Cross-Sectional Imaging in Comparative Vertebrate Morphology - The Intracranial Joint of the Coelacanth Latimeria chalumnae
Computed Tomography - Special Applications
Introduction Vertebrate morphology, including developmental anatomy, has depended on dissection since anatomical study first began, and on microscopy since the 19th century, in particular since the invention of the microtome. These methods have limitations: dissection destroys tissues, and disturbs or destroys three-dimensional relationships. Microscopy is less destructive, in that sections can be preserved for periods of time, but cutting serial sections, as often used in developmental
... velopmental anatomy, is time-consuming, sections are easily lost in processing, and distortion can be a problem. Three-dimensional (3D) reconstruction for interpretation and demonstration of results was done with manual drawing techniques or physical reconstruction with sequential wax plates, but now can be done with image reconstruction software. Microscopy findings still need to be photographed, aligned and segmented -the tissues of interest identified and marked out on 2D slices -before a result can be obtained, all of which is still relatively labour-intensive. Functional morphology became accessible with the invention of the motion-picture camera, and cinefluoroscopy with implanted radiological markers has provided many explanations. These traditional techniques have been supplemented in recent years by cross-sectional imaging and advanced techniques that depend on this imaging. CT (computed tomography) and MRI (magnetic resonance imaging) are best known as medical imaging technologies, but have a range of applications in morphology. Newer and more accurate techniques of imaging, real-time imaging for functional study and methods of image reconstruction are revolutionizing vertebrate morphology, bringing 3D information in a non-destructive manner. Rare museum specimens are often not made available for dissection, and these may be important taxa for biological and phylogenetic reasons; imaging techniques have an important application here. This chapter will review a number of new imaging techniques used in comparative morphology, with examples of recent applications, and will present original research demonstrating a number of these techniques to investigate a morphological mystery, the intracranial joint of the coelacanth Latimeria chalumnae. www.intechopen.com Computed Tomography -Special Applications 260 Imaging techniques in comparative morphology MicroCT MicroCT systems are now available at a size and cost suitable for laboratory equipment, with resolution to several microns. This is much more accessible than use of CT scanners at medical facilities, which are typically committed for medical use during office hours. The ability to produce 3D images of small structures replaces laborious and less accurate histological methods. Rieppel et al. (2009) used microCT to examine the skull of the anomalepidid (blindsnake) Liotyphlops albirostris, and were able to resolve a number of issues in the anatomy of the very small skulls of this snake. Preparation of skulls by the usual methods including digestion by enzymes and cleaning by insects may not give satisfactory results in small vertebrates, as the details are difficult to record, and the individual skull bones tend to disarticulate, making it impossible to study the relationships of the bones to the function of the whole skull. Rieppel et al (2009) have been able to make important functional deductions about adaptation to burrowing, and to compare these with another group of blindsnakes, typhlopids, which adapted the skull to a burrowing lifestyle with different morphological details but a similar result.