THE REACTION-DIFFUSION (RD) THEORY OF WOOL (HAIR) FOLLICLE INITIATIONAND DEVELOPMENT .1. PRIMARY FOLLICLES

A spatial prepattern mechanism based on a biochemical reaction referre d to as a reaction-diffusion (RD) system is able to account for many a spects of the initiation and development of primary wool follicles. Th e RD system can spontaneously produce spatial patterns in the distribu tion of its chemical components within the epidermis and epithelium. T he 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 th e chemical components of the RD system. As the primordial follicles be gin to grow, changing their shape and size, the RD system is able to p roduce a time sequence of spatial prepatterns within the follicles. Th ese 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 gla nd. The same prepatterns which regulate the development of the primord ial follicles also cause the early stage follicles to function as orga nizers. They do this by causing the spatial prepattern in the epidermi s to change, resulting in the initiation of new generations of follicl es. The prepatterns are able to cause the formation of trio groups of primary follicles, the alignment of these groups, and the rapidly incr easing density of primary follicles observed during the early stages o f wool follicle initiation. The prepattern mechanism also predicts the existence of an inverse genetic correlation between the density of sp ots 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 e xist 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.