Lesson 1 - Introduction and the Epidermis

Student Performance Objectives
1. 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 Outline
A. 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
        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
            (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
        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.
            (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
        (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
    c. Stratum granulosum
        (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
        (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.
    e. Stratum corneum
        (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).