I am not an expert on wound healing, but I would like to add my understanding to your post. Perhaps someone who is an expert can step in and correct any further mistakes.
Let me see if my ignorance equals Kumar's:
Some tissues repair because their cells multiply themselves. Like skin, intestines, and bones. Stem cell therapy won't help these types of tissues.
Muscle cells, and I believe tendons and ligaments, use undifferentiated stem cells for growth and repair. When they need repair they latch on to stem cells from the blood stream. Stem cell therapy has a better chance of helping these tissues. Installing stem cells directly into the tissues where they are needed seems to help.
This is incorrect according to my understanding. Whenever you get a wound, no matter the location, inflammation mediated by macrophages (a type of immune cell found in tissues) is the first thing that happens. This is to set up a temporary barrier to aid the immune system when your physical barriers are breached as well as to aid in removing any potential foreign invaders (such as bacteria or fungi) from the location.
Soon after, fibroblasts are recruited to the area. Fibroblasts are a type of cell that specializes mostly in making collagen (kind of a natural glue for knitting the tissue back together). Fibroblasts are what give scarring its distinctive look. Also, during the same time, some factors secreted by fibroblasts bring in endothelial cells to the area. The endothelial cells' job is to form new blood vessels to the site and supply blood to the new tissues. In skin, this makes the scarring area look pinkish.
After the fibroblasts have set up a good collagen foundation, specialization can begin. In bones, the fibroblasts can differentiate to chondroblasts and osteoblasts (making cartilage and bone material). In muscle, they differentiate to myofibroblasts, to form muscle tissue. In skin, keratinocytes (skin cells) are recruited to the site and begin making new skin over the fibroblasts. Other epithelial cells are recruited through chemical signals to begin forming hair follicles, sweat glands etc. Generally though, this happens mostly at the edge of the wound, which is why large scars may not have hair or other skin-like features. This all starts occuring within a few weeks of the injury.
The final stage of healing is maturation, which can take years to complete. Basically, the body tries to organize the scar tissue and remove extraneous blood supply. Occassionally, stem cells migrating to the area are recruited and differentiate to the new tissue types. However, many stem cells simply pass by. Since fibroblasts only job is to knit tissue together, this is why large scarring can result in loss of organ function and may take many years to even get some of the function to return through stem cell differentiation.
About epithelial versus endothelial tissues: any organ directly contacting the "outside" is primarily composed of epithelial cells - skin, mouth, stomach, intestines, nasal passages, urogenital tract. As part of a healthy individual, they constantly shed cells and produce new cells through cell division to prevent infection, since they are constantly exposed to environmental flora and microtrauma. This is why healing seems to occur faster in these areas. Endothelial cells are not exposed to this environment, so by default, they don't shed and don't produce new cells to replace old ones.
The technical terms for the two types are Meiosis and Mitoses, but the two terms are too similar for me to keep track of which is which. Is there a mnomic that would help?
Sorry, this is wrong. Mitosis is when a cell divides to make two copies of itself, each with one full set of DNA (two chromosome copies). Almost all dividing cells in your body occurs through mitosis. Meiosis is a specialized form of mitosis that only occurs in germ cells (sperm and egg cells), where the cell divides to make two copies, but each copy only has half the copy of DNA (one chromosome copy). This is because the function of the germ cell is to combine with another germ cell to produce a full set of DNA during reproduction.
Anyway, I don't think internal organs are helped by stem cells, because they don't repair themselves that way, they use the division of their current cells to make new ones. That is why they are more prone to cancers- bad cells can multiply, where as stem cells are freshly sorted out in the marrow, to eliminate cancer prone ones.
Perhaps the theory is that repairs to these organs, if made by stem cells, would introduce a new family line of cancer-free cells that would be descendants of the stem cells? But hmmmm, do the organ tissues even want to work that way? It would seem to me they have plenty of opportunity, with the endogenous stem cells being available....
Actually, cancer is somewhat independent. Cancer results from the undifferentiated growth of cells. This occurs through many steps and often is the result of DNA mutation. Skin cancers are quite common because of several reasons. First, they are bombarded from environmental factors (such as sunlight) which can mutate DNA. Secondly, they divide a lot, and DNA replication is not perfect. If a mutation occurs in a critical gene, then that can lead to cell transformation and might start the cell down the road to cancer. In some unfortunate individuals, the gene mutations are in the stem cells, and all cells created by the stem cells lead to cancer. This is most common with leukemias and lymphomas. Stem cells are also a catch-all term for undifferentiated cells. There are many types of stem cells. Some of the most common are hematopoietic stem cells, which form new blood cells. Mesenchymal stem cells are those that are intended for forming new organ tissues.
The reason that stem cells are not used for healing in the clinic is manifold. Primarily, scientists do not understand enough about stem cells to make practical use of them yet. The ability for a stem cell to make a particular cell type is dependent on many chemical and molecular factors, and most are not entirely clear. Even with what is understood, the rate of differentiation is poor and inefficient. Only a few percent of all the stem cells they get can be utilized into differentiation to a tissue type, if they know what it is, and therefore requires millions of stem cells. This may be inefficient
in vivo as well, which is why internal damage may take years to heal. Some clinical work has been done with stem cells though to repair wounds and to combat cancer. Nevertheless, it may be many years before they can be fully developed for a clinical setting.
