# Lecture 5: The Integumentary System
## Anatomy and Physiology I
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## Learning Objectives
By the end of this lecture, students will be able to:
1. List the functions of the integumentary system
2. Describe the layers of the epidermis and the process of keratinization
3. Identify the cell types found in the epidermis and their functions
4. Describe the structure of the dermis and its two layers
5. Describe the structure and function of the hypodermis
6. Explain the basis of skin color
7. Describe the structure and function of hair, nails, and skin glands
8. Classify burns by degree and describe the rule of nines
9. Describe the two types of skin cancer and their features
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## Lecture Content
### I. Functions of the Integumentary System
The integumentary system serves the body in numerous ways. It provides **protection** by acting as a barrier against pathogens, chemicals, ultraviolet radiation, mechanical damage, and water loss. It participates in **thermoregulation** through sweat production and by adjusting blood flow to the skin via vasodilation and vasoconstriction. It enables **cutaneous sensation** through receptors for touch, pressure, vibration, temperature, and pain. It performs **metabolic functions**, most notably the synthesis of vitamin D3 (cholecalciferol) upon UV exposure. The dermis serves as a **blood reservoir**, holding up to five percent of total blood volume that can be diverted to other organs when needed. Finally, the skin contributes to **excretion** by eliminating limited amounts of nitrogen-containing wastes and sodium chloride in sweat.
### II. Structure of the Skin
The skin consists of two principal layers: the **epidermis**, an outer avascular epithelial layer, and the **dermis**, an inner vascularized connective tissue layer. Beneath the skin lies the **hypodermis** (subcutaneous layer or superficial fascia), which is not technically part of the skin but is closely associated with it and anchors it to underlying structures.
### III. Epidermis
The epidermis is composed of **keratinized stratified squamous epithelium**. Being avascular, it receives its nutrients by diffusion from the dermis. In thick skin, such as that found on the palms and soles, the epidermis contains four cell types and five distinct layers.
#### Cell Types of the Epidermis
**Keratinocytes** are the most abundant epidermal cells, making up about 90 percent of the total. They produce keratin, a tough fibrous structural protein, and lamellar granules, which are lipid-rich and contribute to the skin's waterproofing. **Melanocytes** account for about eight percent of epidermal cells and are located in the stratum basale. They produce melanin pigment within organelles called melanosomes and transfer these melanosomes to neighboring keratinocytes through their long cytoplasmic processes. Melanin absorbs ultraviolet radiation and thereby protects DNA from damage. **Dendritic cells** (Langerhans cells) are immune cells derived from bone marrow that reside in the stratum spinosum. They phagocytize pathogens and present antigens to T lymphocytes, playing a critical role in immune surveillance. **Tactile (Merkel) cells** are found in the stratum basale in association with sensory nerve endings called tactile discs and function as receptors for light pressure.
#### Layers (Strata) of the Epidermis — Deep to Superficial
The deepest layer is the **stratum basale** (stratum germinativum), a single row of cuboidal to columnar stem cells resting on the basement membrane. This is the site of most active mitosis, continuously generating new keratinocytes. It also harbors melanocytes and Merkel cells and is anchored to the dermis by hemidesmosomes.
Above it lies the **stratum spinosum** (prickly layer), which consists of several layers of keratinocytes connected by desmosomes. These cells appear spiny under the microscope, an artifact of preparation that causes cells to shrink while the desmosomes remain. Langerhans cells are found here, and some mitotic activity still occurs.
The **stratum granulosum** (granular layer) contains three to five layers of flattened keratinocytes filled with dark-staining keratohyaline granules, which help form keratin fibers, and lamellar granules, which release waterproofing lipids into the extracellular space. This layer marks the transition between the deeper metabolically active layers and the dead superficial layers, as cells undergo apoptosis during their upward journey.
The **stratum lucidum** (clear layer) is present only in thick skin on the palms and soles. It is a thin, translucent layer of dead, flattened keratinocytes filled with eleidin, an intermediate form of keratin.
The outermost layer is the **stratum corneum** (horny layer), consisting of 20 to 30 rows of dead, flat, keratinized cells called corneocytes. This layer accounts for about three-quarters of the epidermal thickness and provides the primary barrier against water loss, abrasion, and pathogen penetration. Cells are continuously shed from the surface in a process called desquamation and replaced from below, with the entire epidermis turning over every 25 to 45 days.
A detailed cross-sectional diagram of the epidermis showing all five strata from deep to superficial. The stratum basale sits on a wavy basement membrane with columnar cells, melanocytes (with long processes transferring melanosomes), and Merkel cells. Above it, the stratum spinosum shows several layers of polyhedral cells with visible desmosome connections and scattered Langerhans cells. The stratum granulosum shows flattened cells with dark keratohyaline granules. The stratum lucidum appears as a thin, pale band (labeled "thick skin only"). The stratum corneum shows many layers of flat, dead, anucleate cells being shed from the surface. Arrows indicate the upward movement of cells during keratinization.
### IV. Dermis
The dermis is a strong, flexible connective tissue layer well supplied with blood vessels, lymphatic vessels, nerves, and sensory receptors. It houses hair follicles, oil glands, and sweat glands and is composed of two layers.
#### Papillary Layer (superficial)
The thin papillary layer consists of areolar connective tissue with fine elastic fibers. It features **dermal papillae**, finger-like projections that interlock with epidermal ridges to increase the surface area available for nutrient exchange. Some papillae contain Meissner's corpuscles, tactile receptors that detect light touch, while others contain capillary loops. On the palms, soles, and fingertips, dermal papillae produce **epidermal ridges** (friction ridges) that form the basis of fingerprints. These ridges enhance gripping ability and are genetically determined patterns unique to each individual.
#### Reticular Layer (deep)
The reticular layer is thick, making up about 80 percent of the dermis, and is composed of dense irregular connective tissue containing robust bundles of collagen fibers and coarse elastic fibers. This layer provides the skin's strength, extensibility, and elasticity. **Cleavage lines** (tension or Langer's lines) reflect the predominant orientation of collagen fiber bundles; surgical incisions made parallel to these lines heal better with less scarring. **Flexure lines** are skin creases at or near joints where the dermis is tightly bound to deeper structures. The reticular layer also contains Pacinian corpuscles (lamellated corpuscles) that detect deep pressure and vibration.
### V. Hypodermis (Subcutaneous Layer)
The hypodermis, also called superficial fascia or subcutis, lies beneath the skin. Although not part of the skin itself, it shares several of its functions. Composed of areolar connective tissue and **adipose tissue**, it anchors the skin to underlying muscles and bones, absorbs shock, and provides insulation. The hypodermis tends to be thicker in women than in men due to greater adipose deposition, and its distribution differs by sex. This layer is also the site of subcutaneous (hypodermic) injections.
### VI. Skin Color
Three pigments contribute to skin color. **Melanin**, produced by melanocytes, ranges from yellow to reddish-brown to black. All people have roughly the same number of melanocytes; differences in skin color arise from differences in the amount and type of melanin produced and how it is distributed. Eumelanin is brown-black, while pheomelanin is red-yellow. Melanin production is stimulated by UV exposure (the tanning response) and by melanocyte-stimulating hormone. Freckles and moles represent localized accumulations of melanin. **Carotene**, a yellow-orange pigment obtained from the diet, accumulates in the stratum corneum and hypodermis. **Hemoglobin**, the red pigment in circulating red blood cells, gives a pinkish hue to fair skin as blood flows through dermal capillaries.
Several clinical conditions alter skin color. Cyanosis, a bluish discoloration, results from low oxygen saturation in hemoglobin. Erythema, or redness, reflects increased blood flow as seen in fever, inflammation, or embarrassment. Pallor, or paleness, indicates decreased blood flow or a low red blood cell count. Jaundice, a yellow discoloration, signals excess bilirubin from liver disorders. A bruise (hematoma) results from blood that has escaped and clotted beneath the skin.
### VII. Appendages of the Skin
#### Hair
Hair is present nearly everywhere on the body except the palms, soles, lips, nipples, and parts of the external genitalia. It serves protective and sensory functions: scalp hair guards against UV radiation and heat loss, nostril and ear hairs filter particles, and hair follicle receptors detect light touch.
The visible portion above the skin surface is the **hair shaft**, while the **hair root** is the portion embedded within the follicle. Each hair consists of three concentric layers: the medulla at the core, the cortex forming the bulk and containing pigment, and the cuticle as the outer protective layer. The **hair follicle** comprises an inner epithelial root sheath and an outer connective tissue (dermal) root sheath. At the base, the **hair bulb** contains the hair matrix of actively dividing cells and the **hair papilla**, a dermal tissue supplying blood and growth signals. The **arrector pili muscle**, a smooth muscle attached to the follicle, contracts to pull the hair upright (producing goose bumps) and compresses the associated sebaceous gland.
Hair grows in a cycle consisting of an active growth phase (anagen, lasting 2 to 6 years for scalp hair), a regression phase (catagen, lasting 2 to 3 weeks as the follicle shrinks), and a resting phase (telogen, lasting 1 to 4 months, during which the hair is shed and replaced). On average, 70 to 100 scalp hairs are lost each day.
Hair color is determined by the type and amount of melanin. Dark hair contains large amounts of eumelanin, blonde hair has small amounts of pheomelanin, and red hair is rich in pheomelanin. Gray and white hair result from decreased melanin production and the accumulation of air bubbles within the hair shaft.
#### Nails
Nails are hard, keratinized plates on the dorsal tips of the fingers and toes that protect the distal phalanges, enhance fine touch sensation, and serve as tools for manipulation. The visible, translucent **nail body** (nail plate) appears pink because of underlying capillaries. The **free edge** overhangs the fingertip and appears white due to the absence of an underlying capillary bed. The **nail root** is the proximal portion embedded under the skin. The **nail bed** is the skin beneath the plate, while the **nail matrix**, the thickened proximal portion of the bed, is responsible for nail growth. The **lunula** is the whitish crescent at the proximal end overlying the thickened matrix. The **eponychium** (cuticle) is the fold of skin overlapping the proximal nail border, and the **hyponychium** is the thickened epidermis under the free edge.
#### Skin Glands
**Sebaceous (oil) glands** are found almost everywhere except the palms and soles, with most opening into hair follicles. They produce **sebum**, an oily mixture of lipids, cholesterol, proteins, and electrolytes that softens and lubricates hair and skin, slows water loss, and has bactericidal properties. Secretion is holocrine, meaning the entire cell ruptures to release its product. Sebaceous gland activity increases at puberty under androgen stimulation, and blockage of the glands can lead to acne.
**Eccrine (merocrine) sweat glands** are the most numerous sweat glands, distributed over most of the body with the highest density on the palms, soles, and forehead. These simple coiled tubular glands open directly to the skin surface and secrete sweat, which is about 99 percent water along with sodium chloride, urea, and other solutes. Their primary function is thermoregulation through evaporative cooling, and they secrete by merocrine (exocytosis) mechanisms. **Apocrine sweat glands** are found in the axillary, anogenital, and areolar regions. They are larger than eccrine glands, open into hair follicles, and produce a viscous secretion that is essentially odorless when released but is broken down by skin bacteria to produce body odor. They become active at puberty and are stimulated by the sympathetic nervous system during stress, pain, or sexual arousal. **Ceruminous glands** are modified apocrine glands in the ear canal that produce cerumen (earwax), while **mammary glands** are modified apocrine glands that produce milk.
A cross-sectional diagram of the skin showing all major structures. The epidermis is shown as a thin upper layer with strata labeled. The dermis is divided into the papillary layer (with dermal papillae and Meissner's corpuscles) and the reticular layer (with thick collagen bundles, Pacinian corpuscles, hair follicles, sebaceous glands, and sweat glands). A hair follicle is shown in detail with the hair shaft, root, bulb, papilla, arrector pili muscle, and associated sebaceous gland. An eccrine sweat gland is shown as a coiled structure deep in the dermis with a duct spiraling to the surface. An apocrine sweat gland is shown opening into a hair follicle. The hypodermis (subcutaneous layer) is below with adipose tissue and blood vessels.
### VIII. Burns
Burns are tissue damage caused by heat, chemicals, electricity, or radiation and are classified by depth.
#### Classification by Depth
A **first-degree (superficial) burn** affects only the epidermis, producing redness, swelling, and pain, as in a mild sunburn. It heals in 3 to 5 days without scarring. A **second-degree (partial-thickness) burn** damages the epidermis and part of the dermis, resulting in redness, blistering, swelling, and severe pain. It heals in 3 to 4 weeks if infection is prevented, though scarring may occur. A **third-degree (full-thickness) burn** destroys the entire epidermis and dermis and may extend into the hypodermis, muscle, or bone. The burned area appears white, brown, or black (charred) and is painless because the nerve endings have been destroyed. These burns cannot self-regenerate and require skin grafting.
#### Estimating Burn Severity: Rule of Nines
The rule of nines divides the body into regions, each representing approximately 9 percent (or a multiple) of the total body surface area. The head and neck account for 9 percent, each upper limb for 9 percent, the anterior trunk for 18 percent, the posterior trunk for 18 percent, each lower limb for 18 percent, and the perineum for 1 percent. **Critical burns** are those that are potentially life-threatening, including third-degree burns over more than 10 percent of body surface area, second-degree burns over more than 25 percent, and burns on the face, hands, feet, or perineum, burns involving joints, or inhalation burns.
### IX. Skin Cancer
Skin cancer is the most common type of cancer, primarily caused by UV radiation damage to DNA. **Basal cell carcinoma** is the most common and least malignant form, arising from the stratum basale. It presents as shiny, dome-shaped nodules that may develop a central ulcer. It grows slowly, rarely metastasizes, and has an excellent prognosis with surgical excision.
**Squamous cell carcinoma** arises from keratinocytes of the stratum spinosum and appears as a scaly, reddened, raised lesion. It grows more rapidly than basal cell carcinoma and can metastasize if not removed, though the prognosis is good when caught early and treated with surgical excision and/or radiation.
**Melanoma** is the most dangerous skin cancer, arising from melanocytes. Although it accounts for only about 5 percent of skin cancers, it causes the majority of skin cancer deaths because it is highly metastatic. The **ABCDE criteria** help identify suspicious moles: Asymmetry (one half unlike the other), Border irregularity (notched, blurred, or ragged edges), Color variation (multiple colors within the lesion), Diameter greater than 6 mm, and Evolving (changing in size, shape, or color). Treatment includes wide surgical excision, immunotherapy, and chemotherapy, with prognosis depending on the depth of invasion (Breslow thickness) and stage at diagnosis.
A diagram showing the ABCDE criteria for melanoma identification. Five panels, each showing a close-up of a skin lesion. Panel A (Asymmetry): a mole where one half is distinctly different from the other, with a line of symmetry drawn through it. Panel B (Border): a mole with irregular, notched, scalloped edges compared to a normal mole with smooth borders. Panel C (Color): a mole with multiple shades of brown, black, red, and white within the same lesion. Panel D (Diameter): a mole shown next to a pencil eraser for scale, exceeding 6 mm. Panel E (Evolving): two views of the same mole at different time points showing change in size and shape.