# Polarimetric Detection of Chemotherapy-Induced Cancer Cell Death
## 4 Results and Discussion
### Cell State Assessment: Quantifying Cell Death
The trypan blue test showed a high dispersion in these results, and the diattenuation increase was accordingly noisy. Overall, the evolution was similar to that observed in Figure 7.  
Paying attention to the spatial distribution of \(D_{T}\) in the image (see Figure 12), it seems that the diattenuation enhancement associated with cell death and observed in both U937 and THP1 cells was most likely the consequence of changes in the plasma membrane. As can be seen in Figure 12, where four images of the total diattenuation parameter \(D_{T}\) are shown for different treatment times, the increase of the signal was observed mainly at the cell boundary and clearly outlined its contour. This is in agreement with the fact that plasma membranes suffer major changes during the apoptosis process, including loss of phospholipid asymmetry. This phospholipid redistribution between inner and outer leaflets of the membrane generates interfacial forces able to modify the structure of transmembrane proteins [42], which could be detected by using circular dichroism, a well-known technique that has been employed for studying the structure, dynamics, and interactions of proteins [43] in the UV domain. Another effect that modifies the value of the diattenuation during cell death results from alterations in membrane tension due to modification of membrane-associated molecules and the intracellular and extracellular mechanical stimuli that change the membrane curvature [44]. The morphological changes associated with this effect are in agreement with the appearance offered by U937 cells in the different treatment time points, as shown in Figure 13. Thus, we suggest that the increase of the diattenuation signal could be related to changes in transmembrane protein structure and membrane geometry as cells undergo apoptosis. Figure 9: Mean values of the diattenuation (\(D_{T}\)) obtained from the triplicate experiment with U937 cells as a function of cell death. Values of the diattenuation directly obtained from the image ((\(D_{T}\))\({}_{i}\)) and diattenuation from cells corrected by Equation (12) ((\(D_{T}\))\({}_{c}\)) are represented with \(\Box\) and \(\bullet\), respectively. Error bars represent the mean standard deviation of the three experiments. Figure 11: (**a**) Values of the diattenuation parameter for control and 24-h treated THP1 cells in one of the experiments. The error bar is the mean standard deviation of the five diattenuation images. (**b**) Mean values of the diattenuation (\(D_{T}\)) obtained from the triplicate experiment with THP1 cells as a function of the cell death. The error bar is the mean standard deviation of the diattenuation images over the three experiments. Figure 12: Total diattenuation images (\(D_{T}\)) of U937 cells at different times of the treatment. Figure 10: (**a**) Death of THP1 cells over time treated with 20 \(\upmu\)g/mL of cisplatin evaluated with the trypan blue exclusion test. (**b**) Metabolic activity of THP1 cells under the same treatment using the Alamar Blue assay. a.u., arbitrary units. The error bar is the standard deviation of three measurements.