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largest organ of the body (2sq meters ... 22 sq ft); weights 10-11lbs in adults; it ranges from .5mm to 4mm thick; composed of the epidermis (epithelial tissue), the dermis (connective tissue; fibers that extend down into subcatenous layer), and the subcutaneous layer (not part of the skin; areolar and adipose tissue)


composed of keratinized stratified squamous epithelium; contains 4 principal types of cells: keratinocytes, melanocytes, Langerhans cells, and merkel cells


90% of epidermal cells, which are arranged in four or five layers and produce the protein keratin; produce lamellar granules, which release a water-repellent sealant that decreases water entry and loss and inhibits the entry of foreign materials


a tough, fibrous protein that helps protect the skin and underlying tissues from heat, microbes, and chemicals


8% of epidermal cells; produce the pigment melanin; transfer melanin to keratinocytes via long, slender projections


a yellow-red or brown-black pigment that contributes to skin color and absorbs damaging ultraviolet light

Langerhans cells

arise from red bone marrow and migrate to the epidermis; they participate in immune responses mounted against microbes that invade the skin, and are easily damaged by UV light; their role in teh immune response is to help other cells of the immune system recognize an invading microbe and destroy it

Merkel cells

located in the deepest layer of the epidermis, where they contact the flattened process of a sensory neuron, a structure called a merkel disc; merkel cells and their associated merkel discs detect touch sensations

Five layers of the epidermis

(from deep to superficial) stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, stratum corneum

Stratum Basale

deepest layer of the epidermis; composed of a singel row of cuboidal or columnar keratinocytes; ; some cells in this layer are stem cells that undergo cell division to continually produce new keratinocytes; melanocytes and merkel cells with their associated merkel discs are scattered among the keratinocytes of this layer; is also known as the stratum germinativum to indicate its role in forming new cells

Stratum Spinosum

superficial to stratum basale; arranged in 8 to 10 layers of many-sided keratinocytes fitting closely together

Stratum granulosum

at about the middle of the epidermis, consists of three to five layers of flattened keratinocytes that are undergoing apoptosis (cell death); the nuclei and other organelles of these dieing cells begin to degenerate, and tonofilaments become more apparent; presence of keratohyalin, which converts the tonofilaments of dieing cells into keratin;

Stratum lucidum

is present only in the thick skin of areas such as the fingertips, palms, and soles; consists of 3 to 5 layers of flattened clear, dead keratinocytes that contain large amounts of keratin and thickened plasma membranes

Stratum corneum

consists on average of 25 to 30 layers of flattened dead keratinocytes; these cells are continuously shed and replaced by cells from teh deeper strata; the interior of the cells contain mostly keratin; between the cells are lipids from lamellar granules that help make this layer an efefctive water-repellent barrier


newly formed cells in the stratum basale are slowly pushed to the surface; as the cells move from one epidermal layer to the next, they accumulate mroe and more keratin; then they undergo apoptosis; eventually the keratinized cells slough off and are replaced by underlying cells that in tern become keratinized; process takes 2-4 weeks on average; epidermal growth factors play a role in regulating this


the second, deeper part of the skin; is composed of a strong connective tissue containing collagen and elastic fibers; the few cells present include predominantly fibroblasts, with some macrophages, and a few adipocytes; blood vessels, nerves, glands, and hair follicles (epithelial invaginations of the epidermis) are embedded in the dermal layer; thickest in the palms, soles, posterior aspect of the body, lateral/medial sides of limbs; composed of the papillary region and the reticular region

Papillary region

makes up about 1/5th of the thickness of the dermis; it consists of areolar connective tissue containing thin collagen and fine elastic fibers; its surface area is greatly increased by dermal papillae (nipple-like structures), small, fingerlike structures that project into the undersurface of the epidermis

Reticular region

attached to the subcutaneous layer, consists of dense irregular connective tissue containing fibroblasts, bundles of collagen, and some coarse elastic fibers; a few adipose cells, hair follicles, nerves, sebaceous (oil) glands, and sudoriferous (sweat) glands occupy the spaces between fibers

Subcutaneous layer

consists of areolar and adipose tissue; srves as a storage depot for fat and contains large blood vessels that supply the skin


(pili) present on most skin surfaces except the palms, palmar surfaces of the fingers, the soles, and plantar surfaces of the feet

Anatomy of a hair

each is composed of columns of dead, keratinized epidermal cells bonded together by extracellular proteins; the shaft is the superficial portion of the hair, which projects above the surfeace of the skin; the root is the portion of the hair deep to the shaft that penetrates into the dermis, and sometimes into the subcutaneous layer

Hair shaft

the superficial portion of the hair, which projects above the surface of the skin; composed of 3 layers: cuticle (outside layer), cortex (middle layer), and medulla (inside layer)


single layer of flat, thin, heavily keratinized cells; arranged like "shingles on a house"


major part of the shaft; elongated cells; contain pigments


2-3 rows of polyhedral cells; pigmented; part of the hair shaft

Epithelial root sheath

surrounding the root of the hair is the hair follicle, which is made up of an external root sheath and an internal root sheath; together referred to as an ....

External root sheath

downward extension of epidermis; at base of follicle only found in stratum basale

Internal root sheath

is produceed by the matrix and forms a cellular tubular sheath of epithelium between teh external root sheath and the hair


the base of hair follicle; houses a nipple-shaped indentation, the papilla of the har, which contains areolar connectieve tissue and many blood vessels that nourish teh growing hair follicle; also contains a germinal layer of cells called the hair matrix; the hair matrix cells arise from teh stratum basale, they produce new hairs when old hairs are shed, the matrix also gives rise to the cells of the internal root sheath

Structures associated with hair

sebaceous glands (oil), the arrector pili (smooth muscle), surrounding each hair follicle are dendrites of neurons, called hair root plexuses, that are sensitive to touch

Exocrine glands associated with the skin

sebaceous (oil) glands, sudoriferous (sweat) glands, ceruminous glands, and mammary glands

Sebaceous glands

(oil glands) connected to hair follicles; the secreting portion of the gland lies in the dermis and usually opens into the neck of a hair follicle; in some location, the glands open directly onto the surfeace of the skin (lips, gland penis, labia minora, and tarsal glands of eyelids); these glands are absent in teh palms and soles; they secrete and oily substance called sebum, a mixture of triglycerides, cholesterol, proteins, and inorganic salts; sebum coats the surfeace of hairs and helps keep them from drying and becoming brittle, prevents excessive evaporation of water from teh skin, keeps teh skin soft and pliable, and inhibits the growht of some (but not all) bacteria

Sudoriferous glands

(sweat glands) there are 3-4 million of em; divided into accrine and apocrine glands

Eccrine sweat glands

much more common than apocrine sweat glands; they are distributed throughout the skin of most regions of the body, especially in teh skin of the forehead, palms, and soles; they are not present in teh margin of the lips, nail beds of the fingers and toes, glans penis, glans clitoris, labia minora, and eardrums; the main function of these glands is to help regulate body temperature through evaporation

Apocrine sweat glands

are found mainly in teh skin of teh axilla (armpit), groin, areolae (pigmented areas around nipples) of the breasts, and beareded regions of the face in adults males; these glands do not begin to function until puberty


h2o, ions, urea, uric acid, ammonia, glucose, lactic acid, ascorbic acid

Ceruminous glands

modified sweat glands in teh external ear; produce a waxy lubricating secretion; the secretion, together with hairs in teh external auditory canal, provides a stick barrier that impedes the entracnce of foreign bodies and insects; also waterproofs the canal and prevents bacteria and fungi from entering cells

Mammary glands

(milk) modified sweat glands

Functions of the skin

body temperature regulation, reservoir for blood, protection, cutabeous sensations, excretion and absorption, vitamin D synthesis

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