acne: skin condition due to infected sebaceous glands

albinism: genetic disorder that affects the skin, in which there is no melanin production

anagen: active phase of the hair growth cycle

apocrine sweat gland: type of sweat gland that is associated with hair follicles in the armpits and genital regions

arrector pili: smooth muscle that is activated in response to external stimuli that pull on hair follicles and make the hair “stand up”

basal cell carcinoma: cancer that originates from basal cells in the epidermis of the skin

basal cell: type of stem cell found in the stratum basale and in the hair matrix that continually undergoes cell division, producing the keratinocytes of the epidermis

bedsore: sore on the skin that develops when regions of the body start necrotizing due to constant pressure and lack of blood supply; also called decubitis ulcers

callus: thickened area of skin that arises due to constant abrasion

catagen: transitional phase marking the end of the anagen phase of the hair growth cycle

corn: type of callus that is named for its shape and the elliptical motion of the abrasive force

cortex: in hair, the second or middle layer of keratinocytes originating from the hair matrix, as seen in a cross-section of the hair bulb

cuticle: in hair, the outermost layer of keratinocytes originating from the hair matrix, as seen in a cross-section of the hair bulb

dermal papilla: (plural = dermal papillae) extension of the papillary layer of the dermis that increases surface contact between the epidermis and dermis

dermis: layer of skin between the epidermis and hypodermis, composed mainly of connective tissue and containing blood vessels, hair follicles, sweat glands, and other structures

desmosome: structure that forms an impermeable junction between cells

eccrine sweat gland: type of sweat gland that is common throughout the skin surface; it produces a hypotonic sweat for thermoregulation

eczema: skin condition due to an allergic reaction, which resembles a rash

elastin fibers: fibers made of the protein elastin that increase the elasticity of the dermis

eleiden: clear protein-bound lipid found in the stratum lucidum that is derived from keratohyalin and helps to prevent water loss

epidermis: outermost tissue layer of the skin

eponychium: nail fold that meets the proximal end of the nail body, also called the cuticle

external root sheath: outer layer of the hair follicle that is an extension of the epidermis, which encloses the hair root

first-degree burn: superficial burn that injures only the epidermis

fourth-degree burn: burn in which full thickness of the skin and underlying muscle and bone is damaged

glassy membrane: layer of connective tissue that surrounds the base of the hair follicle, connecting it to the dermis

hair bulb: structure at the base of the hair root that surrounds the dermal papilla

hair follicle: cavity or sac from which hair originates

hair matrix: layer of basal cells from which a strand of hair grows

hair papilla: mass of connective tissue, blood capillaries, and nerve endings at the base of the hair follicle

hair root: part of hair that is below the epidermis anchored to the follicle

hair shaft: part of hair that is above the epidermis but is not anchored to the follicle

hair: keratinous filament growing out of the epidermis

hypodermis: connective tissue connecting the integument to the underlying bone and muscle

hyponychium: thickened layer of stratum corneum that lies below the free edge of the nail

integumentary system: skin and its accessory structures

internal root sheath: innermost layer of keratinocytes in the hair follicle that surround the hair root up to the hair shaft

keloid: type of scar that has layers raised above the skin surface

keratin: type of structural protein that gives skin, hair, and nails its hard, water-resistant properties

keratinocyte: cell that produces keratin and is the most predominant type of cell found in the epidermis

keratohyalin: granulated protein found in the stratum granulosum

Langerhans cell: specialized dendritic cell found in the stratum spinosum that functions as a macrophage

lunula: basal part of the nail body that consists of a crescent-shaped layer of thick epithelium

Meissner corpuscle: (also, tactile corpuscle) receptor in the skin that responds to light touch

Merkel cell: receptor cell in the stratum basale of the epidermis that responds to the sense of touch

medulla: in hair, the innermost layer of keratinocytes originating from the hair matrix

melanin: pigment that determines the color of hair and skin

melanocyte: cell found in the stratum basale of the epidermis that produces the pigment melanin

melanoma: type of skin cancer that originates from the melanocytes of the skin

melanosome: intercellular vesicle that transfers melanin from melanocytes into keratinocytes of the epidermis

metastasis: spread of cancer cells from a source to other parts of the body

nail bed: layer of epidermis upon which the nail body forms

nail body: main keratinous plate that forms the nail

nail cuticle: fold of epithelium that extends over the nail bed, also called the eponychium

nail fold: fold of epithelium at that extend over the sides of the nail body, holding it in place

nail root: part of the nail that is lodged deep in the epidermis from which the nail grows

Pacinian corpuscle: (also, lamellated corpuscle) receptor in the skin that responds to vibration

papillary layer: superficial layer of the dermis, made of loose, areolar connective tissue

reticular layer: deeper layer of the dermis; it has a reticulated appearance due to the presence of abundant collagen and elastin fibers

rickets: disease in children caused by vitamin D deficiency, which leads to the weakening of bones

scar: collagen-rich skin formed after the process of wound healing that is different from normal skin

sebaceous gland: type of oil gland found in the dermis all over the body and helps to lubricate and waterproof the skin and hair by secreting sebum

sebum: oily substance that is composed of a mixture of lipids that lubricates the skin and hair

second-degree burn: partial-thickness burn that injures the epidermis and a portion of the dermis

squamous cell carcinoma: type of skin cancer that originates from the stratum spinosum of the epidermis

stratum basale: deepest layer of the epidermis, made of epidermal stem cells

stratum corneum: most superficial layer of the epidermis

stratum granulosum: layer of the epidermis superficial to the stratum spinosum

stratum lucidum: layer of the epidermis between the stratum granulosum and stratum corneum, found only in thick skin covering the palms, soles of the feet, and digits

stratum spinosum: layer of the epidermis superficial to the stratum basale, characterized by the presence of desmosomes

stretch mark: mark formed on the skin due to a sudden growth spurt and expansion of the dermis beyond its elastic limits

sudoriferous gland: sweat gland

telogen: resting phase of the hair growth cycle initiated with catagen and terminated by the beginning of a new anagen phase of hair growth

third-degree burn: burn that penetrates and destroys the full thickness of the skin (epidermis and dermis)

vitamin D: compound that aids absorption of calcium and phosphates in the intestine to improve bone health

vitiligo: skin condition in which melanocytes in certain areas lose the ability to produce melanin, possibly due an autoimmune reaction that leads to loss of color in patches