The Integumentary System

Comprehensive vocabulary flashcards covering the structure, layers, cell types, appendages, pigmentation, sensory receptors, and clinical burns of the human integumentary system.

Cutaneous Membrane

The skin, making up about $16\%$ of total body weight, composed of two major structural tissue layers: the superficial epidermis and the underlying dermis.

Acid Mantle

A protective, slightly acidic surface film on the skin with a pH of $4.2\text{--}5.6$ that kills invading bacteria before entry, maintains moisture, and combats dehydration.

Vasoconstriction

The narrowing of blood vessels that restricts blood flow into skin capillary beds in order to retain core body heat.

Vasodilation

The widening of blood vessels that increases blood flow to the skin capillary beds, facilitating heat loss across the epidermis alongside sweat gland activation.

Epidermis

The most superficial, avascular layer of the cutaneous membrane made of stratified squamous epithelium ($4\text{--}5$ cell layers thick) that resists abrasion and reduces water loss.

Keratinocyte

The primary cell type found within the epidermis that synthesizes keratin, a tough protective protein that hardens cells, resists abrasion, and forms a barrier against water loss.

Keratinization

The developmental process taking approximately $4\,\text{weeks}$ (range of $3\text{--}6\,\text{weeks}$) wherein living basal cells accumulate keratohyalin and keratin, lose their nuclei and organelles, die, and transition into superficial cornified cells.

Desquamation

The continuous sloughing off and shedding of older, dead keratinized cells from the superficial surface of the stratum corneum.

Stratum Basale

The deepest epidermal layer (also called the germinating layer) consisting of simple cuboidal or columnar keratinocytes that undergo mitosis every $19\,\text{days}$ and give rise to upper strata within $45\text{--}60\,\text{days}$.

Stratum Spinosum

An epidermal stratum consisting of $8\text{--}10$ layers of many-sided cells exhibiting limited mitosis, producing keratin fibers, and forming lipid-filled lamellar bodies that create a water barrier.

Stratum Granulosum

An epidermal layer composed of $2\text{--}5$ layers of flattened, diamond-shaped cells where lamellar bodies excrete lipids into the extracellular matrix, keratohyalin forms a protein envelope, organelles break down, and cells die.

Stratum Lucidum

A clear, thin epidermal stratum consisting of $3\text{--}5$ layers of flat, dead cells containing dispersed keratohyalin, present only in the thick skin of the palms, fingertips, and soles of the feet.

Stratum Corneum

The most superficial epidermal layer composed of $25+$ layers of dead squamous cells completely filled with soft keratin (cornified cells) that provide structural strength and act as a waterproof barrier.

Thick Skin

Skin that contains all five epithelial strata (including a distinct stratum lucidum and heavily thickened stratum corneum), lacks hair follicles, and covers friction-prone areas like the palms, soles, and fingertips.

Thin Skin

Flexible skin covering the majority of the body that lacks a stratum lucidum, possesses a thinner stratum granulosum ($1\text{--}2$ cell layers), and contains hair follicles.

Dermis

The vascularized, dense connective tissue layer beneath the epidermis that contains collagen, elastic, and reticular fibers, provides skin strength and flexibility, and houses nerve endings, glands, and hair follicles.

Dermal Papillae

Small, finger-like projections of loose connective tissue in the upper dermis that interlock with epidermal ridges, increase surface area, supply capillaries, and house touch receptors.

Papillary Layer

The superficial region of the dermis composed of loose connective tissue and dermal papillae, supplying blood vessels to the avascular epidermis and housing Meissner's corpuscles.

Reticular Layer

The deeper, thicker dermal region composed of dense irregular connective tissue arranged in an interlacing, net-like formation that resists multidirectional stretching.

Hypodermis

The subcutaneous layer beneath the dermis (not technically part of the skin) composed of loose connective adipose tissue that anchors skin to underlying muscle and bone, stores about half of body fat, and acts as a shock absorber.

Lanugo Hair

Delicate, unpigmented fetal hair that covers the human fetus by $5\text{ or }6\,\text{months}$ of gestation before being replaced.

Vellus Hair

Short, fine, unpigmented hair that replaces fetal lanugo over most of the body surfaces.

Terminal Hair

Long, coarse, pigmented hair found on the scalp, eyebrows, and eyelids that also replaces vellus hair in the axillary and pubic regions during puberty under androgen stimulation.

Hair Bulb

The expanded, deep bulbous base of the hair root inside the follicle that houses undifferentiated epithelial matrix cells and surrounds the dermal hair papilla.

Hair Papilla

A projection of dermis containing capillaries that protrudes into the hair bulb to supply essential nutrients for mitotic cell division and hair growth.

Cuticle (Hair)

The single outermost layer of the hair shaft composed of hard keratinized cells whose edges overlap like shingles to hold underlying layers together.

Cortex (Hair)

The intermediate layer that forms the bulk of the hair shaft, composed of tightly packed cells containing hard keratin and pigment granules.

Medulla (Hair)

The innermost central axis of the hair shaft, consisting of $2\text{ or }3$ layers of cells containing soft keratin.

Arrector Pili Muscle

A band of smooth muscle extending from the dermal root sheath to the dermal papillae that contracts in response to cold or emotional fright, pulling the hair upright to a $90^\circ$ angle.

Merocrine Gland

An exocrine gland type that secretes its chemical products via exocytosis without any loss of cellular cytoplasm or cell parts (e.g., eccrine sweat glands, pancreas).

Apocrine Gland

An exocrine gland type that discharges secretions by pinching off the apical cytoplasm fragments of the secretory cells (e.g., mammary glands, axillary sweat glands).

Holocrine Gland

An exocrine gland type whose secretions consist of accumulated cellular products released when the entire secretory cell ruptures and dies (e.g., sebaceous glands).

Sebaceous Glands

Alveolar holocrine glands in the dermis with ducts opening into hair follicles that produce sebum, an oily lipid-rich substance that lubricates hair, prevents desiccation, and inhibits bacteria.

Eccrine Sweat Glands

Simple coiled tubular merocrine glands that open directly onto the skin surface via sweat pores, producing hypotonic water and $\text{NaCl}$ sweat for body temperature regulation.

Apocrine Sweat Glands

Coiled tubular glands in the axillary and pubic regions that empty into hair follicles above sebaceous glands, becoming active at puberty and producing secretions broken down by bacteria into body odor.

Ceruminous Glands

Modified apocrine sweat glands located within the external ear canal that synthesize cerumen (earwax) to protect the ear from debris and foreign pathogens.

Nail Matrix

The thickened proximal epithelial tissue of the nail root containing stratum basale that continuously produces hard keratin cells driving nail elongation at $0.5\text{--}1.2\,\text{mm/day}$.

Lunula

The whitish, crescent-shaped region at the proximal base of the nail body whose pale appearance is caused by the thick underlying nail matrix obscuring dermal capillary beds.

Hyponychium

The thickened stratum corneum region located underneath the free distal edge of the nail plate.

Melanocyte

An irregularly branched pigment-producing cell situated in the stratum basale that synthesizes melanin and transfers it to neighboring keratinocytes.

Melanosome

A membrane-bound vesicle packaged by the Golgi apparatus of a melanocyte that carries melanin pigment before being phagocytosed by keratinocytes.

Eumelanin

A class of melanin pigment that imparts dark brown to black hues to human hair and skin.

Pheomelanin

A class of melanin pigment that produces yellow, red, and light brown coloration in human hair and skin.

Erythema

Skin redness resulting from an increase in cutaneous blood flow (vasodilation) through dermal capillary beds.

Cyanosis

A bluish or pale skin discoloration caused by significantly decreased blood flow or reduced oxygen saturation of hemoglobin.

Carotene

A yellow-orange, lipid-soluble plant pigment and source of vitamin A that accumulates in the stratum corneum and hypodermal adipose tissue when ingested in large quantities.

Free Nerve Endings

The simplest and most abundant cutaneous exteroceptors, consisting of unencapsulated neuron dendrites that detect temperature sensations and painful stimuli.

Merkel Discs

Epidermal tactile sensory receptors located in the stratum basale that respond to light touch and superficial pressure.

Meissner's Corpuscles

Encapsulated receptors situated in the dermal papillae that detect light touch, two-point discrimination, and texture evaluation, particularly dense in the fingertips and tongue.

Pacinian Corpuscles

Large, layered, onion-like sensory corpuscles located deep in the dermis or hypodermis that respond to deep cutaneous pressure, body position, and vibrations.

Ruffini End Organs

Cutaneous sensory receptors in the dermis (prominent in the fingers) that detect continuous pressure and tissue stretch to assist in grip regulation.

First-Degree Burn

A partial-thickness burn restricted strictly to the epidermis, characterized by redness, pain, and slight edema, which typically heals within a week without scarring.

Second-Degree Burn

A partial-thickness burn damaging both the epidermis and the upper dermis, resulting in redness, localized pain, edema, and blister formation, usually healing within two weeks without scars.

Third-Degree Burn

A full-thickness burn completely destroying both the epidermis and dermis into deeper structures; it is characteristically painless at its center due to destroyed sensory receptors and regenerates only from wound margins.