TYPE Original Research PUBLISHED 13 September 2023 DOI 10.3389/fvets.2023.1256947 Microanatomical findings with OPEN ACCESS relevance to trigeminal ganglion enhancement on post-contrast EDITED BY Sílvia Sisó, AbbVie, United States REVIEWED BY Luisa Ragionieri, T1-weighted magnetic resonance University of Parma, Italy Cord Brundage, University of Wisconsin–La Crosse, images in dogs United States Viktor Paluš, Koen M. Santifort 1,2*, Eric N. Glass 3, Marti Pumarola 4 and Neurovet, Slovakia Vicente Aige Gil 5 *CORRESPONDENCE Koen M. Santifort 1 IVC Evidensia Small Animal Referral Hospital Arnhem, Neurology, Arnhem, Netherlands, 2 IVC Evidensia koen.santifort@evidensia.nl; Small Animal Referral Hospital Hart van Brabant, Neurology, Waalwijk, Netherlands, 3 Section of koensantifort@gmail.com Neurology and Neurosurgery, Red Bank Veterinary Hospital, Tinton Falls, NJ, United States, 4 Unit of RECEIVED 11July 2023 Compared and Murine Pathology, Department of Animal Medicine and Surgery, Faculty of Veterinary ACCEPTED 28 August 2023 Medicine, Universitat Autònoma de Barcelona, Barcelona, Spain, 5 Department of Sanitat i Anatomía PUBLISHED 13 September 2023 Animal, Faculty of Veterinary Medicine, Universitat Autònoma de Barcelona, Barcelona, Spain CITATION Santifort KM, Glass EN, Pumarola M and Introduction: Trigeminal ganglion contrast enhancement (TGCE) is reported Aige Gil V (2023) Microanatomical findings with relevance to trigeminal ganglion enhancement to be a normal and a common finding on magnetic resonance imaging studies on post-contrast T1-weighted magnetic of dogs, cats and humans. The intent of the present study was to describe the resonance images in dogs. anatomical characteristics of the trigeminal ganglion, its surrounding structures, Front. Vet. Sci. 10:1256947. doi: 10.3389/fvets.2023.1256947 and histological features that are relevant to explain or hypothesize on the reason for TGCE on T1-weighted post-contrast MRI studies of the brain in dogs. COPYRIGHT © 2023 Santifort, Glass, Pumarola and Aige Gil. Methods: Eight dog cadavers were dissected to study the anatomy of the This is an open-access article distributed under trigeminal ganglion. The presence and anatomy of vessels was studied by the terms of the Creative Commons Attribution License (CC BY). The use, distribution or dissection and by histological techniques. Two trigeminal ganglia were isolated reproduction in other forums is permitted, and stained with hematoxylin–eosin (HE). Two other trigeminal ganglia included provided the original author(s) and the in the trigeminal canal and trigeminal cavity were decalcified with formic acid/ copyright owner(s) are credited and that the original publication in this journal is cited, in formalin for 12 weeks and stained with HE to study the related vessels. Additionally, accordance with accepted academic practice. a corrosion cast was obtained from a separate canine specimen. No use, distribution or reproduction is permitted which does not comply with these Results: Leptomeninges and a subarachnoid space were identified at the level terms. of the trigeminal nerve roots and the trigeminal ganglion. No subarachnoid space was identified and leptomeninges were no longer present at the level of the three trigeminal nerve branches. Small arterial vessels ran to and supplied the trigeminal ganglion, passing through the dura mater. No venous plexus was visualized at the level of the trigeminal ganglion in the dissections. A complex arterial vascular network was identified within the leptomeningeal covering of the trigeminal ganglion and was best appreciated in the corrosion cast. Histological examination revealed small-to moderate-sized blood vessels located in the epineurium around the ganglion; from there a multitude of arterioles penetrated into the perineurium. Small endoneurial branches and capillaries penetrated the ganglion and the trigeminal nerve branches. Discussion: Limitations to this study include the limited number of canine specimens included and the lack of electron microscopy to further support current hypotheses included in our discussion. In conclusion, this study provides further support to the theory that TGCE in dogs may be due an incomplete blood-nerve barrier or blood-ganglion barrier at the interface between the central nervous system and the peripheral nervous system. Frontiers in Veterinary Science 01 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 KEYWORDS trigeminal ganglion, gadolinium, blood-nerve barrier, blood-ganglion barrier, contrast enhancement Introduction hypothesis is that vessels supplying the trigeminal ganglion are permeable, thus accounting for gadolinium leakage and presence in Magnetic resonance imaging (MRI) studies of the canine and feline trigeminal ganglia and resulting in TGCE. This is largely based on head offers clinicians the opportunity to diagnose a plethora of human studies regarding specific sections of multiple (branches of) disorders affecting the brain, cranial nerves and surrounding structures. cranial nerves and MRI observations (9, 10). Such vascular Publications describing normal findings, so-called pseudolesions, and permeability suggests that the blood-nerve barrier (BNB) or blood- anatomical features regarding, for instance, cranial nerves exiting from ganglion barrier (BGB) at that level is incomplete. Therefore, an the cranium provide a valuable resource for comparison to pathological incomplete trigeminal BGB may account for TGCE. The other states (1–5). A number of these studies highlight that the canine and hypothesis is based on the presence of perineural/periganglionic feline trigeminal ganglion, ophthalmic, maxillary and mandibular vascular networks that are proposed to cause the appearance of TGCE nerves and/or trigeminal nerve roots show T1-weighted (T1W) post- on MRI in humans (11). There are no published studies that report contrast (gadolinium) enhancement on MRI studies in non-pathological specific anatomical explanations for TGCE in dogs. Some authors states (1–3, 6, 7). Briefly, the presence of gadolinium in anatomical have put forward a hypothesis, mostly based on studies in rabbits (1–3, structures results in increased signal intensity on T1W images (i.e., 7, 12, 13). Researchers observed fluorescence in trigeminal structures structures become more white on post-contrast images compared to after intravenous fluorescein administration, suggesting vascular pre-contrast images). This finding has been reported in 93–100% of permeability (12, 13). Authors of a veterinary publication discussing normal dogs (2, 6, 7). Thus, trigeminal ganglion contrast enhancement these hypotheses have noted that ‘cadaveric studies in dogs would (TGCE) is regarded as a normal finding in dogs (Figure 1). be useful to determine whether a perineural venous plexus is present, It is valuable for clinicians to be aware of this normal finding, as it paralleling what has been documented in humans’ (1). Although may be mistaken for an abnormality (pseudolesion), and it anatomical descriptions of local anatomy regarding the trigeminal be mistakenly thought of as a pathologic process affecting the ganglion in dogs exist (14), no studies specifically address trigeminal ganglion (1). As for other intracranial structures that show microanatomical features with relevance to explanations for TGCE. contrast enhancement in non-pathological or normal states, there The objective of this study was to describe the anatomical should be an anatomical explanation for this finding of enhancement characteristics of the trigeminal ganglion, its surrounding structures, of trigeminal structures. For example, dynamic MRI studies showed and histological features that are relevant to explain or hypothesize the that normal contrast-enhancement of the canine hypophysis is related cause for TGCE on T1-weighted post-contrast MRI studies of the to its (micro)circulatory anatomy (8). brain in dogs. In the discussion, we include a brief literature review In human literature, two hypotheses are put forward to explain regarding TGCE and hypothesize on the reason for TGCE in dogs normal TGCE. These have been based a few articles in particular. One based on this review and our findings. FIGURE 1 Archival magnetic resonance images (1.5 T-Canon Vantage Elan, The Netherlands) of a dog’s brain at the level of the trigeminal ganglion. (A) T1- weighted fast-spin echo pre-contrast. The yellow rectangle depicts the region shown in panels (B–G). The red asterisk point out the right trigeminal ganglion (left of the image). (B) T1-weighted fast-spin echo pre-contrast. (C) T1-weighted fast-spin echo with fat saturation pre-contrast. (D) T1- weighted fast-spin echo post-contrast. (E) Three-dimensional T1-weighted gradient echo post-contrast. (F) T1-weighted fast-spin echo with fat saturation post-contrast. (G) Subtraction image (T1-weighted post-contrast minus T1-weighted pre-contrast). Frontiers in Veterinary Science 02 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 Materials and methods branches into three nerves: the mandibular, the maxillary, and the ophthalmic nerves. These respective nerves are contained within the Eight adult dog cadavers were dissected to study the anatomy of cranial cavity before leaving through their respective foramina/ the trigeminal ganglion. The cadavers used came from the dissection fissures: the oval foramen, round foramen and orbital fissure, room of the Anatomy Unit of the Veterinary Faculty of the Universitat respectively. Autónoma de Barcelona. The dogs were euthanized for medical reasons unrelated to the central nervous system (CNS) and donated by owners following the approved donation program of the University Meninges and used for anatomical dissections. All the specimens were dissected by one author (VAG). Leptomeninges and a subarachnoid space (trigeminal cistern–a Dog breeds included: term derived from human literature but not included in the Nomina Anatomica Veterinaria) were identified at the level of the trigeminal - 3 beagles (females) nerve roots and the trigeminal ganglion (Figure 2). Specifically, - 1 German shepherd (male) between the medulla oblongata and the caudal opening of the - 1 golden retriever (male) trigeminal canal, the trigeminal nerve roots are included in the - 1 Border collie (female) subarachnoid space that contains arteries derived from the basilar - 2 mixed breeds (female) artery as well as veins. At the level of the three trigeminal nerve branches (i.e., the mandibular, the maxillary, and the ophthalmic Cadavers were fixed with a 10% formaldehyde buffered solution nerves), no subarachnoid space was identified and leptomeninges injected via the common carotid artery. The cadavers were then were no longer present. preserved for a few weeks (variable) at 4–6°C. The heads of the eight dogs were isolated by atlantooccipital decapitation. Dorsal craniotomies were performed on 5 heads (beagle, German Vasculature shepherd, Border collie, 2 mixed breeds). The brains were removed by gross dissection. Small arterial vessels, coming from the basilar artery, ran to and The other heads (2 beagles, 1 golden retriever) were supplied the trigeminal ganglion, passing through the dura mater transversally sectioned. (Figure 3). Furthermore, blood vessels that run with the trigeminal For each, dissection of the trigeminal ganglion and related nerve and trigeminal nerve roots were identified. Venous plexuses structures was performed from that point forward with the use of were found at the level of the round and oval foramen near the stereomicroscopy and microsurgical equipment. maxillary and mandibular branches of the trigeminal nerve, The presence and anatomy of vessels was studied by dissection respectively. No venous plexus was visualized at the level of the and by histological techniques. Two trigeminal ganglia were isolated trigeminal ganglion in the dissections. A complex arterial vascular and stained with hematoxylin–eosin (HE). Two other trigeminal network was identified within the leptomeningeal covering of the ganglia included in the trigeminal canal and trigeminal cavity were trigeminal ganglion and was best appreciated in the corrosion cast decalcified with formic acid/formalin for 12 weeks and stained with (Figure 3). HE to study the related vessels. Additionally, a corrosion cast was Figure 4 depicts, schematically, the anatomical arrangement of the obtained from a separate, additional (ninth) canine specimen (beagle, bony, meningeal and trigeminal structures. male) injecting araldite mixed with red die (Monomer CY223, Histological examination revealed small-to moderate-sized blood hardener HY2967–Huntsman Advanced Materials, USA) through the vessels located in the epineurium around the ganglion (Figure 5). carotid artery. Briefly, the injected anatomical sample has been From there a multitude of arterioles emerged penetrating into the dissected and corroded in a basic solution of pancreatin at 37°C that perineurium; the venous components were organized in large destroys the muscles and ligaments while preserving the vascular mold. endothelium-lined cavities associated with the vascular plexuses adjacent to the trigeminal ganglion. Finally, small branches and capillaries penetrated the ganglion and the trigeminal nerve branches. Results These were distributed in the endoneurium (Figure 5). Trigeminal nerve roots, trigeminal ganglion and trigeminal nerves Discussion From the ventrolateral aspect of the brain stem, trigeminal nerve In this canine cadaveric and histological study, we describe the roots course rostrally through the trigeminal canal formed by the presence of complex vascular networks around the trigeminal petrosal crest (Figure 2). At is rostral extent, an osseous trigeminal ganglion in dog specimens. When contrast medium (commonly cave (cavum trigeminale or ‘Merckel cave’–a term derived from gadolinium-based) is administered to canine patients undergoing human literature but not included in the Nomina Anatomica MRI studies of the brain, the presence of vascular structures must Veterinaria) is formed by a subtle depression of bone between the apex be taken into account when assessing TGCE. In theory, enhancement of the petrosal crest and the skull base, internally lined by a double of vascular structures might be mistaken for TGCE, especially when layer of dura mater and the leptomeninges. The trigeminal ganglion is contrast- and spatial resolution are suboptimal. Indeed, a human situated therein. From the ganglion rostrally, the trigeminal nerve cadaveric study described the presence of a perineural/periganglionic Frontiers in Veterinary Science 03 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 FIGURE 2 Photographs taken during microanatomical dissections. (A) Dorsal view of the right trigeminal ganglion surrounded by the leptomeninges inside the trigeminal cave. 1: trigeminal ganglion, 2: mandibular nerve, 3: maxillary nerve, 4: ophthalmic nerve, 5: crista petrosa. (B) Ventral view of the right trigeminal ganglion partially surrounded by the leptomeninges. 1: trigeminal nerve roots, 2: trigeminal ganglion, 3: mandibular nerve, 4: maxillary nerve, 5: ophthalmic nerve, *: optic chiasm. Rostral is left in the image. (C) Close-up of panel (A), after removal of some meninges highlighting the presence of blood vessels (red arrows) that can be appreciated macroscopically in formalin-fixed dissection material. 1: trigeminal ganglion, 2: mandibular nerve, 3: maxillary nerve, 4: ophthalmic nerve, 5: trigeminal canal opened dorsally (petrosal crest removed). vascular plexus and postulated that contrast enhancement is the into account when discussing TGCE. One review by Yousry et al. reason for apparent TGCE (11). The authors reported ‘true’ TGCE in particularly addresses the differences between these studies and their only 4% of human trigeminal ganglia in that study. This study by hypotheses (15). Yousry et al. argued in favor of true TGCE, Williams et al. is referenced frequently in the human literature when particularly evident using contrast enhanced three-dimensional (3D) discussing TGCE. The results thereof contrast the study by Downs constructive interference in steady state (CISS) sequences. et al. who reported clear TGCE in 88% of cases and regional Our findings of a rich and complex vascular supply to the enhancement blending with the dura mater at this site in the trigeminal ganglion contrast the findings in the human cadaveric remaining 12% (10). These authors postulated that TGCE is likely due study reported by Williams et al. (11). In that study, the trigeminal to the presence of an incomplete BNB (or BGB) characterized by ganglion and proximal divisions of the trigeminal nerve were reported fenestrated capillaries at the level of the trigeminal ganglion as in to be devoid of ‘obvious vascularity’. In the authors opinion, such spinal dorsal root ganglia (DRG) (10). Our microanatomical and findings are unlikely to reflect the anatomy of the dog and have indeed histological findings of a complex periganglionic arterial vascular been proven to be inaccurate by other human (16) as well as canine network covering and penetrating the trigeminal ganglion support the (17) studies. Therefore, visual absence of vessels in the study by latter hypothesis. Williams et al. (11) must have been due to limitations of the study Publications from human studies have, without additional methodology. The trigeminal ganglion is populated by neuronal cell evidence, taken the results of either or both of these studies (10, 11) bodies. Neurons are among the most energy-consuming cell Frontiers in Veterinary Science 04 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 FIGURE 3 Photographs of a corrosion cast where arterial vessels are preserved by polymeric resin araldite and red dye. (A) dorsal view at the level of the junction between the middle and caudal cranial fossa. 1: caudal opening of the trigeminal canal, 2: rostral opening of the trigeminal canal, 3: crista petrosa, 4: caudal communicating artery of the circulus arteriosus cerebri, 5: caudal cerebral artery, 6: rostral cerebellar artery, 7: internal carotid artery. (B) Close- up at the level of the yellow arrow in panel (A). It shows the vascular network located at the site of the trigeminal ganglion. Rostral is right in the image. populations in the mammalian body and vascularization is essential finding include images supporting true TGCE rather than mistakenly for provision of substances and clearing of waste products in such a interpreting the presence of TGCE due to enhancement of metabolically active structure (16). Supplementary Figure 1 illustrates periganglionic vascular structures (1, 2, 6, 7). the general anatomy of the vasa nervorum and their derivatives. Our The clinical importance of TGCE as normal feature on MRI own observations in this canine study of a rich and complex vascular studies deserves to be stressed here. Diagnosis of pathological states blood supply, support that this general anatomy is applicable to the of the trigeminal ganglion should not solely be based on contrast- trigeminal nerve roots, trigeminal nerves, and trigeminal ganglion enhancement, though it may well be that contrast-enhancement of, as well. for example, trigeminal ganglionitis or neoplasia affecting the The presence of tight junctions and non-fenestrated endothelium trigeminal ganglion is different from normal TGCE. However, there characterizes the BNB (18–21). But this BNB is not necessarily identical are no specific reports quantifying the extent of TGCE in pathological at every location (22). The term ‘BNB’ is often applied to the ganglion versus normal states in dogs. Reports documenting MRI-based of the nerve in question as well. However, it might be more suitable to diagnoses of trigeminal neuritis focus on contrast-enhancement as speak of a BGB, as there is abundant evidence that this barrier can well as increased size (33). Reports documenting histopathologically be different from the BNB or can at least function differently (23–25). confirmed cases of trigeminal ganglionitis with information on MRI Other reasons for the BGB being incomplete can be postulated and appearance are rare. One report documented diffuse enlargement and include its embryological origin [i.e., the trigeminal ganglion is derived contrast-enhancement of the trigeminal nerve (34). Some quantitative from neural crest cells and the trigeminal placode of the neural tube (26, MRI data regarding size of the canine trigeminal nerve are published 27)] and its neuroendocrine function [e.g., neuropeptides are released (35, 36). Future studies comparing healthy versus affected trigeminal from and enter the trigeminal ganglion (28, 29)]. nerves and ganglia may provide more information on the effects of Whatever the reasons may be, the fact that the BGB is incomplete certain pathological states on size and contrast-enhancement thereof. is evidenced and supported by the finding that the trigeminal ganglion Our findings of leptomeninges and a subarachnoid space is not the only ganglion reported to show contrast enhancement on (trigeminal cistern – a term derived from human literature (37) but MRI in humans. Indeed, other ganglia reported to show contrast not included in the Nomina Anatomica Veterinaria) around the enhancement on MRI in humans include lumbar spinal DRG (30), trigeminal nerve roots and trigeminal ganglion mirror those reported geniculate ganglion of the facial nerve (31), and superior cervical in humans (38, 39). It is reported that the point of transition from ganglion (32). The authors are not aware of specific reports on the meningeal to epi-perineurial covering is variable in people (39). contrast-enhancing features of such structures in dogs. Figure 6 Future studies including more specimens would be needed to verify summarizes our hypothesis regarding the incomplete BGB of the individual variability of this transition in dogs. trigeminal ganglion with relevance to TGCE. The relative lack of The trigeminal cave [(cavum trigeminale or ‘Merckel cave’ – a reports specifically describing non-pathological contrast enhancement term derived from human literature (38–41) but not included in of other (cranial nerve) ganglia in dogs is likely related to the small the Nomina Anatomica Veterinaria] in dogs was subtle in the size of these ganglia and difficulty in visualizing them in MRI studies specimens examined in this study. Others have not named this in general, as well as in assessing contrast enhancement. In other anatomical feature ‘trigeminal cave’ in dogs, as the anatomy differs words, the trigeminal ganglion is the largest and, therefore, TGCE was from that in humans (14). Use of this term may have some merit to the most likely to have been noticed and reported by clinicians. Future facilitate discussion of pathology affecting this location. However, studies looking into the presence or absence of non-pathological we do recognize that the term does not refer to the exact same contrast enhancement of cranial nerve ganglia as well as DRG in dogs anatomical features across species, as there are notable differences are warranted to elucidate this matter. In further support to the as has been previously described (14). Human studies have ‘incomplete BGB-theory’, studies reporting TGCE in dogs as a normal reported asymmetry of the Merckel Cave within individuals and Frontiers in Veterinary Science 05 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 FIGURE 4 Schematic drawings in sagittal plane of the anatomy at the level of the trigeminal canal, trigeminal cave, and ophthalmic, maxillary, and mandibular nerves rostrally. *The ganglion and roots of the trigeminal nerve are covered by arachnoid and dura mater. The mandibular, maxillary, and ophthalmic nerves are not. These are covered by epineurium and perineurium, and contain endoneurium. Rostral is left in the images. (A) Only bone and dura mater are depicted. (B) Leptomeninges, roots of the trigeminal nerve, and the trigeminal ganglion are drawn in, in addition to panel (A). (C) The mandibular, maxillary, and ophthalmic nerves are drawn in, in addition to panel (B). The relationship with regard to the meningeal coverings are depicted. variable morphology between individuals (39). This is likely to anatomical site in humans, affecting structures contained within be true for dogs as well, especially when considering different and the associated with the bony lining itself (41). Future studies breeds. Various pathologic processes have been described at this would be needed to document on the variability in these Frontiers in Veterinary Science 06 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 FIGURE 5 Microscopy photographs taken at the edge of the trigeminal ganglion histology (hematoxylin–eosin stain after decalcification with formic acid/ formalin for 12 weeks). (A,B) [(B):close-up of right top corner of panel (A)]: Panoramic view of the ganglion and bone with the vascular plexus on the right of the image. 1: temporal bone, 2: trigeminal ganglion with neuronal bodies (arrow head), 3: artery, 4: vein, 5: periosteal layer of the dura mater with capillaries (arrows). (C,D) [(D):close-up of right half of panel (C)]: Capillaries in epi-, peri-, and endoneurium (black and white arrows) and capillaries surrounding a group of ganglionic neuronal bodies (arrow head) and collections of myelinated nerve fibers. 1: epineurium and perineurium with arterioles (arrows), 2: myelinated nerve fibers of a trigeminal nerve with capillaries in endoneurium (white arrows), 3: edge of the trigeminal ganglion. (E) Arterioles and capillaries surrounding an isolated ganglionic neuronal cell body (arrow head) and collections of myelinated nerve fibers. 1: epineurium and perineurium with capillaries (arrows), 2: myelinated nerve fibers of a trigeminal nerve, 3: edge of the trigeminal ganglion. FIGURE 6 Photograph of the ventral aspect of a formalin-fixed encephalon of a dog, summarizing the hypothesis supported by findings in this study. dimensions among individuals of the same breed or between presence or absence of, e.g., fenestration, tight junctions or other different breeds of dogs. junctional complexes in capillaries in the trigeminal ganglion will Limitations to this study include the small number of canine be of vital importance to support the ‘incomplete BGB-theory’ as an specimens included and, in particular, the lack of electron microscopy explanation for TGCE in dogs. Immunohistochemistry studies on a to further support current hypotheses included in our discussion. well-fixed non-decalcified tissue could also provide evidence about Future studies including electron microscopy to evaluate for the the presence or absence of tight-junctions and BGB organization. Frontiers in Veterinary Science 07 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 However, various possible markers for such studies are not validated Funding for canine tissues. Finally, future MRI studies to evaluate for the presence or absence of TGCE and compare it to enhancement of the The author(s) declare financial support was received for the complex vascular network surrounding it would be useful. In research, authorship, and/or publication of this article. The publication particular, specific sequences might be incorporated in those studies, fee was covered by IVC Evidensia’s fund for publication of peer- such as a contrast enhanced 3D CISS sequence (15). Together with the reviewed scientific articles. results of our current study, such studies could provide veterinary neurologists and radiologists with valuable information to compare with imaging results of patients. Acknowledgments In conclusion, this study provides further support to the theory that TGCE in dogs may be due an incomplete BNB or BGB at the The authors would like to acknowledge Ester Blasco and Tamara interface between the CNS with an intact BBB and leptomeninges and Rivero for the technical assistance on histological studies. the PNS with a BNB and endo-, peri-, and epineurium rich in vessels. Conflict of interest Data availability statement The authors declare that the research was conducted in the The original contributions presented in the study are included in absence of any commercial or financial relationships that could the article/Supplementary material, further inquiries can be directed be construed as a potential conflict of interest. to the corresponding author. Publisher’s note Ethics statement All claims expressed in this article are solely those of the authors Ethical approval was not required for the studies involving animals and do not necessarily represent those of their affiliated in accordance with the local legislation and institutional requirements organizations, or those of the publisher, the editors and the because the dogs were euthanized for medical reasons unrelated to the reviewers. Any product that may be evaluated in this article, or central nervous system (CNS). Written informed consent was not claim that may be made by its manufacturer, is not guaranteed or obtained from the owners for the participation of their animals in this endorsed by the publisher. study because they were donated by owners following the approved donation program of the University and used for anatomical dissections. Supplementary material Author contributions The Supplementary material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fvets.2023.1256947/ KS: Conceptualization, Formal analysis, Writing – original draft, full#supplementary-material Writing – review & editing. EG: Conceptualization, Formal analysis, SUPPLEMENTARY FIGURE 1 Writing – review & editing. MP: Conceptualization, Formal analysis, Schematic drawing of a nerve with associated blood vessels. Investigation, Methodology, Writing – review & editing. VA: SUPPLEMENTARY VIDEOS 1-2 Visualization, Writing – review & editing, Conceptualization, Formal VAG provides two videos showing parts of the dissection process and analysis, Investigation, Methodology. microanatomy discussion. References 1. Cooper JJ, Young BD, Hoffman A, Bratton G, Hicks DG, Tidwell A, et al. Intracranial 6. Joslyn S, Sullivan M, Novellas R, Brennan N, Cameron G, Hammond G. Effect of magnetic resonance imaging artifacts and pseudolesions in dogs and cats. Vet Radiol delayed acquisition times on gadolinium-enhanced magnetic resonance imaging of the Ultrasound. (2010) 51:587–95. doi: 10.1111/j.1740-8261.2010.01713.x presumably normal canine brain. Vet Radiol Ultrasound. (2011) 52:611–8. doi: 10.1111/j. 1740-8261.2011.01847.x 2. Couturier L, Degueurce C, Ruel Y, Dennis R, Begon D. Anatomical study of cranial nerve emergence and skull foramina in the dog using magnetic resonance imaging and computed 7. Pettigrew R, Rylander H, Schwarz T. Magnetic resonance imaging contrast tomography. Vet Radiol Ultrasound. (2005) 46:375–83. doi: 10.1111/j.1740-8261.2005.00068.x enhancement of the trigeminal nerve in dogs without evidence of trigeminal neuropathy. Vet Radiol Ultrasound. (2009) 50:276–8. doi: 10.1111/j.1740-8261.2009.01533.x 3. Gomes E, Degueurce C, Ruel Y, Dennis R, Begon D. Anatomic study of cranial nerve emergence and associated skull foramina in cats using CT and MRI. Vet Radiol 8. Graham JP, Roberts GD, Newell SM. Dynamic magnetic resonance imaging of the Ultrasound. (2009) 50:398–403. doi: 10.1111/j.1740-8261.2009.01556.x normal canine pituitary gland. Vet Radiol Ultrasound. (2000) 41:35–40. doi: 10.1111/j.1740-8261.2000.tb00423.x 4. Leigh EJ, Mackillop E, Robertson ID, Hudson LC. Clinical anatomy of the canine brain using magnetic resonance imaging. Vet Radiol Ultrasound. (2008) 49:113–21. doi: 9. Tabuchi T, Nakao Y, Sakihama N, Kobayashi T. Vascular permeability to fluorescent 10.1111/j.1740-8261.2008.00336.x substance in human cranial nerves. Ann Otol Rhinol Laryngol. (2002) 111:736–7. doi: 10.1177/000348940211100814 5. Mogicato G, Conchou F, Layssol-Lamour C, Raharison F, Sautet J. Normal feline brain: clinical anatomy using magnetic resonance imaging. Anat Histol Embryol. (2012) 10. Downs DM, Damiano TR, Rubinstein D. Gasserian ganglion: appearance on 41:87–95. doi: 10.1111/j.1439-0264.2011.01106.x contrast-enhanced MR. AJNR Am J Neuroradiol. (1996) 17:237–41. Frontiers in Veterinary Science 08 frontiersin.org Santifort et al. 10.3389/fvets.2023.1256947 11. Williams LS, Schmalfuss IM, Sistrom CL, Inoue T, Tanaka R, Seoane ER, et al. MR 27. Singh S, Groves AK. The molecular basis of craniofacial placode development. imaging of the trigeminal ganglion, nerve, and the perineural vascular plexus: normal Wiley Interdiscip Rev Dev Biol. (2016) 5:363–76. doi: 10.1002/wdev.226 appearance and variants with correlation to cadaver specimens. Am J Neuroradiol. 28. Messlinger K, Balcziak LK, Russo AF. Cross-talk signaling in the trigeminal (2003) 24:1317–23. ganglion: role of neuropeptides and other mediators. J Neural Transm. (2020) 12. Sakihama N. Vascular permeability of fluorescent substance in cranial nerve roots. 127:431–44. doi: 10.1007/s00702-020-02161-7 Nippon Jibi Inkoka Gakkai Kaiho. (1994) 97:684–7. doi: 10.3950/jibiinkoka.97.684 29. Messlinger K, Russo AF. Current understanding of trigeminal ganglion structure and 13. Nakao Y, Sakihama N, Matshumoto K, Ochi M, Kumagami H. Vascular function in headache. Cephalalgia. (2019) 39:1661–74. doi: 10.1177/0333102418786261 permeability to sodium fluorescein in the rabbit cranial nerve root: possible correlation 30. Demondion X, Leroy X, Lapègue F, Drizenko A, Francke JP, Cotten A. Lumbar with normal cranial nerve enhancement on gadolinium enhanced magnetic resonance spinal ganglia enhancement after gadolinium chelate administration: a radio- imaging. Eur Arch Otorhinolaryngol. (1994) 251:457–60. doi: 10.1007/BF00175995 histological correlation. Surg Radiol Anat. (2001) 23:415–9. doi: 10.1007/ 14. Howroyd PC. Dissection of the trigeminal ganglion of nonrodent species used in s00276-001-0415-1 toxicology studies. Toxicol Pathol. (2020) 48:30–6. doi: 10.1177/0192623319854338 31. Hong HS, Yi BH, Cha JG, Park SJ, Kim DH, Lee HK, et al. Enhancement pattern 15. Yousry I, Moriggl B, Schmid UD, Naidich TP, Yousry TA. Trigeminal ganglion and of the normal facial nerve at 3.0 T temporal MRI. Br J Radiol. (2010) 83:118–21. doi: its divisions: detailed anatomic MR imaging with contrast-enhanced 3D constructive 10.1259/bjr/70067143 interference in the steady state sequences. AJNR Am J Neuroradiol. (2005) 26:1128–35. 32. Lee JY, Lee JH, Song JS, Song MJ, Hwang SJ, Yoon RG, et al. Superior cervical 16. Ćetković M, Štimec BV, Mucić D, Dožić A, Ćetković D, Reçi V, et al. Arterial sympathetic ganglion: normal imaging appearance on 3T-MRI. Korean J Radiol. (2016) supply of the trigeminal ganglion, a micromorphological study. Folia Morphol. (2020) 17:657–63. doi: 10.3348/kjr.2016.17.5.657 79:58–64. doi: 10.5603/FM.a2019.0062 33. Musso C, Le Boedec K, Gomes E, Cauzinille L. Diagnostic values of clinical and 17. Okuda H. Blood supply of the trigeminal ganglion of the dog. Okajimas Folia Anat magnetic resonance findings in presumptive trigeminal neuropathy: 49 dogs. J Am Anim Jpn. (1979) 56:205–29. doi: 10.2535/ofaj1936.56.4_205 Hosp Assoc. (2020) 56:106–13. doi: 10.5326/JAAHA-MS-6997 18. Reinhold AK, Rittner HL. Barrier function in the peripheral and central nervous 34. Schultz RM, Tucker RL, Gavin PR, Bagley R, Saveraid TC, Berry CR. Magnetic system-a review. Pflugers Arch. (2017) 469:123–34. doi: 10.1007/s00424-016-1920-8 resonance imaging of acquired trigeminal nerve disorders in six dogs. Vet Radiol Ultrasound. (2007) 48:101–4. doi: 10.1111/j.1740-8261.2007.00212.x 19. Malong L, Napoli I, Casal G, White IJ, Stierli S, Vaughan A, et al. Characterization of the structure and control of the blood-nerve barrier identifies avenues for therapeutic 35. Swain CE, Cherubini GB, Mantis P. Low field MRI measurements of the Normal delivery. Dev Cell. (2023) 58:174–191.e8. doi: 10.1016/j.devcel.2023.01.002 canine trigeminal nerve. Front Vet Sci. (2020) 7:274. doi: 10.3389/fvets.2020.00274 20. Ubogu EE. Biology of the human blood-nerve barrier in health and disease. Exp 36. Kaufman B, Bellon EM. The trigeminal nerve cistern. Radiology. (1973) Neurol. (2020) 328:113272. doi: 10.1016/j.expneurol.2020.113272 108:597–602. doi: 10.1148/108.3.597 21. Weerasuriya A, Mizisin AP. The blood-nerve barrier: structure and functional 37. Blitz AM, Choudhri AF, Chonka ZD, Ilica AT, Macedo LL, Chhabra A, et al. significance. Methods Mol Biol. (2011) 686:149–73. doi: 10.1007/978-1-60761-938-3_6 Anatomic considerations, nomenclature, and advanced cross-sectional imaging techniques for visualization of the cranial nerve segments by MR imaging. Neuroimaging 22. Olsson Y. Topographical differences in the vascular permeability of the peripheral Clin N Am. (2014) 24:1–15. doi: 10.1016/j.nic.2013.03.020 nervous system. Acta Neuropathol. (1968) 10:26–33. doi: 10.1007/BF00690507 38. Janjua RM, Al-Mefty O, Densler DW, et al. Dural relationships of Meckel cave and 23. Allt G. Blood-ganglion barriers. Trends Neurosci. (1979) 2:248–9. doi: lateral wall of the cavernous sinus. Neurosurg Focus. (2008) 25:1–12. doi: 10.3171/ 10.1016/0166-2236(79)90097-3 FOC.2008.25.12.E2 24. Chau YP, Lu KS. Investigation of the blood-ganglion barrier properties in rat 39. Tsutsumi S, Ono H, Ishii H. Meckel cave: an anatomical study using magnetic sympathetic ganglia by using lanthanum ion and horseradish peroxidase as tracers. Acta resonance imaging. J Comput Assist Tomogr. (2021) 45:743–8. doi: 10.1097/ Anat. (1995) 153:135–44. doi: 10.1159/000313647 RCT.0000000000001185 25. Reinhold AK, Rittner HL. Characteristics of the nerve barrier and the blood dorsal 40. Wysiadecki G, Tubbs RS, Iwanaga J, Radek M, Walocha J, Brzeziński P, et al. root ganglion barrier in health and disease. Exp Neurol. (2020) 327:113244. doi: Microanatomical study of arachnoid granulations and meningeal architecture around 10.1016/j.expneurol.2020.113244 Meckel's cave. Neurosurg Rev. (2023) 46:50. doi: 10.1007/s10143-023-01954-0 26. Begbie J. Induction and patterning of neural crest and ectodermal placodes and 41. Malhotra A, Tu L, Kalra VB, Wu X, Mian A, Mangla R, et al. Neuroimaging of their derivatives In: R JLR and P Rakic, editors. Patterning and cell type specification in Meckel's cave in normal and disease conditions. Insights Imaging. (2018) 9:499–510. doi: the developing CNS and PNS. Cambridge, United States: Academic Press (2013). 10.1007/s13244-018-0604-7 Frontiers in Veterinary Science 09 frontiersin.org