Magnetic Resonance Imaging Study of Adipose Tissues in the Head of a Common Dolphin (Delphinus delphis): Structure Identification and Influence of a Freezing–Thawing Cycle

Abstract

Magnetic resonance imaging (MRI) was used to scan the head of a common dolphin (Delphinus delphis) in order to visualize the different adipose tissues involved in echolocation functioning and to precisely delineate their anatomical topology. MRI scans were performed on the head taken from a freshly stranded carcass and repeated after a 2-week freezing time followed by thawing. The main fatty organs of the head, that is the melon, the mandibula bulba, the bursae cantantes, and their different connections with surrounding tissues were identified and labelled. The nasal sacs, other organs of echolocation, were also identified and labelled thanks to different MRI acquisitions. The shape, the location, the type of MRI signal of each organ and of their different connections were successfully analysed on all images, and then, the images of the head fresh or after thawing were compared. No impacts of the freezing/thawing cycle on the fatty tissues of the head were identified. Different parts were distinguished in the melon on the basis of the MRI signal emitted, corresponding most likely to the internal and external melon already identified by other analytical approaches, and linked to differences in lipid composition. MRI is shown here to be a useful tool to study the functional anatomy of the organs responsible for echolocation in odontocetes, with a particularly high level of precision.

Comparative anatomical study of sound production and reception systems in the common dolphin (Delphinus delphis) and the harbour porpoise (Phocoena phocoena) heads

“…In the necropsy sections (especially Figure 4a), we can also discriminate an outer and an inner melon, as already observed using MRI by Arribart, Ognard et al, 2017. Indeed, the caudal part of the melon appears as more yellow and softer (quite liquid, Figure 4a:2), whereas the rostral part of the melon is more fibrous and also firmer (Figure 4a:1).…”

“…In particular, the nasal sacs, not visible on CT scans and collapsed on the MRI images, could be observed. Finally, using necropsies, we highlight an-to our knowledge-unpublished picture of the bursae cantantes We confirmed and highlighted the direct connexions between the right posterior part of the melon and the MLDB complex, already illustrated for the bottlenose and common dolphins (Arribart, Ognard et al, 2017;Cranford et al, 1996;McKenna et al, 2011). In our study, the shape of the melon is, for the first time, visualised di- Computed tomography scans and necropsy sections allowed to determine precisely some measurements of the ears.…”

“…Parasagittal sections were performed to observe the nasal sacs, the bursae cantantes and the mandibula bulba (Arribart, Ognard et al, 2017). Finally, a transverse section cutting the blowhole of the harbour porpoise was performed.…”

“…According to Cranford et al (1996), soundwaves are originated from the nasal respiratory system which the Monkeys Lips/Dorsal Bursae (MLDB) complex is part. The soundwave reception is performed in part by the mandibles and the mandibula bulba (fatty tissues medially positioned in the mandibles) which is directly linked to the middle ear (Arribart, Ognard et al, 2017).…”

“…Recently, Arribart, Ognard et al, 2017 proved that MRI can be used to decipher with a high level of precision the complex anatomy of the adipose tissues, responsible of the echolocating sound emission and reception in the head of a common dolphin. Here we aimed to study the main anatomical differences betweens common dolphin and harbour porpoise sound emission and reception systems.…”