Epithelial cells line the surface of various organs
and
are responsible for many important biological functions including
secretion, absorption, and protection. As the first
line of defense in frequent contact with environmental carcinogens,
epithelium-derived neoplasms (the carcinomas) account for over 90% of all
human tumors. It is thus critically important to better understand the
structure and function of epithelial cells. Our laboratory has shown
previously that keratins, a group of intermediate proteins that were
thought to be uniquely associated with skin epidermis and its appendages
such as hair and nail, are present in almost all epithelial cells and
their neoplasm. We showed that specific basic and
acidic keratins form
keratin pairs that represent markers for different pathways and stages of
epithelial differentiation. By studying keratin expression in corneal
epithelium, we showed that corneal epithelial stem cells are not evenly
distributed in the entire corneal epithelium. Rather, corneal epithelial
stem cells are found to be limited to peripheral conrea in a narrow zone
known as the limbus. In collaboration with Robert Lavker and co-workers,
we have shown that hair follicular epithelial stem cells are not located
at the bottom of hair follicle in the hair root. Rather such hair
follicular stem cells reside in upper follicle in the so-called bulge
area. More recently, our group has focused our attention to a group of
epithelial membrane proteins, called uroplakins, that are made by
mammalian bladder urothelium as major differentiation products.
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