Integumentary System: Functions and Components

• The integumentary system is accessible, large, and underappreciated.
• The integumentary system makes up about 16% of total body weight.
• The integumentary system has a 1.592 m² surface area, is continually abraded, attacked by microorganisms, irradiated by sunlight, and exposed to environmental chemicals.
• The integumentary system is the first line of defense.

Components of the Integumentary System

• Cutaneous membrane (epidermis, dermis)
• Accessory structures (hair shaft, pore of sweat gland duct, tactile corpuscle, sebaceous gland, arrector pili muscle, sweat gland duct, hair follicle, lamellar corpuscle, nerve fibers, sweat gland)

General Functions of the Integumentary System

• Protection of underlying tissues and organs against impact, abrasion, fluid loss, and chemical attack
• Excretion of salts, water, and organic wastes by glands
• Maintenance of normal body temperature through either insulation or evaporative cooling, as needed
• Production of melanin protects underlying tissue from ultraviolet (UV) radiation
• Production of keratin protects against abrasion and repels water
• Synthesis of vitamin D3, a steroid that is converted to calcitriol, a hormone important to normal calcium ion metabolism
• Storage of lipids in adipocytes in the dermis and in adipose tissue in the subcutaneous layer
• Detection of touch, pressure, pain, vibration, and temperature stimuli, and the relaying of that information to the nervous system
• Coordination of the immune response to pathogens and cancers in the skin.

Cutaneous Membrane

• Epidermis: Protects the dermis, prevents water loss and the entry of pathogens, and synthesizes vitamin D3, sensory receptors detect touch, pressure, pain, and temperature
• Dermis Papillary Layer: Nourishes and supports epidermis.
• Dermis Reticular Layer: Has sensory receptors that detect touch, pressure, pain, vibration, and temperature; blood vessels assist in thermoregulation.

Accessory Structures

• Hair Follicles: Hairs protect the skull and provide delicate touch sensations on the general body surface.
• Exocrine Glands: Assist in temperature regulation and waste excretion.
• Nails: Protect and support tips of fingers and toes.

The Cutaneous Membrane

• The cutaneous membrane consists of two components; the epidermis (epi: above), or superficial epithelium, and the dermis, an underlying area of connective tissues.

Accessory Structures

• These include hair and hair follicles, exocrine glands, and nails.
• Accessory structures are embedded in the dermis and project up to or above the surface of the epidermis.

Epidermis Organization

• It consists of stratified squamous epithelium.
• The epidermis is separated from the dermis by a basement membrane.
• The stratum basale and the underlying dermis interlock, strengthening the bond between the two.
• The epidermis forms epidermal ridges, which extend into the dermis and are adjacent to dermal papillae that project into the epidermis.
• Thin skin contains four layers of keratinocytes and is about as thick as the wall of a plastic sandwich bag (about 0.08 mm).
• Thick skin contains a fifth layer, the stratum lucidum.
• Thick skin also has a much thicker superficial layer (the stratum corneum), and is about as thick as a standard paper towel (about 0.5 mm).

Layers of the Epidermis, Characteristics, and Surface

• Stratum corneum: Multiple layers of flattened, dead, interlocking keratinocytes; water resistant, but not waterproof.
• Stratum lucidum: Appears as a glassy layer in thick skin only.
• Stratum granulosum: Keratinocytes produce keratin, keratin fibers develop as cells become thinner and flatter.
• Stratum spinosum: Gradually, the plasma membranes thicken, the organelles disintegrate, and the cells die and keratinocytes are bound together by desmosomes.
• Stratum basale: Deepest basal layer attachment to the basement membrane, contains basal cells (stem cells), melanocytes, and tactile cells (Merkel cells).
• Fingerprints reveal the pattern of epidermal ridges

Epidermal Growth Factor (EGF)

• EGF is produced by the salivary glands and glands of the duodenum (initial segment of the small intestine)
• EGF has widespread effects on epithelia throughout the body.
• The roles of EGF are promoting the divisions of basal cells in the stratum basale and stratum spinosum.
• EGF accelerates the production of keratin (keratinization) in differentiating keratinocytes.
• EGF stimulates the secretory product synthesis and secretion by epithelial glands.
• EGF stimulates epidermal development and epidermal repair after injury.

Dermis

• The dermis lies between the epidermis and the subcutaneous layer.
• Accessory structures of epidermal origin, such as hair follicles and sweat glands, extend into the dermis
• The dermis contains networks of blood vessels and nerve fibers.
• Layers of the dermis: a superficial papillary layer and a deeper reticular layer

Papillary Layer

• Consists of areolar tissue, contains the capillaries, lymphatic vessels, and sensory nerve fibers that supply the surface of the skin.
• This layer gets its name from the dermal papillae that project between the epidermal ridges.

Reticular Layer

• Lies deep to the papillary layer
• Consists of an interwoven meshwork of dense irregular connective tissue containing both collagen and elastic fibers

Subcutaneous Layer (Hypodermis)

• Quite elastic, consisting of primarily adipose tissue (areolar tissue dominated by adipocytes)
• This layer is also called the subcutaneous fat and makes about 80% of all body fat.

Repair of Injury to the Integument

• Inflammatory Phase: Bleeding occurs at injury site and mast cells in the region trigger an inflammatory response
• Migration Phase: A scab has formed and cells of the stratum basale are migrating along the edges of the wound and phagocytic cells are removing debris while clotting around the edges partially isolates the region.
• Proliferation Phase: About a week after the injury, the scab is undermined by epidermal cells migrating over the collagen fiber meshwork produced by fibroblast proliferation and activity with phagocytic activity almost ended.
• Scarring Phase: After several weeks, the scab sheds, and the epidermis is complete, and fibrolasts continue to create scar tissue.

Effects of Aging

• The epidermis thins as basal cell activity declines.
• The number of dendritic cells decreases to about 50% of levels seen at about age.
• Vitamin D3 production declines by about 75%.
• Melanocyte activity declines.
• Glandular activity declines.
• The blood supply to the dermis is reduced.
• Hair follicles stop functioning or produce thinner, finer hairs.
• The dermis thins, and the elastic fiber network decreases in size.
• Skin repairs proceed more slowly.
• With changes in levels of sex hormones, secondary sexual characteristics in hair and body fat distribution begin to change.

Explore the integumentary system, a large and vital organ system. Learn about its components, including the cutaneous membrane and accessory structures. Understand its general functions, such as protection, excretion, temperature maintenance, and melanin production.