 This video will cover part two of an introduction to the integumentary system. We will cover the following objectives, list the types of cells found in the epidermis, and describe the five layers of the epidermis, and describe how melanin, keratin, and hemoglobin pigments contribute to skin color, what might cyanosis indicate, and what might jaundice indicate. The most common type of cell in the epidermis is called a keratinocyte. Keratinocytes produce the fibrous intermediate filament protein keratin that provides strength to the epidermis. Melanocytes are cells that produce a brown pigment called melanin, which is important for protecting us against the damaging effects of ultraviolet light in the sun. Melanocytes are found in the deep layers of the epidermis, and melanocytes produce melanin, which is then taken up by keratinocytes. Longer hand cells are a type of leukocyte found in the epidermis. Longer hand cells are important for monitoring the skin for infections and helping to protect against skin cancer. And merkle cells are a specialized type of neuroepithelial cell, a sensory receptor cell, important for the sense of touch, the text changes in the shape of the epidermis, and then relays that information to sensory neurons. There are five layers of the epidermis. Stratum basale is the deepest layer, basal layer, which is attached to the basal lamina, and is adjacent to the papillary layer of the dermis. It's also known as the stratum germinativum, because the Latin word germinare means to sprout. The stratum basale or stratum germinativum contains stem cells, and these stem cells have the capacity to divide and differentiate into keratinocytes in order to regenerate the epidermis after damage, after injury, and to constantly regenerate the layers of the epidermis as cells are continuously being lost from the surface of the epidermis. Melanocytes are also found in stratum basale, and they produce the melanin, the brown pigment, which is important for protecting against UV light. The next layer of the epidermis is called stratum spinosum, which means spiny layer. Stratum spinosum is a layer of approximately 8 to 10 cells. Stratum basale is just the one layer of cells adjacent to the papillary layer of the dermis, then the next 8 to 10 cells form stratum spinosum. Stratum spinosum has a spiny appearance under the light microscope. This is a result of the web-like system of intermediate filaments of the pre-keratin protein, which are attached to desmosomes in between the keratinocytes. And as we fix the tissue to prepare it for histology, cells are dehydrated, leaving the keratin filaments poking through the plasma membrane, giving the cells of stratum spinosum a spiny appearance. Stratum granulosum is 3 to 4 cells thick and contains cells that are becoming flattened and disintegrating their organelles as they're preparing to go through a programmed cell death mechanism. Cells are excreting lipid granules that fill the cytoplasm and surround keratin, allowing to create a water-resistant barrier in the epidermis. So stratum granulosum can be distinguished by its dark, granular appearance with flattened cells that are starting to go through cell death. The next layer towards the surface is stratum lucidum, which literally means clear layer. Stratum lucidum is a thin layer of only a few cells and these cells are very thin and flat. And because they have a large amount of lipid granules, they don't stain well for light microscopy. When tissue is prepared for histology with a stain, the stratum lucidum cells don't absorb the stain, so they appear clear. So stratum lucidum is a thin, clear layer, just superficial to stratum granulosum. And stratum lucidum is only found in thick skin. So this is a fifth layer of the epidermis that's only present in thick skin, whereas there's four layers in the epidermis in thin skin. Thick skin is found in the palmar and plantar regions and as the name suggests has a much thicker epidermis, including this extra layer, the stratum lucidum. The most superficial layer of the epidermis is called stratum corneum, literally translated as the horny layer. The stratum corneum contains dead keratinocytes, which are filled with keratin and surrounded with glycolipids that fill the extracellular space, creating a water-resistant barrier. Stratum corneum is the thickest layer of the epidermis. Cells are constantly being shed from stratum corneum and new cells migrate up from stratum basale through stratum spinosum, stratum granulosum, and stratum lucidum in order to replace the cells that are lost from the surface of stratum corneum. The dead keratinocytes that are attached to each other within stratum corneum stay held onto the epidermis for approximately two weeks before they're normally broken away and shed from the surface. Here we have a slide of thin skin that has been specially stained to allow us to visualize the langurhan cells, the specialized lucocytes that are found in primarily stratum spinosum. We can see there are also some langurhan cells scattered through the papillary layer of the dermis, maybe one or two in stratum basale, but the majority of the langurhan cells are distributed throughout stratum spinosum. The function of langurhan cells, like other lucocytes, involves defense against infection. Remember that lucocytes are the white blood cells, however they're not always found in the blood, such as these langurhan cells are a type of lucocyte found in the cutaneous membrane or in the skin. They help defend against infection if you are cut. If the barrier of the epidermis is compromised, then the langurhan cells are there to immediately recognize any pathogens and help defend against those pathogens preventing an infection from spreading. The langurhan cells can also monitor other cells in the cutaneous membrane that are part of our own body in order to help recognize if those cells are abnormal and can help to trigger a mechanism that will remove cancerous cells or abnormal cells that are not functioning properly. Here on the right in the top we can see a light micrograph of thin skin, where there's a much thinner layer of stratum corneum and no stratum lucidum layer in between stratum granulosum and stratum corneum. Below that on the bottom right here we can see an image of thick skin. Thick skin has much more exaggerated epidermal ridges and a much larger layer of stratum corneum and an extra layer in the epidermis stratum lucidum is found in between stratum corneum and stratum granulosum. Melanocytes found in the deep layer of the epidermis in stratum bisale have extensions that travel through the epidermis and melanocytes produce melanin and store melanin in secretory vesicles that are released from the extensions of melanocytes and then taken up by keratinocytes. The number of melanocytes in our skin is not related to the pigmentation, the darkness of the coloration of our skin. Instead the activity of melanocytes contributes to darker skin pigment where melanocytes can produce more melanin and there will be a thicker layer of the epidermis that contains melanosomes that are the granules of melanocytes inside of keratinocytes. A tan is a result of an increased melanin production in response to light exposure, melanocytes will produce more melanin leading to darker pigmentation of the skin. Freckles and moles are examples of local increase in melanin accumulating within a region of the skin. Hemoglobin is another pigment that contributes to the color of the skin. Hemoglobin is a red pigment found in erythrocytes within the red blood cells inside of our blood. Increased blood flow can turn the skin red. For example, when a person is blushing and their face turns red there's an increase in blood flow through the cutaneous membrane leading to the red color of the skin. Or when you're very hot, if you've been exercising and your skin becomes more reddish colored this is a result of an increased blood flow. The binding of oxygen to hemoglobin affects its color so hemoglobin is bright red when it's saturated with oxygen but as it becomes deoxygenated the color shifts to a darker red and this darker red appears blue when viewed through the skin. Cyanosis is a term referring to the blue color of the skin indicating a lack of oxygen when a person is suffocating or if they have poor circulation to a region of the body as is shown in the image here. Poor circulation to the fingers to the tips of the fingers has led to cyanosis to this blue coloration. Bilirubin is a chemical that's produced from the breakdown of hemoglobin and jaundice is a condition where the skin turns yellow as a result of the accumulation of bilirubin. You can also see jaundice by looking at the whites of the eyes the sclera of the eyes as is shown in the image on the bottom here you can see the yellow coloration to the sclera of the eyes resulting from jaundice. Jaundice may indicate impaired liver function however it's fairly common in newborns because the liver is overwhelmed by the breakdown of fetal hemoglobin which is replaced by a mature form of hemoglobin after birth and this jaundice in a newborn is commonly treated with phototherapy which helps break down bilirubin into a form that's water soluble and can be excreted by the kidneys. Keratin is a pigment that has an orange color which comes from vegetables. Beta keratin is a precursor to vitamin A that gives the orange color to kerats and as we consume vegetables that contain keratin it contributes to the color of our skin giving an orange pigment color to our skin however if we consume a large amount of keratin it can lead to a excessive orange coloration of the skin known as keratinosis as is shown in this image this infant has keratinosis from excessive consumption of kerats.