okay in this video ring go into more detail on the structures and functions of the epidermis remember the epidermis is the superficial layer of skin and it's an epithelial tissue specifically keratinized stratified squamous epithelium it makes sense than a stratified squamous because it's protective in that regard and it makes sense that it's keratinized because keratin is actually a really tough fibrous protein that gives skin its protective properties the major cells of the epidermis are actually called keratinocytes this is what these are the cell types that you find in stratified squamous epithelium and they are called that because they make keratin now these cells are very tightly connected by desmosomes remember Devi's ohms are strongest of the inter site of junctions so it makes sense that desmosomes would be found between these keratinocytes that make up the protective layer of your skin now millions of these cells slough off every day and that actually is consistent with the idea that epithelial tissues have a high regenerative capacity so you actually replace your carrots in a stratified squamous epithelium about once every two months now in terms of other cell types of the epidermis we also have melanocytes melanocytes are these spider shaped cells located within the epidermis and they actually called melanocytes because they produce a pigmented molecule called melanin melanin is a type of organic molecule that's packaged into these little vesicles called melanosomes and these Milano's ohm's actually transferred into the growing keratinocytes so the keratinocytes are packed full of melanin from the melanocytes now the function of melanin is actually involved with UV protection so these melanocytes help to protect some of the cells of your skin from the damaging effects of UV radiation from the Sun or other sources now dendritic cells you also find in the epidermis and there are the star-shaped mobile macrophages that patrol deep in your epidermis they're also called Langerhans cells and essentially these cells you could say walk around deep in the epidermis and they essentially scavenge for microorganisms and debris and play a really key role in your immune system now the another cell type you find the epidermis are the tactile cells or Merkel cells and these are specialized sensory cells that can respond to light touch so very light touch on your skin can excite receptors on these cells that actually can you know eventually be indiv nerve impulses that your brain can interpret is touch now in terms of the layers of epidermis we find that it's actually made of four to five distinct layers now we have thick and thin skin thick skin has all five layers or strata strata means layers so we have five of these and thick skin thick skin is non-hairy skin and you defined thick skin in areas of high abrasion like the you know palms of your hands and the soles of your feet thin skin only has four layers only four strata and we also think of thin skin as big hairy skin so skin that has hair is gonna be thin skin now there are five layers of skin total members strata we have so stratum assaleh spinosum granulosa m-- lucidum and corneum now lucidum is the only one you find in thick skin right so it turns out that thin skin would only have passaglia spinosum granulosa and then we would skip lucidum and go to corneum but salah is the deepest corneum is the most superficial and it makes sense because think of a bus Allah is like basement deeper so Casali is gonna be a single layer of actively mitotic stem cells that can divide and then give rise to the these other layers as I get pushed upward towards the most superficial layer which is the cornea now the B solid is this deepest layer and it's firmly attached to the dermis by the way of the basement membrane and it consists of a single row of actively mitotic stem cells that produce daughter cells that men are pushed up superficially into the epidermis one daughter cell kind of journeys from the basal layer towards the surface and this could take up to 45 days to reach the surface which is why we say that on on average you know you can replace your skin about what's every two months or so now cells actually dies they move towards the surface because your epidermis is so thick that the cells towards the superficial layer actually too far from their nutrient supply so they start to undergo a pitocin which is programmed cell death now the other daughter selector remains in the masala and remains as that stem cell which can regenerate inform noose new cells that can help regenerate your epidermis now we also call the stratum assaleh stratum germinativum because it has its the germinating layer of actively mitotic cells and about 10 to 25% of these cells in this layer are melanocytes remember these are the ones that make melanin stratum spinosum is also called a prickly layer and these are several cell layers thick this is just superficial to the masala and these cells contain web-like system of intermediate pre keratin filaments so in the stratum spinosum these cells are already the keratinocytes here already starting to pack themselves full of pre keratin it allows them to resist things like tension and pulling now i because of these shape of these cells they'll appear spiky you can say Mizpah no 'some and they're scattered you know with abundant melanosomes and dendritic cells so these Milano zones are gonna incorporate into the keratinocytes and then dendritic cells actually kind of can scavenge around and look for microorganisms or debris to remove through phagocytosis stratum granulosum is called the granular layer no it's about for two cells thick and these cells are flattened so the cells layers a little thinner these start to undergo changes in appearance because the cells flatten nuclei and organelles start to disintegrate this is where keratinization begins which is what gives these cells kind of a granular appearance so these cells accumulate hirato hyland granules that actually helped form keratin fibers and because of this appearance we call them granulosa now these cells also accumulate lamellar granules which are a water-resistant glycolipid that slows water loss in cells above this layer begin death you know because if the organelles disintegrate and your nuclei disintegrates there's no way to regenerate you know tissue or damaged cells rather and they're too far from the dermal capillaries like for new nutrients supply to survive stratum lucidum is the clear layer hence the name lucidum so lucid means clear and you only find this in thick skin so consists of kind of a thinner layer a translucent band of several cell layers of clear flat dead keratinocytes they lies just superficial to the stratum granulosum and hence the name lucidum remember remember you only find stratum lucidum in thick skin now stratum corneum is what you find in both thin and thick skin let's call this the the horny layer because it's hardened now it's consisted of twenty to thirty rows of flat a nucleated keratinized dead cells so what's interesting is the most superficial layer of epidermis is actually made of dead cells and it accounts for about three-quarters of epidermal thickness so it's very thick now though these cells are dead they still function to protect deeper cells from your environment they prevent water loss they protect from abrasion and act as an ice barrier against biological chemical and physical assaults now as these cells are pushed more superficially the cells undergo a process of a Potosi switch is programmed cell death now these dead cells slough off as dandruff or dander and humans can shed up to 50,000 keratinocytes every minute which is why our epidermis needs to have a high regenerative capacity and you're gonna replace these cells about once every 45 days so what this slide shows is essentially a pictorial view of these cell layers remember we have five major layers in thick skin and four major layers and thin skin this is showing an example of thin skin because we're actually missing stratum lucidum but there's actually a really nice pneumonic to memorize these and it goes best skin gets love and care now we don't have the love layer here because lucidum is not present but it would be here so best is stratum Beausoleil skin is stratum spinosum stratum granulosum and then love is loosen them which isn't here and then core diem is care now let's just start a new mnemonics memorize the cell layers now Basava member is the single layer of actively dividing stem cells you can see that part the dermis here which has your nutrients supply and it makes sense the cells that are dividing are closest to the nutrients stratum spinosum is called that because the cells kind of take on a spiny appearance and they also start to pack themselves full of pre keratin this is also where you find dendritic cells that can move around and kind of scavenge and look for microorganisms stratum granulosum is the granular appearance here you can see these cells kind of a granular sort of you know appearance and their cytoplasm this is actually where keratinization begins hence the granular appearance here what's missing is the lucidum layer and there should be a kind of a thin clear layer here if this were a thick skin because it's a thin skin you don't have loosen instead we go straight from granulosa to corneum so all of this is corneum you can identify that this cornea because there are no nuclei do you notice that none of these cells here have you know the distinct nucleus because essentially these cells are dead after granulosa m-- the cells undergo a pitocin degradation of the nuclei and organelles so what you're left with are essentially just bags of keratin that are you know high tightly bound together by intercellular junctions and this outer corneum layer is kind of like the bark of the tree you know on your skin which is essentially an outer layer dead cells just like tree bark and in fact that's what cornea means like cortex means bark and this is actually gonna make up most of the thickness of your skin and it's essentially made of dead cells which are protective and they prevent water loss and this is just showing the artist's representation of of what we see over here which is light microscopy image so over here we see our dermis we see our stratum assaleh we have extreme spinosum with dendritic cells in it stratum granulosum and then corneum remember our stratum Messala has got three major cell types in there Messala has got your keratinocytes that are dividing and replacing keratinocytes as your skin pushes upward we have our melanocytes which are spider-like projections that can inject melanosomes into growing carrots in Oh sites and we have our tactile or Merkel cells which are involved with touch