Lighting up Neuroanatomy

" Contemporary neuroscience is in urgent need of a new generation of neuroanatomical techniques allowing scalable, reliable, specific, and quantitative analysis of macroscopic portions of brain tissue with cellular or sub-cellular resolution. " In this first sentence of their Discussion, Silvestri et al. (2015) succinctly set forth the key issues of their contributing article; namely, visualization of Purkinje cells in 3-D volumes, and the methods for doing so. The importance of rigorous quantitative analysis in neuroscience has long been recognized as an essential baseline against which to compare and interpret species differences, developmental changes, and pathological changes, among other processes. Counting, however, is not easy, and incorrect sampling methods have more than once produced fundamentally incorrect results—notably, the now discredited belief that there is no neuronal loss in normal aging (reviewed in Guillery, 2002), or that there are " ten times " more glia than neurons (reviewed in Hilgetag and Barbas, 2009). A particular challenge in neuroanatomy has been the problem of compiling accurate quantitative data about inherently 3-dimensional structures from essentially 2-dimensional histological sections. The standard way of dealing with this problem has been the application of specific stereological procedures. Even when combined with some degree of automation, however, stereology is notoriously labor intensive, and provides only limited information about spatial localization and pattern. A rapidly emerging alternative approach in quantitative neuroanatomy is direct 3-dimensional visualization. In itself, this is not new; for example, whole mount preparations reacted for various antigens have demonstrated an intricate compartmentalization of the cerebellar cortex (White et al., 2012; Cerminara et al., 2015); and reconstructed stacks of consecutive histology slides have successfully demonstrated the complex striosomal architecture (Mikula et al., 2009). The newer techniques, although not without their own problems, offer faster processing, better standardization, and the potential for integration across cellular and subcellular spatial scales. Several volumetric approaches are available. Serial two-photon tomography is a block-face approach developed in the early 2000's and now in routine use for whole mouse brains (Ragan et al., 2012; Kim et al., 2015). The main features (reviewed in Amato et al., 2016) include straightforward specimen preparation, minimal tissue processing, automated and robust data acquisition, and high resolution 3-D reconstructions. In confocal light sheet microscopy (LSM), the approach used by Silvestri et al., the sample is illuminated with a thin sheet of light and optically sectioned, typically in a wide-field detection scheme. Millions of pixels are collected simultaneously …