PATTERN AND MORPHOGENESIS IN SKIN

Models dealing with the development of hair and feather follicles comm only predict that the factors initiating morphogenesis also specify pa tterns of follicle distribution. The factors have been postulated as c hemical or mechanical instabilities which, at certain threshold concen trations, determine both the location of follicles and their form. The models tend to focus on the earliest waves of induction, where follic les develop at separate, non-randomly spaced initiation sites in skin. However, in many animals, there are later waves of initiation, some o f which give rise to compound follicles. These are bundles of follicle s that arise by branching from the necks of those formed earlier and s hare a common pilary canal when mature. In some species, compound foll icles make the greatest numerical contribution to the follicle populat ion. Measurements of the frequencies of single and branched follicles in sheep selection lines with different follicle densities (from previ ous studies by Moore et al.) revealed that the follicles which formed first during foetal life (primary and original secondary populations) were established at separately identifiable sites in the skin, called here ''initiation sites''. However, there were also later waves of dev elopment, contributing follicles exclusively by the process of branchi ng (the derived secondary population). Final follicle densities were n ot correlated with the densities of initiation sites. The observations suggested that mechanisms specifying the positional values of initiat ion sites' differed from those determining follicle number. The final densities of the follicle populations in the sheep lines were also hig hly negatively correlated with the diameters of the wool fibres grown. The close statistical relationship suggested that the two parameters were developmentally linked. However, whereas fibre characteristics ar e realised when the follicle is mature, density is established earlier , during foetal life. We have reconciled these observations with the f ollowing hypothesis: a population of cells, committed to a follicular pathway of development, differentiates in the skin at or before the fi rst wave of initiation. Subpopulations of the committed cells subseque ntly participate in each follicle initiation event, the number in each subpopulation ultimately determining fibre dimensions. Follicle initi ation continues until most or all of the original population have been utilised. Transplantation and skin recombinant studies have demonstra ted that the cells forming the dermal papilla of the follicle particip ate in follicle initiation and have inductive effects on epidermal tis sue. Papilla size is also correlated with fibre diameter in the mature follicle. These attributes are consistent with those described for th e committed cell population. (C) 1998 Academic Press Limited.