Figure 4. Cellular proliferation and death are kept in balance in normal, healthy tissues. This scenario might appear in any one of the tissues, such as the epithelium of the respiratory tract or the colon, where cells turn over and are replaced frequently in the course of normal use. Cells are bound on one end by the basal lamina and stroma, layers of collagen and other connective-tissue proteins that form a wall around the tissue. On the other end is the lumen, or the tube of tissue such as an airway. In this tissue, immature cells, also known as stem cells, lie dormant near the basal part of the epithelium until they receive a growth signal. Then they undergo the cell-division cycle and proliferate. The membrane of the cell nucleus remains intact inside cells preparing to divide. The nuclear membrane disappears during cell division itself when individual chromosomes become visible. Dividing stem cells differentiate and produce one of two mature cell types in this tissue. One type is the ciliated epithelial cell. Another type is the mucus-secreting goblet cell, which produces the mucus coating the lumen of this air passage.