7 - A dynamic framework for morphogenesis

Summary

The last four chapters have contained a great deal of information about the molecular, the cellular and the structural events that take place during morphogenesis. Here, I want to assert that we cannot hope to understand the significance of all these facts if we lack a framework within which they can be examined. The purpose of this brief interlude is to provide such a framework and it comprises a series of statements about the processes underpinning morphogenesis. Their order reflects the sequence of events that takes place as a new tissue forms. Their choice derives from the view that the essence of development is change and, hence, that we must look to dynamics for insights into how new structures arise. The statements are, to a great extent, self-evident, but will be discussed and justified in the next chapter. They are intended not only to impose some order on the data, but also to focus attention on the processes that we have to explain if we are to understand any example of morphogenesis.

1. The starting signal for morphogenesis sets in train events that cause the existing cellular organisation to become unstable. This signal is less important than the processes which it initiates.

2. The response to the initiating signal usually changes the state of cell differentiation or activates molecular activity in a fairly simple way. Cells may make new or break old adhesions (CAM expression changes), a new environment may become available for colonisation (ECM swells locally or fibronectin is laid down), intracellular activity may commence (microfilaments start to contract), or global events may be set in train (pressure builds up).

3. […]