Spatial organization of fibroblast nuclear chromocenters: component tree analysis

The nuclei of mouse connective tissue fibroblasts contain chromocenters which are well-defined zones of heterochromatin that can be used as positional landmarks to examine nuclear remodeling in response to a mechanical perturbation. This study used component tree analysis, an image segmentation algorithm that detects high intensity voxels that are topologically connected, to quantify the spatial organization of chromocenters in fibroblasts within whole mouse connective tissue fixed and stained with 4',6-diamidino-2-phenylindole (DAPI). The component tree analysis method was applied to confocal microscopy images of whole mouse areolar connective tissue incubated for 30 min ex vivo with or without static stretch. In stretched tissue, the mean distance between chromocenters within fibroblast nuclei was significantly greater (vs. non-stretched, P < 0.001), corresponding to an average of a 500-nm increase in chromocenter separation (~10% strain). There was no significant difference in chromocenter number or average size between stretch and no stretch. Average chromocenter distance was positively correlated with nuclear cross-sectional area (r = 0.78, P < 0.0001), and nuclear volume (r = 0.42, P < 0.0001), and negatively correlated with nuclear aspect ratio (r = -0.65, P < 0.0001) and nuclear concavity index (r = -0.44, P < 0.0001). These results demonstrate that component trees can be successfully applied to the morphometric analysis of nuclear chromocenters in fibroblasts within whole connective tissue. Static stretching of mouse areolar connective tissue for 30 min resulted in substantially increased separation of nuclear chromocenters in connective tissue fibroblasts. This interior remodeling of the nucleus induced by tissue stretch may impact transcriptionally active euchromatin within the inter-chromocenter space.