Development of histochemical and functional properties of baboon respiratory muscles.

We assessed morphological, histochemical, and physiological characteristics of respiratory muscles of a non-human primate, Papio cynocephalus, from midgestation through adult life. Samples were taken of diaphragm muscles for histochemical analysis, electron microscopy, and assessment of contractile properties and fatigability. Histochemical analyses were also performed on samples of intercostal muscles. Initially, developing fibers are type IIc but differentiate into types I and IIa fibers by term. We observed no IIb fibers in respiratory muscles of premature baboons. Beginning late in gestation, muscle fibers grew rapidly. After term, IIb fibers were found, and fiber size ranked by increasing mean fiber area became types I, IIa, and IIb. After term, we rarely observed type IIc fibers. In electron micrographs we observed large numbers of interfibrillar mitochondria in all muscle fibers of premature baboons but not in all IIb fibers of adults. Histochemical observations were supported by contractile properties. Muscles of premature baboons had significantly longer contraction and relaxation times than adult muscles. Muscles from premature baboons were more resistant to fatigue than those of adult baboons. We conclude that the fibers of respiratory muscles are high in oxidative capacity and are resistant to fatigue during gestation. Fatigue of the respiratory muscle fibers secondary to low oxidative capacity is not a likely cause of respiratory distress in premature baboons.