Embryonic Insight: Mouse Histology in 3-D
Josephson, Julian, Environmental Health Perspectives
Histology, the study of how cells are organized into tissue, is a keystone of the biological sciences. For decades, histologists have depended upon methods such as the tried-and-true slides of stained tissue sections. Although much has been learned through these methods, how much more would be learned if histologists could view tissues in three dimensions?
Today this advance is becoming a reality thanks to a newly developed ability to combine ultra-high resolution/microscopic computed tomography (microCT) scans with high-tech computer protocols to produce detailed 3-D images of mouse embryos. Besides the marked advantage of a 3-D perspective, the technique also achieves improvements in resolution, time required, and costs for studying developmental patterning effects attributable to genetically engineered mutations and chemically induced embryotoxicity.
The research team that reported the development of this method, known as microCT-based virtual histology (MBVH), was led by Charles Keller, an assistant professor of cellular and structural biology at the Children's Cancer Research Institute, University of Texas Health Sciences Center at San Antonio. The innovative technique was reported in PLoS Genetics in April 2006.
A Better Mouse Sample
Mouse embryos are commonly used to study gene function. The team's original aim was to develop a method for rapidly assessing birth defects associated with targeted disruptions of each gene in the mouse embryo. According to the authors, this called for "systematic, interdisciplinary approaches to analyzing patterning defects in the mouse embryo."
What emerged, the authors wrote, was a "novel, rapid, and inexpensive method for obtaining high-resolution virtual histology for phenotypic assessment of mouse embryos." Besides determining how genetic polymorphisms may contribute to end points such as birth defects and cancer, this new technique can perhaps be used to assess the safety of new medicines and other chemicals.
The traditional method used in histology is difficult and time-consuming. Mouse embryos that have genetic mutations or that have sustained damage from chemicals are killed, embedded in wax, and sliced into thin sections. Next, they are stained and placed on slides for microscopic examination. "[The new] technique allows us to get at a lot more tissues other than bone, such as internal organs, which [conventional] microCT scans of unstained tissue can't pick up," says coauthor Chris Johnson, a distinguished professor of computer science who directs the University of Utah's Scientific Computing and Imaging Institute.
The technique uses a 1% solution of osmium tetroxide (Os[O.sub.4]), a major fixative of choice for electron microscopy, to stain tissues differentially. Images of whole embryos are then made by volumetric X ray microCT, with as little as 2 hours needed to achieve isometric resolutions of 27 [micro]m, or 12 hours to achieve resolutions of 8 [micro]m.
The X ray microCT scans of mouse embryos thus generated are converted using computer visualization techniques into detailed 3-D images that show the mouse's exterior and interior. Instead of being physically sliced, the special dyes or fixatives permeate the skin and other membranes of a mid-gestation embryo; in older embryos and fetuses, the skin must be removed for the stain to penetrate.
What's in There?
Johnson and his group wrote an algorithm that distinguishes and visualizes various organs and structures in the mouse embryo based on the microCT scan data. This produces a virtual rendering of the scan data that also includes a virtual light source. The 3-D embryo image can therefore be rendered with shadows, which makes it easier for the human eye to understand and interpret the image.
The new technique furthermore allows users to create transparent images or even produce cutaways, so that internal organs and body parts become visible. …