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