Optical Projection Tomography - a better way to view tissues and genes
A new type of microscopy has revolutionised the way scientists look at specimens such as embryos and mouse body organs. The technique, developed and commercialised by the MRC, produces 3D images that give unprecedented insights into the structure of tissues and the activity of genes. The method will aid research on the way genes work and may improve the accuracy of medical diagnosis using tissue biopsies.
Optical Projection Tomography (OPT) was invented in 2001 by an MRC molecular and developmental biologist, Dr James Sharpe at the MRC Human Genetics Unit (HGU) in Edinburgh. He was trying to build 3D maps of genes and proteins in mouse embryos. The only method available was confocal microscopy (invented at the MRC Laboratory of Molecular Biology), which focuses a laser beam to a fine point, but cannot be used on samples larger than 1mm. To make a 3D image of this type of specimen, it was instead necessary to fit together by computer hundreds of thinly-cut serial sections; this was laborious, time-consuming and introduced distortions in the final image.
An old idea, a new application
Taking pictures of an object at different angles in order to reconstruct a 3D image was not a new idea – this approach has been used in X-ray computed tomography (CT) scanners for the last two decades, but no one had yet tried this idea with light microscopy. Dr Sharpe began to build such a system using pieces of equipment lying around in the lab. He took multiple images of the specimen at different angles using a large depth of field – so that the whole length of the specimen was in focus as opposed to a fine point. He also developed the computer software to turn the raw data into a 3D image.
Dr Sharpe found that he was able to construct images of whole specimens of between 1 and 15 millimetres this way1. The technique could also be combined with staining methods to label internal structures, and most importantly the complex activities of genes in different tissues. OPT can image specimens that are too large for conventional confocal microscopy and too small for Magnetic Resonance Imaging (MRI) [cross-ref]. It is also cost-effective because it is able to image these types of structures without requiring the specimen to be destroyed.
The world market
MRC Technology, the MRC’s own technology transfer company, helps turn MRC inventions into products and services that have the potential to deliver healthcare benefits. It helped Dr Sharpe to develop the patent for OPT and to set up Bioptonics – MRCT’s OPT scanning service. Dr Sharpe’s group began collaborating with scientists all over the world, using OPT for a variety of applications, including mouse embryos, plants, and, increasingly, organs of adult animals.
OPT has been described as a ‘disruptive technology’, which is one that revolutionises an existing technology. MRCT and scientists have been testing, marketing and selling a commercially viable OPT instrument, OPT3001, and by October 2007 had sold 13 of them in eight countries. The technology has the potential to be taken up by a major manufacturer that has the expertise to maximise OPT’s commercial value and to market it around the world.
Many uses
In collaboration with Dr. Sharpe’s lab, a group of researchers in Sweden has used OPT to study the pancreas in adult mice, which have been tagged with antibodies against insulin. The researchers compared diabetic mice with healthy mice and found a link between the total volume of cells that produce insulin in the pancreas and the onset of type-1 diabetes2.
The main application of OPT has been in the study of embryo development. An MRC group in Newcastle has put together atlases of the human [www.ncl.ac.uk/ihg/EADHB] and mouse brain to investigate the genetic changes that drive development. The advantage of using OPT in this project is that embryos are not damaged and can be re-used.
As well as the pancreas, various other adult mouse organs have been imaged by OPT, including the brain2, heart3, lungs and kidney. Researchers expect adult organ imaging to become a very important area for future OPT studies building on its role in studying rodent models of disease. OPT may also improve biopsy analysis in medical diagnosis, because it could be used to scan a whole tissue sample. The current approach is to cut a sample into thin sections and examine a limited number of them, which may lead to some abnormalities being missed.
In addition, plant scientists at the Biotechnology and Biological Sciences Research Council (BBSRC) John Innes Centre in Norwich are applying OPT to image the process of germination, and gene expression patterns in stems, flowers and seeds4.
References
1. Sharpe et al. (2002). Optical Projection Tomography as a tool for 3D microscopy and gene expression studies, Science, 296, 541
2. Alanentalo et al. (2007). Tomographic molecular imaging and 3D quantification within adult mouse organs. Nature Methods, 4, 31
3. Lickert et al. (2004). Baf60c is essential for function of BAF chromatin remodelling complexes in heart development. Nature, 432,107
4. Lee et al. (2006). Visualizing plant development and gene expression in three dimensions using Optical Projection Tomography. Plant cell, 18, 2145