Microvascular adaptations during maturation of striated muscle

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Abstract
Capillary density in striated muscle changes during maturation. Capillary density is an important determinant of tissue O2 supply, the other principal determinants being capillary erythrocyte flow and capillary hematocrit. The microcirculation of the hamster cremaster muscle was studied at different stages of development. The microcirculation of juvenile animals was characterized by small intercapillary distances, short capillary lengths and tortuous vessels. During maturation, the capillaries elongated and developed the more typical parallel pattern. Capillary density decreased from 1626 .+-. 60 capillaries .cntdot. mm-3 at 35 days of age to 696 .+-. 65 capillaries .cntdot. mm-3 at 132 days; erythrocyte flow per capillary decreased from 1441 .+-. 135 to 583 .+-. 47 .mu.m3 .cntdot. s-1; and capillary hematocrit decreased from 21.5 .+-. 0.7 to 14.6 .+-. 0.6%. Concomitant with these decreases, the functional reserve increased in adult muscles, capillary density could increase by 42%, erythrocyte flow per capillary by 457.2%, and capillary hematocrit by 112.4%, compared with 7.7, 20.3 and 24.1%, respectively, in immature animals. Age significantly modifies microvascular parameters related to tissue O2 supply and provides an explanation for some conflicting observations in the literature.