Lecture 07

Epidermis

Epidermis

-outermost layer

-composed of epithelium

-avascular

-protects underlying layers

Dermis

-underlying layers

-vascular & innervated

-provides strength and resilience

-composed of CT

-contains smooth muscles (associated w/ hair follicles) and nerve fibers to detect and monitor sensory input

Subcutaneous layer (hypodermis)

-composed of areolar and adipose CT

-not technically part of the skin

-anchors the skin to underlying structures

-acts as a shock absorber & insulator

Function of the ___

Protection -chemical barrier -physical barrier -biological barrier -temp. barrier -radiation (UV) barrier

Prevention of Water Loss -water resistant

Metabolic Regulation -vitamin D production -other minor metabolic roles

Secretion & Absorption -small amounts of metabolic waste (water, salts, urea, etc.) -selectively permeable

Immunity -identify and destroy pathogens -alerts immune system & initiates immune response

Epidermis

Function of the ___

Temp. Regulation -blood vessels -sweat glands -adipose

Sensory Reception -mechanoreceptor (touch, pressure, vibration, etc.) -nociceptor (pain) -thermoreceptor (temperature)

Dermis

Keratinocytes (cells of epidermis)

-most abundant

-produces keratin (fibrous protein that forms intermediate filaments) that provides the protective properties of the skin

-tightly connected to one another by desmosomes

Melanocyte (cells of epidermis)

-have long, branching spiderlike processes

-synthesize and store up the pigment of melanin in response to UV light

-melanin granules are taken up by keratinocytes & accumulate on the superficial side of the nucleus to protect the nucleus from UV light

From outer-to innermost layer

stratum corneum stratum lucidum stratum granulosum stratum spinosum stratum basale

Epidermis consists of

-keratinized stratified, squamous epithelium

-3 innermost layer consists of living cells

-two outermost layers are dead cells

Stratum Basale

-deepest layer

-attached to a basement membrane that separates it from the underlying dermis

-epidermal ridges increase grip

-single layer of cuboidal to low columnar cells *(most are keratinocytes) *(small population of melanocytes)

Stratum Spinosum

-overlies the stratum basale

-several layers thick

-keratinocytes from basale differentiate into non-dividing, specialized keratinocytes

-attached to neighbors by desmosomes results in spiny appearance

Stratum Granulosum

-3 to 5 layers thick

-keratinization *(keratinocytes produce keratohyalin (precursor to keratin) granules) *(nucleus and organelles begin to disintegrate) *(cells begin to die, due to lack of organelles & being pushed further away from source of nutrients)

-won't be complete until after keratinocytes rise to superficial layer

Stratum Lucidum

-thin, clear layer about 2-3 cell layers thick *(protects against friction) *(found only in thick skin on palms of hands and soles of feet)

-cells have become flattened and featureless

-filled with eleidin (an intermediate protein formed by keratohyalin) during keratin maturation *(helps protect skin from UV light)

Stratum Corneum

-most superficial layer

-individual keratinocyte from stratum basale to strata corneum exists for about 4 weeks *(lost due to desaquamation)

-15 to 30 layers of dead, scaly interlocked keratinized cells *(cells are anucleate) *(tightly packed together) *(contain large amounts of keratin)

-protective, durable overcoat *(thickened plasma membrane enhanced by special glycoproteins waterproofs the strata corneum) *(relatively insensitive to biological, chemical, and physical assault)

Thick Skin

-5 layers (contains stratum lucidum

-found on palms of hands and soles of feet

-contains sweat glands

-lacks hair follicles and sebaceous glands

Thin Skin

-4 layers (lacks stratum lucidum)

-covers most of the body

-contains sweat glands, hair follicles, and sebaceous glands

Epidermal dendritic (Langerhans')

-found within strata spinosum and granulosum

-phagocytic cell capable of stimulating immune response

Melanin

-only pigment made in the skin

-delivered to keratinocytes within melanosomes

-everyone has the same relative number of melanocytes

-varying shades of skin color reflects the amount of melanin produced and retained

-freckles and pigmented moles are local accumulations of melanin

Eumelanin

-black/brown pigment

Pheomelanin

-red/yellow pigment

Carotene

-yellow/orange pigment found in certain plants

-accumulates in the stratum corneum & fatty tissues of the hypodermis

Tactile discs

-Merkel (tactile) cells (in stratum basale)

-mechanoreceptors

-small receptive fields

-response to tactile stimulation help determine shape and texture of object *(fine touch) *(pressure)

Free nerve endings

-responsible for pain

-tickling (light touch)

-itching (noxious stimuli)

Epidermis produces Vitamin D

-vitamin D3 produced in the skin on exposure to UV light from a cholesterol precursor *(may also be absorbed in diet)

-converted to its active form by 2 enzymatic reactions *(one in liver) *(one in kidney)

-increases intestinal reabsorption of Ca2+ and (PO4)3-

Dermis

-cells are typical of those found in CT proper

-richly innervated, vascularized

-possesses lymphatic vessels

-2 layers *(papillary layer) *(reticular layer)

Papillary layer

-located near the superior surface

-areolar CT

Dermal papillae

-small projections indenting into epidermis

-contains free nerve endings = pain receptors, touch receptors

-interdigit with epidermal ridges to increase area of contact and interlock the 2 layers

Reticular layer

-dense irregular CT underlying the papillary layer of the dermis *(mostly collagen fibers running parallel to the skin surface) -> creates "cleavage lines" *(contains elastin providing elasticity)

-highly vascularized and innervated *(nerves allow us to distinguish diff. types of sensory stimuli) *(blood vessels supply nutrients for dermis and epidermis) *(blood vessels also play a role in temp. regulation)

-possesses flexure lines *(dermal folds where dermis is tightly secured to deeper structures)

Meissner's (tactile) corpuscle

-dermal papillae -light touch, pressure, and vibrations

Pacinian (lamellated) corpuscle

-reticular layer of dermis -deep pressure & vibration

Ruffiini corpuscle

-reticular layer among collagen bundles -pressure and skin distortion

Hair

-flexible strands of dead keratinized cells produced by the hair follicle *("hard keratin") *(tougher and more durable) *(individual cells do not flake off)

Function of ___ -protection *(head) sunburn and injury *(nasal and ear) traps particles *(eyes) prevent sweat and particles from endangering the eye

-heat retention *(acts like a cap to prevent heat from escaping scalp)

-sensory reception *(tactile receptors that detect light touch)

-visual identification *(determining age and sex) *(identifying individuals)

-chemical signal dispersion *(pheromones in axillary and pubic regions)

Hair

Shaft (structure of hair)

-projects above skin surface

-dead epithelial cells

Root (structure of hair)

-lies below the skin surface

-dead epithelial cells

Hair bulb (structure of hair)

-region at base of hair follicle

-located deep in dermis

-consists of living epithelial cells

-origin of hair

Hair papilla (structure of hair)

-possessing capillaries supplying nutrients and nerves (root hair plexus) of hair

-hair matrix contains cells that become the hair shaft

-contain hard keratin proteins

Medulla (structure of hair)

-innermost portion

-remnant of the matrix

-loosely arranged cells containing soft, flexible keratin and air pockets

Cortex (structure of hair)

-external to the medulla

-several layers of flattened cells

-hard keratin

Cuticle (structure of hair)

-outer layer consisting of a single layer of cells

-overlap one another (like shingles) away from the root

Hair follicle

-folds down from the epidermis into the dermis

-wall has 2 layers *(connective tissue root sheath) *(epithelial root sheath)

-arrector pili muscle

Connective tissue root sheath

-derived from the dermis

Epithelial root sheath

-derived from the epidermis

Arrector pili muscle

-responsible for contraction of the hair follicle

Anagen

-active phase

-producing new hairs

-pushes club hair out of follicle

Catagen

-end of active phase

Telogen

-resting phase for the follicle

Club hairs

-hair follicle is inactive

Lanugo hair

-fine, downy hair found on a fetus (third trimester)

Vellus hair

-replaces lanugo hair

-fine, pale body hair

Terminal hair

-darker, thicker hair located on the head, axillary, and pubic regions

-face and chest of adult males

-hair growth is dependent on nutrition and hormones

Shaft shape determines the

-appearance of the hair

Ribbonlike

-kinky hair

Round

-straight and coarse

Oval

-wavy hair

Hair pigment is made in the melanocytes

-found at the base of the follicle

Gray hair

-diminished melanin production

White hair

-no melanin production

Nails

-scalelike modifications of the stratum corneum *(compromised of hard keratin)

-protect the distal tips of the digits

-assist in grasping objects

3 parts of the nail

-nail plate

-nail bed *(underlies the nail plate)

-nail matric *(actively growing part of the nail) *(produces nail bed)

Lunula (structure of the nail)

-"white crescent"

-most proximal portion of the nail plate

Eponychium ("Cuticle")

-thin strip of epithelium that protects the matrix

Sebaceous glands

-secretions *(secrete an oily substance called sebum) *(bactericidal) *(stimulated by hormones)

-function as holocrine glands

-usually secrete into hair follicles

-secretions soften and lubricate hair and skin *(prevent excessive water loss) *(prevent bacterial growth)

Sudoriferous glands (sweat glands)

-scattered over the whole body (excluding nipples & genitals)

-two types *(Merocrine (Eccrine) sweat glands) *(Apocrine sweat glands)

Structure of Aprocrine sweat gland

-located in axillary & anogenital regions

-ducts empty into hair follicles

Secretion of Apocrine sweat gland

-same basic components as eccrine sweat + some fatty substances & proteins

-secretions are odorless but when decomposed by bacteria on the skin = body odor

Merocrine sweat glands

-abundant on palms, soles, and forehead

-sweat: hypotonic filtrate of the blood *(99% H2O)

-3 functions

-completely sympathetically regulated *(emotionally-induced sweating)

3 functions of Merocrine sweat glands

-thermoregulation *(primary role of sweat is to prevent overheating of body)

-excretion *(salts & metabolic wastes, such as urea)

-protection *(antibodies & derminidin (anti-bacterial peptide))