Development of the facial nerve and its specific features

Abstract

Background. The data reported in the specialty literature on the embryogenesis of the facial nerve mainly refer to the initial stages of its development. Nevertheless, the intrauterine development of the facial nerve is a complex and insightful process, characterized by a range of peculiarities. The goal of our study was to highlight the specific developmental features of the facial nerve for a better understanding of its morphology in adults. Material and methods. Fifty-two groups of serially sectioned human embryos and fetuses from the embryo-fetal collection of the Department of Normal Anatomy at the Belarusian State Medical University in Minsk were investigated. The embryonic samples were classified according to Carnegie stages (day of gestation and crown-rump length). For protocols description, an OLYMPUS CX31 binocular microscope (eyepiece 10x, objectives 4x; 10x; 40x; 100x) and a Nikon DS-Fi1 camera were used. Results. The primary and secondary divisions of the facial nerve trunk, the chorda tympani, the greater petrosal nerve, and the intracerebral and peripheral connections of the facial nerve were distinguished at Carnegie stage 15. The first appearance of intraplexal connections within the parotid plexus was identified at Carnegie stage 16. At stage 20, thin connections between the geniculate ganglion and the vestibular ganglion were revealed. Macro- and microstructural changes in the geniculate ganglion were noted throughout its development. The plexiform nature of the peripheral divisions of the facial nerve and the formation of the pes anserinus minor were observed at stage 21. At Carnegie stage 23, the motor nucleus was well developed, and the internal knee of the facial nerve was identified. Conclusions. Early specific features of facial nerve development include: 1) intracerebral and peripheral connections with neighboring cranial nerves; 2) intraplexal connections between branches of the parotid plexus; 3) distinguishable intracerebral pathways; 4) connections between the geniculate and vestibular ganglia. Late developmental peculiarities include: 1) progressive macro- and microstructural changes in the geniculate ganglion; 2) appearance of the pes anserinus minor; 3) formation of the motor nucleus, internal knee, and intracerebral pathways. These specific developmental features of the facial nerve provide clear evidence of its complex morphology in adults.