The genome is packed into the cell nucleus in the form of chromatin. Biochemical approaches have revealed that chromatin is packed within domains, which group into larger domains, and so forth. Such domain-within-domain packing, also called hierarchical packing, is equally visible in super-resolution microscopy images of large-scale chromatin organization. While previous work has suggested that chromatin is partitioned into distinct domains via microphase separation, it is unclear how these domains organize into a hierarchical packing. A particular challenge is to find an image analysis approach that fully incorporates such hierarchical packing, so that hypothetical governing mechanisms of euchromatin packing can be compared against the results of such an analysis. Here, we obtain 3D STED super-resolution images from pluripotent zebrafish embryos labeled with improved DNA fluorescence stains, and demonstrate how the hierarchical packing of euchromatin in these images can be described as multiplicative cascades. Multiplicative cascades are an established theoretical concept to describe the placement of ever-smaller structures within bigger structures. Importantly, these cascades can generate artificial image data by applying a single rule again and again, and can be fully specified using only four parameters. Here, we show how the typical patterns of euchromatin organization are reflected in the values of these four parameters. In particular, we can pinpoint the values required to mimic a microphase-separated configuration of euchromatin. We suspect that the concept of multiplicative cascades can also be applied to images of other types of chromatin. In particular, cascade parameters could serve as test quantities to assess whether microphase separation or other theoretical models accurately reproduce the hierarchical packing of chromatin.

中文翻译：

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常染色质的域内域包装可以描述为乘积级联

基因组以染色质的形式堆积在细胞核中。生化方法表明，染色质堆积在多个区域中，这些区域又分为较大的区域，依此类推。这种域内域填充，也称为分层填充，在大规模染色质组织的超分辨率显微镜图像中同样可见。尽管先前的工作表明染色质通过微相分离被划分为不同的结构域，但尚不清楚这些结构域如何组织成分层包装。一个特殊的挑战是找到一种完全结合了这种分层堆积的图像分析方法，以便可以将常染色质堆积的假设控制机制与这种分析的结果进行比较。这里，我们从具有改进的DNA荧光染料标记的多能斑马鱼胚胎中获得了3D STED超分辨率图像，并展示了如何在这些图像中将常染色质的分层堆积描述为乘法级联。乘法级联是一个已建立的理论概念，用于描述大型结构中越来越小的结构的位置。重要的是，这些级联可以通过一次又一次地应用单个规则来生成人工图像数据，并且仅使用四个参数就可以完全指定它们。在这里，我们展示了常染色质组织的典型模式如何在这四个参数的值中得到反映。特别是，我们可以查明模拟常相染色质的微相分离构型所需的值。我们怀疑乘法级联的概念也可以应用于其他类型的染色质图像。特别是，级联参数可以用作测试量，以评估微相分离或其他理论模型是否准确地再现了染色质的分层堆积。