THE STRUCTURE AND FUNCTION OF THE SKIN The skin • • •largest organ of the body •protects us from microbes and the elements • helps regulate body temperature •and permits the sensations of touch, heat, and cold • Skin of an adult person • • •surface 1,5 -2 m² •weight 18-20 kg (15-19 % of total weight) •epidermis and dermis thikness - 2 mm (0,5 - 3,5 kg) •subcutis thikness - 8-25 mm Layers of the skin •Epidermis - the outermost layer of skin, provides a waterproof barrier and creates our skin tone •Dermis - beneath the epidermis, contains connective tissue, museles, senzory neurons, blood vesseles, hair follicles, hairs, and sweat glands •Subcutaneous tissue (hypodermis) - made of fat and connective tissue. • Layers of the skin 152-5282_IMG Ep idermis Dermis Subcutaneous fat 1. Epidermis •the thinnest part of the skin - thickness 0,3 – 1,5 mm •the outermost layer of skin •formed by cells: • - keratinocytes - arranged into 5 layers • - melanocytes • - Langerhans cells • - Merkels cells 1. Epidermis Layers of epidermis and keratinocytes •Stratum basale • – the deepest part of the epidermis, prolifaration of keratinocytes occurs there, the keratinocytes are connected by molecules called desmosomes to each other and to the basal membrane (layer between epidermis and dermis) are connected by hemidesmosomes •Stratum spinosum • – above str. basale, layer where process of differentiation starts (change in morphology of keratinocyte and production of keratin) •Stratum granulosum • – an area where the keratinization process is completed and keratohyalin granules become visible (precursors for keratin) •Stratum lucidum – amorphous band between str. granulosum and str. corneum (visible by microscope only on palms and soles of feet) •Stratum corneum • – consists of corneocytes (remnants of keratinocytes) made up of keratin and cell walls without nuclei • •Keratinocytes of stratum basale and lower part of stratum spinosum have ability to divide – this part is called stratum germinativum Malpighi • • 152-5291_IMG Keratinization •the epidermis is a self-renewing structure • •keratinocytes are formed by mitosis in the stratum basale •as they move up through the stratum spinosum and stratum granulosum, they differentiate to form a rigid internal structure of keratin, microfilaments and microtubules (keratinization) •the outer layer of the epidermis, stratum corneum, is composed of layers of flattened dead cells (corneocytes) that have lost their nucleus. T •corneocytes are shed from the skin in process called desquamation • •this process takes approximately 28 days, the change from a basal layer keratinocytes to a corneocyte takes 14 days and then the loss of this cell remnant as scale occurs after another 14 days Horny layer Granular layer Spindle cell layer Basal cell layer 152-5284_IMG Horny layer - epidermal barrier •surface od epidermis is called horny layer (stratum corneum) or epidermal barrier •stratum corneum can be compared to a brick wall •the keratinocytes are connected together due to epidermal lipids and adhesion molecules •the lipids stabilize the epidermis and help to seal the barrier but also allow the passage of substances throught the epidermis in both directions • Other cells in the normal epidermis •Melanocytes – synthesize melanin (main photoprotective factor) •Langerhans cells – antigen-processing cells of the skin •Merkel cells – neuroendokrine cells • that function as mechanoreceptors • Cells and Layers of the Epidermis – Earth's Lab Melanocytes • clear cells located in the stratum basale • •are derived from the neural crest and migrate into epidermis • •main function of melanin is to absorb ultraviolet (UV) radiation to protect us from its harmful effects 152-5285_IMG Epidermal melanin unit Tyrosine –> Dopa –> Dopaquinone –> Eumelanin, pheomelanin Tyrosinase - synthesis of melanin is complex process starting with tyrosine, the most important enzyme is tyrosinase An important intermediate is DOPA (a precursor of dopamine) - the melanin is packaged into melanosomes in the Golgi apparatus and transferred to keratinocytes by dendrites - melanocytes have long cell extension denrites and can provide 30-40 keratinocytes with melanin They produce the pigment melanin, manufactured from tyrosine, which is an amino acid, packaged into cellular vesicles called melanosomes, and transported and delivered into the cytoplasm of the keratinocytes (Graham-Brown and Bourke, 2006). Skin colour is determined not by the number of melanocytes, but by the number and size of the melanosomes (Gawkrodger, 2007). It is influenced by several pigments, including melanin, carotene and haemoglobin. Melanin is transferred into the keratinocytes via a melanosome; the colour of the skin therefore depends of the amount of melanin produced by melanocytes in the stratum basale and taken up by keratinocytes. Melanin occurs in two primary forms: •Eumelanin – exists as black and brown; •Pheomelanin – provides a red colour. Skin colour is also influenced by exposure to UV radiation, genetic factors and hormonal influences (Biga et al, 2019). Cells and Layers of the Epidermis – Earth's Lab Langerhans cells •antigen-presenting dendritic cells •part of the body’s immune system •constantly on the lookout for antigens • in their surroundings so they can trap • them and present them to T-helper • lymphocytes, thereby activating an • immune response Merkel cells •present in very small numbers in the stratum basale •closely associated with terminal filaments of cutaneous nerves and seem to have a role in sensation, especially in areas of the body such as palms, soles and genitalia Cells and Layers of the Epidermis – Earth's Lab Types of cell junctions in the epidermis •desmosomes – complex structures with many proteins holding cells together – the most important are the desmogleins •adherent junctions - connect actin filaments and involve cadherins and catenins •gap junctions • 152-5287_IMG Dermo epidermal junction –basal membrane zone - narrow, undulating, multi-layered structure situated between the epidermis and dermis, which serve as cohesion between these two layers - key components of the basal membrane zone – hemidesmosomes – junctional complexes, attach keratinocytes of str. basale to underlying basal membrane sharing many features with desmosomes - - is made up of lamina lucida and lamina densa - barrier functions – allowing molecules to diffuse to and from the dermis 2. Dermis •thickness: 0,6 – 3 mm •connected to the epidermis at the level of the basal membrane •consists of two layers, the papillar and reticular layer which merge together without clear demarcation •contains: connective tissue • senzoric neurons blood vessels • hair follicles and hairs • sweat glands • Layers of dermis •Stratum papillare – formed into papilles orientated to epidermis, connective tissue with cells, elastic filaments, nerve endings, senzoric corpuscules - Meissner, Ruffini, ... •Stratum reticullare – bigger part of the dermis, thick network of colagen and elastic filaments, less amount of cells, Vater-Pacini corpuscules • 152-5288_IMG 2. Dermis - the major structural component of the skin - composed of collagen (strength), elastin (elasticity), blood/ lymph vessels and specialized cells called fibroblasts and mast cells. - work together in a mesh like network. - the network is surrounded by a gel-like substance called ground substance, which is made mostly from glycosaminoglycans (composed of hyaluronan, glycoproteins and proteoglycans), gel substance plays a key role in hydration and moisture level of the skin. Types of cells in dermis •3 type of cells: -Fibroblasts - the synthesis of collagen and elastin -Histiocytes – created from monocytes (from blood vessels), active form call as mastocytes -Mastocytes – fagocyte antigens, release mediators (histamine, heparin, prostagalndins, leukotirens, tryptase etc.) Inervation of skin •Nerve endings • •Merkels cells • •Sensitive corpuscles •heat •cold •pressure •vibrations •touch •itch •pain • 152-5290_IMG Adnexal structures •Hair •nails •glands (eccrine, apocrine, sebaceous) • •most of these adnexal structures is localized in dermis The adnexal glands •Eccrine glands • – widely distributed over the body, mostly concentrated on palms and soles, they can´t be found on the lips, external ear canal, clitoris and labia minora • - „skin kidneys“ – sweat is usually clear and odorless •Apoccrine glands • – associated with hair follicles, their secretory duct empties into the upper part of the hair follicule • - sweat is odoriferous due to bacteries • - found in armpits, anogenital area and areolas The adnexal glands •Sebaceous glands • - assotiated with hair follicules • - sebaceous localization • - their secretion lubricates the follicle to allow the hair shaft to growth outwards against less resistence • - androgen sensitive • • Hairs •first hairs start to grow in the 20.th. week of pregnancy - lanugo •the number of hair follicules is final after birth, new follicules aren´t produced •there is 5 milion hair follicules on human body (on head approx. 100 000) 152-5289_IMG Pilosebaceous unit •- The lower part of the follicule is extended • into the bulbus, there is the papila with • capillaries, above the papila germinative • matrix and its cells diferentiate to other • layers of the follicule and • from this part to the growing hair • •- Hair is Inside of the follicule, • consisting of medula, cortex a cuticula • •- Around hair there is inner root sheath, • which ends under opening of ductus • of sebaceous gland to follicule • •- Musculus arrector pili – is under the • ductus of sebaceous gl., • isn´t on beard, axillar and pubic hair • The hair cycle •Include: • anagen – growing phase (2-6 years) • katagen – (days – weeks) • telogen - (2-4 months) • •Colour of the hair – depends on numbers and activity of melanocytes in hair follicule Types of hairs •Lanugo - starts to grow in the 20.th. week of pregnancy •Velus hair – change from lanugo after birth •Terminal hair – has more pigment, is stronger, contain medula • pili longi – capillus (pilus capitis) • barba • hirci • pubes • body hairs • pili breves – cilia • supercilium • vibrisae • tragi 152-5292_IMG The nail •consists of the nail matrix, nail plate, • nail bed and periungual skin • (paronychium) •the nail matrix – growth zone of the nail •the nail plate – is sealed proximally • by the cuticule (eponychium) and • laterally by the nail folds • •nails grow slowly, a finger nail requires • 4-6 months to replace itself, toenail 2-18 • months, grow faster at night, in summer, • in young than in the elderly, and in men • than women • 3. Subcutis •lies between the dermis and muscle fascia, tendons or ligaments •consists of adipocytes and connective tissue •the subcutis contains numerous connective tissue septae which carry lymphatics and blood vessels and nerves •the network of septae keeps the lobules of fat in place and provides support •adipocytes are metabolically active 3. Subcutis Skin functions •Protection •Thermoregulation •Storage •Sensation •Metabolism •Immunologal processes •Detoxification •Social function • The function of the skin •Protection against •chemicals •physical factors (mechanical, thermic, actinic) •Biological factors (Infections, antigens) • •Preservation of balanced internal environment •Loss of water, electrolytes, macromolecules • •epidermal barrier • • • • • • • •horny layer, water reservoir, sweat gland,… • • • The function of the skin •Temp erature regulation • - p ers p iration • - vasodilatation • - vasoconstriction • • •Sensation • • • • • • •Storage (fat, water) •blood vessels, eccrine sweat glands • • • • • •thermoreceptors – cold and heat • mechanoreceptors – touch, pressure, vibrations • nociceptors – pain and itch • • •energy reservoir - subcutaneous fat • • The function of the skin •Metabolic – vitamin D synthesis, metabolism od sacharides, li p ids and p roteins, secretion of keratin, eanin, sebum, sweat • •Immunological function • • • •Psychosocial function •Keratinocytes • • • • •Langerhans cells, T-lymphocytes, macrophages, mastocytes, keratinocytes • •Cosmetic quality, lips, hair • THANK YOU FOR YOUR ATTENTION • • •You can find the overview here: https://www.youtube.com/watch?v=MPLV4h0Tr8c •