M6: Integumentary System

Primary Organ:

Skin:

• Encloses the body -> creates protective barrier between the internal body + external environment.

• contributes to immunity + thermoregulation.

Accessory Organs

• Hair.

• Nails.

• Oil glands.

• Sweat glands.

• Sensory receptors

Skin: Layers

• Skin = Cutaneous Membrane: largest organ in the body by surface area + weight.

Layers:

• Subcutaneous Layer (Hypodermis): Layer beneath the skin (not part of the skin itself), composed of loose fatty connective tissue containing large blood vessels supplying the skin.

• Skin Structural Layers:
  Consists of two layers:

  Epidermis: the superficial, thinner
  Dermis: deeper, thicker

Epidermis: Structure + Cell Types

Structure:

• stratified squamous epithelium arranged in rows and organized into distinct strata (layers)

Principal Cell Types

• Keratinocytes: Most abundant cell type, packed with tough keratin protein.

• Melanocytes: Synthesise melanin, a pigment that colors the skin and protects against ultraviolet (UV) radiation.

• Langerhans Cells: Subtype of dendritic cell involved in immune defense.

• Merkel Cells: Specialized cells involved in tactile sensation.

Epidermal Strata (Layers)

• Stratum Basale: Innermost basal layer of cells attached to the basement membrane.

• Stratum Spinosum: Second layer up, consisting of ~3–15 rows of cells.

• Stratum Granulosum: Layer containing 3–5 rows of keratinocytes filled with lamellar granules.

• Stratum Lucidum: Present only in thick skin, situated between the stratum granulosum and stratum corneum.

• Stratum Corneum: Outermost layer consisting of 3–50 rows of dead keratinocytes.

Thick vs. Thin Skin

• Thick Skin: Located on the anterior hand/fingers (palms, finger pads) and soles of feet/toes.

  • Features five epidermal layers (includes the stratum lucidum).

  • Hairless and contains a higher proportion of sweat glands.

Thin Skin:

• Covers most of the body except palms, soles, anterior fingers, and toes;

• Features four epidermal layers.

Dermis: Structure and Cell Types

Structural Regions

• Deeper, thicker portion of the skin composed of connective tissue.

• Papillary Region: Superficial region where dermal papillae and epidermal ridges interlock.

• Reticular Region: Much thicker, deeper region composed of dense connective tissue.

Contents & Cell Types

• Houses blood vessels, nerves, and various accessory organs of the skin.

• Fibroblasts: Predominant cell type responsible for synthesising and secreting the extracellular matrix.

• Immune Cells: Contains resident immune cells including mast cells and macrophages.

Hair:

HAIR

Distribution & Structure

• Shaft: Visible portion extending out of the skin.

• Root: Portion embedded within the dermis.

• Hair Follicle: Surrounds the root in the dermis -> essential for hair growth.

Functions of Hair

• Sensation: Hair movement activates root sensory receptors -> detects tactile stimuli near or on the skin.

• Protection:

  • Scalp hair shields the head from injury and UV radiation damage.

  • Eyebrows and eyelashes catch foreign particles before entering the eye.

  • Nostril and ear canal hair traps foreign particles before entering the respiratory system or deeper ear canal.

• Warmth: Contributes to body warmth maintenance.

Nails:

Structure & Functions

• Structure: Plates of hardened, dead keratinocytes located on the dorsal aspects of distal digits.

• Functions:

  • Protects the distal ends of digits.

  • Free edge enables scratching and grasping of objects.

    
    

Skin Glands: Sebaceous Glands

Overview

• Main types: sebaceous glands and sweat glands.

Sebaceous Glands

• Secretes sebum (oily substance) into hair follicles -> exits skin via the same pore as hair shafts.

• Sebum Functions:

  • Coats hair to maintain strength and prevent drying out.

  • Forms a thin lipid barrier on skin -> prevents skin dehydration.

  • Inhibits growth of certain bacteria.

Skin Glands: Sweat Glands

1. Eccrine Sweat Glands

• Most numerous sweat glands, located across most body areas.

• Secretes watery, salty fluid directly onto the skin surface via independent pores.

• Functions: Contributes to thermoregulation and emotional sweating.

2. Apocrine Sweat Glands

• Located primarily in the skin of the armpits and groin.

• Secretes a milkier product into hair follicles -> exits via the same pore as hair shafts.

• Secretions are initially odourless -> skin bacteria metabolise fluid -> produces body odour.

3. Ceruminous Glands

• Modified sweat glands located in the skin of the ear canal.

• Secretions contribute to cerumen (ear wax) -> protects the ear canal.

Integumentary System Functions

• thermoregulation

• protection

• serving as a blood reservoir

• cutaneous sensation

• excretion & absorption

• activation of vitamin D

ISF: Thermoregulation

• The skin acts to cool the body down when hot and heat or reduce heat loss when cold.

• Thermoregulation is controlled by altering three main physiological elements,:

  • Activity of eccrine sweat glands.

  • Diameter of blood vessels situated near the skin surface.

  • Angle of the skin's hair shafts.

ISF: Protection

Skin Protective Components

• Keratinocytes and their internal keratin protein.

• Lipid sebum coating the skin surface.

• Chemical composition of sweat.

• Melanin pigments.

• Resident immune cells, including Langerhans cells and macrophages.

ISF: Blood Reservoir

Vascular Storage & Redistribution

• Extensive blood vessel networks in the dermis and subcutaneous tissue hold large volumes of blood when dilated, functioning as a blood reservoir.

• Vessel Constriction: Constriction of these skin vessels -> reduces blood volume in the skin -> increases blood available for distribution to other body regions.

ISF: Cutaneous Sensation

Sensory Perception

• skin contains multiple types of specialized sensory receptors.

• Enables the body to detect external environmental stimuli —> touch, temperature, and pain.

ISF: Excretion

EXCRETION & ABSORPTION

Absorption

• Absorbs select substances into the body, particularly lipid-soluble substances capable of diffusing through the lipid layers created by sebum and lamellar granule secretions.

Excretion

• Assists in excreting metabolic waste products from the body.

• Excretes a portion of urea (produced during protein breakdown) via sweat, although most urea is excreted in urine.

ISF: Activation of Vit D

UV-Mediated Activation Pathway

• Vitamin D requires multiple biological transformations within the body to become fully activated.

• vitamin D precursors can be absorbed from foods

• alternative synthesis pathway involves altering a precursor molecule in the skin through exposure to solar UV rays.

Types of Skin Wounds

Epidermal wounds

Deep Wounds

Epidermal Wounds

• Affect exclusively the epidermis without penetrating the basement membrane.

  • The epidermis is avascular -> epidermal wounds do not bleed.

  • Absence of pain-sensing neurons in the epidermis -> results in minimal pain (e.g. minor red scratch).

Deep Wounds

•  Penetrate through the basement membrane into the dermis.

  • Cause damage to blood vessels, nerves, glands, fibroblasts, and accessory structures -> result in bleeding.

Epidermal Wound: Healing Process

Regeneration Process

• Epidermal epithelial cells regenerate easily —> Simple healing process

• Operates via a two-step mechanism:

  1. Basal epithelial cells adjacent to wound edges migrate inwards.

  2. Migrated basal cells continually proliferate -> replace cells lost in the wound.

     
     

Deep Wound: Healing Process (1)

three non-sequential, overlapping phases:

1. Inflammation & Haemostasis:

• Haemostasis: Blood clot forms within the wound -> superficial clot surface dries -> forms a protective scab.

• Scab Function: Maintains a moist tissue environment -> keeps healing cells alive + allows cytokine and nutrient diffusion throughout the wound.

• Inflammatory Response: Inflammatory mediators (histamines from mast cells, bacterial toxins) -> cause vasodilation, increased capillary permeability, and chemotaxis.

• Immune Cell Recruitment: Neutrophils migrate into the wound -> phagocytose invading pathogens.

  • Neutrophils release cytokines -> attract monocytes/macrophages.

  • Macrophages clear cellular debris + assist neutrophils in fighting infection.

  1. Proliferation & Migration.

  2. Maturation & Remodeling.

Deep Wound: Healing Process (2)

Proliferation & Migration.

• Extracellular Matrix Synthesis: Macrophages secrete growth factors -> attract and stimulate nearby fibroblasts.

  • Fibroblasts synthesise and secrete extracellular matrix -> replaces the dissolving blood clot.

  • Collagen is deposited in a random crisscross arrangement -> resulting tissue is not as strong as original linear tissue.

• Angiogenesis: Macrophages release endothelial growth factor -> triggers angiogenesis to replace damaged blood vessels.

• Granulation Tissue: New blood vessels + extracellular matrix form granulation tissue.

• Re-Epithelialisation: Macrophages release epidermal growth factors -> stimulate epidermal cell migration and mitosis -> scab falls off once complete re-epithelialisation occurs.

Deep Wound: Healing Process (3)

Maturation & Remodelling

• Healing Pathways:

  • Regeneration: Normal tissue cells regenerate.

  • Fibrosis: Excess collagen deposited in a random pattern with high vascularity -> forms scar tissue.

  • Approximating wound edges (e.g. using stitches) -> reduces likelihood of scar tissue formation.

• Maturation Stage: Can continue for years -> scar tissue gradually loses vascularity -> collagen fibres redistribute into linear arrangements -> restores tissue consistent with pre-wound structure.