The scale of pattern formation.

There are many kinds of patterns in biology and a number of quite different mechanisms for generating them. For example, patterns due to pigment in some cases depend largely on variations in the movement of melanocytes, as in the case of chicken feathers. Other patterns are likely to be produced, at least in part, by lineage mechanisms. This is probably true in the formation of bristles in insects. In Rhodnius, for example, the single bristle mother-cell divides twice, thus producing a small group of four cells, which become the trichogen, the tormogen, the nerve cell and the neurilemma cell (Wigglesworth, 1953). In this case, the relative movement of the cells also appears to play some part in forming the spatial pattern of the group. A pattern may exist even when it is not immediately obvious by visual inspection. Such a case is the retina of the amphibian eye, studied by Gaze and Jacobson (see the review by Gaze, 1967), in which some mechanism fairly early in the development of the retina instructs the cells where they lie in the tissue, so that they know to which part of the optic tectum to send their axons. It is this latter type of pattern which I shall be particularly concerned with here. Wolpert (1969) has suggested the use of the term ‘positional information’ to describe such cases. It is, of course, not limited to cases where the pattern is invisible. Most of the examples of such systems which have been studied show regulation. Moreover, as far as can be determined, the relative movement of cells does not seem to occur to any appreciable extent. Thus the pattern is unlikely to be due entirely to lineage. There must be a mechanism, involving some sort of communication between cells, which enables each cell to discover its location in the tissue. This paper is concerned with the general nature of such communication. The very important topic of the junctions between cells in a tissue is not dealt with here, except by implication.