The purpose of this study was to analyze the postnatal development of the intrinsic laryngeal muscles and the nucleus ambiguus motoneurons. The following measurements were taken : the number of intrinsic laryngeal muscle fibers; diameter of intrinsic laryngeal muscle fibers; the number of nucleus ambiguus motoneurons; and diameter of the nucleus ambiguus motoneurons. Measurements were taken at postnatal days (PND) 1, 3, 7, 14 and in mature Sprague-Dawley rats.
The results are as follows : 1) The number of intrinsic laryngeal muscle fibers is about 6800 at birth and does not increase thereafter. 2) The diameter of intrinsic laryngeal muscle fibers increases gradually; diameter at PND 14 is 1.5 times larger than that at PND 1, while diameter at mature age is 2.5 times larger. 3) The number of nucleus ambiguus motoneurons is 850 at birth and decreases to 740 at PND 7.4) The diameter of the nucleus ambiguus motoneurons increases gradually after birth, but remains unchanged after PND 14.
These results suggest that postnatal cell death in the nucleus ambiguus occurs within PND 7 and innervation of the intrinsic laryngeal muscles is completed by that time.
