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Description
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TCS is a CompuCell3D steppable that dynamically changes cell types during a simulation. Cell types change as a result of Delta-Notch signaling. Delta and Notch are membrane bound proteins that can bind when cells touch. When this happens, the Notch intracellular domain (NICD) is released and that inhibits tip cell fate. 

The TCS steppable first computes the amount of NICD in each cell:
	NICD(cell) = Notch(cell)/Area(cell)*(Sum(Delta(neighbor) for all neighbors of cell))
The Notch and Delta level of a cell are associated with its cell type. When all NICD levels are computed, the cell types are updated according to:
	type = stalk if NICD > NICD_threshold; tip if NICD < NICD_threshold

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Usage
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To use the TCS steppable it should be specified in the CC3DML. For this the following syntax is used:

<Steppable Type="TCS">
	<!-- definition of a stalk cell: 
		dll = the Delta level of a stalk cell; 
		notch = the Notch level of a stalk cell;
		type = the name (defined in CellType plugin) of the stalk cell type
	-->
    <stalk dll="1" notch="0.5" type="stalk"/>
	<!-- definition of a tip cell: 
		dll = the Delta level of a tip cell; 
		notch = the Notch level of a tip cell;
		type = the name (defined in CellType plugin) of the tip cell type
	-->	
    <tip dll="4" notch="0.3" type="tip"/>
	<!-- NICD_thresold -->
    <threshold>0.125</threshold>
</Steppable>