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WonderBody fact

The epidermis, the outer layer of the skin, renews itself constantly. New cells form at its base, move up to the surface over several weeks, harden and flake away. Classic estimates put a full turnover at roughly 40 to 56 days.

Human biology

Health

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The epidermis is thin, about a tenth of a millimetre over most of the body and about half a millimetre on the soles of the feet, and it has no blood vessels of its own. About 90% of its cells are keratinocytes, which fill with a tough protein called keratin as they rise. A cell takes about two weeks to climb from the base to just below the surface, then roughly four more weeks to cross the outermost layer: about 15 layers of flat, dead cells, sealed with fats, that keep water in and germs out. The pace varies with age and body site and tends to slow as we get older. In psoriasis, a skin condition, cells make the journey in a few days instead of weeks, which causes thick, scaly patches.

Türkçe oku
Derinin en dış katmanı olan epidermis, yani üst deri, kendini sürekli yeniler. Yeni hücreler bu katmanın tabanında oluşur, birkaç haftada yüzeye çıkar, sertleşir ve sonunda dökülür. Klasik tahminlere göre katmanın baştan sona yenilenmesi 40 ila 56 gün sürer.

Epidermis çok incedir: Vücudun çoğu yerinde milimetrenin onda biri kadardır, ayak tabanında ise yaklaşık yarım milimetre. Kendine ait kan damarı yoktur. Hücrelerinin yaklaşık %90'ı keratinositlerdir; bu hücreler yukarı çıktıkça keratin adlı sert bir proteinle dolar. Bir hücrenin tabandan yüzeyin hemen altına çıkması yaklaşık iki hafta sürer; en dıştaki katmanı geçmesi için de yaklaşık dört hafta daha gerekir. Bu dış katman, yağlarla birbirine tutturulmuş yaklaşık 15 kat yassı, ölü hücreden oluşur; suyu içeride, mikropları dışarıda tutar. Yenilenme hızı yaşa ve vücut bölgesine göre değişir; yaş ilerledikçe genellikle yavaşlar. Sedef hastalığında ise hücreler bu yolu haftalar yerine birkaç günde tamamlar; deride kalın, pullu lekeler bu yüzden oluşur.

Sources and further reading

Background reading: Epidermis
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