Histology and 3D microanatomy of the brain of Sardina pilchardus (Teleostei, Clupeiformes)
In order to understand the mechanisms of vision and the processing of visual information in the central nervous system of a teleost fish, it is a first step to get an idea of the histology and 3D microanatomy of the brain.
In this study several slice series of the brain of Sardina pilchardus (Clupeidae, Clupeiformes), embedded in different mediums were made. These have been treated with different histological dyes, captured digitally, registered with Amira ® and processed in Adobe ®Photoshop. Based on these data a 3D surface model (microanatomy) and a histological 2D atlas (histology) of the brain have been composed. Therefore the regions and structures of the brain were appointed and named. Furthermore the histological structure and layers of the optic tectum were examinated. 6 Layers could be identified, a design that is consistent with literature. Throughout this work a specialty of the brain of S. pilchardus was found: the partition of the optical tectum into an anterior and a posterior area.
The Data of the 3D surface model and the 2D atlas form the basis of orientation for tracing experiments on the retinofugal tracts of the brain. For this DiX dyes (lipophilic fluorescent membrane tracers), that autonomously diffuse along cell membranes, were applied to the retina of the fish. It was possible to trace the fluorescent dye as far as the optic chiasm. The final target areas couldn`t be labeled in the scope of this study.
Preliminary studies showed that retinotopic maps are conserved in the optic tectum. To prove this on the European sardine, thickness measurements were made on the tectum. This also was a first approach to demonstrate that known concentrations of photoreceptive structures in the retina (area) are represented in the tectum by certain denseness in circuitry. First local measurements of the strength of the tectal layers responsible for the converting of visual information took place. As relevant magnitude differences could be shown, a global map was made. This map shows significant maxima and minima, which encourage further studies on this aspect.
The 3D data sets of the brain of Sardina pilchardus present, with its possibilities of free scaling, rotatability as well as volume and distance measurement, a completely new approach for functional and morphological comparison of fish brains. Together with the results and experiences from the tracing experiments and the magnitude measurements on the tectum, they provide the fundamental means for the path to the understanding of the retinfugal circuitry in the brain of the Clupeiformes.