Nigrosomes are small clusters of dopaminergic cells within the substantia nigra that exhibit calbindin D28K negativity on immunohistochemistry. Five nigrosomes measuring up to a few millimeters in size have been described with the largest labeled as nigrosome-1 1.
Nigrosome-1
Nigrosome-1 within the posterior third of the substantia nigra is able to be visualized on high resolution T2*/SWI weighted MRI, although its exact radiographic appearance is debated in the literature. It was initially thought that nigrosome-1 returns high signal on axial SWI in a linear or comma shape, leading to the swallow tail sign in the substantia nigra, however, this is not shown to be correct in subsequent autopsy studies where it was instead found that nigrosome-1 either only partially accounted for this swallow tail appearance or actually demonstrated low (rather than high) signal on axial SWI 3.
Nigrosome-1 contains the largest proportion of neurons affected in Parkinson disease 1,3. It has been reported that in Parkinson disease that there is absence of the high SWI signal within the substantia nigra, known as the absent swallow tail sign 1. However, given findings of subsequent studies, this absence likely correlates to more than just nigrosome-1 loss.
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