Ultrastructural characteristics of glomus cells in the external carotid artery during larval development and metamorphosis in bullfrogs, Rana catesbeiana

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Abstract
Electron microscopic observations of the external carotid artery in the larvae of the bullfrog, Rana catesbeiana, showed that glomus cells are present in the subendothelial stroma of the septum between the expanded region of the external carotid artery and the carotid arch. There were some differences in the ultrastructure of the glomus cells at each stage of larval development. At the early stages (stages I, III, V, X), most glomus cells were isolated and free from the covering of a supporting cell. The cytoplasm of the glomus cells contained fewer dense-cored vesicles. No synaptic junctions were observed. At the middle stages (stages XV, XX, XXI), some glomus cells showed a tendency to form small clusters. Between adjacent cells in a cluster, gap junctions were often observed. The number of dense-cored vesicles increased remarkably. Intimate apposition of the glomus and smooth muscle cells (g-s connection) was also observed. Nerve terminals containing clear vesicles were observed in synaptic contact with glomus cells at this phase. At the metamorphic climax (stages XXII–XXV), in addition to g-s connections, the glomus cells made intimate apposition to the cells around the glomus cells. The afferent synapses described in other amphibians were not encountered in this study. These findings suggest that the glomus cells at the early stages of development are nonfunctional, the vascular regulation via the g-s connection starts at the middle stages, and the chemoreception starts after metamorphosis.