Are cell affinity relationships for insect epidermal tissues evidence for combinatorial control?

THE segregation of determined, yet undifferentiated, cells in aggregates consisting of cells from various vertebrate embryonic structures represents a high degree of specificity in cellular recognition^1-3. Steinberg's differential adhesiveness model² has been particularly successful in accounting for this sorting out of cells in terms of differences in their adhesiveness. Although arranged in monolayers, insect epidermal cells seem to behave similarly when their normal spatial organisation is disrupted. The regrouping of disaggregated Drosophila imaginal disk cells according to epidermal type and the failure of grafts from one epidermis to form stable contacts with cells of another epidermal tissue have both been attributed to sorting out of unlike cells^4,5. Yet some insect epidermal cell types fail to segregate completely from one another; even when disaggregated and mixed, cells from genetically marked leg and antenna disks are known to form some integrated patterns⁶. It is possible that different degrees of adhesiveness exist for various combinations of two different epidermal types, the adhesive properties not being uniquely specified for each tissue. With this possibility in mind, exchanges between grafts and hosts of various pupal epithelia of the moth, Manduca sexta, were performed. The affinity relationships observed reflected the existence of covert cell surface differences in these epidermal populations whose component cells at the time of grafting are determined but undifferentiated. The findings suggest that the cell surface properties are not tissue type specific but are shared to various degrees by the different epidermal tissues.