The Integumentary System

Dermatology

The scientific study and medical treatment of the integumentary system.

Epidermis

The superficial, avascular layer of the skin composed of keratinized stratified squamous epithelium.

Dermis

The deeper connective tissue layer of the skin beneath the epidermis, composed mainly of collagen and ranging from $0.2\,\text{mm}$ to $4\,\text{mm}$ in thickness.

Hypodermis

A layer of areolar and adipose connective tissue beneath the skin that pads the body, binds skin to underlying tissues, and serves as a common site for drug injection.

Thick Skin

Skin covering the palms, soles, and corresponding surfaces of digits, featuring an epidermis about $0.5\,\text{mm}$ thick, sweat glands, a stratum lucidum, and lacking hair follicles or sebaceous glands.

Thin Skin

Skin covering most of the body with an epidermis about $0.1\,\text{mm}$ thick, possessing hair follicles, sebaceous glands, and sweat glands, but lacking a stratum lucidum.

Acid Mantle

A protective acidic film on the surface of the skin with a $\text{pH}$ range of $4\text{ to }6$ that inhibits bacterial growth.

Keratinocytes

The great majority of epidermal cells, responsible for synthesizing the fibrous protective protein keratin.

Epidermal Stem Cells

Undifferentiated cells located in the stratum basale of the epidermis that divide and give rise to keratinocytes.

Melanocytes

Cells located only in the stratum basale that synthesize the brown-to-black pigment melanin to shield cellular DNA from ultraviolet radiation.

Tactile (Merkel) Cells

Touch receptor cells found in the basal layer of the epidermis that are associated with underlying dermal nerve fibers.

Dendritic (Langerhans) Cells

Phagocytic immune cells found in the stratum spinosum and stratum granulosum that guard against toxins and microbes.

Stratum Basale

The deepest epidermal layer, consisting of a single layer of cuboidal to columnar stem cells and keratinocytes resting on the basement membrane.

Stratum Spinosum

An epidermal layer composed of several strata of keratinocytes attached by desmosomes and tight junctions, named for the spiny histological appearance produced during preparation.

Stratum Granulosum

An epidermal layer consisting of three to five layers of flat keratinocytes containing dark-staining keratohyalin granules.

Stratum Lucidum

A thin, clear, featureless zone of the epidermis found only in thick skin, containing cells densely packed with the clear protein eleidin.

Stratum Corneum

The most superficial layer of the epidermis, comprising up to $30$ layers of dead, scaly, keratinized cells that resist abrasion, penetration, and water loss.

Dander

Tiny specks of dead keratinocytes that exfoliate (flake off) from the epidermal surface.

Filaggrin

A protein released by keratohyalin granules in the stratum granulosum that bundles keratin filaments into tough bundles.

Epidermal Water Barrier

A functional barrier created by tight junctions, cell envelope proteins, and an extracellular lipid mixture released in the stratum granulosum that prevents body dehydration.

Dermal Papillae

Upward, finger-like extensions of the dermis that interlock with the downward waves of the epidermis.

Epidermal Ridges

Downward wave-like projections of the epidermis that interlock with the dermal papillae.

Papillary Layer

The superficial one-fifth of the dermis composed of areolar tissue, allowing mobility for leukocytes and rich in small blood vessels.

Reticular Layer

The deeper, thicker four-fifths of the dermis composed of dense irregular connective tissue.

Striae

Stretch marks formed by tears in the collagen fibers of the dermis resulting from severe stretching during obesity or pregnancy.

Subcutaneous Fat

Subcutaneous tissue composed predominantly of adipose tissue that provides thermal insulation and energy storage.

Autograft

A tissue graft taken from another location on the same person's body to repair skin loss.

Allograft

A skin graft obtained from an unrelated person, most commonly a deceased donor.

Eumelanin

The dark brown to black form of melanin pigment responsible for tan, brown, and black skin and hair colors.

Pheomelanin

A reddish-yellow, sulfur-containing melanin pigment responsible for yellow to red tones in hair and skin.

Cyanosis

A blueness of the skin resulting from an oxygen deficiency in the blood.

Erythema

Abnormal redness of the skin caused by increased blood flow in dilated cutaneous blood vessels.

Pallor

An ashen or pale skin color that occurs when there is significantly diminished blood flow through the skin.

Albinism

A genetic condition characterized by a total lack of melanin synthesis, resulting in white skin, hair, and unpigmented eyes.

Jaundice

Yellowing of the skin and whites of the eyes caused by high bilirubin levels in the blood, frequently associated with compromised liver function.

Hematoma

A visible bruise produced by a mass of clotted blood trapped under the skin.

Friction Ridges

Markings on the fingertips, palms, and soles formed during fetal development that leave oily fingerprints and enhance grip and touch sensitivity.

Flexion Lines

Grooves on the flexor surfaces of digits, palms, wrists, and elbows marking sites where skin folds during joint movements.

Hemangiomas

Patches of discolored skin (birthmarks) caused by benign tumors of the dermal blood capillaries.

Pilus

A slender filament of keratinized cells that grows from an epidermal tube called a hair follicle.

Lanugo

Fine, downy, unpigmented hair that appears on the fetus in the last $3\,\text{months}$ of prenatal development.

Vellus Hair

Fine, pale hair that replaces lanugo by the time of birth, constituting two-thirds of the hair of women and one-tenth of the hair of men.

Terminal Hair

Longer, coarser, and more heavily pigmented hair forming eyelashes, eyebrows, scalp hair, and post-pubertal axillary and pubic hair.

Hair Bulb

A dilated swelling at the deep base of the hair root where hair originates in the dermis or hypodermis.

Hair Matrix

The mitotically active growth center of a hair situated immediately superior to the dermal papilla.

Hair Medulla

The innermost central core of a hair, composed of loosely arranged cells and air spaces.

Hair Cortex

The bulk of the hair shaft surrounding the medulla, composed of several layers of elongated, keratinized cells.

Hair Cuticle

The outermost layer of a hair composed of multiple layers of thin, scaly cells that overlap like shingles on a roof.

Arrector Muscle

A bundle of smooth muscle cells connecting the dermal collagen fibers to a hair follicle that contracts to make the hair stand erect.

Anagen

The active growth stage of the hair cycle lasting $6\text{ to }8\,\text{years}$, during which stem cells multiply and follicle cells form hair matrix.

Catagen

The degenerative stage of the hair cycle lasting $2\text{ to }3\,\text{weeks}$, during which mitosis ceases and the hair base keratinizes into a club hair.

Telogen

The resting stage of the hair cycle lasting $1\text{ to }3\,\text{months}$, where the dermal papilla ascends to the bulge and the club hair eventually sheds.

Club Hair

A hair whose base has keratinized into a hard knot, detached from the hair matrix, and is easily pulled out during brushing.

Alopecia

Thinning of the hair or baldness across the scalp or body.

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