A Study of the Head during Prenatal and Perinatal Development of Two Fetuses and One Newborn Striped Dolphin (Stenella coeruleoalba, Meyen 1833) Using Dissections, Sectional Anatomy, CT, and MRI: Anatomical and Functional Implications in Cetaceans and Terrestrial Mammals

Abstract

Simple Summary: The head region of the dolphin has been studied widely to identify its anatomical structures and to compare it with other marine and terrestrial mammals. In this study, specimens stranded off the Spanish coast were used. Our study analyzes four dolphin heads during fetal and perinatal development. All specimens were scanned using modern imaging techniques to study their internal organs and to preserve the specimens, which are difficult to obtain. Only one fetus was transversely cross-sectioned to help us to identify critical organs. The developmental study shows several anatomical structures that are compared with cetaceans and terrestrial mammals. During development of the oral cavity, it was observed that the rostral maxillary and mandible teeth (incisive area) had not completely erupted, in contrast with the rest of teeth, which have done so. Also, the main chewing muscle (masseter) was not observed. In addition, we describe the absence of major salivary glands during these developmental stages. Furthermore, we explain the characteristics of the orbit and its relation to the eyeball. In addition, the fetal dolphin's ear is connected with pharynx in a way similar to that in horses. We conclude that these developmental studies will help cetacean conservation. Abstract:Our objective was to analyze the main anatomical structures of the dolphin head during its developmental stages. Most dolphin studies use only one fetal specimen due to the difficulty in obtaining these materials. Magnetic resonance imaging (MRI) and computed tomography (CT) of two fetuses (younger and older) and a perinatal specimen cadaver of striped dolphins were scanned. Only the older fetus was frozen and then was transversely cross-sectioned. In addition, gross dissections of the head were made on a perinatal and an adult specimen. In the oral cavity, only the mandible and maxilla teeth have started to erupt, while the most rostral teeth have not yet erupted. No salivary glands and masseter muscle were observed. The melon was well identified in CT/MRI images at early stages of development. CT and MRI images allowed observation of the maxillary sinus. The orbit and eyeball were analyzed and the absence of infraorbital rim together with the temporal process of the 2 of 24 zygomatic bone holding periorbit were described. An enlarged auditory tube was identified using anatomical sections, CT, and MRI. We also compare the dolphin head anatomy with some mammals, trying to underline the anatomical and physiological changes and explain them from an ontogenic point of view.

Comparative Anatomy of the Nasal Cavity in the Common Dolphin Delphinus delphis L., Striped Dolphin Stenella coeruleoalba M. and Pilot Whale Globicephala melas T.: A Developmental Study

“…These diverticula present a pigmented mucosa as in other species [18] and also in the dissections, sections and histology of our study. The stratified squamous epithelium is similar to that of the equine nose and corroborates our idea about its function, which is protection from external agents and water [17,28]. The vestibular fold shows a very different mucosa from that of the respiratory region, probably related to is proposed function in sound production.…”

“…This region delivering air towards the lungs is characterized by the absence of nasal conchae or cornettes and paranasal sinuses except for the maxillary sinus [ 28 ] which is absent in the adult. The maxiloturbinate, well developed in terrestrial mammals, is vestigial in odontocetes, though there are differences between mysticetes and odontocetes [ 31 ].…”

“…We have not seen this described by other authors using other techniques, including endoscopy [ 34 , 35 ]. The endoscope allowed us to examine the incisive recess (premaxillary sac according to other authors [ 8 , 12 , 14 ]) that our study suggests serves to store, along with the vestibular sac [ 19 ], water escaping from the nasal vestibule which is expelled when the animal emerges to breathe [ 28 ]. The extent of the recesses could be seen in the nasal casts, as was their growth during fetal development, findings supported by the MRI data, anatomical sections and even dissection.…”

“…Most of those articles are about adult specimens, either odontocetes [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ] or mysticetes [ 21 , 22 , 23 , 24 ] and many of them focused on sound production and biosonar [ 14 , 25 , 26 ]. The few papers on fetal specimens [ 27 , 28 , 29 , 30 , 31 , 32 ] do not cover different stages of development and only show one fetus without comparison with other fetal stages, essential to the understanding of cetacean ontogeny and its application in the fields of biology and veterinary medicine [ 29 ]. The development of the bones and other nasal structures continues post-partum with opening of the blowhole, expansion of the lungs and independent breathing.…”

Advances in Animal Anatomy

“…This Special Issue includes three research articles that address the anatomical study of the whole head of dolphin fetuses and newborn dolphins, and the developmental and comparative study of some species of dolphins focused on different regions of the head (oral, pharyngeal and nasal cavities). Regarding García de los Ríos et al’s [ 10 ] the developmental study on the head of striped dolphins during the fetal and perinatal periods, they used different techniques, from gross dissection and sectional anatomy to diagnostic imaging (CT and MRI), to study the anatomical and physiological inferences affecting the heads of marine and land mammals. The development of the head reveals the evolutionary changes of Cetaceans that helped them adapt to the marine environment.…”