Have you ever wondered how animals like fish, and chameleons rapidly change their color to camouflage or communicate? Our recent work shows that Zebrafish quickly alter their structural colors by changing the spacing between their intracellular crystals using motor proteins

To change color, animals often rearrange their optical materials: redistribute pigments, change crystal spacing, or tilt reflective structures. In our new study, just published in Advanced Materials, zebrafish reveal another mechanism: changing the molecular order within those materials. We studied xantholeucophores, cells in the light stripes of the zebrafish anal fin, originally identified by the @parichylab lab as developing from yellow, xanthophore-like progenitors. During development, these cells become highly efficient visible-light scatterers while retaining strong UV absorption. What changes inside them? Their pteridine-containing organelles undergo a transition from relatively disordered assemblies to highly ordered, liquid-crystal-like structures. This molecular reorganization transforms how the cells interact with light. The gatekeeper is potassium! We found that as the cells mature, K⁺ levels fall within their organelles and the pteridines become ordered. In vitro, removing K⁺ alone was sufficient to trigger ordering and crystallization. A change in the ionic environment can therefore control the assembly's molecular order and its optical function. The chemistry held another surprise. These organelles contain a composite of isoxanthopterin and 7-oxo-biopterin, which we identify as a major constituent. This adds 7-oxo-biopterin to the repertoire of small molecules known to participate in ordered, functional biological assemblies. The result is one cell doing two optical jobs: efficiently scattering visible light and absorbing UV. Molecular ordering makes the fin more conspicuous while preserving the pigments' UV absorption. Biology doesn't always need to move its optical materials around. Sometimes it just needs to convince the molecules to get their act together. This work was led by the terrific @SourabhBera4 , together with @ZoharEyal1, Yael Noy, @yuval_barzi, @Siddharth Sahoo, Avi Baram, and Tali Lerer-Goldshtein from our lab. With amazing contributions from our collaborators and friends: Venkata Jayasurya Yallapragada, Srikant Moharana, Ido Hadar, Bar Bader, Andrea Sorrentino, Neta Varsano, Lothar Houben, Uwe Heinig, Iddo Pinkas, Moshe Goldsmith, Tamar Unger, Shira Albeck, Maxim Itkin, Sergey Malitsky, and Ziv Porat. @Weizmann Institute of Science Check it out: advanced.onlinelibrary.wiley…

There is a saying that it takes a village to raise a child, each person playing a specific role. In our latest study, we found that forming a molecular crystal inside a cell is not so different. It takes an entire organelle: a lysosome-related organelle (LRO) equipped with a specialized molecular machinery, where every molecular component plays its own role. In this work, we show that guanine crystals form within dedicated organelles, iridosomes, that belong to the LRO family. These organelles assemble rapidly and in large numbers within zebrafish iridophores in the eye and skin, enabling vision and camouflage early in development. The study sheds new light on how cells orchestrate the formation of hundreds of crystal-forming organelles with remarkable precision. The movie below shows time-lapse imaging of crystal formation dynamics in zebrafish eye iridophores. Read the full manuscript here: lnkd.in/d6rF2Dz4 This project was spearheaded by the incredibly talented Anna Gorelick-Ashkenazi, alongside many outstanding collaborators. Grateful to the whole team for this journey! @yuval_barzi Tali Lerer-Goldshtein, @TsviyaO, @ZoharEyal1, @May_Gleser, Yonatan Broder, @nadavmishol, @RachaelDeis, Merav Kedmi, @WeizmannScience

How do you tell a molecular crystal how to grow? In materials science, crystal morphology is usually an outcome. In biology, it’s often a design feature, tuned to deliver specific optical and other functional properties. In our new paper in Small, we dissect how crystal growth can be steered at the molecular level, and show that simple synthetic polymers can not only reproduce biogenic plate-like morphologies, but also expand the accessible shape space. By systematically varying polymer chemistry, we uncover three key principles: • Multivalent interactions are important for sustained growth control • Carbonyl-bearing functional groups selectively adsorb to the crystal stacking face, suppressing growth along the stacking direction • Subtle changes in polarity and sterics redirect growth into plates, prisms, or even needle-like morphologies This work was led by Dolev Brenman-Begin, who submitted her first first-author manuscript just before giving birth, and then handled the revisions with a newborn at home. A remarkable scientific and personal milestone. 💪🏽 The study was a close collaboration with (@barak_hirshberg) Barak Hirshberg‘s lab and his talented postdoc Jonatan Church (Tel Aviv University), and with (@LielS5) Liel Sapir Sapir’s lab (Bar-Ilan University). Other authors and collaborators - thank you all: Siddharth Sahoo, (@ZoharEyal1) Zohar Eyal , (@IdanBiran) Idan Biran, PhD , Nir Kampf, (@yuval_barzi) Yuval Barzilay, Anna Kossoy-Simakov, Lothar Houben. @WeizmannScience onlinelibrary.wiley.com/doi/…

Super proud of our postdoc @SourabhBera4 and PhD student @RachaelDeis on receiving Travel Awards at ESPCR 2025. Huge congrats-well deserved!

A new article by @ZoharEyal1 @RachaelDeis @DvirGur from @lab_gur at @WeizmannScience reveals that dynamic pH changes allow the formation of guanine crystals within specialized organelles, so that animals display brilliant colors nature.com/articles/s41589-0…

In materials science, crystallization is usually driven by supersaturation and tuned by factors like temperature and solvent composition. In biology, however, crystals must form under physiological conditions and ambient temperatures, which drastically narrows the “playground” for crystallization. In our new study, just published in @nchembio, we used zebrafish crystal-forming cells as a model to uncover what drives biogenic crystal formation - and found that pH dynamics play a central role. By integrating cryo-electron microscopy, spectroscopy, and pharmacological perturbations, we revealed how the iridosome microenvironment orchestrates guanine crystal formation. Early-stage iridosomes are highly acidic, which likely facilitates the buildup of protonated amorphous guanine and the assembly of macromolecular scaffolds. As the organelles mature and crystals expand, the pH gradually neutralizes, enabling further growth. When these pH changes are disrupted, crystal formation is severely impaired, highlighting the importance of tightly regulated organellar environments in biological crystallization. This work was brilliantly led by @ZoharEyal1 , together with @RachaelDeis and Anna Gorelick Ashkenazi, and made possible through the contributions of many wonderful collaborators and friends: @yuval_barzi, Yonatan Broder, @KellumAshe74214, @NetaVarsano, Michal Hartstein, Andrea Sorrentino, Ron Rotkopf, Ifat Kaplan-Ashiri, Katya Rechav, Rebecca Metzler, Lothar Houben, Leeor Kronik, Peter Rez. #Crystallization, #Biomineralization, #MaterialsScience, #CellBiology, #StructuralBiology, #CryoEM, #Spectroscopy, #Zebrafish Check it out: nature.com/articles/s41589-0…

אנחנו נמצאים בנקודת זמן קריטית לשיקום מכון ויצמן למדע, וזקוקים לתמיכה שלך. בעזרתך נוכל לבנות מחדש מעבדות, לבסס מחדש מחקרים פורצי דרך ולהוביל את המדע לגבהים חדשים. התרומה שלך היום תניע את התגליות של מחר. עכשיו זה הזמן. לתמיכה >>

Just as Batman has Robin and Mario has Luigi, melanophores—the black pigment cells—have their counterpart: leucophores (from the Ancient Greek λευκός, meaning white). These often-overlooked cells play a key role in generating the remarkable colors and patterns we see in many animals. In our new study, published in PNAS (@PNASNews ), we investigated how leucophores generate their white appearance by crystallizing uric acid—a compound commonly known for its involvement in human diseases such as gout and kidney stones To understand how this process compares to other pigment cell types, we used a combination of hashtag#scRNA-Seq, hashtag#metabolomics, and hashtag#cryoEM to study leucophores alongside iridophores, which create iridescent effects using guanine crystals, and xanthophores, which carry yellow pterin pigments. Our findings reveal that while these pigment cells share underlying cellular machinery, their distinct pigment chemistries give rise to specialized molecular programs. Most strikingly, we uncovered star-like fibrous scaffolds within leucosomes. These structures likely serve as templates for the growth of one-dimensional uric acid crystals, which radiate outward and scatter light with high efficiency, shedding new light on the molecular strategies organisms use to construct whiteness from molecular building blocks. This project was led by our outstanding PhD student @yuval_barzi , with valuable contributions from a wonderful team of collaborators and friends. Read the full paper here: lnkd.in/e94XEKxj The movie below provides a high-resolution view of the ultrastructure of distinct pigment organelles, captured using cryo-electron tomography. From left to right: leucosomes (uric acid), pterinosomes (pterin), and iridosomes (guanine). @WeizmannScience @ZoharEyal1, YaelNoy, @NetaVarsano ,@TsviyaO ,@SOURABHBERA12, Tali Lerer, Merav Kedmi, @PoratZiv,@IddoPinkas, Smadar Zaidman, @dezorellaNili

My seminar yesterday at the Pasteur Institute was dedicated to the memory of Prof. Benny Shilo, who passed away earlier this week. Benny was an extraordinary scientist and a gifted photographer, with a rare ability to beautifully bridge these two worlds. Since the day I opened my lab, Benny has been a mentor to me—always generous with his time, insights, and guidance. He will be deeply missed.

RT @lab_gur: Thrilled to see our work on the"Genetic Control Over Biogenic Crystal Morphogenesis in Zebrafish" featured on the cover of Nat…

Our new website is live! Check it out, and don’t miss the 3D virtual tour 😀 weizmann.ac.il/molgen/gur/

Looking forward for tomorrow!

For an ERC-funded project, we are searching for talented postdocs/PhD students to carry out exciting studies involving genetic engineering, functional crystals, and optical and electron microscopy at the @WeizmannScience. Prior experience working with yeast is an advantage.