Epithelial tissues undergo turn-over throughout adult life: cells die and are replaced. Consequently,most if not all epithelia contain stem cells that <span>(i) are competent for cell division, </span> <span>(ii) self renew:regeneration of a "mother" stem cell with each division, and </span> (iii) produce differentiated cell types specific to each epithelia. -Stem cells produce differentiated cell types by cell division coupled with specific pathways of cell specification imposed on some of their daughter cells. In most stem cell-tissue systems, cell division and specification goes through several steps. Many stem cells produce daughters that also proliferate themselves (undergo cell division cycles), often at faster rates; these transitional intermediates are called transit amplifying cells. These transit amplifying cells then produce differentiated cells, either directly or through several steps of specification. At each step,specific changes in protein expression patterns and activities occur. -Stem cell in gland crypt bottom (tightly control stem cell, have to divide longutdinal) <span>-Stem cell in skin basal lamina (divide in apical direction (change in phenotype) form squamous layer at apical, highly control (reactivate/reuse), secrete ligand that core signaling molecule (WINKS) in early develop than reuse and reactivate for stem cell in skin... also use in cancer cell (lost of control=tumor))</span>
