The Integumentary System, skin, hair, oil and sweat glands, and fingernails, serves to protect our bodies from water loss, invasion of bacteria and chemicals, and the physical abuse of the external environment. It also helps with temperature regulation, sensation, excretion and serves as a blood reservoir. Your skin may also be considered an endocrine organ because of its role in the production of vitamin D.
Skin has two regions, the epidermis and the dermis. Refer to figures 5.1, 5.2, and 5.3 in the text.
The epidermis is the outermost part, made of stratified squamous epithelial tissue. So, are there blood vessels in the epidermis? Nope. It's epithelium and epithelial tissue is avascular.
The epidermis has 5 (or 4) layers. We discuss them from the deepest to the superficial because that corresponds to the process that is taking place in which the epidermal cells are being produced in this deep layer and then pushed upward to become the more superficial layers. This process takes two to four weeks.
Stratum Basale - The deepest epidermal layer, it's a single row of cells (usually cuboidal) that are always rapidly dividing, producing the superficial layers.
Stratum Spinosum - The cells in this layer (mostly keartinocytes) are living and they have bundles of tonofilaments (part of the cytoskeleton) that give them strength.
Stratum Granulosum - 3-5 layers of cells that are beginning to die and whose membranes are thickening. They are loaded with granules that will produce keratin in superficial layers. The granules consist of keratohyalin.
Stratum Lucidum - Appears clear (hence the name) because of dead, eleiden- filled cells. Apparent only in the thick skin of the palms, soles and fingertips.
Stratum Corneum - 20-30 cells thick but the cells are all flat and filled with keratin filaments. The filaments are made of tonofilaments of the Stratum Spinosum and the granules of the Stratum Granulosum. Remember that there are no blood vessels in the epidermis so the cells get their nutrients by diffusion from the connective tissue below, therefore the cells of this outermost layer are dead. Stratum Corneum cells flake off. This is known as dandruff and there can be 40 pounds of it produced in a lifetime.
Cells of the epidermis (see figure 5.2).
Most of the cells of the epidermis are keratinocytes, but there are others, too.
Keratinocytes - 90% of the epidermal cells are keratinocytes, cells which produce keratin, a fibrous protein. They are formed in the stratum basale and get pushed up toward the surface. They manufacture keratin precursors and keratin as they age. Eventually their nuclei degenerate and the cells die. They're strong because they're loaded with keratin and because they're tightly connected to each other by glycoprotein filaments called desmosomes.
Melanocytes - They make melanin. About 8% of your epidermal cells are melanocytes. They live in the stratum basale but they have long branches that go in between cells of the stratum basale and the stratum spinosum. Keratinocytes phagocytize chunks of melanin produced by melanocytes and incorporate the melanin in the tops of their cells. This shields the cell's nucleus from ultra violet light. UV light is very destructive and we don't want it screwing up the genetic material of our skin cells.
Langerhans' Cells - Arise in bone marrow and migrate to the epidermis. They also have long branches that extend between the cells. They are part of the immune system and will be covered later.
Merkel Cells - Hemispherical cells at the boundary between the epidermis and the dermis. Combined with its associated nerve ending, it is a touch receptor. Sometimes referred to as a Merkel Disc.