A group of researchers at Yale University lead by Valerie Horsley found that stem cells in the fatty layer of mice skin send signals that trigger hair growth. Hair loss researchers know that men who are going bald still have a full supply of hair follicle stem cells in an area of the follicle called the bulge. These stem cells remain dormant, however, in men who are losing their hair because they are not getting the proper signals to initiate hair growth. The Yale team thinks that they have found the source of molecular signals that will trigger the hair follicle stem cells into action, at least in mice.
Valerie Horsley said, “If we can get these fat cells in the skin to talk to the dormant stem cells at the base [the bulge area] of the hair follicles, we might be able to get hair to grow again.”
The skin of mammals, both mice and men, is a complex organ. There are many types of cells in the various layers of the skin - the epidermis, the dermis, and the subcutaneous layer. And there is a lot of dynamic activity among the cells in these layers because cells signal to each other with chemical messages.
An important signal is the one that generates the hair cycle - anagen, catagen, and telogen. In people with pattern baldness problems, the hair follicles have miniaturized and remain in the resting (telogen) stage of the hair cycle much too long. A previous theory - the bulge activation hypothesis - states that signals from the follicular papilla at the very base of the hair follicle trigger the activation of the stem cells in the bulge area.
The new theory based on the research at Yale University and appearing in the September 2, 2011, issue of the journal Cell, states that these molecular signals come from adipose precursor cells. These are cells that are essential in the creation of new fat cells in the skin. The Yale team noticed that when the hair follicles miniaturize, the layer of fat that accounts for the much of the thickness of the scalp skin gets thinner. If and when the hair follicles enter the growing (anagen) stage of the hair cycle, the layer of fat thickens, sometimes up to four times at thick. This creation of new fat cells is called adipogenesis.
The researchers studying mice in the laboratory discovered that these adipose precursor cells were essential in the growth of new hair. These cells, also called immature or lineage cells in the article, emit a molecule that is thought to be responsible for hair regeneration. This signaling molecule is called platelet derived growth factor or simply PDGF.
This statement is from the article in Cell: “Intradermal adipose lineage cells [and the expressed molecule PDGF] are necessary and sufficient to drive follicular stem cell activation.”
This research group from Yale is now trying to find other signals that these adipose precursor cells may be sending to stimulate hair growth. They also want to find out if these same signals apply to human hair follicles.
There is a major difference in the hair growth patterns of humans and mice though. The following two paragraphs are from Kevin McElwee’s website, Keratin.com. I think this is a clear explanation of what researchers might encounter when going from mice to men:
"The normal cycle of growth and rest in human hair follicles occurs in a mosaic pattern. That is, each individual hair follicle determines its own anagen growth time period and cycles without paying much attention to the growth activity of neighboring hair follicles. In other species a different pattern of hair growth may occur where the hair growth activity in one hair follicle affects the growth activity in neighboring follicles. For example, in rodents (except guinea pigs which have a mosaic hair growth pattern), hair growth over the skin occurs in a wave pattern. From birth onwards rodent hair follicles go through a "Mexican wave" of growth and rest. Starting at the head, the wave of growth moves backwards to the tail and from the belly to the back. This happens every few weeks. This is very different to what occurs in humans.
"Unlike humans rodent hair follicles communicate with each other. One hair follicle that is growing takes note of what its neighbors are doing and responds accordingly. After a period of growth the hair follicle returns to its resting state. Because of this difference in hair growth pattern, rodents' hair follicles are mostly in the telogen state. Only around 10% of rodent hair follicles are in anagen at any one time and the period of growth only lasts a week or two. As the growth wave passes, the hair follicles enter the resting telogen stage but unlike humans the hair does not fall out. It stays in place wedged in the hair follicle. The hair then gets pushed out as the next wave of growth runs by. This difference in growth pattern has important consequences for any laboratory tests on hair growth treatments that involve rodents - a fact many pharmaceutical companies seem to ignore."
The study done by Valerie Horsley and her team was funded by the National Institutes of Health and the Connecticut Stem Cell Research Program.