These diagrams show the nine stages of hair follicle development as defined by embryologists. The diagram shows the accumulation of dermal papilla cells below undifferentiated epidermis which is then stimulated to grow down into the dermis as a hair "peg". Interaction between the hair peg and the dermal papilla cells promotes differentiation into a mature hair follicle: 1 - epidermis; 2 - dermis; 3 - hipodermis; 4 - subcutaneous fat layer, 5 - pregerm, 6 - bazal keratin cells (keratinocytes); 7 - hair germ; 8 - dermal fibroblasts; 9 - hair peg; 10 -- dermal papilla; 11 - inner root sheath cone; 12 - hair bulb; 13 - sebacous cells (sebocytes); 14 - bulge, 15 - melanin; 16 - outer root sheath; 17 - hair canal; 18 - hair shaft; 19 - sebaceous glands.

 

The establishment of a dermal papilla is vital to the development of all hair follicles and associated modified structures like sebaceous glands. The dermal papilla is a group of specialized dermal fibroblast cells, derived from the embryonic mesoderm. These dermal papilla cells begin to aggregate in the dermis just below the epidermis. For humans, this initial aggregation begins when the embryo is approximately 60 days old. The dermal papilla cells at this stage are only loosely collected and are long, spindle shaped cells. The development of a dermal papilla marks the site for future development of a hair follicle. Above the dermal papilla an epidermal plug, or peg, of cells develops and proliferates growing into the dermis towards the dermal papilla. The epidermal plug of cells does not grow straight down into the dermis but at a slight angle. The mesoderm derived dermal papilla and the ectoderm derived epidermal plug "communicate" with the result of further proliferation of epidermal matrix cells and eventual differentiation into the various sheath and hair fiber structures. This gradual differentiation of the hair plug first begins with the development of three distinct buds of cells on the same side of the down growth one above the other. The cell bud closest to the epidermis may develop into an apocrine (sweat type) gland. However, this only happens in a minority of hair follicles, primarily those on the scalp, genitals and anus. Isolated apocrine glands may develop in connection to hair follicles elsewhere but they are usually few and far between. Most of these upper cell buds regress as the hair follicle matures. The cell bud in the middle will gradually develop into the oil producing sebaceous gland while the cell bud below it forms what is called the hair follicle "bulge". The bulge is the area of the hair follicle to which a small muscle will attach. The "arrector pili" muscle develops separately from the hair follicle within the mesoderm/dermis. It grows up towards the epidermis and down towards the hair follicle bulge region. Eventually it will attach to both the epidermis at one end and the hair follicle at the other. The bulge is also important as this is the location where the stem cells reside in adult hair follicles. As the epidermal plug of cells penetrates down into the dermis, mesodermal cells congregate around it and develop into a fibrous follicular sheath and/or collagen capsule to encase the epidermal cells. As the epidermal plug comes into contact with the dermal papilla the growth is still downward. The epidermal plug seems to push the dermal papilla down into the dermis as it grows to its full size. It may penetrate up to 5mm below the skin surface (more in adults). The dermal papilla develops into a more identifiable structure of rounded cells containing organelles vital for product synthesis, although the cells themselves at this later stage of development are non-proliferative. The dermal papilla cells communicate with the plug cells and these epidermal cells begin to arrange themselves into concentric layers above the dermal papilla. The layers eventually differentiate into the hair fiber and the inner and outer root sheaths encasing the fiber. These layers begin to keratinize higher up the hair follicle while the cells close to the dermal papilla remain undifferentiated and continue to multiply. The multiplying cells can’t push out into the dermis because of the surrounding fibrous capsule. The only way out is up. The cells push up and away from the dermal papilla pushing other cells in front of them. As the cells mature they are incorporated into the layers of the hair follicle, become keratinized, die and eventually are extruded from the surface of the skin. Hence, development of a hair follicle requires a continuum through induction, initiation, elongation and differentiation. The basic hair follicle structures are complete throughout the skin of an embryo by 160 days.