Early development of the facial nerve in human embryos at stages 13-15
M. Weglowski , W. Woźniak , A. Piotrowski, M. Bruska , J. Weglowska , J. Sobański , M. Grzymisławska , J. Łupicka
Department of Anatomy, Poznan University of Medical Sciences, Poznan, Poland
Study was made on 16 human embryos at developmental stages 13-15 ( fth week). The facial nerve was traced on serial sections made in three planes (sagittal, frontal and horizontal) and stained with routine histological methods and impregnated with silver. In embryos at stage 13 the facial ganglion forms a complex structure with the vestibulocochlear ganglion. It is of fusiform shape in contact with epipharyngeal placode and is located anteriorly and ventrally to the vestibulocochlear ganglion. In embryos at stage 14 the facial ganglion separates from the vestibular and cochlear ganglia and the chorda tympani as the rst branch appears. During stage 15 the main trunk of the facial nerve elongates and the greater petrosal nerve originates at the level of the facial ganglion and above the origin of the chorda tympani. (Folia Morphol 2015; 74, 2: 252-257)
RESULTS
Embryos at stage 13 (32 postovulatory days)
Both neuropores of the neural tube are closed and its canal, forming the future ventricular system, is separated from the amniotic cavity. All three layers in the wall of the neural tube are distinguishable. The motor nuclei of the cranial nerves separate from the common efferent tract. The sensory nuclei develop in the common afferent tract. The motor nucleus of the facial nerve is medial to the abducent nucleus. In the secondary brain vesicles, the neuromeres are indicated. Ganglia of the cranial nerves and their relations to the pharyngeal arches are visible (Fig. 1). The external surfaces of the rhombencephalic neu - romeres are well indicated. All three branches of the trigeminal ganglion are discerned. The otic vesicle is separated from the surface and its dorsal part, as the primordium of the endolymphatic appendage, is seen. The facial ganglion forms a common complex with the vestibulocochlear ganglion (Fig. 2). It pre sents a fusiform structure with cells arranged in rows and is traversed through nerve bres (Fig. 3). The ganglion is in the contact with epipharyngeal placode of the second pharyngeal arch and is located ventrally and anteriorly to the vestibulocochlear ganglion at the level of the rhombencephalic neuromere 4. The facial neural crest is still visible.
Embryos at stage 14 (33 postovulatory days)
Future cerebral hemispheres are delineated and the telencephalon medium is separated from the diencephalon through the torus hemisphericus . With- in the brain, 16 neuromeres are well de ned . The epipharyngeal placode and neural crest contributing cells to the trigeminal and facial ganglia are present. All ganglia forming facial-vestibulocochlear complex are distinguishable (Fig. 4). The nuclei of the cranial nerves in the brain stem are well formed. The mes - encephalic and spinal trigeminal tracts develop. From the trunk of the facial nerve originates the chorda tympani as the rst branch of this nerve (Fig. 5). It passes anteriorly whereas the main trunk of the facial nerve courses ventrally and posteriorly in the direction to the second pharyngeal arch.
Embryos at stage 15 (35 postovulatory days)
The cerebral hemispheres are well developed. The diencephalon is divided into future ve zones. The motor and sensory nuclei of the cranial nerves are distinct and several tracts in the brain stem appear. The motor nucleus of the facial nerve lies medially to the abducent nucleus in the future dorsomedial nuclear column. The intracranial part of the facial nerve elongates. At the level of the facial ganglion arises greater petrosal nerve. It is directed anteriorly (Figs. 6, 7). Ventrally to the greater petrosal nerve aris - es the chorda tympani which also passes anteriorly. The main trunk of the facial nerve forms a slide arch bent anteriorly. This segment may be considered as the primordium of the horizontal part of the nerve.
CONCLUSIONS
The present study, based on international Carnegie staging showed that in embryos aged 5 weeks the facial ganglion separates from the vestibulocochlear ganglia and 2 branches of the nerve appear: the chorda tympani at stage 14 and the greater petrosal nerve at stages 15.
Strategies to Prepare