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integument system

skin plus its appendages


covers surface of body, composed of epidermis and dermis

epidermis and dermis interdigitate with each other to form irregular contour, dermal papillae project into epidermis to produce epidermal ridges--can be seen on finger tips with naked eye

hypodermis is not part of skin

appendages of skin

includes sweat glands, hair follicles, sebaceous glands, nails


stratified squamous keratinized epithelium, composed of several strata (basale, spinosum, granulosum, lucidum, corneum), forms superficial layer of skin

constantly regenerated by keratinocytes via mitotic activity (mostly at night)

stratum basale (germinativum)

deepest layer of cells in epidermis, attached directly to basal lamina by hemidesmosomes, cuboidal to columnar in shape, frequently undergoing division, also have melanocytes and merkel cells

stratum spinosum

above stratum basale, consists of few layers of polyhedral (prickle) keratinocytes, have extensions=intercellular bridges where desomosomes attach to each other, cells mitotically active, upper regions are keratinocytes with membrane-coating granules-->released into intercellular spaces-->cement cells together="water proofing" skin, also have langerhan cells

stratum malpighii

refers to stratum basale together with stratum spinosum

stratum granulosum

layer of epidermis where cells accumulate keratohyalin granules and bundles of intermediate keratin filaments aka tonofilaments, above stratum spinosum, become flattened, produce membrane-coating granules

keratohyalin granules

found in cells in stratum granulosum, enclosed in membrane, composed of histidine and cysteine rich proteins that appear to bind keratin filaments

stratum lucidum

above stratum granulosum in epidermis, clear homogenous layer, nuclei and organelles not present, cells contain eleidin--believed to be transformation product of keratohyalin

stratum corneum

outermost layer of epidermis, composed of scale-like dead cells (squames), each squame=14-sided polygon, surface of layers constantly being desquamated, desquamated region referred as stratum disjunctum


present in stratum basale, derived from neural crest, synthesize brown melanin pigment in melanosomes--oval shaped organelles, contain tyrosinase--UV sensitive enzyme involved in melanin synthesis

contain long processes-->extend between cells and transfer melanosomes to keratinocytes of stratum spinosum, process know as cytocrine secretion, melanosomes in keratinocytes degraded by lysosomes

cytocrine secretion

process where melanocytes transfer melanosomes to keratinocytes

Langerhans cells

dendritic shaped cells derived from bone marrow, present mainly in stratum spinosum, contain paddle-shaped membrane-bounded granules (Birbeck granules), present antigen to lymphocytes-->play role in contact allergic responses

Merkel cells

present in small numbers in stratum basale, near areas of connective tissue containing vessels and nerves, contain small dense-cored granules similar in appearance to those in adrenal medulla, receive afferent nerve terminals believed to function as sensory mechano-receptors


layer of skin under epidermis, consists of dense irregular connective tissue, contains collagen Type 1 and thick elastic fibers, divided into superficial papillary layer and deeper reticular layer, no distinct boundary between papillary and reticular layer, deep to reticular layer is superficial fascia=hypodermis=loose connective tissue containing fat cells-->not considered part of skin

normal cell turnover=2-4 weeks

papillary layer

superficial layer of dermis, uneven, forms dermal papillae that interdigitate with basal surface of epidermis, composed of thin loosly arranged fibers and cells, have Meissner's corpuscles--fine touch receptors, have capillary loops going with papillae

reticular layer

deep (and major) layer of dermis, contains thick, dense, irregular arrays of collagen fiber bundles, thick elastic fibers, have encapsulated nerve endings consisting of Pacinian corpuscles (pressure receptors) and Krause's end bulbs (cold and pressure receptors), have arrector pili muscle

Pacinian corpuscles

pressure receptors found in reticular layer of dermis

Krause's end bulbs

cold and pressure receptors found in reticular layer of dermis

arrector pili muscle

bundle of smooth muscle attached to hair follicles in dermis, extend superficially to underlie sebaceous glands, insert into papillary layer of dermis, contraction elevates hair, produce "goose bumps"

thick skin

thick epidermis with prominent stratum corneum, lines palms of hands and soles of feet, lacks hair follicles, sebaceous glands, arrector pili muscle bundles

thin skin

has thin epidermis, less prominent stratum corneum, present over most of body surface, contains hair follicles, sebaceous glands, arrector pili muscle bundles, seldom have stratum lucidum and stratum granulosum but have individual cells that show characteristic of these layers, dermis usually thicker than in thick skin

hair follicle

tubular invagination of epidermis extending deep into dermis, shaft of hair projects above surface of epidermis, hair consists of medulla, cortex, and cuticle, root of hair embedded in expanded hair bulb deeply indented by papilla of dermis

cells in hair bulb form inner epithelial root sheath and medulla, internal root sheath=epithelial structure lying deep to entrance of sebaceous gland composed of Henle's layer, Huxley's layer and cuticle, outer root sheath is direct continuation of stratum Malphighii of epidermis

next superficial layer=glassy (basement) membrane, outermost layer=connective tissue sheath (dermis)


composed of hard keratin lying on nail bed, eponychium=cuticle, hyponychium=keratinized epithelial layer under nail

cells in nail matrix at root responsible for growth of nail

eccrine sweat glands

distributed in skin throughout body, simple tubular coiled glands with secretory unit, composed of dark cells, clear cells, myoepithelial cells

dark cells line lumen, contain secretory granules

clear cells passes intercellular canaliculi-->extend to lumen of secretory unit

myoepithelial cells lie on basal lamina scattered beneath clear cells, underlie deark cells, rich in mitochondria and glycogen

apocrine glands

large specialized sweat glands located in various area of body: axilla, areola of nipple, circumanal region, do not begin to function until puberty, responsive to hormonal influences, have large coiled secretory portion enveloped by scattered myopithelial cells

empty viscous secretory products into hair follicles at location superficial to entry of ducts from sebaceous glands, innervated by adrenergic fibers, portion of cytoplasm becomes part of secretion

sebaceous glands

consist of several sacs (alveoli) that empty into a short duct-->empties into neck of hair follicle, cells at periphery of alveoli are flattened and inactive, near ducts cells undergo mitosis, daughter cells migrate to alveolus, produce secretory product (sebum) secreted by holocine secretion

ceruminous (wax) glands

glands of the external auditory canal

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