Electron microscopic study of cardiac ganglia in human fetuses

The early events in the development of the heart ganglia and nerves in human fetuses ranging from 5 to 12 weeks of gestation age were studied by transmission electron microscopy. The first neuroblasts in the atrial mesenchyme differ from surrounding cells in the presence of short cisternae of rough endoplasmic reticulum and in the absence of glycogen particles. The most valuable criterion for identification of neuroblasts is the presence of contacts with preganglionic nerve terminals. Only ganglia composed of compact aggregations of neuronal cells and nerve terminals have a complete glial sheath. The first signs of synapse formation were seen in 5-week-old fetuses; well developed synapses with many synaptic vesicles were found from the 8th week of gestation onwards. These were predominantly axodendritic synapses. It is proposed that synaptogenesis begins with the appearance of osmiophilic zones at the sites of interneuronal contacts, then synaptic vesicles move by axonal transport to reach the preformed specialized junctions and stop in axonal presynaptic varicosities.