The reaction-diffusion (RD) theory of wool (hair) follicle initiation and development. I. Primary follicles

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A spatial prepattern mechanism based on a biochemical reaction referred to as a reaction-diffusion (RD) system is able to account for many aspects of the initiation and development of primary wool follicles. The RD system can spontaneously produce spatial patterns in the distribution of its chemical components within the epidermis and epithelium. The first pattern produced in the epidermis by the chosen RD system is a spotted pattern which gives rise to the first generation of primary central follicles. Follicles initiate at the location of the spots, i.e. the localized regions where maxima exist in the concentration of the chemical components of the RD system. As the primordial follicles begin to grow, changing their shape and size, the RD system is able to produce a time sequence of spatial prepatterns within the follicles. These spatial prepatterns have the capacity to regulate the development of the follicles, causing them to grow at an angle to the skin surface and providing a temporo-spatial cue for the initiation of a sweat gland. The same prepatterns which regulate the development of the primordial follicles also cause the early stage follicles to function as organizers. They do this by causing the spatial prepattern in the epidermis to change, resulting in the initiation of new generations of follicles. The prepatterns are able to cause the formation of trio groups of primary follicles, the alignment of these groups, and the rapidly increasing density of primary follicles observed during the early stages of wool follicle initiation. The prepattern mechanism also predicts the existence of an inverse genetic correlation between the density of spots and the size (diameter) of the spots produced by the RD system in the epidermis. As a result we expect a strong genetic correlation to exist between the density of primary follicles and the diameter of the fibres which those follicles produce. The same prepattern mechanism is able to account for many aspects of secondary follicle initiation and development, as described in an accompanying paper.