Читать книгу Applied Oral Physiology - Robin Wilding - Страница 39

The rate of cell division (mitosis) of the basal layer of cells normally keeps pace with the rate of desquamation from the surface. The time taken for a recently divided basal cell to reach the surface and exfoliate is the turnover time for one cell. The rate of mitosis is reduced with increasing age but increased by stress, infection, and changes in the levels of female hormones. Apart from these influences, the rates of turnover vary according to skin and mucosa types. The cells of a keratinized epithelium exfoliate after about 60 days for skin and after 45 days for gingiva. In comparison, the nonkeratinized lining mucosa turns over in 25 days. However, even this is a short period compared to the gut epithelium, where cells only last 10 days. The junctional epithelium around the cervical margin of the tooth is very fragile and turns over in 4 to 6 days. The rate of turnover of epithelium is related to its functions. Those which are primarily protective (skin) have a tough layer of impervious keratin and turn over relatively slowly. Mucosa, which must be flexible and stretch during function, is not protected by keratin but is quite thick. The shedding of the surface cells is an important protective device which compensates for the lack of a protective keratin layer. Mucosa which must be permeable to allow for food absorption (e.g., gut) or to allow secretions of fluid to combat bacteria (e.g., junctional epithelium) must be very thin and is therefore easily damaged. Its turnover rate is high.