New way to image whole organisms in 3D brings key skin color pigment into focus

To understand the biological underpinnings of skin and hair pigmentation and related diseases such as albinism or melanoma, scientists and doctors need quantitative, three-dimensional information about the architecture, content and location of pigment cells. Penn State College of Medicine researchers have developed a new technique that allows scientists to visualize every cell containing melanin pigment in 3D, in whole zebrafish.
Studying melanin is challenging because it blocks the light used in traditional microscopy. So the researchers turned to X-ray imaging, which can pass through optically opaque matter like melanin.
In 2019, a team led by Dr. Keith Cheng, distinguished professor of pathology, pharmacology and biochemistry and molecular biology, developed "X-ray histotomography" -- a cellular form of CT imaging, as a method for investigating the 3D architecture of cells and tissues in biological samples at unprecedented resolution and clarity. Spencer Katz, an MD/PhD medical scientist training program student on Cheng's team, has modified this micro-CT technique to specifically investigate melanin, a pigment scientists are studying in human skin color and melanoma research, in whole zebrafish.
Melanin is a brown to black pigment that gives the zebrafish its characteristic stripes and humans dark skin, hair and eyes. More than 15 years ago, Cheng and his lab discovered a key gene in the evolution of light skin color in humans by studying a particular mutant zebrafish line, golden, that has lighter stripes. This discovery demonstrated the relevance of zebrafish models for studying critical questions about human biology and disease, including albinism and melanoma.
Micro-CT, like human CT, uses a series of X-rays taken at slightly different angles to compute, or reconstruct, 3D representations of the original object. For micro-CT, the samples are smaller and the resolution developed by Cheng's team is 2000-fold higher. Katz used silver to stain the melanin, which allowed the researchers to determine the 3D location and density of melanin from scans of whole zebrafish.
To perform the imaging, the Cheng Lab partnered with Dilworth Parkinson at the Advanced Light Source at the Lawrence Berkeley National Labs in Berkeley, California -- home of one of America's most powerful synchrotron X-ray sources, where he directs a micro-CT resource suitable for Cheng's X-ray histotomography. The lab's new X-ray detector system was designed to achieve resolutions unprecedented for samples the size of whole zebrafish or human biopsies. The team scanned zebrafish with both normal and altered pigmentation, including golden.