Morphometric properties of the facial nerve in fetal temporal bones
Introduction
The facial nerve, including a) motor fibers, which innervate the muscles developed from second brachial arch, b) sensory fibers, which transmit the special sense of taste, and c) parasympathetic fibers, which supply the sublingual, submandibular, nasomucosal, and lacrimal glands, is divided into six separate sections: cisternal, meatal (MeS), labyrinthine (LS), tympanic (TS), mastoid (MaS), and extracranial or intraparotid segments, throughout its course [1]. Within the temporal bone, four of all segments are located: a) MeS from internal acoustic porous to the fundus of the internal auditory canal (IAC), b) LS from the fundus of IAC to the geniculate ganglion, c) TS from the geniculate ganglion to the pyramidal eminence, and lastly d) MaS from the pyramidal eminence to the stylomastoid foramen [1,2]. The complex course, variations and abnormalities of those segments and their relationship with different structures such as greater superficial petrosal nerve (GSPN), semicircular canals, cochlea, oval window, round window, or ear ossicles have clinical and surgical importance for otosurgeons and neurosurgeons to prevent iatrogenic injury during the applications such as decompression, grafting or rerouting of the facial nerve, cochlear implantation or tumour resections [[2], [3], [4], [5], [6], [7], [8], [9]]. In addition, the facial nerve course and its relation with adjacent structures are important to understand the developments of IAC, fallopian canal, and temporal bone in terms of anatomists and embryologists [2,[10], [11], [12], [13], [14]]. This feature makes the course, length and angulation of the facial nerve within the fetal temporal bone an important area of interest [3].
The fetal development of the facial nerve from the internal acoustic porous to the stylomastoid foramen has been examined by many authors using different techniques such as histological, radiological and dissection methods [[10], [11], [12], [13], [14]]. It is generally accepted that, before fourth month (approximately 16 weeks) of the fetal life, the facial nerve is observed to lie in a narrow sulcus within the primordial otic capsule and Reichert's cartilage [2,10,14]. Between 16 and 21 weeks of gestational ages, multiple endochondral and intramembranous ossifications appear in the cartilaginous otic capsule [10]. In this period, an osseous superior clasp replaces the primitive fibrous covering in the proximal part of the facial groove, but the fibrous tissue persists in the distal part [10]. At 24 weeks, the ossification and fusion of the superior and inferior clasp result in neural enclosure of the facial nerve proximally [10]. Between 25 and 28 weeks of gestation, the enclosure continuous distally [10]. At 40 weeks, approximately 83% of the facial canal is formed and this process continues after birth [10,13,14]. The deviations that may occur during this developmental period give rise to the anomalous facial nerve tracts [3]. In this regard, further information on the fetal development including the course, length and angulation of the facial nerve may be useful for pediatric surgeons, otosurgeons and neurosurgeons.
Despite there are plenty of studies on the fetal facial nerve development in the current literature, existing inventory based on its morphometric assessment appear to be limited [13]. From this point of view, the main objective of this study is to display the quantitative analysis of the growth dynamics of the facial nerve in fetal temporal bones.
Section snippets
Materials and methods
The study was conducted on 16 fetuses (7 females, 9 male) between 20 and 34 weeks of gestation (at mean, 26.75 ± 4.36 weeks) from the collection of the Anatomy Department of Medicine Faculty,Mersin University. The ethical approval was taken from the Clinical Research Ethics Committee of Mersin University (2018/151). Due to limited knowledge about the cause of death of fetuses donated to our department between 2000 and 2005, fetuses with structural defects were excluded from the study.
Results
The fetal demographic values such as sex, age, foot length, and side numbers were given in Table 1. The measurements made by the independent researchers (ICC = 0.986–0.998, p < 0.001) and the same researcher (O.B. or T.K., p > 0.05) proved that reliability of the data was quite excellent. The means ± standard deviations of the segment lengths and angles considering the gestational weeks were presented in Table 2. The scatter plot of parameters (segment lengths) versus age, regression line, and
Discussion
The study's data contributes to the limited knowledge in the current literature on algebraic anatomy of fetal facial nerve. The growth patterns of the segment lengths and angles of the facial nerve within the temporal bones were revealed in the transition from 20th to 34th gestational weeks.
The facial nerve anomalies may be associated with congenital hearing loss, Turner's syndrome, Treacher-Collin's syndrome, congenital stapes fixation, congenital aural atresia, CHARGE syndrome, congenital
Conclusion
The linear functions calculated by this study can be adapted in the estimation of its lengths and in understanding the facial nerve's growth dynamics. The regression equations and the scatter plot with increment curve, representing the growth dynamics of the facial nerve segment lengths without side and sex differences displayed a linear change from 20th to 34th weeks of gestation. In the light of our data, we suggest that there is a dramatic change transition from fetal life to adulthood in
Conflicts of interest
The authors declare no conflict of interest.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Acknowledgement
We are grateful to all the individuals who participated in the current study.
This study was presented orally on the 6th National Congress of Otology and Neurotology on April 28, 2018.
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