Student Performance Objectives1. Distinguish between the cutaneous membrane and the skin's accessory structures.2. Explain the meaning of thick and thin skin.3. Explain how the epidermis is attached to the tissue layers below it.4. Explain the functions of the superficial fascia. 5. Name three cell types found in the epidermis.6. Name the 5 layers of the epidermis in order from deepest to most superficial.7. Explain the functional difference between the 5 layers of the epidermis
Lesson OutlineA. Introduction 1. The integumentary system has two parts: the cutaneous membrane (epidermis and dermis), and the accessory structures - glands, hair and nails. For an overview of skin see: https://www.youtube.com/watch?v=4zKjzl0futI 2. Skin, taken as a single covering organ of the body, is the largest of all the body's organs. a. Area of skin - 1.5 - 2 m2. Go to the black board and draw a square using a meter stick; measure out 1.5 meters for each of the 4 sides. b. Weight of skin - approximately 15% of body weight; e.g., a 135 person has 20 lbs of skin. For every 100 pounds of body weight, about 15 pounds is skin. c. Thickness of skin - the skin's thickness not generally given as the overall thickness of the entire cutaneous membrane, which would be the combined thickness of both epidermis and dermis. Thickness is generally measured as the thickness of the epidermis only. (1) Thick skin, as on the palms and soles, is about a 0.5 mm thick - which is about the thickness of a bathroom paper towel for drying one's hands. (2) Thin skin is about 6 times thinner - about 0.08 mm thick - and is the typical thickness of all skin regions, except for the thick areas. 3. In general, the system protects us in many ways outlined in a later lesson, provides us with sensation, produces vitamin D, and helps to maintain normal body temperature. 4. Recall from Unit 5, Lesson 6, that superficial fascia (or tela subcutanea) is a loose, irregular connective tissue with a high fat (adipose) content. It forms a contin sheet surrounding all parts of the body just under the skin. B. Epidermis 1. Composed of stratified squamous epithelial tissue. 2. This layer is avascular; it derives its nutrition by diffusion from the underlying dermis. 3. As epidermal cells move toward more superficial regions, more distant from the dermis, their metabolism slows and the cells forming the skin surface we actually see are dead. 4. Epidermal cell types a. Keratinocytes - the most abundant epidermal cell type found in the epidermal layers. b. Melanocytes - produce the pigment, melanin that eventually is incorporated into each keratinocyte. Responsible for the skin's pigmentation. c. Langerhans cells - these are a type of white blood cell that protect the epidermis from microorganisms and potential cancer cells. d. Merkel's cells, which provide for touch sensations. 5. Epidermal layers - from deepest to most superficial, the epidermal layers are: a. Stratum germinativum (also called the stratum basale). Go to the following site and then click on the epidermal layer you wish to see. http://www.meddean.luc.edu/lumen/MedEd/medicine/dermatology/melton/skinlsn/sknlsn.htm (1) Undergoes mitosis - stem cells in this layer, mostly dividing at night, produce the keratinocytes that gradually push upward toward the skin's surface. It takes 2 weeks to a month for cells to reach the surface from this deepest germinative layer. (2) This deepest layer of epidermal cells rests on a basement membrane (also called the basal lamina) and is firmly attached to it by hemidesmosomes (see Unit 5, Lesson 2, under desmosomes). (3) This layer grows in a way that helps to interlock it with the underlying dermis: epidermal ridges and dermal papillae interlock like linking the fingers of two hands. (a) The ridges and papillae are doubled up in the palms and soles for an extra-firm attachment. [It would be quite embarrassing to try to twist off a tight lid on a jar and have your skin rip off before the jar lid did]. (b) The epidermal ridge patterns are unique to each individual and constitute one's fingerprint. (4) Melanocytes in or just below this layer secrete melanin which is taken up by surrounding keratinocytes and used by these cells to shield the cell's nucleus and DNA from the damaging effects of ultraviolet radiation that penetrates into the layers of skin. (5) Merkel's cells are also present in this layer providing touch sensations. (6) Free nerve endings are also present that provide pain sensations. b. Stratum spinosum http://www.meddean.luc.edu/lumen/MedEd/medicine/dermatology/melton/skinlsn/sknlsn.htm (1) This is the 8-10 cell thick region forms as dividing stem cells of the stratum germinativum push upward toward the skin's surface. (2) Mitosis continues in this region also. (3) Cells of this region are held tightly together by desmosomes. (4) Langerhans cells wander around looking for abnormalities and microbes. c. Stratum granulosum http://www.meddean.luc.edu/lumen/MedEd/medicine/dermatology/melton/skinlsn/sknlsn.htm (1) This 3-5 cell thick region marks the area of epidermis where cellular production of the protein, keratin (and keratohyalin) is of major importance as this protein waterproofs the cells as its concentration increases. (2) Mitosis has ended. (3) Cells flatten out, become dehydrated, and lose their organelles. d. Stratum lucidum - consists of the end product of the processes going on in the stratum granulosum: cells are tightly adherent to each other, flattened, and filled with keratin. http://www.meddean.luc.edu/lumen/MedEd/medicine/dermatology/melton/skinlsn/sknlsn.htm e. Stratum corneum http://www.meddean.luc.edu/lumen/MedEd/medicine/dermatology/melton/skinlsn/sknlsn.htm (1) 15-30 layers of dead, fully keratinized, dehydrated cells firmly locked together by desmosomes. (2) This protective layer of dead cells forms the skin's surface. (3) Cells will eventually fall off in clumps (generally in about 2 weeks).