Development of the carotid body and the glomus cell groups in the wall of the common carotid artery and its branches was studied in chickens at various developmental stages by electron microscopy. At 8 days of incubation, the carotid body anlage consisted of mesenchyme-like cells, whereas the clusters of epithelial cells, which occasionally contained a few dense-cored vesicles and were accompanied by unmyelinated nerve fibers, were located in the region surrounding the carotid body anlage and in the wall of the common carotid artery. Subsequently, the granule-containing cells together with nerve fibers were detected in the periphery of the carotid body anlage. At 12 days of incubation, a large number of granule-containing cells (glomus cells) were dispersed throughout the carotid body parenchyma and were also widely distributed along the common carotid artery and its branches. The cells frequently extended long cytoplasmic processes that made contact with other glomus cells and nerve fibers. Synaptic junctions which showed desmosome-like thickening of pre- and postsynaptic membranes and accumulations of small clear vesicles (around 50 nm in diameter) were first detected along the contact between the long axons and glomus cells at 12 days of incubation. In the wall of the common carotid artery, interdigitations between the cytoplasmic processes of glomus cells and smooth muscle cells began to form. Sustentacular cells investing partly the glomus cells were also discerned both in the carotid body and around the arteries at this stage. At 14 days of incubation, the glomus cells expressed most of the characteristics of the mature cells, and the synaptic junctions displaying afferent morphology appeared; the secretory granules of glomus cells were accumulated near and attached to the desmosome-like thickening of apposed membranes. © 1994 Wiley-Liss, Inc.