SKIN STRUCTURE

As I mentioned in the previous post, our skin is composed of 3 main layers (from inner to outer):

1. SUBCUTANEOUS TISSUE

• Crucial for skin appearance, containing clusters of fat (adipocytes) partitioned by connective tissue - INTERWOVEN WITH: --> BLOOD AND LYMPH VESSELS, NERVES, PARTS OF HAIR FOLLICLES AND SMALL SWEAT GLANDS, AND SKIN MUSCLES

• Functions as:

• Thermoisolator - fat as thermal insulation, deep vascular network as the body's cooler

• Stores energy and lipophilic vitamins (A, D, E, K)

• Protects internal organs from external influences/damage

• Anchors the skin to underlying muscle tissues (attached by skin ligaments or ligaments) and enables skin movement

• Gives the body shape - accumulation of subcutaneous fat with gender and sex hormones gives the body a gender-specific shape

• Acts as an endocrine organ

• Thickness and distribution of subcutaneous tissue influenced by age, gender, and lifestyle

• Represents the bulk of body fat - clusters of fatty tissue located between connective partitions - a specialized form of connective tissue and intercellular substance between them -

• Its task is to take up excess fatty acids from the blood, produce and store fats, and release them into the bloodstream when the body needs these fats

2. DERMIS

• Lies beneath the epidermis and represents a 2-3 mm thick layer of skin

• Composed of dense connective tissue with blood vessels, nerves, muscles, various defense systems, and skin appendages (sweat glands, sebaceous glands, hair follicles, nails)

• Provides the skin with firmness, elasticity, tension

• Divided into 3 layers: --> UPPER OR NIPPLE LAYER / STRATUM PAPILLARE --> MIDDLE LAYER / STRATUM SUBPAPILLARE --> DEEP PART OR NET LAYER / STRATUM RETICULARE

• WHAT IS CONNECTIVE TISSUE?

• Composed of connective cells and intercellular substance (basic substance, intercellular fluid, and connective fibers) a) CONNECTIVE CELLS: star-shaped cells responsible for producing fibrous proteins like collagen, elastin, and other composite proteins - mucopolysaccharides b) INTERCELLULAR SUBSTANCE: composed of clear, gelatinous protein mass from mucopolysaccharides - the basic substance binds a lot of water and provides tension to connective tissue / tissue fluids, facilitating substance transport through the intercellular space - connective and other dermal cells reside here - connective cells exchange substances with blood and lymph and release their products into it, including collagen and elastin, forming collagenous and elastic connective fibers / collagen fibers provide connective tissue with firmness, strength, resistance / elastic fibers provide connective tissue with elasticity

TYPES OF CONNECTIVE TISSUE:

• Loose connective tissue: abundant intercellular fluid and basic substances with few connective fibers

• Dense connective tissue: less intercellular fluid and basic substances and many connective fibers --> if collagen fibers predominate, it is dense collagenous connective tissue --> if elastic fibers predominate, it is dense elastic connective tissue

Collagen, the most abundant protein in our body and dermis, represents approximately 70% of its dry weight. Collagen fibers, mainly composed of types I, III, V collagen, provide the skin with the necessary tensile strength. Their main characteristic is high tensile strength. They, along with keratin, influence the firmness and elasticity of the skin. Collagen type I is most important in skin aging. Collagen type III contributes to skin flexibility (found below the epidermis). Collagen type V is evenly distributed throughout the dermis.

Elastic fibers form a three-dimensional network with hyaluronic acid and collagen fibers. Composed of elastic microfibrils from various glycoproteins, they encircle elastin fibers. Elastic fibers are differentially differentiated - more mature ones are deeper in the reticular layer.

3. EPIDERMIS:

• the epidermis is the outermost layer of the skin

• it ranges in thickness from 0.1 - 1 mm

• thinnest on the eyelids (hence emphasizing not to "pull" the skin when removing eye makeup) and thickest on the palms and soles

• covered on the surface by a protective oily acidic mantle

• its texture, moisture, and color significantly influence the external appearance of the skin

• healthy skin and an attractive appearance are linked to the structure and function of the epidermis, reflected in numerous cosmetic products aimed at improving the condition of the epidermis and the appearance of the skin

• as it lacks blood vessels and connective tissue, it receives all necessary substances from the dermis through diffusion across the basement membrane

• protects deeper layers of the skin and the body from harmful physical, chemical, and biological influences from the external environment

• there are 6 different layers of the epidermis (from top to bottom):

* stratum corneum (horned layer)

* stratum lucidum (clear layer)

* stratum granulosum (granular layer)

* stratum spinosum (prickle cell layer)

* stratum basale (basal layer)

* the basal membrane, which separates the epidermis and the dermis, is considered a special layer

• it consists of a multilayered epithelium, with keratinocytes as the main building blocks Like any epithelial tissue, the epidermis has two parts: EPIDERMAL CELLS / INTERCELLULAR EPIDERMAL SUBSTANCE (product of epidermal cells)

• Epidermal cells:

• vary in different layers of the epidermis

• proliferate in the deepest, basal layer, then undergo changes, maturation, and preparation for specific protective tasks in higher layers of the epidermis --> we have living, immature epidermal cells: the process of cornification/maturation of the epidermis occurs - KERATINOCYTES (Gr. keras: horn, Gr. kytos: cell) and dead, cornified, mature epidermal cells: in the upper layer of the epidermis - cornified cells, performing the protective functions of the epidermis - CORNEOCYTES (Lat. corneus: horny, Gr. kytos: cell) --> ON THE SURFACE OF THE EPIDERMIS, CORNEOUS EPIDERMAL CELLS DRY INTO HORN-LIKE SCALES AND FALL OFF

--> Intercellular substance:

• surrounds cells and allows access of necessary substances to them while binding them

• in the deeper layers, the intercellular substance is immature, watery, gelatinous; in the upper layer, it becomes mature, dense, fatty, impermeable - on the surface, it dries up and falls off!

a) BASAL LAYER:

• the deepest layer of epidermal cells

• keratinocytes, producing keratin, originate from these basal cells --> keratin is a strong, supportive fibrous protein / the most numerous cells in the epidermis / undergo constant maturation - differentiation - during differentiation, they travel from the basal layer to the surface of the skin, undergoing morphological changes and giving a characteristic appearance to each layer of the epidermis - they die in the horned layer, turning into corneocytes - this process is called keratinization, which ends with the shedding of individual cells (desquamation) - in healthy skin, desquamation is inconspicuous and takes 26 - 42 days for the shed cells to be replaced by new cells coming from the basal layer

• here, we also find:

• LANGERHANS CELLS (involved in the body's immune response), MELANOCYTES (contain organelles, melanosomes, producing melanin), MERKEL CELLS (act as touch receptors and are found at the ends of nerve endings)

b) PRICKLE CELL LAYER:

• lies above the basal layer

• keratinization (production of keratin) takes place here - epidermal maturation --> cells produce keratin, forming a rigid cytoskeleton --> these cells are connected by protein structures (watery, gelatinous intercellular substance or cytoplasm) --> DESMOSOMES

• numerous keratin filaments in the cytoplasm, attached to desmosomes

• no more cell division here

• desmosomes on the outer side of the cell membrane connect neighboring cells to each other, and inside the cell, it is connected to keratin fibers, serving as an anchor within the intracellular framework - these connections form a strong network that is ESSENTIAL FOR THE MECHANICAL RESISTANCE AND INTEGRITY OF THE EPIDERMAL STRUCTURE, and they also maintain a sufficient distance between cells so that substances necessary for cells can reach them through the intercellular substance

• the intercellular substance is composed of proteins, which, in addition to their proteinaceous part, also have polysaccharide components: MUCOPOLYSACCHARIDES (produced by keratinocytes) --> they also act as water binders, natural moisturizers of skin tissues, and their presence in skin tissues provides tension

c) GRANULAR LAYER:

• composed of 2 - 3 layers of granular keratinocytes and watery, gelatinous intercellular substance

• here, the epidermal maturation continues and ends

• contains keratohyalin granules and is connected with the gluing of keratin fibers into a dense and impermeable mass of keratin

• keratohyalin granules are still present on the surface of granular cells in the deeper layers of the granular layer --> these granules are attached to the outer surface of prickle and granular cells --> in the upper layer of granular cells, they begin to break down and release their contents - changing keratinocytes and intercellular substance between them --> in granular cells, substances are released that dissolve the nucleus and cell components --> keratinocyte transforms into a dead, cornified cell --> keratin fibers, glued into a solid, impermeable network, completely fill the interior of the cornified cell, and thus, the process of cornification is completed - THEY BECOME MATURE EPIDERMAL CELLS --> the watery intercellular substance turns into a dense, fatty, sticky intercellular substance

BARRIER:

• is the boundary between the living, cornifying, and dead, cornified parts of the epidermis

• it consists of a thin, dense, and fatty layer of complex proteins, which also contain lipids

• has a protective purpose, and its main role is water impermeability

• its lipid-hydrophobic part is impermeable to water and water-soluble substances necessary for the body, preventing them from escaping from the deeper parts of the epidermis and skin and from deeper tissues of the body into the surroundings

• its protein-hydrophilic part of the barrier is impermeable to fats and fat-soluble substances, preventing them from passing through the barrier into the depth of the skin and into the interior of the body

• prevents water from evaporating from the body through the skin

• the humidity in the epidermis below the barrier and in the deeper layers of the skin is 70%, the humidity in the epidermis above the barrier gradually decreases towards the surface of the skin

• although the horned layer of the epidermis and the protective oily acidic mantle prevent water evaporation from the body through the skin, the influences of the external dry environment above the barrier are too strong, and the humidity above the barrier gradually decreases towards the surface of the skin

d) CLEAR LAYER:

• between the granular and horned layers is the clear layer, which is located on the palms and soles

• the thick, inflexible, and hard horned layer of the epidermis on the palms and soles could detach from the deeper parts of the epidermis due to constant and significant skin stress, but the sticky and flexible clear layer firmly connects it to the deeper parts of the epidermis

e) HORNED LAYER:

• is the top, visible layer of the epidermis

• the final stage of keratinocyte differentiation occurs here - transformation into corneocytes

• it is most simply described as a brick and mortar structure, where bricks represent corneocytes surrounded by mortar or intercellular lipids

• it is metabolically inactive

• protects against dehydration, loss of water-soluble substances from the body, penetration of harmful substances into the skin and the body, invasion of microorganisms into the skin, harmful effects of UV and thermal rays

• the horny layer has 2 parts:

DEEPER PART IS A COMPACT, DENSE LAYER - the deep part of the horny layer, consisting of flattened, dead, mature corneocytes, closely packed together, having a thick protein cell wall, and their interior filled with densely glued keratin fibers. Corneocytes and intercellular substance significantly contribute to the protective abilities of the epidermis Due to the effects of the dry environment, mucopolysaccharides in it bind less water, and the humidity of this part of the horny layer is around 10% The compact layer of the epidermis is similar to mortar - intercellular substance gluing together corneal cells

THE OTHER PART IS THE PEELING LAYER OF THE EPIDERMIS - this is the topmost part of the horny layer of the epidermis. Here, corneocytes dry into horn-like scales or squames because, due to the effects of the dry environment, they lose most of their water. On the surface of the skin, they are closely stacked next to each other so that their edges overlap like roof tiles, and the intercellular substance, due to the effects of the dry environment, gradually loses water and stickiness, breaks down, and the scales fall off unnoticed. This is called peeling or physiological desquamation. One interesting fact: during a person's lifetime, there are about 20 kg of these scales.

EPIDERMAL RENEWAL (EPIDERMATOGENESIS)

• is a natural process that begins with the division of basal cells in the basal layer of the epidermis, continues with the cornification or maturation of cells and intercellular substance in the prickly and granular layers, ends with mature corneal cells and fatty intercellular substance in the horned layers, and finishes with dried corneal scales and the breakdown of intercellular substance, and the shedding of dry corneal scales from the surface of the skin

• this process takes 28 - 60 days in healthy skin

• when the multiplication of basal cells and the shedding of corneal scales are disturbed for various reasons, basal cells multiply faster, and corneal scales shed more slowly, e.g., callus formation

• when, for pathological reasons, the rapid multiplication of basal cells occurs, and at the same time, the process of cornification and thus the shedding of scales is disrupted, the horned layer of the epidermis thickens significantly, and corneal scales on the surface of the skin do not shed, forming a thick scaly coating, e.g., psoriasis

f) BASEMENT MEMBRANE:

• connecting the epidermis and dermis is a thin, undulating basement membrane, consisting of a complex network of extracellular matrix proteins that firmly connects them

• the undulation of the basement membrane increases its surface area and thus provides an even firmer and more flexible connection between the two parts of the skin

• this membrane covers the papillary extensions of the dermis into the epidermis - papillae and thus brings blood vessels from the dermis closer to the epidermis

• as the epidermis is devoid of blood vessels, necessary substances reach the epidermal cells through the basement membrane from the dermis

• the larger the surface area of the basement membrane, the better the supply of the epidermis with necessary substances

• due to harmful effects from the environment, lifestyle, or incorrect skin care, the basement membrane may be flattened, reducing its surface area, and the consequences are a poorer supply of the epidermis with necessary substances and oxygen, and the loosening of the previously firm and flexible connection between the epidermis and dermis

ZGRADBA KOŽE

Kot sem že omenila v prejšnji objavi, je naša koža sestavljena iz 3 glavnih plasti (od notranje proti zunanji):

- PODKOŽJE (hipodermis)

- USNJICA (dermis)

- POVRHNJICA (epidermis)

1) PODKOŽJE

* pomembno prispeva k videzu kože, saj se tukaj nahajajo skupki maščob (adipociti), ki pregrajujejo vezivno tkivo - JE PREPREDENO S:

--> KRVNIMI IN LIMFNIMI ŽILAMI, ŽIVCI, DELI LASNIH MEŠIČKOV IN MALIH ZNOJNIC TER KOŽNIMI MIŠICAMI

* služi kot:

- termoizolator - maščevje kot termoizolator, globoki žilni splet kot hladilnik telesa

- skladišči energijo in lipofilne vitamine (A, D, E, K)

- ščiti notranje organe pred zunanjimi vplivi/poškodbami

- pritrjuje kožo na spodaj ležeča mišična tkiva (pritrjeni so s kožnimi vezmi ali ligamenti) in tako omogoča premikanje kože

- telesu daje obliko - s spolom in spolnimi hormoni povezano kopičenje vrhnjega sloja podkožnega maščevja daje telesu obliko za spol značilno obliko

- deluje kot endokrini organ

* na debelino in porazdelitev podkožja vplivajo starost, spol in življenjski slog

* predstavlja glavnino telesne maščobe - skupki maščobnega tkiva se nahajajo med vezivnimi pregradami - je specializirana oblika vezivnega tkiva in medceličnine med njimi -

- naloga je sprejemanje viška maščobnih kislin iz krvi in proizvodnja ter skladiščenje maščob in njihovo sproščanje v krvni obtok, kadar telo potrebuje te maščobe

2) USNJICA

* leži pod povrhnjico in predstavlja 2 - 3 mm debelo plast kože

* sestavljena je iz gostega vezivnega tkiva in v njej se nahajajo žile, živci, mišice, prisotni so različni obrambni sistemi ter kožni dodatki (znojnice, lojnice, lasni mešički, nohti)

* daje koži čvrstost, prožnost, napetost

* delimo jo v 3 plasti:

--> VRHNJA ALI BRADAVIČASTA PLAST / STRATUM PAPILLARE

--> SREDNJA PLAST / STRATUM SUBPAPILLARE

--> GLOBOKI DEL ALI MREŽASTA PLAST / STRATUM RETICULARE

* KAJ JE VEZIVNO TKIVO?

- sestavljen je iz vezivnih celic in vezivne medceličnine (je iz osnovne snovi, medcelične tekočine in vezivnih vlaken)

a) VEZIVNE CELICE: so celice zvezdaste oblike, zadolžene za proizvodnjo nitastih beljakovin kolagena, elastina in drugih sestavljenih beljakovin - mukopolisaharidi

b) VEZIVNA MEDCELIČNINA: sestavljena je iz:

prozorne, želatinaste beljakovinske mase, ki je iz mukopolisharidov - je tudi osnovna snov, veže veliko vode in daje vezivnemu tkivu napetost / tkivne tekočine, ki zagotavlja transport snovi po medceličnem prostoru - tukaj živijo vezivne in druge celice usnjice - po njej vezivne celice izmenjujejo snovi s krvjo in limfo in vanjo izločajo svoje produkte, kolagen in elastin, iz katerih se oblikujejo kolagenska in elastična vezivna vlakna / kolagenska vlakna, ki dajejo vezivnemu tkivu čvrstost, trdnost, odpornost / elastična vlakna, ki dajejo vezivnemu tkivu prožnost

VRSTE VEZIVNEGA TKIVA:

- rahlo vezivno tkivo: veliko medcelične tekočine in osnovne snovi ter malo vezivnih vlaken

- gosto vezivno tkivo: vsebuje manj medcelične tekočine in osnovne snovi in veliko vezivnih vlaken

--> če prevladujejo kolagenska vlakna, je to gosto kolagensko vezivno tkivo

--> če prevladujejo elastična vlakna, je to gosto elastično vezivno tkivo

Kolagen, ki je najbolj zastopan protein našega telesa in tudi usnjice, predstavlja pribl. 70% njene suhe mase. Kolagenska vlakna so zlasti sestavljena iz kolagena tipov I, III, V, ki dajejo koži ustrezno natezno moč. Njihova glavna lastnost je velika natezna trdnost. Je družina proteinov z značilno strukturo trojne vijačnice, ki se močno razlikuje v tipu verig.

Skupaj s keratinom vplivajo na čvrstost in elastičnost kože.

Kolagen tip I je najbolj pomemben pri staranju kože. Kolagen tipa III prispeva h gibljivosti kože (najdemo ga pod povrhnjico). Kolagen tipa V je enakomerno porazdeljen po usnjici.

Elastična vlakna tvorijo trodimenzionalno mrežo s hialuronsko kislino in kolagenskimi vlakni. Sestavljena so iz elastičnih mikrofibril iz različnih glikoproteinov in tvorijo ovoj okoli elastinskih vlaken. Elastinska vlakna so različno diferencirana - bolj zrela so globlje v retikularni plasti.

3) POVRHNJICA:

- je vrhnjica del kože

- debela je od 0,1 - 1 mm

- najtanjša na očesnih vekah (zato poudarjam, da kadar si iz oči čistimo makeup, ne "vlečemo" kože) in najdebelejša na podplatih in dlaneh

- na površini je prekrita z zaščitnim mastnim kislim plaščem

- s svojo teksturo, navlaženostjo ter barvo odločilno vpliva na zunanji videz kože

- zdrava koža in privlačen videz sta povezana s strukturo in delovanjem povrhnjice, kar se tudi odraža v številnih kozmetičnih izdelkih, namenjenih izboljšanju stanja povrhnjice in videza kože

- ker je brez žil in veziva, dobiva vse potrebne snovi z usnjice z difuzijo preko bazalne membrane

- ščiti globlje dele kože n notranjost telesa pred škodljivimi fizikalnimi, kemijskimi, biološkimi vplivi iz zunanjega okolja

- poznamo 6 različnih plasti vrhnjice (od zgornje proti spodnji):

* roževinasta plast ali stratum corneum

* svetla plast ali stratum lucidum

* zrnata plast ali stratum granulosum

* trnasta plast ali stratum spinosum

* zarodna plast ali stratum basale

* posebna plast je osnovna ali bazalna membrana, ki loči povrhnjico in usnjico

- sestavlja jo večplasten epitelij in glavni gradniki so keratinociti

Kot vsako epitelijsko tkivo je tudi vrhnjica sestavljena iz 2 delov: EPITELJISKIH CELIC VRHNJICE / EPITELIJSKE MEDCELIČNINE (produkt epitelijskih celic)

- Epiteljiske celice vrhnjice:

* so različne v različnih plasteh vrhnjice

* razmnožujejo se v najgloblji, zarodni plasti, ki se nato v višjih plasteh vrhnjice spreminjajo, zorijo in se pripravljajo za opravljanje specifičnih zaščitnih nalog vrhnjice -->

poznamo žive, nezrele celice vrhnjice: poteka proces poroženevanja / zorenja vrhnjice - KERATINOCITI (gr. keras: roževina, gr. kytos: celica)

in

odmrle, poroženele in zrele celice vrhnjice: so v vrhnji, roževinasti plasti vrhnjice - poroženele ali roževinaste celice vrhnjice, ki opravljajo zaščitne naloge vrhnjice - KORNEOCITI (lat. corneus: roževinast, gr. kytos: celica)

--> NA POVRŠINI VRHNJICE SE ROŽEVINASTE CELICE VRHNJICE POSUŠIJO V ROŽEVINASTE LUSKE IN NEOPAZNO ODPADAJO

- Epitelijska medceličnina:

* obdaja celice in omogoča dostop potrebnih snovi do njih ter jih zleplja

* v globljih plasteh je med živimi celicami vrhnjice nezrela, vodena, želatinasta, v vrhnji, roževinasti plasti vrhnjice pa je med odmrlimi celicami vrhnjice zrela, gosta, mastna, neprepustna medceličnina - na površini se posuši in odpade!

a) ZARODNA / BAZALNA PLAST:

- najgloblja plast celic vrhnjice

- iz izvornih celic vrhnjice nastajajo keratinociti - proizvajajo roževino ali keratin --> keratin je čvrsta in oporna beljakovina nitaste oblike / so najštevilčnejše celice vrhnjice / podvrženi so procesu stalnega zorenja - diferenciacija - med diferenciacijo potujejo od zarodne plasti do površine kože, pri tem pa se morfološko spreminjajo in dajejo značilno podobo vsaki posamezni plasti vrhnjice - odmirajo v roženi plasti v korneocite - temu procesu rečemo keratinizacija, ki se konča z luščenjem posameznih celic (deskvamacija) - pri zdravi koži je deskvamacija neopazna in traja od 26 - 42 dni, da se odluščene celice rožene plasti nadomestijo z novimi celicami, ki pripotujejo iz bazalne plasti

- tukaj najdemo še:

* LANGERHANSOVE CELICE (sodelujejo pri imunskemu odzivu telesa), MELANOCITE (vsebujejo organele, melanosome, ki proizvajajo melanin), MERKLOVE CELICE (imajo vlogo receptorjev za dotik in jih najdemo na koncih živčnih končičev)

b) TRNASTA PLAST:

- leži nad zarodno plastjo

- tukaj poteka poroženevanje ali keratinizacija (proizvodnja keratina) - zorenje vrhnjice --> celice proizvajajo keratin, ki tvori tog citoskelet --> te celice so povezane z beljakovinskimi strukturami (vodena, želatinasta medceličnina ali citoplazma) -->

--> DEZMOSOMI

- v citoplazmi so številni keratinski filamenti, pritrjeni na dezmosome

- tukaj ni več celične delitve

- jedra celic so presnovno zelo aktivne

- dezmosomi na zunanji strani celične membrane med sabo povezujejo sosednje celice, znotraj celični del pa je povezan s keratinskimi vlakni in služi kot sidrišče znotrajceličnega ogrodja - te povezave tvorijo močno mrežo, ki je BISTVENA ZA MEHANSKO ODPORNOST IN CELOVITOST ZGRADBE POVRHNJICE, poleg tega pa vzdržujejo dovolj veliko razdaljo med celicami, da lahko do njih po medceličnini prihajajo k celicam potrebne snovi

- medceličnino sestavljajo beljakovine, ki imajo v svoji molekuli poleg beljakovinskega še polisahardini del: MUKOPOLISAHARIDI (proizvajajo jih keratinociti) --> so tudi vezalci vode, naravne vlažilce tkiv kože in njihova prisotnost v tkivih kože daje napetost

c) ZRNATA PLAST:

- sestavljajo jo 2 - 3 sloji zrnatih keratinocitov in vodena, želatinasta medceličnina

- tukaj se poroženevanje vrhnjice nadaljuje in konča

- vsebujejo keratohialna zrnca in so povezana z zlepljanjem keratinskih vlaken v gosto in neprepustno keratinsko maso

- na površini zrnatih celic so v globljih plasteh zrnatega sloja še vedno roževinasta telesca -->

so telesca prilepljena na zunanjo površino trnastih in zrnatih celic --> v vrhnjem sloju zrnatih celic pričnejo razpadati in sproščati svoje sestavine - spreminjajo keratinocite in medceličnino med njimi --> v zrnate celice se sprostijo snovi, ki raztopijo jedro in sestavine celice --> keratinocit se spremeni v odmrlo, poroženelo celico --> keratinska vlakna, zlepljena v trdno, neprepustno mrežo so povsem napolnila notranjost roževinaste celice in s tem je proces poroženevanja končan - SO ZRELE CELICE VRHNJICE --> vodena medceličnina se pretvarja v gosto, mastno, lepljivo medceličnino

PREGRADA ALI BARIERA:

- je meja med živim, poroženevajočim in odmrlim, poroženelim delom vrhnjice

- tvori jo tanka, gosta in mastna plast sestavljenih beljakovin, ki ima poleg tega še lipide

- ima zaščitni pomen in njen glavni pomen je neprepustnost za vodo

- njen lipidno - hidrofobni del je neprepusten za vodo in v vodi topne, telesu potrebne snovi, ki ne morejo uhajati iz globljih delov vrhnjice in kože in iz globljih tkiv telesa v okolico

- njen beljakovinski - hidrofilni del pregrade pa je neprepusten za maščobe in v njih topne snovi, ki tako ne morejo skozi pregrado v globino kože in v notranjost telesa

- preprečuje izhlapevanje vode iz telesa skozi kožo

- vlažnost v vrhnjici pod pregrado in v globljih delih kože je 70%, vlažnost v vrhnjici nad pregrado pa se proti površini kože postopoma manjša

- izhlapevanje vode iz vrhnjice nad pregrado sicer preprečujeta roževinasta plast vrhnjice in zaščitni mastni kisli plašč, vendar so nad pregrado vplivi zunanjega suhega okolja premočni in vlažnost v vrhnjici nad pregrado se znižuje postopoma proti površini kože

č) SVETLA PLAST:

- med zrnato in roževinasto plastjo je še svetla plast, ki se nahaja na dlaneh in podplatih

- debela, neprožna in trda roževinasta plast vrhnjice na dlaneh in podplatih bi se zaradi stalnih in velikih obremenitev kože lahko odlepila od globljih delov vrhnjice, vendar jo lepljiva in prožna svetla plast trdno poveže z globljimi deli vrhnjice

d) ROŽEVINASTA PLAST:

- je vrhnja, vidna plast vrhnjice

- tukaj poteka zadnja stopnja diferenciacije keratinocitov - sprememba v korneocite

- najbolj enostavno se jo opiše kot opeko in malto, kjer opeko predstavljajo korneociti, obdani z malto oz. medceličnimi lipidi

- je presnovno neaktivna plast kože

- varuje pred izsušitvijo, izgubo vodotopnih snovi iz telesa, vdorom škodljivih snovi v kožo in notranjost telesa, vdorom mikroorganizmov v kožo, škodljivimi učinki UV in toplotnih žarkov

- roževinasta plast ima 2 dela:

* GLOBLJI DEL JE KOMPAKTNA ZGOŠČENA PLAST - je globoki del roževinaste plasti, ki je sestavljen iz sploščenih, odmrlih, zrelih roževinastih celic (korneociti), ki so tesno skupaj in imajo debelo beljakovinsko celično steno, njihovo notranjost pa napolnjujejo v gosto, roževinasto maso zlepljena keratinska vlakna.

Korneociti in medceličnina bistveno pripevajo k zaščitnim sposobnostim vrhnjice

Mukopolisaharidi v njej zaradi vplivov suhega okolja vežejo manj vode in vlažnost tega dela roževinaste plasti vrhnjice zato znaša okoli 10%

Zgoščena plast vrhnjice je podobna z malto - medceličnina zlepljenim opekam - roževinaste celice

* DRUGI DEL JE LUŠČEČA PLAST VRHNJICE - to je najbolj vrhnji del roževinaste plasti vrhnjice. Tukaj se roževinaste celice posušijo v roževinaste luske ali skvame, ker zaradi vplivov suhega okolja izgubijo večino vode.

Na površini kože so zložene tesna ena zraven druge tako, da se njihovi robovi prekrivajo kot strešne opeke, medceličnina pa zaradi vplivov suhega okolja postopoma izgublja vodo in lepljivost ter razpade in luske neopazno odpadajo.

Temu rečemo luščenje ali fiziološka deskvamacija.

Ena zanimivost: v času človekovega življenja naj bi bilo teh lusk za okoli 20kg.

OBNAVLJANJE VRHNJICE (EPIDERMOGENEZA)

- je naravni proces, ki se začne z delitvijo zarodnih celic v zarodni plasti vrhnjice, se nadaljuje s poroženevanjem oz. zorenjem celic in medceličnine vrhnjice v trnasti in zrnati plasti, se zaključi z zrelimi roževinastimi celicami in mastno medceličnino v roževinastimi plasti vrhnjice in konča s posušenimi roževinastimi luskami in propadom medceličnine in z odpadanjem suhih roževinastih lusk s površine kože

- ta proces pri zdravi koži traja od 28 - 60 dni

- kadar je množenje zarodnih in odpadanje roževinastih lusk iz različnih razlogov moteno, se zarodne celice hitreje množijo in roževinaste luske počasneje odpadajo, npr.: otiščanec

- kadar pa je iz bolezenskih razlogov pospešeno razmnoževanje zarodnih celic in hkrati moten proces poroženevanja in s tem odpadanja lusk, roževinasta plast vrhnjice se zelo odebeli, roževinaste luske pa na površini kože ne odpadajo in napravijo debelo luskavo oblogo, npr.: luskavica

e) BAZALNA MEMBRANA:

- to povezuje povrhnjico in usnjico tanka, valovita bazalna membrana, ki jo tvori kompleksna mreža proteinov zunajceličnega ogrodja in ju čvrsto povezuje

- valovitost bazalne membrane poveča njeno površino in s tem zagotavlja še bolj čvrsto in tudi bolj prožno zvezo med obema deloma kože

- ta membrana pokriva bradavičaste podaljške usnjice v vrhnjico - papile in tako približa žile usnjice vrhnjici

- ker je vrhnjica brez žil, prihajajo potrebne snovi do celic vrhnjice skozi bazalno membrano iz usnjice

- čimvečja je površina bazalne membrane, tem boljša je preskrba vrhnjice s potrebnimi snovmi

- zaradi škodljivih učinkov iz okolja, načina življenja ali napačne nege kože izravnava osnovno membrano in se s tem njena površina zmanjša in posledice so slabša preskrba vrhnjice s potrebnimi snovmi in kisikom in rahljanje prej trdne in prožne veze med vrhnjico in usnjico