[Morphological development of the intrinsic laryngeal muscles in rat--a scanning electron microscopic and...

The postnatal development of the subneural apparatus (SNA) and differentiation of muscle fibers of the intrinsic laryngeal muscles (ILMs) in the rat were investigated on the 1st, 5th, 15th, 30th and 90th days after birth. The cricothyroid (CT), thyroarytenoid (TA) and posterior cricoarytenoid (PCA) muscles in the ILMs were examined. Furthermore, we compared the development of these muscles with that of the skeletal muscles of the hindlimb, the extensor digitorum longus (EDL) and soleus (SOL) muscles. The SNA was observed with a scanning electron microscope. The SNA was quantitatively analyzed by use of two parameters: the ratio of the length of the secondary synaptic cleft to its maximal width (L/W ratio) and the ratio of the area of the secondary synaptic clefts to that of the primary synaptic cleft(s) (ASC/APC ratio). The muscle fiber types were classified according to the density of each muscle fiber by the ATPase stain. On the 15th postnatal day, the SNAs of the CT and TA muscles had a tendency to show more advanced development than those of the PCA muscle. The SNAs of the ILMs appeared to develop earlier than those of the EDL and SOL muscles. The L/W and ASC/APC ratios of the ILMs inclined to be higher than those of the hindlimb muscles. On the 30th day, the SNAs of the PCA muscle were still immature, while those of the CT and TA muscles had become nearly mature. The EDL muscle showed completely mature development and outstripped the ILMs. In contrast, the SNA of the SOL muscle was the least mature of all muscles. The L/W ratios of the ILMs were not significantly different from each other, while the ASC/APC ratio of the PCA muscle was significantly lower than those of both the CT and TA muscles. The L/W and ASC/APC ratios of the EDL muscle were the highest of all muscles examined and those of the SOL muscle were the lowest. The muscle fiber types were not significantly different in the ILMs throughout the course. On the 30th day, the EDL muscle had reached the mature adult proportion of the muscle fiber types, while the ILMs and SOL muscle needed further development. Phylogenetically, the TA muscle appeared as a sphincter of the lower airway and then the PCA muscle, as a dilator, appeared. The CT muscle arose after separation of the thyroid and cricoid cartilages. These findings infer that the lag in development of the PCA to the TA muscle in early postnatal life was due to the importance of the protective function of the lower airway and the phylogenetical difference. Moreover, these results suggest that the ILMs, which are indispensable for sucking, develop early in life, followed by rapid development of the EDL muscle to start quadruped walking after the 15th day. Thus, different muscles need to develop as an animal grows because distinct functions of muscles are necessary for normal living. In conclusion, the ILMs may have a specific mode of postnatal development in contrast with those of the hindlimb muscles.