| Popis: |
The nucleus, although it has been studied for many decades, remains the body of many mysteries. In eukaryotic cells, DNA is found in two coexistent readable forms, an open, gene-rich region (euchromatin) that allows transcription of DNA into RNA, and a compacted region (heterochromatin) that contains silenced genes. In response to numerous extracellular stimuli, the cell must express specific proteins, which means that the DNA coding for these proteins must be in a transcriptionally active form. Consequently the balance between euchromatin and heterochromatin must be regulated during cell differentiation. Understanding this regulation requires fine-tuned biochemical and biophysical analyses and cutting-edge imaging techniques. Thanks to the soft X-ray microscope of the National Center for X-ray Tomography (Uchida et al., 2010; Larabell et al. 2010), high-resolution images of the 3D organization of the nucleus in the native state are now achievable. Cryo-immobilization of the cells (fast freezing) for x-ray imaging avoids chemical fixation artifacts associated with TEM. Imaging with X-rays in the water window energy range allows a natural contrast between water and biomolecules. X-ray imaging avoids perturbations caused by chemicals and osmotic changes associated with dehydration required for TEM, which cause deformation of nuclear structures (Finan and Guilack, 2010). As a result, x-ray tomography yields the first 3D views of nuclear organization in intact cells with a resolution of 50 nm. Our study indicates that heterochromatin forms a continuous network in 3D space, with no evidence of the chromatin patches described in TEM. Likewise, euchromatin regions are continuous. In addition distinct 3D chromatin patterns are linked to the differentiation/maturation state of the cell. Studying the 3D pattern of the nucleus using soft x-ray tomography is shedding new light on our understanding of cell differentiation. |
