The outer epithelial layer of zebrafish retinae contains a crystalline array of cone photoreceptors, called the cone mosaic. As this mosaic grows by mitotic addition of new photoreceptors at the rim of the hemispheric retina, topological defects, called “Y-Junctions”, form to maintain approximately constant cell spacing. The generation of topological defects due to growth on a curved surface is a distinct feature of the cone mosaic not seen in other well-studied biological patterns like the R8 photoreceptor array in the Drosophila compound eye. Since defects can provide insight into cell-cell interactions responsible for pattern formation, here we characterize the arrangement of cones in individual Y-Junction cores as well as the spatial distribution of Y-junctions across entire retinae. We find that for individual Y-junctions, the distribution of cones near the core corresponds closely to structures observed in physical crystals. In addition, Y-Junctions are organized into lines, called grain boundaries, from the retinal center to the periphery. In physical crystals, regardless of the initial distribution of defects, defects can coalesce into grain boundaries via the mobility of individual particles. By imaging in live fish, we demonstrate that grain boundaries in the cone mosaic instead appear during initial mosaic formation, without requiring defect motion. Motivated by this observation, we show that a computational model of repulsive cell-cell interactions generates a mosaic with grain boundaries. In contrast to paradigmatic models of fate specification in mostly motionless cell packings, this finding emphasizes the role of cell motion, guided by cell-cell interactions during differentiation, in forming biological crystals. Such a route to the formation of regular patterns may be especially valuable in situations, like growth on a curved surface, where the resulting long-ranged, elastic, effective interactions between defects can help to group them into grain boundaries.

斑马鱼视网膜的外上皮层包含锥体光感受器的晶体阵列，称为锥体马赛克。当这种马赛克通过有丝分裂在半球视网膜边缘添加新的感光细胞而生长时，会形成称为“Y 形连接”的拓扑缺陷，以保持大致恒定的细胞间距。由于在曲面上生长而产生拓扑缺陷是锥形镶嵌体的一个显着特征，而在其他经过充分研究的生物模式（如果蝇复眼中的 R8 感光器阵列）中未发现这一特征。由于缺陷可以深入了解负责图案形成的细胞间相互作用，因此我们在这里描述了各个 Y 型连接核心中视锥细胞的排列以及 Y 型连接在整个视网膜上的空间分布。我们发现，对于单个 Y 结，核心附近锥体的分布与物理晶体中观察到的结构密切对应。此外，Y 形结从视网膜中心到周边组织成线，称为晶界。在物理晶体中，无论缺陷的初始分布如何，缺陷都可以通过单个颗粒的迁移性合并到晶界中。通过对活鱼进行成像，我们证明了锥形镶嵌体中的晶界在最初的镶嵌体形成过程中出现，而不需要缺陷运动。受这一观察的启发，我们证明了排斥细胞间相互作用的计算模型会产生带有晶界的马赛克。与大多数静止细胞堆积中的命运规范范式模型相反，这一发现强调了细胞运动在形成生物晶体中的作用，在分化过程中由细胞间相互作用引导。这种形成规则图案的途径在某些情况下可能特别有价值，比如在弯曲表面上生长，缺陷之间产生的长程、弹性、有效的相互作用可以帮助将它们分组为晶界。