An ultrastructural study of the cardia ganglia in the bulbar plexus of the developing chick heart.

The development of the chick bulbar cardiac ganglia has been studied ultrastructurally from the 3rd day of incubation to hatching. For descriptive purposes, their development has been divided into three phases. The first phase includes migration and aggregation of undifferentiated neuroblasts, stages 21-26 (day 3.5-5). Cardiac branches of the vagus first grow toward the heart at 3.5 days of incubation. The nerves are accompanied by migrating neural crest neuroblasts. The neuroblasts begin to aggregate at 4.5 days around the aortic arch arteries and truncus arteriosus. Some of these cells sprout axonal processes by day 5. A sparse population of developing supporting cells can be found near some neuroblasts and nerves. During the second phase of development, stages 27-36 (days 5-10), the neuroblasts assume a globular shape and have eccentric, indented nuclei. Supporting cells become more numerous as this phase of development progresses. Axodendritic synapses appear first at stage 34 in the ganglia. The neurons and supporting cells in the ganglia undergo maturation during the final phase from stage 37 to hatching (11-21 days). The immature/mature ganglion cells are large ovoid to pyriform neurons which become rounder as the cytoplasm accumulates. Axosomatic synapses are seen first at stage 38. Small supporting cells become more numerous. Clusters of catecholamine-containing cells near the cardiac ganglia are present during the final phase of development.