Cell Signalling [be sure to review the figures in your text and 
those from lecture]

1.  The coordination of cellular activities is an essential element during 
development because it allows a  coordinated and regulated unfolding 
of the information contained in each cell's DNA.

2.  This coordination is achieved through a molecular system of signals 
and receptors which act as  information processors.

3.  Signalling molecules are either secreted or displayed by cells.  They 
convey information about the state of the cells that secrete them 
as well as instructions for the behaviour of other cells.

4.  Receptors have a basic structure that allows them to recognize signals 
and to relay the information contained in them to the cell. This is usually 
achieved through conformational changes, that may be 
associated with an enzymatic activity.	

5.  The interactions between signals and receptors can be modulated by 
other molecules that interfere in a  positive or negative way with 
the informational exchange.

6.  The programs of gene expression determine dynamic distributions of 
signals and receptors during development that contribute to the generation
of pattern.

7.  Signalling molecules, receptors, and transcription factors are key 
players in the information processing that underlies the unfolding of the 
developmental program, and in the coordination of the program in  
different cells.

8. These elements function as components of signal transduction pathways 
that are linked and integrated to form information-processing networks.

9. The components of signal transduction networks use molecular currencies 
(such as second messengers and phosphorylation) to exchange information, 
and their activities are linked through conformational changes and 
interactions involving specinc molecular interfaces.  Some of these 
molecular interfaces are used to group sets of signal transduction 
elements into functional complexes, or to localize them to specific 
cellular compartments.

10. Some signalling routes from the cell surface to the nucleus employ 
only a few signal transducers, while others involve multiple 
intermediaries or use second messengers, offering many opportunities for 
interactions with other pathways.

11. Signal transduction networks are subject to complex regulatory 
mechanisms that can be effected, for  example, by changes in the concentration,
activity or stability of signalling components,or in their compartmentalization 
within the cell.  Feedback control systems are particularly important.

12.  Integration of signalling pathways can occur in the nucleus but also 
at the level of the signal transduction networks and even at the cell 
surfacce. 
[Signalling, by a number of growth factors has been shown to be 
regulated by specific proteoglycans, and it may be that most if not all 
growth factors may be sequestered at the cell surface by these ECM molecules. 
 For example, binding of FGF to a heparan sulphated proteoglycan, called 
syndecan, is  required for signalling by FGF, and the proteoglycan glycan can 
bind TGF and present it in high  concentration to low-affinity receptors 
or increase its local concentration in regions where there is little of it.] 

13.  Cells can respond to different concentrations of the same signalling 
molecule by activating different  genes. If the signalling molecule is secreted
from a localized source in a field of cells and can diffuse, it will form a 
concentration gradient that can be used to pattern the field.  Cells will be 
exposed to  different concentrations depending on their distance from the 
source and will respond by activating different genes.

14.  The term 'positional information' refers to the ability of cells to 
acquire fates depending on their  position relative to the source of a 
diffusible signalling molecule.

15.  Morphogens are secreted signalling molecules that can elicit at least 3
different cellular responses in a direct and concentration-dependent manner.

16.  The ability of a cell to respond to a signal is called its competence.  
This can mean the presence of  receptors for the signal, the availability of
a signal transduction pathway or an open transcriptional  state for that signal.

17.  A signalling molecule can alter the pattern of response of the target 
cells by altering their competence (i.e. changing the expression of receptors, 
state of nuclear activity or modulating the interactions 
between the signal and the receptor).

18.  Gradients of signalling molecules are often used to establish 
coarse-grained patterns of responses that are refined through interactions 
between the original signalling molecule and its targets.

19.  Short-range signals also play a role in pattern formation and, in 
vivo, patterns must result from an interplay between long- and 
short-range signals. 
(excerpted from MBDB)