beta-amyloid deposition and neurofibrillary tangle formation in the olfactory bulb in ageing and Alzheimer's disease

Impaired olfaction, hyposmia or anosmia are part of the clinical phenotype in neurodegenerative disorders including Alzheimer's disease (AD). It has b een proposed that the most severely affected areas are interconnected with the central olfactory system in contrast to the relative sparing of other s ensory areas which lack olfactory connections. The pathology of the first s ynaptic relay in the olfactory pathway, the olfactory bulb (OB), has been s tudied in AD, but the results have been inconsistent. In order to define mo re fully the pathology of the OB, we analysed 15 AD and 15 control cases, u sing amyloid and tau immunohistochemistry on serial sections. This study de monstrates for the first time that all layers of the OB are severely affect ed in AD and in normal ageing. The principal effector cells of the OB, the mitral cells, developed neurofibrillary tangles (NFTs) both in AD and in co ntrols. All the cases, with the exception of two of the controls, contained NFTs. Amyloid immunoreactivity was detected in diffuse, primitive, classic al and compact deposits in AD, while five control cases contained mainly di ffuse deposits. We did not find a correlation between amyloid deposition an d NFT formation. Among the control cases, two contained neither amyloid nor NFTs, eight had NFTs but no amyloid and only five had both NFTs and amyloi d. All the AD cases had NFT and amyloid deposition. Our data suggest that t he earlier pathology in the OB is NFT formation and more than ten NFTs/sect ion is compatible with 93.3% diagnostic accuracy for AD.