Skin and its appendages form the integumentary system. It is the body’s largest organ and serves as a major component of the structural framework and the internal environment regulation.
Primary functions include support and protection of the body.
Skin has a role in defining the internal environment and contributing to homeostasis through various mechanisms.
Skin: Structure and Components
The integumentary system comprises the skin and its appendages: hair, nails, and skin glands.
The skin is the body’s largest organ and covers the entire surface.
Skin area in an average-sized adult is approximately A≈1.6–1.9m2.
Appendages include hair, nails, and skin glands that extend from or originate in the skin.
Layers of the Skin
Three main layers:
Epidermis
Dermis
Hypodermis (Subcutaneous Layer)
Additional components seen in the diagram include arrector pili muscles, sebaceous glands, hair follicles, adipose tissue, sensory nerve fibers, eccrine sweat glands, Pacinian corpuscles, and a cutaneous vascular plexus.
The Epidermis
Composed of keratinised, stratified squamous epithelium.
Has four or five layers depending on location on the body.
Thin skin vs. thick skin distinction:
Most skin is thin.
Thick skin is found on palms of the hands and soles of the feet.
Cell types in epidermis:
Keratinocytes are present in all layers except the stratum basale.
Keratinocytes synthesize and store keratin, a fibrous protein that provides hardness and water-resistance to hair, nails, and skin.
Keratin and Skin Water-Resistance
Keratin is a fibrous protein that contributes to the hardness of hair, nails, and skin and provides water resistance.
The Dermis
The dermis contains:
Blood vessels, nerves, hair follicles, and sweat glands.
Two layers of connective tissue: the superficial Papillary layer and the deeper Reticular layer.
Papillary layer (superficial):
Projects into the stratum basale to form dermal papillae (finger-like projections).
Contains fibroblasts, some adipocytes, and a rich network of small blood vessels.
Contains immune and sensory components: phagocytes, lymphatic capillaries, nerve fibers, and touch receptors (Meissner corpuscles).
Reticular layer (deep):
Rich in blood vessels and a dense network of sensory and sympathetic nerves.
Appears reticulated due to a tight mesh of fibers.
Contains elastin (elasticity) and collagen (tensile strength).
The Hypodermis (Subcutaneous Layer)
Located directly below the dermis; connects skin to underlying fascia of bones and muscles.
Not strictly part of the skin; border with dermis can be indistinct.
Contains blood vessels, loose areolar connective tissue, and adipose tissue.
Functions: fat storage, insulation, and cushioning for the integument.
Pigmentation of the Skin
Skin color is influenced by pigments: melanin, carotene, and haemoglobin.
Melanin is produced by melanocytes, located in the stratum basale of the epidermis.
Melanin is transferred to keratinocytes via melanosomes (vesicles).
UV exposure increases melanin production in keratinocytes; UV stimuli melanocytes via chemical signaling.
Excessive sun exposure can cause: wrinkle formation due to degradation of cellular structure and potential DNA damage leading to skin cancer.
Irregular melanocyte accumulation leads to freckles; larger masses of melanocytes form moles (nevi) that are usually benign but should be monitored for changes indicating cancer.
Accessory Structures of the Skin
Hair, nails, and glands are accessory structures arising from the skin.
Hair and nails derive from epidermal structures and extend into deeper dermal components.
Hair: Development and Structure
Development:
Hair coverage throughout most of the body except palms, soles, and a few specific areas.
Initial fine, soft hair coat present before birth.
Pubic and axillary hair develops at puberty.
Hair follicles and hair develop from the epidermis.
Hair Follicle Anatomy (simplified):
Hair shaft sits within the follicle; the root is enveloped by the dermal root sheath and the internal epithelial root sheath.
Hair bulb contains germinal matrix (growth zone) and papilla (nourishment).
Medulla, cortex, and cuticle are structural components of the hair shaft.
Associated structures: arrector pili muscle, sebaceous gland, and various vascular components.
Appearance:
Hair color results from melanin type and quantity in the cortex.
Growth cycles alternate between growth and rest periods.
Sebaceous glands secrete sebum into follicles; intake of oils can influence hair condition.
Male pattern baldness (androgenic alopecia) arises from genetic susceptibility and male hormones.
Hair greying involves changes in pigment deposition with time.
Nails
Nails consist of epidermal cells converted to hard keratin.
Nail body: visible part of the nail.
Nail root: part embedded in a groove under the cuticle.
Lunula: the moon-shaped white area near the nail root.
Glands of the Skin
Eccrine glands
Most numerous sweat glands.
Distributed over nearly the entire body surface, with some small areas excluded.
Secrete perspiration (sweat) for thermoregulation and excretion of some wastes.
Apocrine glands
Open into hair follicles and surface of the skin.
Develop in areas with abundant hair follicles (scalp, armpits, groin).
Secrete perspiration (sweat) that is typically associated with body odor when metabolized by skin bacteria.
Sebaceous glands
Secrete sebum (lipid-rich oily substance).
Lipids have antifungal activity and help lubricate skin and hair.
Simple, branched glands located in the dermis except on palms and soles.
Secretion increases during adolescence.
Ceruminous glands
Modified apocrine sweat glands.
Simple, coiled, tubular glands that secrete into the external ear canal; may work with sebaceous glands.
Cerumen (earwax) helps protect against dehydration.
Functions of the Skin
Sensation: Somatic sensory receptors detect pressure, touch, temperature, pain, and other general senses.
Flexibility: Skin is supple and elastic to accommodate body contour changes without injury.
Excretion: Sweat and urinary by-products (e.g., urea, ammonia, uric acid) are expelled via skin/secretions where applicable.
Immunity: Epidermal dendritic cells trigger immune responses; phagocytic cells also help destroy bacteria.
Body temperature homeostasis: Skin helps regulate temperature; heat production vs heat loss maintainance; sweating and shivering contribute to heat exchange.
Protection: Chemical, physical, and UV-protection roles; melanin contributes to UV defense; barrier against microorganisms and chemical hazards; protection against mechanical trauma.
Vitamin D production: Skin exposure to UV converts 7-dehydrocholesterol to cholecalciferol (vitamin D3), which is transported to the liver and kidneys for final activation; vitamin D is hormone-like in function.
Vitamin D Production (Skin-initiated)
UV exposure converts 7-dehydrocholesterol to cholecalciferol (vitamin D3):
7-dehydrocholesterolUVcholecalciferol (D3)
The liver and kidneys convert to the active hormone form of vitamin D.
This process is part of the endocrine functions of the skin and supports calcium homeostasis systemically.
Homeostasis of Body Temperature
Primary determinants:
Heat production, largely from muscular work.
Heat loss via evaporation, radiation, conduction, and convection; evaporation is the key mechanism for cooling.
The skin contributes to thermoregulation through:
Blood flow adjustments (vasodilation/vasoconstriction) and sweating.
Evaporation of sweat and radiation of heat from the skin surface.
Diagrams (conceptual): Heat conservation and heat loss mechanisms via epidermis and dermis through precapillary sphincters and vascular control.
Normal core temperature setpoint around 37°C; fever can raise setpoint toward higher values (e.g., 38°C during fever) and triggers cooling responses when needed.
The Skin Regulates Body Temperature: Feedback Loop
Senses temperature changes via receptors in the skin (temperature receptors) and senses the core temperature monitored by the hypothalamus.
When overheating is detected, effectors respond to restore setpoint:
Sweat glands secrete to promote cooling via evaporation.
Blood vessels dilate in the skin to increase heat loss.
When cold, precapillary sphincters reduce blood flow to the skin to conserve heat, while metabolic heat production may increase through shivering.
Integration of signals occurs via the hypothalamus, which receives input from skin sensors and sends corrective signals through nerves to the effector organs.
Ageing and Diseases of the Skin
Learning objectives include describing age-related changes and briefly describing burns and melanoma.
Cycle of skin life:
Children: Skin is smooth, elastic, and flexible; few sweat glands; rapid healing.
Adults: Development/activation of sebaceous and sweat glands; increased sebum production; potential acne.
Implications: Age-related reductions in gland activity lead to drier skin and reduced thermoregulation; structural changes affect wound healing and immune responses.
Burns
Burns occur when skin is damaged by heat, radiation, electricity, or chemicals.
Consequences include death of skin cells and massive fluid loss, leading to dehydration, electrolyte imbalance, and potential renal and circulatory failure.
Treatment: Intravenous fluids and nutrients to offset dehydration and support tissue repair.
Infections risk: Burned skin is highly susceptible to bacterial and other pathogens due to loss of protective barrier.
Size assessment: Burns are sometimes measured by total surface area affected using the Rule of Nines.
Rule of Nines: specific anatomical areas correspond to percentages of body surface area (translates to an estimation method for burn size).
Burn Severity and Degree Classifications
First-degree burn: Superficial; affects only the epidermis (e.g., mild sunburn); heals in a few days.
Second-degree burn: Deeper injury; affects epidermis and part of the dermis; causes swelling and painful blisters; heals in several weeks.
Third-degree burn: Extends through epidermis and dermis; destroys tissue and nerve endings; may appear red, white, or black; requires medical attention; heals slowly without treatment.
Fourth-degree burn: Extends beyond skin to underlying muscles and bones; most severe.
Melanoma
Cancer characterized by uncontrolled growth of melanocytes; often develops from a mole.
Most fatal of skin cancers due to metastatic potential and sometimes late detection.
Early-stage melanoma diagnosis often uses the ABCDE mnemonic:
A: Asymmetry – two sides not equal
B: Borders – irregular edges
C: Color – varied shades of brown/black
D: Diameter – larger than 6mm
E: Evolution – changes in shape, size, or color over time
Important: Monitor moles for changes and seek medical evaluation for suspicious lesions.
Quick Reference: Key Numbers and Facts
Skin area in adults: A≈1.6 to 1.9m2
Common cancers (Australia, 2020):
Males: Prostate 16,700 cases; Melanoma of the skin 9,500; Colorectal cancer 8,300; Lung cancer 7,200.
Females: Breast 19,800; Colorectal cancer 7,200; Melanoma of the skin 6,700; Lung cancer 6,000.
Major skin components and their roles include: keratinocytes, melanin production, dermal papillae, Meissner corpuscles, Pacinian corpuscles.
UV exposure effects: stimulates melanin production; excessive exposure risks DNA damage and cancer.
Vitamin D synthesis: UV converts 7-dehydrocholesterol to cholecalciferol (D3) in the skin; liver and kidneys complete activation; vitamin D acts hormonally.
Normal core body temperature: around 37ext°C; fever can raise setpoint to around 38ext°C; cooling relies on evaporation and skin blood flow changes.
Hair growth cycle includes growth and rest periods; grey hair results from pigment changes.
Moles vs freckles: freckles are irregular melanocyte patterns; moles are aggregations of melanocytes; monitor for changes.
Gland functions: eccrine (sum sweat for cooling), apocrine (sweat in hair-rich areas; odor associated), sebaceous (sebum production; greasy hair; adolescence increases activity), ceruminous (earwax for dehydration protection).
Burn classifications and management emphasize hydration, infection prevention, and gradual tissue repair; Rule of Nines is used to estimate affected body surface area.