FROM STRIPES TO SPOTS - PREPATTERNS WHICH CAN BE PRODUCED IN THE SKINBY A REACTION-DIFFUSION SYSTEM

A key question in the area of spatial pattern formation in development al biology is: how do groups of cells in a homogeneous tissue suddenly differentiate along entirely different developmental paths compared t o neighbouring cells? Although experiments are now beginning to provid e answers to this question, the mechanisms responsible for the develop ment of repeated or periodic structures and spatial patterns, e.g. hai r follicles and pigmentation patterns, are still unknown. Theoretical biologists and applied mathematicians have suggested various prepatter n mechanisms as the primary cause of repeated or periodic spatial patt erns. A class of biochemical reactions referred to here as reaction-di ffusion (RD) systems, having the capacity to spontaneously generate st able stationary wavelike spatial patterns (Turing, 1952), has been sug gested as a possible prepattern mechanism, e.g. during hair follicle i nitiation and development (Nagorcka, 1989), and pigmentation patterns (Murray, 1989). Spatial patterns arising during development of the ver tebrate skin are frequently complex. Spatial patterns in the skin can be seen to vary within an individual from one region of the skin to an other. One pattern change commonly observed across the skin is from st ripes to spots. An RD system is defined which is able to generate diff erent spatial patterns depending on the value of a single parameter. T he parameter varied controls the transport of the chemical components of the RD system across the basement membrane separating the epidermis and dermis. The patterns produced range from stripes to an irregular array of spots. Not only are different patterns produced, but a differ ent time sequence of prepatterns is expected to arise in the different skin regions depending on whether the first prepattern is an array of spots or stripes. As a consequence it is possible to account for hair follicle initiation in the hair-bearing regions of the mammalian skin as well as the sequence of events required for the formation of derma toglyphics in the volar regions.