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 within the posterior third of the substantia nigra is able to be visualized on high resolution T2*/SWI weighted MRI. This nigrosome returns high signal on axial SWI in a linear or comma shape. It is surrounded anteriorly, laterally and medially by low SWI signal intensity which resembles the distinctive split tail of a swallow and hence is referred to as the swallow tail appearance.
Nigrasome-1 contains the largest proportion of neurons affected in Parkinson's disease, a condition that causes the high SWI signal within nigrosome-1 to be absent. Absence of the normal swallow tail appearance of nigrosome-1 is reported to be a reliable sign of Parkinson's disease 1. There are two main possible mechanisms for this, increased iron content or decreased neuromelanin content with decreased iron storage capacity leading to more free iron with paramagnetic properties 2.
- 1. Schwarz ST, Afzal M, Morgan PS et-al. The 'swallow tail' appearance of the healthy nigrosome - a new accurate test of Parkinson's disease: a case-control and retrospective cross-sectional MRI study at 3T. PLoS ONE. 2014;9 (4): e93814. doi:10.1371/journal.pone.0093814 - Free text at pubmed - Pubmed citation
- 2. Zecca L, Casella L, Albertini A et-al. Neuromelanin can protect against iron-mediated oxidative damage in system modeling iron overload of brain aging and Parkinson's disease. J. Neurochem. 2008;106 (4): 1866-75. doi:10.1111/j.1471-4159.2008.05541.x - Pubmed citation