240

16

Environment and Ecology

Fig. 16.1 Plot of the timet 90t90 taken to reach 90% of the maximum incoupling peak position shift

Delta alpha Subscript normal m normal a normal x BaselineΔαmax after cell deposition on a planar silica substrate v. Delta alpha Subscript normal m normal a normal x BaselineΔαmax, for the standard differentiation

protocol (open symbols) and a slightly modified protocol (filled symbols), which results in a novel

striated cell morphology. Circles, 7 days after starting differentiation with retinoic acid; squares, 14

days; circles, 21 days after starting treatment (from Aref et al. 2010; reproduced with permission)

upper A equals bold italic upper X upper CAAA = XCCC

(16.2)

whereupper AAAA is a vector of different attributes andbold italic upper XX is a tensor giving the susceptibilities

of the different attributes of the cell to the different chemicals. There are clearly

immense difficulties in progressing along this path. One difficulty is that attributes

can often not be expressed as a Boolean variable, let alone a single number. For

example, Aref et al. (2010) showed that neurons could be characterized by letting

them spread on a planar surface and measuring the kinetics and extent of spreading; ²

they could then show that slightly modifying the protocol of retinoic acid treatment

(prior to depositing the cells on the waveguide) changed their attributes, ³ as can

be strikingly seen in a kind of phase plot showing the transformation from stem

cell to neuron (Fig. 16.1). “Neuronness” presumably has many attributes such as

2 The planar surface is also an optical waveguide; interaction of the cell with the evanescent field

of guided light modes is quantified via the angle alphaα of incidence (of an external light ray onto an

ultrashallow or planar grating incorporated into the surface) required to couple the external ray

into the waveguide;alphaα is almost exactly linearly proportional to the “contact” area between cell and

substrate.

3 It is well known that treatment of stem cells with all-trans retinoic acid transforms them into

neurons.