Introduction

The purpose of this chapter and the next is to lay out the morphogenetic properties of the two main types of cells that participate in organogenesis: those of the mesenchyme and those that comprise epithelial sheets. We shall thus examine in some detail the individual and social behaviour of fibroblasts, the most accessible type of mesenchymal cell, the general properties of epithelial sheets, and, of course, how the two populations cooperate together. The information on how these properties are used will be taken from studies made both in vivo and in vitro and mainly from phenomena where our knowledge is not as substantial as it is for the case studies that were detailed in Chapter 3. These examples point by implication to areas where there is more work to be done.

There will, unfortunately, be some overlap between these and the preceding chapter, but this will be kept to a minimum by emphasising phenomenological properties here and referring the reader back for information on molecular mechanisms. By the end of these chapters, the reader should have a strong sense of the morphogenetic events that take place as tissue organisation emerges and will thus be prepared for the final section which attempts to place in their dynamical context the many events that are necessary for the formation of any tissue.

The term mesenchyme has been loosely used for a long time to describe embryonic cells that are found as individuals or in groups rather than in sheets, and that have not differentiated into a specialised cell type (e.g. Davenport, 1895; see Fig. 6.14).