Rothkegel Jannik, Kaufmann Sylvia, Wilsch-Bräuninger Michaela, Lopes Catarina, Mateus Rita
Max Planck Institute of Molecular Cell Biology and Genetics, 01037, Dresden, Germany.
Cluster of Excellence Physics of Life, Technische Universität Dresden, 01062, Dresden, Germany.
Small Methods. 2025 Aug 21:e01956. doi: 10.1002/smtd.202401956.
Across phyla, many organisms self-organize crystals, for functions like vision, pigmentation, and metabolite storage. In zebrafish, a vertebrate known for its crystal-based color patterns, iridophores concentrate purines in membrane-bound organelles, the iridosomes. Inside these vesicles, crystals assemble into large, flat, and thin hexagons following unknown mechanisms that defy typical thermodynamic interactions. Here, we investigate the development of zebrafish iridosomal crystals by using live imaging, cryoFIB-SEM, and novel morphometric analysis pipelines. In doing so, we find that crystal growth predominantly occurs along the b-crystallographic axis, producing their characteristic anisotropic shape. By performing comparative genetic analyses in vivo and reproducing such conditions in silico, we uncover that the zebrafish crystals' in-plane hydrogen bond molecular structure is the main determinant for the observed crystal anisotropy. Macroscopically, the b-axis anisotropy is controlled by the ratio of guanine-to-hypoxanthine in the iridosome, without affecting the other axes. At the atomic level, the extent of the (100) facet anisotropy depends entirely on the type, number, and strength of molecular H-bonds within the crystal lattice. Mechanistically, our work shows that purine diversity and availability inside the zebrafish iridosome is key to form an anisotropic crystal lattice, leading to the observed functional crystal shapes.
在不同的生物门类中，许多生物体能够自我组织形成晶体，用于视觉、色素沉着和代谢物储存等功能。在斑马鱼中，这种以基于晶体的颜色模式而闻名的脊椎动物，虹彩细胞将嘌呤浓缩在膜结合细胞器——虹膜小体中。在这些囊泡内部，晶体通过未知机制组装成大的、扁平的薄六边形，这些机制违背了典型的热力学相互作用。在这里，我们通过使用实时成像、冷冻聚焦离子束扫描电子显微镜（cryoFIB-SEM）和新颖的形态计量分析管道来研究斑马鱼虹膜小体晶体的发育。通过这样做，我们发现晶体生长主要沿着b晶轴发生，产生其特征性的各向异性形状。通过在体内进行比较遗传分析并在计算机上再现这些条件，我们发现斑马鱼晶体的面内氢键分子结构是观察到的晶体各向异性的主要决定因素。宏观上，b轴各向异性由虹膜小体中鸟嘌呤与次黄嘌呤的比例控制，而不影响其他轴。在原子水平上，（100）面的各向异性程度完全取决于晶格内分子氢键的类型、数量和强度。从机制上讲，我们的工作表明斑马鱼虹膜小体内嘌呤的多样性和可用性是形成各向异性晶格的关键，从而导致观察到的功能性晶体形状。
