Postnatal changes in the types of muscle fibre in the canine inferior pharyngeal constrictor.

Deglutition is considered to be immature in infants and to mature postnatally. We evaluated postnatal changes in muscle fibre type composition in the canine inferior pharyngeal constrictor muscle, which consists of the thyropharyngeal (TP) and cricopharyngeal (CP) muscles, using ATPase staining with respect to the maturation of deglutition. In the TP muscle type IIA and type IIB fibres, the main components in the adult, were already predominant at 1 week postnatally. The percentage of primitive type IIC fibre showed a rapid reduction and reached the adult level within 6 weeks. In the CP muscle, the majority of fibres were type IIC at 2 weeks. At 2 months, more than 20% of the fibres were still type IIC and the proportion of type I fibres as a main component in the adult was smaller than that of the adult. None of the puppies younger than 9 weeks old had a fibre type composition similar to that of the adult. In the extensor digitorum longus and flexor digitorum superficialis, the compositions of muscle fibre types became similar to that of the adult at 6 and 9 weeks of age, respectively. Thus, the TP muscle matured more rapidly than the limb muscles, while the CP muscle matured more slowly. We speculated that the TP and CP muscles have specific individual differentiation patterns associated with their functional roles before and after birth, compared with the limb muscles.