Date Published: April 1, 2019
Publisher: Public Library of Science
Author(s): Martin J. Schmidt, Carola Knemeyer, Helmut Heinsen, Timothy JP Bray.
In this study, the morphology of the horse brain (Equus caballus) is decribed in detail using high field MRI. The study includes sagittal, dorsal, and transverse T2-weighted images at 0.25 mm resolution at 3 Tesla and 3D models of the brain presenting the external morphology of the brain. Representative gallocyanin stained histological slides of the same brain are presented. The images represent a useful tool for MR image interpretation in horses and may serve as a starting point for further research aiming at in vivo analysis in this species.
Neuroimaging is increasingly important in veterinary large animal neurology. Magnetic resonance imaging (MRI) is more and more used to evaluate intracranial diseases in horses with neurological signs [1–12]. High magnetic field strengths (3 Tesla) are now available in veterinary medicine. Images collected at this field strength provide improvements in image clarity and detail. Despite considerable progress in the technical adaptions of scanners and detection coils to the practical requirements in equine medicine, the description of brain morphology in the horse has been somewhat neglected. Currently, studies describing equine brain anatomy are only available at reduced resolution and from the brain of foals [13,14]. As the number of MRI-investigations of the equine brain grows, so does the need for detailed information about brain structures and special characters of the equine brain, as existing for a number of domestic species [15–19]. The aim of this study was therefore to examine the brain of the horse using high field MRI and to describe the morphology of the equine brain as shown in these images.
Histological slides in transverse orientation were obtained to support image analysis. After scanning, the brain was removed and fixed by immersion in 10% formalin for 2 weeks. Due to the large size of the brain, the brainstem with the cerebellum was severed at the level of the rostral pons and the hemispheres were divided. Brain tissue was dehydrated in 96% alcohol and embedded with 8% celloidin (180 g of commercial alcohol-moistened celloidin dissolved in a mixture of 750 ml 100% ethanol and 750 ml diethyl ether) for 48 h. The brain together with an excess of 8% celloidin were placed into a big transparent PVC embedding form. The celloidin was concentrated to 16% in a dessicator by a slight vacuum (150 millibar). The 16% celloidin was finally hardened by chloroform vapors and one day prior to cutting with 70% ethanol. The celloidin blocks were serially sectioned on a sliding microtome (Polycut, Leica Mikrosysteme Vertrieb GmbH Wetzlar, Germany) with section thickness of 350 μm. Celloidin served as a support, guiding the microtome knife through the tissue block and preventing tangentially cut gyri from floating away during subsequent staining procedures. Slicing of the histology slides was performed perpendicular to the brainstem axis to match the same orientation as the MR transverse image slices. The slices were then stained free-floating in gallocyanin-chromalum  quenched between two filter papers and two perforated stainless steel plates to prevent distortion of the single slices during dehydration in ethanol, ispopropanol:xylene 1:1, and finally xylene. The sections were coverslipped with Permount.
A 3D rendered model (Fig 1A–1E) demonstrates the morphology of the brain as a whole from a dorsal, lateral, frontal, and ventral as well as from a midsagittal-medial view. T2-weighted high-resolution images with detailed structural identification are provided in transverse, sagittal and dorsal planes shown in Figs 2–34. Based on comparison with histological slices, morphological structures of the adult equine brain are labelled in detail in the MR images. A list of anatomical terms and the figures, in which they are displayed, are shown in the Supporting information (S1 Table). The transverse images correspond to a reference line presented along with the 3D model in the right upper corner of the image. Selected examples of histological slides corresponding to the transverse MR-images are presented as Supporting material (S1–15 Figs). Anatomical structures were named according to the Nomina Anatomica Veterinaria . A description of the brain parts, their substructures and connections is given in the following.