INTERSTITIAL SPACE AND COLLAGEN ALTERATIONS OF THE DEVELOPING RAT DIAPHRAGM

The effect of growth on the relative interstitial space [%total cross- sectional area (CSA)] and collagen content of the rat diaphragm muscle was examined at postnatal ages of 0, 7, 14, and 21 days as well as in adult males. The proportion of interstitial space relative to total m uscle CSA was determined by computerized image analysis of lectin-stai ned cross sections of diaphragm muscle. To assess collagen content and extent of collagen maturation (i.e., cross-linking), high-pressure li quid chromatography analysis was used to measure hydroxyproline concen tration and the nonreducible collagen cross-link hydroxylysylpyridinol ine (HP), respectively. At birth, interstitial space accounted for app roximately 47% of total diaphragm muscle CSA. During postnatal growth, the relative contribution of interstitial space decreased such that b y adulthood the interstitial space accounted for approximately 18% of total muscle CSA. The change in relative interstitial space occurred w ithout a concomitant change in hydroxyproline concentration. However, the concentration of HP markedly increased with age such that the adul t diaphragm contained approximately 17 times more HP than at birth. Th ese results indicate that during development the relative CSA occupied by interstitial space decreases as muscle fiber size increases. Howev er, the reduction in relative interstitial space is not associated wit h a change in collagen concentration. Thus collagen density in the int erstitial space may increase with age. It is possible that the observe d changes in relative interstitial space and collagen influence the pa ssive length-force properties of the diaphragm.