A morphogenetic study of the facial nerve in the mouse fetus

Abstract

Normal development of the facial nerve in the mouse was investigated morphologically and histologically, especially at the middle ear region, with a light microscope. The materials were 59 mouse fetuses, belonging to ddN strain, from the day 8 to 20 of pregnancy. Their heads were serially sectioned and stained with hematoxylin and eosin. The results were as follows: 1) On the day 9, the anlage of the acoustic ganglion was developing just anterior to the otic vesicle and the cells of the superficial ectodermal thickening around the region of the 1st branchial cleft appeared to differentiate into the neuroblasts of the future geniculate ganglion. 2) The geniculate and acoustic ganglia became clearly differentiated on the day 10. The main trunk of the facial division located posteriorly to the anlage of the stapes and ran across the mesenchyme of the 2nd branchial arch vertically. 3) The peripheral and central nerve trunks from the geniculate ganglion were recognizable on the day 11. The chorda tympani and the greater petrosal nerve were also present at the day 11. 4) The identification of the horizontal and vertical portions of the facial nerve was clear on the day 12. 5) The development of the otic capsule and the stapedial footplate seemed to have much influence upon the running course of the horizontal portion of the facial nerve and the development of Reichert's cartilage upon the running course of the vertical portion between the day 12 and 13. 6) The development of the facial canal belonging to the horizontal and vertical portions began as a sulcus formed with the precartilaginous otic capsule and the laterohyale at the day 13. 7) The development of the lateral wall of the horizontal facial canal began by forming the connective band at the day 16 and the vertical segment at the day 18.