Cranial Pair 0: The Nervus Terminalis ANGEL PE ~ NA-MELIAN , 1 * JUAN PABLO CABELLO-DE LA ROSA, 2 MARIA JOSE GALLARDO-ALCA ~ NIZ, 2 JULIA VAAMONDE-GAMO, 2 FERNANDA RELEA-CALATAYUD, 3 LUCIA GONZALEZ-LOPEZ, 3 PATRICIA VILLANUEVA-ANGUITA, 4 ALICIA FLORES-CUADRADO, 4 DANIEL SAIZ-SANCHEZ, 4 AND ALINO MARTINEZ-MARCOS 4 1 Departamento de Anatom ıa y Embriolog  ıa Humana, Facultad de Medicina, Universidad Complutense de Madrid, Madrid 28040, Spain 2 Servicio de Neurolog  ıa, Hospital General Universitario de Ciudad Real, Ciudad Real 13005, Spain 3 Servicio de Anatom ıa Patol ogica, Hospital General Universitario de Ciudad Real, Ciudad Real 13005, Spain 4 Laboratorio de Neuroplasticidad y Neurodegeneraci on, Facultad de Medicina de Ciudad Real, Centro Regional de Investigaciones Biom edicas, Universidad de Castilla-La Mancha, Ciudad Real 13071, Spain ABSTRACT Originally discovered in elasmobranchs by Fritsh in 1878, the nervus terminalis has been found in virtually all species, including humans. After more than one-century debate on its nomenclature, it is nowadays recognized as cranial pair zero. The nerve mostly originates in the olfac- tory placode, although neural crest contribution has been also proposed. Developmentally, the nervus terminalis is clearly observed in human embryos; subsequently, during the fetal period loses some of its ganglion cells, and it is less recognizable in adults. Fibers originating in the nasal cavity passes into the cranium through the middle area of the cribiform plate of the ethmoid bone. Intracranially, fibers joint the telencephalon at several sites including the olfactory trigone and the primordium of the hippocampus to reach preoptic and precommissural regions. The nervus terminalis shows ganglion cells, that sometimes form clusters, normally one or two located at the base of the crista galli, the so-called ganglion of the nervus terminalis. Its function is uncertain. It has been described that its fibers facilitates migration of luteinizing hormone-releasing hor- mone cells to the hypothalamus thus participating in the development of the hypothalamic–gonadal axis, which alteration may provoke Kallmann’s syndrome in humans. This review summarizes current knowledge on this structure, incorporating original illustrations of the nerve at different developmental stages, and focuses on its anatomical and clinical rele- vance. Anat Rec, 00:000–000, 2018. V C 2018 Wiley Periodicals, Inc. Abbreviations: CG 5 crista galli; CP 5 cribiform plate; GTN 5 ganglion of the nervus terminalis; H 5 heart; Hi 5 hippocampus; LF 5 longitudinal fissure; LHRH 5 luteinizing hormone- releasing hormone; LOS 5 lateral olfactory stria; MN 5 mandible; MOS 5 medial olfactory stria; MRM 5 medial rectus muscle; MX 5 maxilar; NC 5 nasal cavity; NS 5 nasal septum; OB 5 olfactory bulb; OE 5 olfactory epithelium; ON 5 olfactory nerve; OT 5 olfactory tract; OTR 5 olfactory trigone; OV 5 olfactory ven- tricle; SG 5 straight gyrus; SOM 5 superior oblique muscle; T 5 tongue; TN 5 nervus terminalis; TV 5 telencephalic vesicle; VN 5 vomeronasal nerve; VNC 5 vomeronasal cartilage; VNO 5 vomeronasal organ. Grant sponsor: Spanish Ministry of Economy and Competitiveness-FEDER; Grant number: SAF2016-75768-R Relevant conflicts of interests/financial disclosures: The authors declare no conflicts of interest. *Correspondence to:  Angel Pe~ na-Meli an, Departamento de Anatom ıa y Embriolog  ıa Humana I, Facultad de Medicina, Uni- versidad Complutense de Madrid, Spain. Tel.: 91 394 1212; Fax: 913941373 E-mail: apmelian@med.ucm.es Received 28 July 2017; Revised 15 November 2017; Accepted 13 December 2017. DOI 10.1002/ar.23826 Published online 00 Month 2018 in Wiley Online Library (wileyonlinelibrary.com). THE ANATOMICAL RECORD 00:00–00 (2018) V V C 2018 WILEY PERIODICALS, INC.