What factors stimulate new hair growth?

Brand new growth of hair does not spring from the soil like a weed; it requires encouragement. So, what specific conditions must be present at the site of each potential hair in order to activate its growth?

Two local factors, either together or individually, have the capacity for directly initiating or accelerating the growth of hair: one is INCREASED BLOOD SUPPLY; the other is HORMONE STIMULATION.

1) Increased Blood Supply:

If a vellus (or lanugo) hair is already growing from, say, the lobe of the sebaceous (oil) gland, an increase in the blood supply to the immediate area greatly enhances its growth. As if the hair were being “fertilized” by the influx of this unaccustomed nourishment, it tends to grow deeper and coarser, becoming an “accelerated” lanugo hair (i.e., one which is just beginning to acquire a bulb and take on pigmentation, it is not yet a “terminal hair”). A plentiful supply of blood is necessary for the development of these deeper kinds of hair. Nevertheless, increasing the blood supply in an area where there are no hairs at all will not create new hairs (example palms of your hands) Blood supply merely nourishes existing hairs.

2) Hormone Stimulation:

Certain hormones, on the other hand, seem to have the capacity for initiating the growth of new hairs where none existed previously. Of course, an excess of hormones does not cause hairs to leap out of the flesh wherever it flows. If that were the case we would have bristles sprouting from our tongues and our eyeballs. The fact is that hormones have no “hair-growing” power in themselves. What they do is stir to life certain cells near the sebaceous (oil) glands. These hair-germ cells (as some refer to them) need only be “turned on” by the chemical action of the hormone. They then begin to multiply and divide, creating a lanugo hair structure which may continue to accelerate in growth. Thus, the action of certain hormones is capable of eventually bringing about a course, pigmented hair where no hair at all had existed.
Information referenced from the Electrolysis Society of Alberta: http://www.onlineesa.com/ and Electrolysis Thermolysis and the Blend: The Principles and Practice of Permanent Hair Removal by Arthur Ralph Hinkel