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Anatomy andAnatomy andPhysiology of skinPhysiology of skin
Anatomy andAnatomy andPhysiology of skinPhysiology of skin
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Basic structure and function
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SKIN
Largest, Thinnest, Important organ. Self-Repairing and protective boundary.
Surface area: 2m (approx).
Depth 0.5 -4.0mm: thickness dependson location.
It is classified into:-1. EPIDERMIS2. DERMIS
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EPIDERMIS Epithelial layer derived fromectodermal germ layer. Superficial and thinner layer. Includes Also hairs, sweat glands,
sebaceous glands. Avascular tissue, made up of
keratinocytes that germinate in the
lowest layer and migrate towards thesurface whilst changing in character. 4-5 layers depending on location. Protective and regulatory functions.
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THICK AND THINSKIN
-The hairless skin covering the palms ofhands, soles of feet and other bodysurface subjected to friction is calledthe thick skin.
-Hairs are not found in thick skin.
-Each of 5 strata of epidermis arepresent.-Outermost stratum- stratum corneum is
especially noticeable in thick skin.
-In thick skin underlying dermal papillaeare raised in curving ridges to formFINGER PRINTS or FOOTPRINTS.
-These helps in pick up and manipulatesmall objects with hands and provide
slip resistance to soles of feet.
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THIN SKIN
- Most of the body surface iscovered by thin skin.
- Number of cell layers of epidermisare less than that of thick skin.
- One or more strata may be absent.
- Raised curved papillae are notpresent.
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CELL TYPES
K
ERATIN
OC
YTES
become filled witha tough, fibrous protein calledkeratin.
-More than 90% of epidermal cells.-Principal structure element of outer
skin. MELANOCYTES contribute color to
the skin.
-Decrease the amount of UV rays thatcan penetrate into deeper layers ofskin.
- More than 5% of epidermal cells.- Melanocytes may be completely
absent from skin called VITILIGO.
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- LANGERHANS CELLS dendritic cells,branched cells that play role inimmunity.
- These cells originate in bone marrow
but migrate to deep cell layers ofepidermis early in life.
- Merkel cells: touchsensitive.
- Merkel discs: tactile disc.
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CELLLAYERS1. STRATUM BASALE ( base layer ) :-
-primary layer-Single layer of columnar cells.-Cells of deepest stratum of epithelial undergo
MITOSIS.-As a result cells transfer or migrate from basallayer to the other layer until they are shedfrom the skin surface.
-contains stem cells connected to the papillary
layer of the dermisregenerating layercontains some sensory cells (Merkelsdiscs)contains melanocytes (25%)
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2. STRATUM SPINOSUM(spiny layer)
-Formed from 8-10 layers of irregularlyshaped cells with very prominent bridgesor desmosomes.
-The desmosomes appear to pull points of
the plasma membrane of adjoining cellstoward are another.-This gives cells of this layer a spiny or
prickly appearance, prickly layer.
- Rich in RNA.- The term stratum germinativum (growth
layer) is often used to describe thestratum basale and stratum spinosum
together.
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3. STRATUM GRANULOSM(granular layer) :-
granular layer contain keratohylin-a precursor to
keratin. Process of surface keratinformation begins
cells take on protective function in
this layer degeneration of cell nucleus begins-some cell death occurs.
Cells are arranged in a sheet.
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4. STRATUM LUCIDUM(clear layer)
friction layer.The kertinocytes in this layer are
flats, closely packed and clear.
Typically nuclei are absent.Filled with a substance called ELEIDIN,
which will be transformed to keratin.This layer is absent in thin skin but is
apparent in section of thick skin.3 -5 cells deephigh friction, non-haired areas -eg
soles,
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5. STRATUM CORNEUM(horny layer)
- Superficial layer.- composed of very thin squamous cells.- Continually being shed and replaced.- Cytoplasm in these cells has been replaced by a dense
network of keratin fibers.
-The process by which cells in this layer are formedfrom cells in deeper layer of epidermis and thenfilled with keratin and moved to the surface is calledas KERATINIZATION.
-The stratum corneum is sometimes called the barrierarea of the skin because it function as a barrier to
water loss and to many microorganism and harmfulchemicals. Once this barrier layer is damaged, theeffectiveness of skin as a protective covering isgreatly reduced.
- Sometimes certain diseases of skin cause thethickening of this layer more than normal limits
called hyperkeratosis.
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EPIDERMALGROWTH ANDREPAIR
- Turnover time and regeneration time areterms used to describe the period requiredfor a population of cells to mature andreproduce.
- Lost cells are replaced by mitotic activity.- New cells are formed at the same ratethat old keratinized cells flake off fromthe stratum corneum, to maintain a
constant thickness of the epidermis. Current research suggest that regenerationtime required for completion of mitosis, isabout 35 days.
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The process can be accelerated byabrasion of skin surface, which
tend to peel off a few of the celllayers of stratum corneum.
If the abrasion continues over a
prolonged time, the increase inmitotic activity and shortenedturnover time will result in anabnormally thick stratum corneum
and development of callus. Normally about 10% to 12% of allcells in the stratum basal entermitosis each day.
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DERMAL-EPIDERMALJUNCTION
Area between the epidermis anddermis.
The junction glues the 2 layers
together and provides mechanicalsupport for the epidermis. Partial barrier for passage of somecells and large molecules.
Any detachment of large area ofepidermis from dermis is an extremelyserious condition that may result indeath.
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DERMIS- Deeper and thicker layer.- Derived from mesoderm.- Dense and vascular connective tissue layer- Thickness 4mm- Thinnest on eyelids and penis where thickness
is about 0.5mm.- Provide mechanical strength to skin.-A specialized network of nerves and nerve
endings in dermis is called as somatic sensory
receptors, which gives the sensory informationsuch as pain, pressure touch and temperature.- It is rich in vascular supply.-Plays important role in regulation of body
temperature.
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RETICULAR LAYER Consists of dense reticulum or network of
fibers. Most of the fibers are of collagenous type
which gives the toughness to skin. Elastic fibers are also present. It is point of attachment for skeletal &
smooth muscle fibers It helps in giving facial expression &
voluntary movement of scalp. In this area arrector pili muscles are
present, Contraction of these muscle makethe hair stand on end-as in extreme fear& cold we called it as Goosebumps.
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DERMALGROWTH AND
REPAIR Dermis does not continually shed and
regenerate. In wound healing, the regeneration of dermis
takes. In dermis, fibroblast quickly reproduce & formdense mass of new connective tissue fiber.
If the dense mass is not replaced by normal
tissue, it remain as a scar. If the elastic fibers in the dermis arestretched too much these fibers will weaken &tear that will result as a stretch marks
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Physiology of Skin
PROTECTIVE FUNCTIONSSkin forms the covering of all the
organs of the body & protectsthese organs from the followingfactors:
Bacteria and toxic substances.
Mechanical blow. U.V. rays.
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Protection from Bacteria andToxic Substances
Skin covers the organs of the bodyand protects the organs from having
direct contact with external
environment.
The keratinized stratum corneum
layer of epidermis is responsible for
the protective function of skin. This
layer also offers resistance to the
skin against toxic chemicals like
acids and alkalies.
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Protection from Mechanical Blow
The skin is not tightly placed over
the underlying organs or tissues.
It is some what loose and movesover the underlying subcutaneous
tissues. So the mechanical impact
of any blow to the skin is nottransmitted to the underlying
tissues.
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Protection from U.V. Rays.
Exposure to sunlight causes increased production of
melanin pigment in skin. This absorb U.V. rays.
At the same time the thickness of stratum corneum
is increased. This layer also absorbs U.V. rays.
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General sensation
Skin is considered as the largest sense
organ in the body. It has many nerveendings which are specialized to formcutaneous receptors .
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Storage
The dermis as well as thesubcutaneous tissue can store fats,water ,salts, glucose and suchother substances.
It can also store the blood by thedilation of blood vessels.
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Vitamin D productionThe first step in the production of vitamin D in the
body occurs when the skin in exposed to U.V. light.7- dehydrocholestrol
Cholecalciferol ( Vit D3 )
25- hydroxyl cholecalciferol
1,25-hydroxycholecalciferol
Active form
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ExcretionThrough sweat and insensibleperspiration, salts and metabolitesare excreted to some extent.
Excess sweating may lead to lowblood Na levels (hyponatremia)
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Secretion
Sweat
Sebum
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Absorption
Water absorption causing swelling ofthe stratum corneum.
Lipids are easily permeable to skin likevitamins.
Some drugs in transdermal patchs eg.Hormone replacement therapy during themenopause
Some toxic chemicals eg. mercury
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IMMUNITY
The epidermis containsspecialised immune cells calledLANGERHANS CELLS.
They phagocytose interdusingAntigens & travel to lymphoidtissue where they present
Antigen to T-lymphocytesthus stimulating an immuneresponse.
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REGULATION OF BODY
TEMPERATURE A Set point of body temp. is 37c . It
may increase to 37.6C by lateafternoon & decrease to 36.2C byearly morning.
The homeostatic regulation of bodytemp. in two ways:
By liberating sweat at its surfaceBy adjusting the flow of blood indermis
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HEAT PRODUCTION
BY MEANS OF
Metabolism of foodsDuring exercise
S
hivering
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HEATLOSS
Heat loss can be regulated by alteringthe flow of blood in the skin .
If heat must be conserved, dermal bloodvessels constrict to keep the warm bloodcirculating in body.
Heat can be transferred to the externalenvironment through the physical process
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EVAPORATION
Evaporation of waterconstitute one
method by whichheat is lost from thebody, especially fromthe body
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RADIATION
Heat radiate from the body surfaceto nearby object that are coolerthan skin.
CONDUCTION Conduction means the transfer of
heat to any substance actually incontact with the body; To clothingor jewelry, Even to cold foods.
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CONVECTION
It is transfer of heat away from a surfaceby movement of heated air or fluidparticles.
For examples:-If you have ever stepped fromyour shower into even slightly moving air
from an open window.
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HOMEOSTATIC REGULATION OF
HEATLOSS Temperature receptors in a part of the
brain called Hypothalamus detectchanges in the body the bodys internal
temperature. Above the set point i.e.370 C.
The hypothalamus acts as an integratorand sends a nervous signal to the sweat
glands and blood vessels of the skin. Sweat glands increase their output
sweat and blood vessels increase theirdiameter.
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Skin Color Melanin: The main determinant of
skin color is the quantity of themelanin deposited in the cells ofthe epidermis.
Two groups of melanin are made bythe melanocytes of the body.
Eumelanin: true black substance. Pheomelanin: dusky back substance.
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FACTORS AFFECTING SKINCOLOR
GE
NE
Sprolonged exposure ACTH
to sunlight
Melanocytes synthesis of
melanin
type & quality of melanin deposited inepidermis
Unoxgen. Hb skin color vol. Of blood
F ti f ki
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Functions of skin
Function Example Mechanism
Protection From microorganism
From dehydrationFrom u.v. rays
From mechanicaltrauma
Surface film/
mechanical barrier.Keratin
Melanin
Tissue strength
Sensation PainHeat &coldPressureTouch
Somatic sensoryreceptors.
Permit movement andgrowth without injury
Body growth andchange in body contoursduring movements
Elastic and recoilproperties of skin andsubcutaneous tissue.
Endocorine Vitamin D production Activation of precursorcompound in skin cell byU.V. rays
Excretion WaterUreaAmmoniaUricacid
Regulation of sweatvolume and content
Immunity Destruction of microbes Phagocytic cells andlangerhans cells
Temperature regulation Heat loss or retention Regulation of blood flowto skin and evaporation
of sweat.
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