Small clusters of granule-containing cells at the lateral side of the posterior cricoarytenoid muscle of the young adult rat.

Small clusters consisting of granule-containing cells, sustentacular cells and capillaries around them, similar in structure to the carotid body-like paraganglia, sometimes existed at the lateral side of the posterior cricoarytenoid (PCA) muscle of young adult (3 months) rats. Differing from the paraganglia, however, these cell clusters were discontinuously invested by slender cytoplasmic processes of fibroblasts. In individual granule-containing cells, granules varied in size and had a concentrically or eccentrically arranged, electron-dense material, resembling those of chromaffin cells of the adrenal medulla. A series of desmosome-like structures were frequently observed between adjacent granule-containing cells, but synapses between them were not necessarily clear. Nerve endings containing clear synaptic vesicles and occasional granulated vesicles, being possibly cholinergic in nature, sometimes formed synapses with the granule-containing cells, probably indicating that the granule-containing cells receive the efferent nerve innervation. On the other hand, the sustentacular cells lacked cytoplasmic granules and sent their cytoplasmic processes around the granule-containing cells. Capillaries in and around clustered cells were of the fenestrated type. From these findings, it is suggested that unlike the carotid body-like paraganglia, the noncapsulated cell clusters at the lateral side of the PCA muscle of the young adult rat may be identical to groups of extra-adrenal chromaffin tissues.