Cellular interactions in tissue and organ development.

Developmental patterns result from combinations of interactions and intracellular programmes. The purpose here is to define the roles of interactions wherever possible and to consider their major parameters, such as the timing of their action, their specificity and the distances over which they occur. The approach is one of a board survey, attempting to outline major interaction systems on the basis of information from different sources. The evidence concerning interactions that control development must come primarily from development itself. Both disturbed, as during regeneration and grafts, and normal development are relevant. Growing apices interact over relatively long distances. They reduce the development of similar apices and induce the development of axial tissues that connect them to the plant. Young shoot tissues also induce the development of root apices and vice versa. These various effects can be understood on the basis of a hormonal feedback involving auxins and cytokinins. Vascular differentiation, furthermore, is a cellular expression of these interactions. It occurs along the flow of auxin from the young tissues of the shoot towards the root apices. This flow is canalized by a positive feedback between cell polarization and the polar flow that both controls and results from this polarization. Structural relations between pholem and xylem, limitations of regeneration and the formation of rays all indicate the existence of additional, radial interactions. The fate of individual cells in development is varied and often follows no recognizable rules. This indicates interactions that operate on the size of a tissue or organ rather than on its precise cellular development. On the other hand, the continuity of plasmodesmata, wall thickenings and cytoplasmic strands demonstrate local interactions between neighbouring cells. It is concluded that, though the possibilities of developmental interactions may be bewildering, the list, when known, may not be all that long. The interactions specify orientation and quantity rather than the precise fate of cells. Their effects are gradual and they involve feedback loops. Growth factors, even known growth factors, play surprisingly large roles in cellular interactions. Such controls whose effect is gradual and general could provide a basis for developmental plasticity.