, hair follicle, hair root, dermis, and hypodermis.

  • Nervous Structures: Includes sensory nerve fibers with free nerve endings, lamellar corpuscles, and hair follicle receptors.