Cell nuclei have lower refractive index and mass density than cytoplasm

Abstract

Common perception regards the nucleus as a densely packed object with higher refractive index (RI) and mass density than the surrounding cytoplasm. Here, the volume of isolated nuclei is systematically varied by electrostatic and osmotic conditions as well as drug treatments that modify chromatin conformation. The refractive index and dry mass of isolated nuclei is derived from quantitative phase measurements using digital holographic microscopy (DHM). Surprisingly, the cell nucleus is found to have a lower RI and mass density than the cytoplasm in four different cell lines and throughout the cell cycle. This result has important implications for conceptualizing light tissue interactions as well as biological processes in cells.

Three‐dimensional correlative single‐cell imaging utilizing fluorescence and refractive index tomography

“…In a first example, we measured the nuclear RI of an HL60 cell. The nuclear RI was reported to be lower than the complete cell in recent studies on isolated nuclei , which our results shown in Figure confirm. To further verify that our tomographic technique yields correct quantitative RI values for cells, we validated it with the 2D phase analysis algorithm used in (see Figure S10).…”

“…The nuclear RI was reported to be lower than the complete cell in recent studies on isolated nuclei , which our results shown in Figure confirm. To further verify that our tomographic technique yields correct quantitative RI values for cells, we validated it with the 2D phase analysis algorithm used in (see Figure S10). Note that the Hoechst stain used could only affect either the imaginary part of the RI (absorption of light) or increase the real part of the nuclear RI shown (due to an increase of dry mass).…”

“…By colocalizing the 3D RI with the 3D fluorescence data, we are able to resolve and characterize intracellular structures which we showcase by segmenting and quantifying the RI of the nucleus. For an exemplary HL60 cell, we measure a nuclear RI that is lower than the RI of the cytoplasm, which confirms previous results . In addition, a 3D RI analysis of an adult mouse photoreceptor cell shows an increased RI at the center of the cell as a result of terminal differentiation associated with an inversion of the chromatin structure .…”

“…A recent debate in the biophotonics community centers on the question, whether the nucleus of a cell has a lower RI than the cytoplasm . Adding to this debate, we were able to show that the RI of isolated nuclei is lower compared to the cell average RI for several cell lines , which was confirmed by an independent study . However, these studies measured the RI of isolated nuclei, raising the question whether the nuclear RI in situ is also lower.…”

Topography and refractometry of sperm cells using SLIM

“…4a). Interestingly, the nucleus of the cell appears to have a lower refractive index than the rest of the cell head, which seems to confirm the recent findings on eukaryotic cells 24,25 . In order to perform averages across different cells, without artifacts due to the curvature, the tail of each cell was numerically "straighten" (Fig.4) using an ImageJ plugin developed by Meijering et al 26 .…”