Ultrastructural observations of the fetal rat heart during prolonged gestation.

The ultrastructure of myocardial cells was examined from fetal rat hearts during experimentally induced prolonged gestation (GD 23, 24 and 25), and compared to those from term fetuses (GD 22) and newborn animals (N1, N2, N3). Gestation was prolonged by daily subcutaneous injections of 5 mg progesterone to gravid females on days 20 through 24. The 4 chambers of the heart were assessed individually for structural differences by morphometric analysis. Comparison between the 2 atria at each stage revealed no differences as was the case between the 2 ventricles. At term and throughout the newborn period, atrial and ventricular cells were characterised by numerous mitochondria, moderate amounts of glycogen, few lipid droplets and membrane-bounded granules, which, moreover, were peculiar to atrial tissue. During prolonged gestation (GD 24 and 25), there was a statistically significant increase in the size of mitochondria in both atria and ventricles and a reduction of membrane-bounded atrial granules. Whole heart glycogen content was also assessed, biochemically, and found reduced on GD 24 and 25. In addition to the morphological and biochemical findings, fetal rat heart weights were obtained from each designated day of prolonged gestation and compared to newborn rat hearts of comparable age. A statistically significant increase in heart weight was noted on GD 24. Meconium staining, which is indicative of fetal distress was also observed on GD 24 and 25. The morphological and biochemical changes observed suggest a response by the fetal heart to stressful conditions, namely hypoxia and hypoglycemia, which are known to exist during prolonged gestation.