The stratum corneum consists of up to 30 layers of dead, scaly, keratinized cells. Those at the surface flake off (exfoliate or desquamate) as tiny scales called dander (see insight 6.1). Dandruff is composed of clumps of dander stuck together by oil from the scalp.
As we surveyed the five layers of epidermis, we followed keratinocytes from their birth to their death. They are produced by mitosis in the stratum basale and lower level of the stratum spinosum. Mitosis occurs here mainly at night, and most skin specimens are taken during the day, so mitotic cells are rarely seen in prepared slides of skin. Over the course of 30 to 40 days, a keratinocyte is pushed farther and farther toward the surface by the dividing cells below, until it finally exfoliates from the surface. This process is slower in old age, but faster in skin that has been injured or stressed. Injured epidermis regenerates more rapidly than any other tissue in the body. Mechanical stress from manual labor or tight shoes accelerates keratinocyte multiplication and results in calluses on the hands or feet.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.