Metaplasia Explanation of metaplasia: This is a segment of mucous membrane of bronchia. The left is the normal structure — ciliated columnar epithelium, but right is squamous epithelium, which is not exist here. The concept of metaplasia is: one adult cell type is replaced by another adult cell type. Metaplasia means “convert”. Note that the cell types before and after converting are matured. Whether the cell is mature is the landmark of metaplastic cell and tumor cell. The commonest example is respiratory columnar epithelium transfer to squamous epithelium. Look this figure. This is respiratory columnar epithelium, this is basement membrane. Note this cell, it is small, irregular, located base of the epithelium. It is stem cell， or you can call it reserve cell. What is stem cell? Yes, the cell keeps undifferentiated state and has the ability of reproduce. It is this cell in the tissue that the cell type transformation is possible. When the outside stress threaten the cell s...