Facial On Matures
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processing.... Drugs & Diseases > Anatomy Facial Nerve Embryology Updated: Jul 06, 2016 Author: Timothy Els, MBChB; Chief Editor: Arlen D Meyers, MD, MBA more... Share Email Print Feedback Close Facebook Twitter LinkedIn WhatsApp webmd.ads2.defineAd(id: 'ads-pos-421-sfp',pos: 421); Sections Facial Nerve Embryology Sections Facial Nerve Embryology Overview The Mature Facial Nerve Overview of Hindbrain Development The Hindbrain Nuclei of the Facial Nerve The Neural Crest and Hyoid Arch Invasion The Development of the Fallopian Canal Development of the Facial Nerve in Weeks Congenital Facial Paralysis Show All Media Gallery Tables References Overview Overview In order to appreciate the complex embryology of the facial nerve, one has to have a basic understanding of cranial embryology as a whole. Although it may seem daunting to the casual reader, revisiting cranial embryology allows for a more comprehensive understanding of the final 3-dimensional structure of the nerve, as well as the inherent logic in its development. To this end, this article briefly discusses numerous important processes in head and neck embryology, namely the implications of patterning in hindbrain development, the diverse roles of neural crest cells, migration of the neural crest cells into the branchial arches (particularly the hyoid arch), and the genetic control of these processes. This may help prepare the otolaryngologist to comprehend and anticipate variations encountered in clinical practice, such as anticipating facial nerve anomalies in congenital stapes fixation.  However, the main objective of this article is to outline the embryology of the facial nerve and its common clinical implications. The reader is referred to Embryology and Anomalies of the Facial Nerve and Their Surgical Implications, 2nd Ed for a more comprehensive review of the development of the facial nerve and the associated development of the ear (see table 1). 
While studying the embryology of the facial nerve, keep in mind the mature course and structure that is the end result of developmental events. The motor nucleus of the facial nerve is located in the reticular formation of the caudal pons. Upon leaving the motor nucleus, axons extend dorsally and medially, cranially and superficially, to bend around the abducens (sixth cranial nerve) nucleus. The fibers then exit the central nervous system (CNS) between the olive and the inferior cerebellar peduncle.
After exiting the internal auditory canal, the facial nerve enters the middle ear, where it bends posteriorly (first, or medial, genu) and courses horizontally through the middle ear. Just anterior to the lateral aspect of the horizontal semicircular canal, the facial nerve curves gently (the second genu) to form the vertical, or mastoid, segment that exits via the stylomastoid foramen.
The development of the hindbrain (rhombencephalon) and the subsequent delamination of the neural crest cells are interrelated processes that need to be understood to appreciate the development of the branchiomotor cranial nerves in general and the facial nerve in particular.  Primary neurulation is a process that leads to the development of the neural tube from the neural plate. During the first 4 weeks of embryogenesis, the notochord induces axial ectoderm to form the neural plate, which then folds along its long axis to form the neural tube. The tube subsequently develops vesicles at its rostral end, which give rise to the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).  The latter then divides into the metencephalon and caudal myelencephalon. 
The rostral division of the neural tube into its 3 main sections falls under the control of homeobox (Hox) family of genes.  The Hox genes, well described as the master regulators of development, encode a set of transcription factors that specify the identity of particular segments during embryogenesis.  The hindbrain is subseq