Development of nervous tissue in the heart of the fetal and neonatal pig and the effect of decapitation in utero.

The development and distribution of the nerves in the heart of the pig was studied macroscopically and by light microscopy. Hearts were collected from 86 fetuses between 31 and 114 days of gestation (term = 114 days), from 12 neonatal pigs aged 9 and 20 days and from 6 adult sows of the Dutch Landrace breed. The effect of vagotomy produced by decapitation in utero at 40-43 days was studied in an additional 24 hearts from fetuses aged between 51 and 114 days of gestation. The amount of acetyl-thiocholine reactive fibres increases in the atria, A-V node and ventricles throughout gestation. At every age the amount of nervous tissue is highest in the A-V node and lowest in the ventricles. Hearts from decapitated fetuses have smaller amounts of nerve tissue than those from intact fetuses at every age studied. Ganglia are present in both intact and decapitated fetuses. Fluorescent cells containing catecholamines are observed in hearts from fetuses as young as 35 days gestation. Although fluorescent nerve fibres are rarely seen in hearts at 70 days gestation, more fibres are present near birth and thereafter there appears to be a considerable increase in the number of fibres and in the intensity with which they fluoresce. These results show that there is substantial nerve growth into the heart of the pig during gestation and that catecholamine containing nerve fibres develop later than those reactive to acetyl-thiocholine.