MATERNAL CONSTRAINT INFLUENCES MUSCLE-FIBER DEVELOPMENT IN FETAL LAMBS

The objective was to examine myogenesis in two situations expected to be characterized by maternal constraint: (i) in fetuses due to be born in spring (n = 10) or autumn (n = 10); and (ii) in single (n = 16) an d twin (n = 20) fetal lambs. Maternal constraint operating through lim itation of placental size, as measured by placentome weight per fetus, was evident in each study. Although a lower placental weight did not influence body and muscle weights of fetuses due to be born in the spr ing or autumn, twins had lower body and muscle weights than singles. F ibre number and average fibre cross-sectional (CS) area were different ially affected by season and fetal number. The differences in muscle f ibre morphology between spring-and autumn-born fetuses suggest that mu scle fibre development was influenced by maternal constraint in the ab sence of an effect on fetal weight. The differences in muscle fibre nu mber and CS area in particular muscles from twin and single fetuses su ggest that more severe maternal constraint, reflected in a lower place ntal size per fetus, not only influences fetal weight but can also aff ect muscle development.