Shades of gray in human white matter.

Anatomists have long expressed interest in neurons of the white matter, which is by definition supposed to be free of neurons. Hypotheses regarding their biochemical signature and physiological function are mainly derived from animal models. Here, we investigated 15 whole-brain human postmortem specimens, including cognitively normal cases and those with pathologic Alzheimer's disease (AD). Quantitative and qualitative methods were used to investigate differences in neuronal size and density, and the relationship between neuronal processes and vasculature. Double staining was used to evaluate colocalization of neurochemicals. Two topographically distinct populations of neurons emerged: one appearing to arise from developmental subplate neurons and the other embedded within deep, subcortical white matter. Both populations appeared to be neurochemically heterogeneous, showing positive reactivity to acetylcholinesterase (AChE) [but not choline acetyltransferase (ChAT)], neuronal nuclei (NeuN), nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d), microtubule-associated protein 2 (MAP-2), somatostatin (SOM), nonphosphorylated neurofilament protein (SMI-32), and calcium-binding proteins calbindin-D28K (CB), calretinin (CRT), and parvalbumin (PV). PV was more richly expressed in superficial as opposed to deep white matter neurons (WMNs); subplate neurons were also significantly larger than their deeper counterparts. NADPH-d, a surrogate for nitric oxide synthase, allowed for the striking morphological visualization of subcortical WMNs. NADPH-d-positive subcortical neurons tended to embrace the outer walls of microvessels, suggesting a functional role in vasodilation. The presence of AChE positivity in these neurons, but not ChAT, suggests that they are cholinoceptive but noncholinergic. WMNs were also significantly smaller in AD compared to control cases. These observations provide a landscape for future systematic investigations.