Topographical and structural characterization of the V1–V2 transition zone in the visual cortex of the long‐finned pilot whale Globicephala melas (Traill, 1809)

Abstract

The visual system of cetaceans is at best poorly understood. With a handful of electrophysiological studies and a limited number of histological preparations from well‐preserved specimen, the investigation of the principles underlying the cortical organization in cetaceans remains a challenge. In the course of our current investigation, we identified the transition from V2 to V1 in the long‐finned pilot whale Globicephala melas, only recognizable through immunocytochemistry, and a similar if not homologue transition in the sheep Ovis aries. Our results emphasize the importance of differential pattern recognition in which the application of different markers uncovers a diversity in a delphinid’s cortex, formerly widely considered as uniform and archetypal. In fact, the evidence that we present suggests the existence of relatively unacknowledged areas beyond the well‐known sensory territories in cetaceans.

The primary visual cortex of Cetartiodactyls: organization, cytoarchitectonics and comparison with perissodactyls and primates

“…We are aware that CR is a powerful tool to detect GABAergic neurons, especially in the central nervous system (Glezer et al 1992b). Former comparative studies in Cetartiodactyls and primates noted that CR-containing neurons were much more abundant and brightly stained than CB-ir ones (Hof et al 2000;Graïc et al 2021), as we also report. However, in our experimental series most CR cells were distributed in the upper layers L1-L2, but also in L3 and L5.…”

“…Comparison of immunohistochemical results with Nissl images strongly suggests that the cetacean visual cortex, composed of large pyramidal-like cells, is indeed furtherly subdivided in additional functional layers (Graïc et al 2021). These, in turn, suggest potentially different connection patterns within the column and towards other areas.…”

“…The cytoarchitectonics of the cetacean brain has been the subject of numerous studies (Breathnach 1960;Morgane et al 1986;Ridgway 1990;Glezer et al 1992bGlezer et al , 1995Poth et al 2005;Kern et al 2011;Butti et al 2011Butti et al , 2014avan Kann et al 2017; for general reference see Cozzi et al 2017a;Huggenberger et al 2019). Specifically, the cetacean visual cortex has been thoroughly studied by relatively few authors (Kesarev and Malofeeva 1977;Morgane et al 1985Morgane et al , 1988Morgane et al , 1990Oelschläger et al 1987;Garey et al 1985;Garey and Leuba 1986;Garey and Revishchin 1986;Garey et al 1989;Glezer et al 1992aGlezer et al , 1993Manger et al 1998;Graïc et al 2021), sometimes with a broad comparative scope (Glezer et al 1998;Hof et al 2000;Raghanti et al 2014;Cozzi et al 2017a;van Kann et al 2017). In general, the organization of the cetacean cerebral cortex differs from that of terrestrial mammals, with predominance of phylogenetically older L1 and L6, a poorly differentiated population of relatively Fig.…”

“…According to (Morgane et al 1988), the primary visual cortex of the striped dolphin is heterolaminar and contains a weak ("incipient") L4 with distinct granules, which has recently been found in the long-finned pilot whale (Graïc et al 2021), and differentiate V1 from V2. In the same study, Morgane and colleagues (1988) found by Golgi staining that L2 contained extraverted neurons with widespread spinous apical dendrites directed towards L1.…”

Systematic validation and assessment of immunohistochemical markers for central nervous system pathology in cetaceans, with emphasis on auditory pathways

“…Table 1 summarizes pertinent information on the investigated dolphins, with additional information on their sex, body length, full histopathological findings of the CNS, and the most probable cause of death provided in the supplementary materials. Brains had been extracted within 24 hours after death, cut into 1 cm-thick coronal sections, fixed in 10% neutral-buffered formalin and washed in phosphate buffer (0.1 M, pH: 7.4) prior to paraffin embedding [ 36 , 37 ]. When available, the right VCN (n = 11) and IC (n = 17) of each animal were sampled according to the protocol in Supplementary Materials, and routinely processed for histological and IHC analyses as previously reported [ 38 ].…”