in addition to the cells destined for keratinization, the deep layers of the epidermis include small numbers of cells (not shown) that invade this tissue and have quite different origins and functions. these immigrants include dendritic cells, called langerhans cells, derived from bone marrow and belonging to the immune system; melanocytes (pigment cells) derived from the neural crest; and merkel cells, which are associated with nerve endings in the epidermis. (b, from R.v. krsti´c, ultrastructure of the mammalian Cell: an atlas. berlin: Springer-verlag, 1979.) tissue Renewal that does not depend on Stem Cells: insulin-Secreting Cells in the pancreas and hepatocytes in the liver
Some types of cells can divide even though fully differentiated, allowing for renewal and regeneration without the use of stem cells. The insulin-secreting cells (β cells) of the pancreas are one example. Their mode of renewal has a special importance, because it is the loss of these cells (through autoimmune attack) that is responsible for type 1 (juvenile-onset) diabetes; they are also a significant factor in the type 2 (adult-onset) form of the disease. The β cells are normally sequestered in cell clusters called islets of Langerhans. These islets contain no obvious subset of cells specialized to act as stem cells, yet fresh β cells are continually generated within them. Lineage tracing studies, similar to those described above for the gut, show that the renewal of this population normally occurs by simple duplication of the existing insulin-expressing cells, and not by means of stem cells.
Another tissue that can renew by simple duplication of fully differentiated cells is the liver. The main cell type in the liver is the hepatocyte, a large cell that performs the liver’s metabolic functions. Hepatocytes normally live for a year or more and renew themselves through cell division at a very slow rate. Powerful homeostatic mechanisms operate to adjust the rate of cell proliferation or the rate of cell death, or both, so as to keep the organ at its normal size or restore it to that size in case of damage. A dramatic effect is seen if large numbers of hepatocytes are removed surgically or are killed by poisoning with carbon tetrachloride. Within a day or so after either sort of damage, a surge of cell division occurs among the surviving hepatocytes, quickly replacing the lost tissue. If two-thirds of a rat’s liver is removed, for example, a liver of nearly normal size can regenerate from the remainder by hepatocyte proliferation within about two weeks.
Both the pancreas and the liver contain small populations of stem cells that can be called into play as a backup mechanism for production of the differentiated cell types in more extreme circumstances. This imparts resilience to the mechanisms of renewal and repair.
Some tissues lack Stem Cells and are not Renewable
The variety among tissues in the capacity for self-renewal is illustrated in a striking way by comparing the olfactory epithelium in the nose, the auditory epithelium of the inner ear, and the photoreceptive epithelium of the retina. These three sensory structures, which like the epidermis develop from the ectodermal layer of the early embryo, differ radically in their self-renewal capabilities. The olfactory epithelium contains a population of stem cells that give rise to differentiated cells that have a limited life-span and are continually replaced. But unlike the epidermis, these differentiated cells (the olfactory receptor cells) are neurons, with cell bodies lying in the olfactory epithelium and axons that extend back to the olfactory lobes in the brain. The continual renewal of this epithelium therefore involves continual production of fresh axons, which have to navigate back to the appropriate sites in the brain.