Re: What is the life span of a normal skin cell?

Hi Faith,
	Skin or epidermis refers to a specific kind of epithelium- a continuous 
sheet of cells that separates the blood-supplied side of the tissue from the 
"outside" environment (which, for example, includes the cavity within 
esophagus/food-pipe or within trachea/wind-pipe). Skin is the kind of 
epithelium that has evolved to protect us from the dry and pathogen-filled 
environment on the surface of the body. It is a layer of epithelial cells 
that originate at the base and move toward the surface continuously, at a 
rate that replenishes the constantly shedding older cells from the surface. 
The life-span of the skin cell therefore depends on how fast the surface 
cells are shed and how many layers thick the skin is over a particular part 
of the body. Another interesting fact about the skin cells that determines 
their life-span is their "keratinization". Keratin is a protein found in 
hair, nails or skin. This protein starts accumulating in a skin cell as it 
moves toward the surface through the layers. By the time this cell is shed 
from the surface, it is dead and full of keratin. These layers of dead 
keratinized cells at the surface (called "squames") are flat (squamous), 
scaly and tough, protecting the underlying live layers. The constantly 
dividing cells at the base of skin that make new ones of these Kamikaze skin 
cells are called basal cells or stem cells. Although the stem/basal cells 
last a lifetime, a typical epidermal cell has a life span of about 2 to 4 
weeks depending upon the part of the body. You can read more about skin, 
other epithelia and all about cells, in "Molecular Biology of the Cell" by 
Alberts, Bray, Lewis, Raff, Roberts and Watson (yes the one who co-
discovered the structure of DNA!), published by Garland Publishing, NY. I 
have an older version, so the page numbers differ, but you can look up 
"epidermis", "dermis" or "epithelium" in the index. Other cell Biology texts 
at the undergraduate level also have similar chapters.