3.1 The Skin: Structure and Function
Overview of the Integumentary System
The skin is your body's largest and heaviest organ, serving as a protective barrier, a sense organ, and a regulator of temperature.
It accounts for about 15\%\text{ of body weight}.
It contains multiple components beyond a simple covering: epidermis, dermis, hypodermis (subcutaneous tissue), plus accessory structures like hair, glands, and nails.
The skin’s layers and structures work together to protect, sense, regulate, and communicate with the environment.
Layers of the Skin (Structure and Organization)
Two main structural layers: epidermis (outer) and dermis (underneath).
The hypodermis (subcutaneous tissue) lies beneath the dermis and is not technically part of the skin.
Epidermis is avascular (no blood vessels); its nutrients come from diffusion from the dermis.
Dermis is vascular and contains most of the skin’s accessory structures (sweat glands, sebaceous glands, hair follicles, nerve endings).
Hypodermis stores fat, provides insulation and cushioning, and anchors the skin to underlying fascia via areolar connective tissue and adipose tissue.
The skin with its accessory structures is part of the integumentary system, essential for defense, sensation, thermoregulation, and more.
The Epidermis (outermost layer)
Composition: keratinized stratified squamous epithelium.
Avascular: depends on diffusion of nutrients from the underlying dermis.
Main cell type: keratinocytes (produce keratin, strengthening skin and contributing to water resistance).
Other epidermal cell types:
Melanocytes: produce melanin, providing pigmentation and protection against UV radiation.
Langerhans cells: act as antigen-presenting cells, playing a role in immune response.
Merkel cells: function in touch sensation, aiding in the detection of pressure and texture.
Epidermal layers (from deep to superficial):
Stratum basale (deepest): single layer of cuboidal/low columnar mitotically active keratinocytes; contains epidermal stem cells; melanocytes and Merkel cells scattered here.
Stratum spinosum: several layers of keratinocytes bound by desmosomes; thickest layer in most skin; as cells move upward, they synthesize more keratin and flatten; contains Langerhans cells and melanocyte projections.
Stratum granulosum: 3–5 layers of flattened keratinocytes undergoing apoptosis; nuclei/organelles degenerate as they move away from nutrient source; features dark staining keratohyalin granules.
Stratum lucidum: clear layer present only in thick skin (palms and soles); 4–6 layers of flattened dead keratinocytes.
Stratum corneum: outermost layer; up to ~30 layers of dead, flattened keratinocytes filled with keratin; provides durable protection against abrasion, microbes, and water loss; cells continuously slough off and are replaced from beneath.
Key functional implications:
Avascularity of the epidermis explains why superficial cuts that stay in the epidermis do not bleed, whereas deeper cuts through the dermis do bleed.
The epidermis thickness varies by region; thick skin contains the stratum lucidum, while thin skin does not.
The Dermis (the supportive, vascular layer beneath the epidermis)
Composition: connective tissue rich in collagen with elastic and reticular fibers; contains fibroblasts and macrophages.
The dermis is well supplied with blood vessels and houses sweat glands, sebaceous glands, nerve endings, hair follicles, and nail roots.
It is organized into two main layers:
Papillary layer (superficial): thin layer of loose areolar connective tissue with collagen and elastic fibers and a rich capillary network.
Reticular layer (deeper and thicker): dense irregular connective tissue rich in collagen fibers; contains the majority of the skin’s strength and elasticity.
Dermal papillae: upward finger-like projections of the dermis that extend into the epidermis, increasing surface area for nutrient exchange and helping anchor the layers.
Some papillae contain tactile corpuscles (Meissner corpuscles) that are sensitive to light touch.
Other dermal papillae contain free nerve endings that detect temperature, pain, and other stimuli.
Epidermal ridges: formed where the epidermis (stratum basale) interacts with the dermal papillae; create fingerprint patterns that enhance grip and tactile sensitivity; patterns form before birth and are influenced by genetics and intrauterine environment; fingerprints are unique to each individual.
Reticular layer specifics:
Thicker than the papillary layer and provides strength and elasticity via collagen and elastin fibers.
Stretch marks (striae) occur when collagen fibers tear due to overstretching; over time, scar tissue forms and the affected area may appear lighter.
Other cells in the dermis:
Fibroblasts: produce collagen, elastin, and other fibers.
Macrophages: wander the dermis as part of the immune defense.
Accessory structures located in the dermis:
Hair follicles and nail roots, as well as glands (sweat and sebaceous), are embedded here.
The Hypodermis (Subcutaneous Layer)
Not technically part of the skin, but closely associated and often discussed with it.
Functions: insulation, energy storage (fat), cushioning, and anchoring the skin to underlying fascia.
Composition: primarily adipose tissue with some areolar connective tissue; contains larger blood vessels and nerves.
Clinical note: subcutaneous injections are delivered to this site just beneath the skin.
Hair (Pili) and Hair Follicles
Hair is made of dead keratinized cells and serves protection, sensory input, and thermoregulation.
Hair shaft: portion that projects above the skin surface.
Hair root: portion embedded within the skin.
Hair follicle: tubular structure that extends into the dermis and houses the hair root.
Hair bulb: onion-shaped enlargement at the base of the follicle; contains the hair matrix (germinal layer of actively dividing cells) where new hair is produced.
Hair layers (in both shaft and root):
Medulla: innermost core; may contain pigment granules depending on hair color.
Cortex: thickest layer of elongated keratinized cells.
Cuticle: outermost layer of flat, heavily keratinized cells protecting the hair.
Arrector pili muscle: small band of smooth muscle attached to the follicle; contraction causes hair to stand up (goosebumps) in response to cold or fear.
Hair root plexus: network of dendrites from sensory neurons surrounding the follicle; enables sensation when hair is disturbed, even though the hair shaft itself is dead tissue.
Accessory Glands of the Skin
Sebaceous glands (oil glands): secrete sebum to lubricate and waterproof skin and hair; secretion typically empties into a hair follicle.
Sudoriferous (sweat) glands:
Eccrine (merocrine) glands: the most numerous; produce a watery sweat primarily for thermoregulation. Sweat composition includes water, ions (notably Na+ and Cl−), urea, uric acid, ammonia, amino acids, glucose, and lactic acid. Evaporation of sweat removes heat from the body.
Apocrine glands: large coiled tubular glands located in areas such as the axillae and groin; secrete a thicker, milkier fluid containing lipids and proteins. They become active at puberty and are influenced by hormonal and emotional factors; their secretions can contain pheromones and contribute to body odor when metabolized by surface bacteria.
Ceruminous glands: specialized sweat glands in the external ear canal; produce cerumen (ear wax) which traps dust, debris, and microorganisms and provides antibacterial properties to protect the eardrum.
Nails
Nails are made of hard keratin and protect the tips of fingers and toes, aiding in grip and manipulation.
Nail components:
Nail plate: visible hard portion of the nail.
Free edge: portion that extends beyond the fingertip; appears white because there are no capillaries beneath it.
Nail root: embedded part of the nail at the proximal end.
Nail matrix: tissue at the nail root where cells divide to form new nail cells; site of nail growth.
Lunula: the whitish crescent-shaped area at the base of the nail plate, indicating the visible portion of the actively growing region.
Embryology and Integration with Other Systems
Many accessory structures (hair, nails, glands) originate from the embryonic epidermis but are located within the dermis.
The integumentary system integrates with the nervous system (sensory receptors in skin and hair), circulatory system (blood vessels in the dermis and hypodermis), and immune system (Langerhans cells).
Functional Significance and Practical Implications
Barrier function: protects against pathogens, chemical exposure, and physical injury; reduces water loss.
Thermoregulation: sweat production (eccrine) and blood flow adjustments; evaporative cooling.
Sensation: numerous receptors (Meissner corpuscles, free nerve endings) provide touch, temperature, pain, and other sensory inputs.
Immunity: Langerhans cells contribute to pathogen recognition and immune responses in the skin.
Aesthetic and diagnostic value: epidermal ridges and fingerprints reflect developmental biology and can indicate genetic and environmental influences.
Clinical notes mentioned in the video:
Stretch marks (striae) arise from tearing of collagen in the reticular dermis due to overstretching.
The unique patterns of epidermal ridges are formed before birth and persist throughout life.
Quick References and Key Terms
Epidermis: avascular, keratinized stratified squamous epithelium; layers from deep to superficial: basale, spinosum, granulosum, lucidum (thick skin), corneum.
Dermis: vascular connective tissue; papillary and reticular layers; dermal papillae; Meissner corpuscles; free nerve endings; hair follicles; glands; nerves.
Hypodermis: subcutaneous tissue; adipose and areolar tissue; insulation and energy storage; injection site.
Cells in the epidermis: keratinocytes, melanocytes, Langerhans cells, Merkel cells.
Hair structures: shaft, root, follicle, bulb, matrix, medulla, cortex, cuticle, arrector pili, hair root plexus.
Glands: sebaceous (sebum), eccrine (sweat), apocrine (sweat with pheromones), ceruminous (ear wax).
Nails: plate, root, matrix, lunula, free edge.
Closing
The skin is a remarkably integrated organ system that performs protective, sensory, thermoregulatory, and communicative roles, combining layers, specialized cells, and accessory structures to interact with the external environment. The structure–function relationships across the epidermis, dermis, and hypodermis underpin everyday phenomena from gripping objects to sensing heat and feeling touch, as well as responses like goosebumps and skin healing after injury.