Development
The muscles of facial expression in addition to the buccinator, platysma, stapedius, stylohyoid, digastric posterior belly, develop from the second brachial arch, also known as the hyoid arch. The facial nerve is responsible for innervating all of these muscles.
Embryology timeline
The rhombencephalon (or hindbrain) divides into the myelencephalon, which becomes the medulla oblongata, and the metencephalon, which becomes the pons and cerebellum. The facioacoustic primordium appears during the third week of life and is attached to the metencephalon just cranial to the otic vesicle.
By the end of the fourth week of gestation, the facial nerve splits into 2 parts, caudal and rostral trunks. The chorda tympani nerve exits rostrally and courses ventrally to the first pharyngeal pouch to enter the mandibular arch.
During weeks 5-6 of gestation the chorda tympani nerve enters the mandibular arch and terminates just proximal to the submandibular ganglion. By 7 weeks the nervus intermedius is smaller than the motor root and enters the brain stem between the vestibulocochlear nerve and the motor root of the facial nerve. At this time the five terminal roots of the motor aspect divide to follow separate paths.
At 8 weeks some of the important muscles of facial expression that are innervated by the facial nerve appear, such as the orbicularis oris, levator anguli oris, and orbicularis oculi muscles.
Within another week additional muscles including auricularis anterior, corrugator supercilii, occipital and mandibular platysma, and levator labii superioris alaeque nasi muscles appear and nerve connections reflected the adult state occur.
Extensive branching and the formation of nerve connections develop and the facial nerve passageway through the parotid gland is established. By the fifteenth week the geniculate ganglion is fully developed, and the facial nerve's relationship to middle ear structures is more fully developed.
All definitive communications of the facial nerve are established by the sixteenth week and by birth the anatomy of the facial nerve closely resembles that of an adult.
Image copyright details: