Three-dimensional development of the facial nerve path through the ear region in human embryos.

The goal was to determine how the facial nerve path forms normally through the ear region. Five three-dimensional computer reconstructions and eight wax plate-graphic, two-dimensional reconstructions were made from a group of 47 serially sectioned human embryos that were in the blastemal period of ear development, when most of the definitive arrangement is established. The size, shape, and positional changes of the facial nerve relative to the external ear and notochord were studied in both lateral and frontal views. The vertical (dorsoventral) position of the external ear region remains constant but shifts caudally. After formation, the geniculate ganglion assumes a position that gradually becomes rostrodorsal to the external ear. Facial nerve branches form and grow into regions expanding peripherally. The superficial petrosal branch appears to hold the ganglion in position as the horizontal segment of the facial nerve forms when the external ear shifts caudally. Possible growth movements explaining abnormal nerve paths are discussed.