Hair color

Look at this image, specifically the papilla. Cells of the papilla multiple to form hair. The papilla cells contain pigment (color molecules). This pigment mixes in with the keratin (protein), which comprises the hair shaft.



At the bottom of the follicle the epithelium swells to form a hair bulb. This is the dilated end from which the growth of the hair and the inner root sheath occur. The hair bulb in an active hair follicle is usually large enough to enclose a bit of the dermis. This is the dermal papilla of the follicle, which plays an important role in the hair growth cycle.

The dermal papilla itself isn't the source of the hair, the active epithelium lying over it is. But the papilla is necessary to hair growth, in that its presence and close proximity to this germinal matrix induces it to proliferate by sending chemical signals. When the signal is received, the germinal matrix proliferates and produces the hair and the inner root sheath (see below). You can think of the germinal matrix as the homolog of the stratum basale of the epidermis, because it's here that everything is produced. Another similarity is that it contains melanocytes; just as in the epidermis, the melanocytes of the germinal matrix transfer melanin to hair to give it pigmentation.

The triggering of hair growth by proximity of the dermal papilla is a classic example of the phenomenon of induction, the response of one cell population to signals from a second population. You may notice that the bulbs and papillae in a given section of skin are not all the same size. You can expect to see smaller papillae in hairs which are being shed. The catagen phase is characterized by a shrinkage and withdrawal of the papilla away from the bulb. This removes the necessary induction and the hair stops growing.