Material to be transferred between the cells

stimuli can affect both the internal state and the pattern of gene expression (and the cell's responsiveness to other signals). The converse is also true. Both pattern of gene expression and internal state affect the cell's responsiveness to stimuli.

This is because a cell's capacity to respond to a stimulus depends on the presence and the condition of the relevant signalling pathway components. If one or more pathway components have not been made, there will be no response to the stimulus. Each component comprises one or more gene products (proteins). So the cell's capacity to respond to a stimulus depends on the pattern of gene expression. If the pattern of gene expression changes, the cell might become responsive to new stimuli or lose its ability to respond to old ones; or it might respond in a different way.

However, each component must not only be present; it must be in the right place and in the right condition. If, because of a particular internal state, a signalling pathway component is chemically modified or locked up in a store and unable to participate in the pathway, the stimulus will elicit an attenuated response or no response at all. So the capacity of the cell to respond to a stimulus depends on the internal state. Once again we find reciprocal dependences:- A cell's response to a stimulus depends on the pattern of gene expression; and the stimulus might alter the pattern of gene expression.

- A cell's response to a stimulus depends on the internal state; and the stimulus might alter the internal state.

Internal state affects both responses to stimuli and the pattern of gene expression more or less instantaneously (Fig. 9-4). However, the effects of stimuli are slightly delayed. The internal state at time ti affects the responses to stimuli (R1) and gene expression pattern (G1) at that time. But the effects of R1 on internal state (and on pattern of gene expression - not shown in Fig. 9-4) are not seen until later, t2. The effects of G1 on internal state (and on the pattern of responses to stimuli - not shown in Fig. 9-4) are not seen until even later, time t3. It is as though the S directs R and G by telephone, R directs S and G by fax and G directs R and S by snail-mail.

In chapter 10 we shall discuss the relevance of this to our definition or characterisation of "livingness". To finish the present chapter, let us consider the implications of the three-way dialogue for cell differentiation.

Suppose that at time t0, two cells send signals to each other. By time t1, these signals have induced alterations in the internal states and patterns of gene expression in both cells. Because of these alterations, both cells might change the signals they send, or they might respond to such signals in different ways. In principle, this is how two cells can direct each other's

Fig. 9-4: time delays between gene expression, responses to signals and internal state.

differentiation. Cell A might instruct cell B to keep dividing, or make it move to another location; simultaneously, cell A might make cell B anchor itself and over-express certain genes.

In a multicellular organism there are not just two cells, or two kinds of cells, but many. They talk to one another and direct each other's activities. Details of these cell-cell interactions are being uncovered in many areas of research, including embryo development and immunology. Embryo development and the immune system provide dramatic illustrations of the complexity of cell-cell interactions. The three-way dialogue we have outlined in this chapter is manifested less dramatically elsewhere, but it is fundamental to multicellular life. At the whole-body level, homeostasis (chapter 6) depends on cell-cell communications of the sort we have touched on here. Cells respond to signals from one another. Thus, homeostasis is extended from the single cell, as described in chapters 6 and 8, to the vast multicellular assemblies that constitute organisms such as ourselves.

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