Cell Density and Cell Division in the Early Morphogenesis of the Chick Wing

Abstract

THE early development of the chick wing involves cell differentiation, pattern formation and growth¹. In general terms, its morphogenesis can be seen in terms of how the growth of the mesenchyme and ectoderm is controlled, so that the very simple initial protrusion is transformed into an elongated paddle-like structure. At the same time a spatial pattern of differentiation must be specified within the mesenchyme, the cells forming cartilage and muscle so that the major skeletal and muscular features are laid down. In a previous paper² we described the changing pattern of the distribution of mitoses in the mesenchyme. We found that there was an overall fall in mitotic index from early stages (18–19 Hamburger-Hamilton) but that this occurred more rapidly in the proximal regions after about stage 23 so that a graded proximo-distal increase in mitotic index became established. We also suggested that the overall form was determined by the ectoderm and not the mesenchyme. This raised specific problems about the control of growth of the mesenchyme: we could not account for the observed distribution of mitoses but wondered whether this involved a temporal programme or was related to positional information. Our investigations of the so-called mesenchymal condensations which are supposed to be the prelude to cartilage formation³ have led to detailed analysis of cell density during early morphogenesis. We show here that the cell density varies in a very regular manner, and is closely correlated with mitotic index: mitotic index is inversely proportional to cell density. This finding is not only important in its own right because it may be the first demonstration of density dependent growth control⁴ in vivo, but because it provides a new mechanism for the control of growth and pattern formation in limb morphogenesis.