Cytoarchitectonic Atlas of the Brain of a Fetal Common Bottlenose or Atlantic Dolphin (Tursiops truncatus)
Introduction
Please see the Introduction to the Cytoarchitectonic Atlas of the Brain of the Common Bottlenose or Atlantic Dolphin (Tursiops truncatus) for information on this species.
Materials and Methods
The fetal brain depicted here is Y DOL-PHD-68 from the Yakovlev collection. The Nissl-stained sections are held at the National Museum for Health and Medicine in Silver Springs, Maryland, USA. The brain (following dehydration for embedding) is 3 cm wide and approximately 3.5 cm in the rostrocaudal direction. Estimated body length is 20 to 25 cm.
Selected frontal plane sections in a regular sequence (please see section numbers on plates) were scanned with the aid of an Epson 11000. The images were cropped and adjusted for optimal contrast levels with Adobe Photoshop (2023), before 22 sections were placed into 20 Adobe Illustrator (2023) files for mapping. In most plates only one side of the brain (right side of the section) has been labelled to leave the other side unobscured. Regions of the brain were labelled using terminology adapted from that used in the Paxinos and Watson system of brain atlases. The section number is given in the top right-hand corner of the plate. A small line-diagram finder image in each atlas plate shows the position of the frontal section (grey line) within the fetal brain.
Description
This fetal dolphin brain shows many similarities to fetal human brains of the early third trimester (Bayer and Altman, 2004). A prominent subventricular zone (SubV) is present in the inner curvatures of the lateral ventricles adjacent to the head, body and tail of the caudate nucleus (plates 3 to 14). This proliferative region would be producing microneurons and macroglia for the cerebral cortex and subpallial parts of the telencephalon. External to this SubV germinal zone is a stratified transitional zone (stz in plates 3 to 16) containing paused migrating neurons destined for the cingulate, frontal, parietal, temporal and occipital cerebral cortex.
The cerebellar cortex is surmounted by the transitional external granule cell (or germinal) layer (egl in plates 18 to 20) that produces the microneurons for the internal granule cell layer (IGL) of the cerebellar cortex.
Distinctively enlarged components of the auditory pathway can be seen even at this early stage. These include parts of the superior olivary nuclear complex that play a critical role in echolocation (LSO, LVPO, MSO, SPO in plates 16, 17), inferior colliculus components processing ascending auditory information (DCIC, ECIC, CIC in plates 14, 15) and the thalamic auditory nucleus (MG in plates 12 and 13). The auditory cerebral cortex would be on the lateral convexity of the cerebral hemisphere, but its boundaries cannot be defined at this stage.
As in the adult dolphin (see plate 17 of that atlas), the medial nucleus of the inferior olivary complex (IOM) is larger than the principal nucleus (IOPr), reflecting the importance of spinocerebellar systems serving proximal limb and axial musculature in motor learning in this species. Contrast this with human fetuses and adults, where the IOPr predominates, reflecting the importance of motor learning for distal musculature, particularly of the upper limb.
The pontine nuclei have yet to reach the large size noted in the adult dolphin brain (see adult brain, plates 15 to 18), reflecting the immaturity of the corticopontocerebellar system at this age.
Acknowledgements
I would like to thank Elizabeth Lockett of the National Museum of Health and Medicine in Silver Springs, Maryland, USA for the opportunity to study the marine mammal brains in the NMHM collection and for use of the Epson 11000 scanner.
References
Bayer SA, Altman J (2004) The Human Brain during the Third Trimester. CRC Press, Boca Raton.
