in the human brain , the white matter ( wm ) underlying the cerebral neocortex is highly developed and occupies a much larger volume than in other mammals . although the dominant components of the wm are the complex fiber tracts , their ensheathing myelin and supporting glia , there are also large numbers of neurons dispersed among the fibers , termed the  interstitial neurons  ( in ) . the species differences may reflect a direct correlation between the size of the cortical gray matter , the amount of wm connecting the neocortex , and the number of in . in human , the border between gray and white matter is sharply defined at the bottom of the sulci and along the flanks of the gyri , but more difficult to delimit at the crowns or apices of the gyri , where radial fiber fascicles intermingle with radial rows of layer vib neurons and in seem to be continuous with neurons of layer vib ( see von economo and koskinas , 1925 ) . the highest density of in is in the wm immediately subjacent to the gray matter , in the zone that contains the association or fibers of the cortical convolutions , and then gradually decreases with increasing distance from the gray matter . very few neurons lie among the long fiber tracts in the deep wm , such as internal capsule , superior and inferior longitudinal fasciculi , or corpus callosum . however , there is no sharp boundary between the superficial wm , rich in in , and the deep wm , where in are sparse . there may also be regional differences in the density of in , with lowest numbers in the visual cortex , and higher numbers in the frontal and prefrontal cortex ( meyer et al . the in display a variety of morphologies ranging from pyramidal - like to bipolar and multipolar . they can be classified into the two main neuronal categories also present in the gray matter , namely excitatory glutamatergic cells and inhibitory gabaergic neurons . a previous golgi study in the wm of adult human cortex ( meyer et al . , 1992 ) revealed the presence of neurons with the morphology of pyramidal cells displaying apical and basal dendrites covered with dendritic spines , which may represent the excitatory component of the wm . the second category of in corresponds to non - pyramidal neurons similar to those described in the gray matter . unfortunately , there are only few studies on in of the adult human cortex , and most experimental data stem from nonprimate brains . in the following sections , we summarize the available literature on in and point out the limitations of generalizing nonprimate data on the primate , and specifically , the human brain . the figures of this review were taken from our human brain material in the department of anatomy , university of la laguna , which was obtained from autopsies under the supervision of the ethical committees of our institutions . in of the cortical wm are often referred to as  subplate  cells . during development , the subplate is a transient cell compartment just below the future layers vi - ii , or birthdating studies in rodents and carnivores revealed that subplate neurons are generated at the same time as cajal - retzius cells in the marginal zone ( or future layer i ) , and prior to the birth of cortical plate neurons ( chun and shatz , 1989a ; luskin and shatz , 1985 ) . subplate cells perform multiple developmental functions : they extend pioneer fibers into the internal capsule and direct thalamo - cortical pathfinding , serve as transient synaptic targets for thalamocortical fibers , and provide a substantial glutamatergic input into the maturing cortical plate , helping in the establishment of ocular dominance columns in the primary visual cortex ( reviewed by allendoerfer and shatz , 1994 ; finney et al . , 1998 ; , 1990 ; kanold and shatz , 2006 ; kanold et al . , 2003 ; mcconnell et al . , 1989 ) . as the cortical plate matures , many subplate neurons degenerate and undergo programmed cell death ( allendoerfer and shatz , 1994 ; kostovic and rakic , 1990 ; wahle and meyer , 1987 ) . the survivors continue into adult life as in of the wm ( chun and shatz , 1989b ; kostovic and rakic , 1980 , 1990 ) . . the gabaergic subpopulations may express a variety of peptides such as neuropepide y , somatostatin and cholecystokinin , or contain nitric oxide synthase ( chun and shatz , 1989a ; finney et al . 1992 ; torres - reveron and friedlander , 2007 ; uylings and delalle , 1997 ; wahle and meyer , 1987 ; wahle et al . , 1987 ) . it is not known if developmental cell death affects specific cell classes within the subplate , or whether all subpopulations are equally reduced . to what extent is the subplate of rodents and carnivores comparable to the human subplate ? in human fetuses , an initial cell condensation , the  pioneer plate  , appears at 7/8 gestational weeks ( gw ) and is almost immediately split into superficial and deep pioneer neurons by the arrival of the first cortical plate cohorts at 8/9  gw ( meyer et al . , 2000 ) . the deep pioneer cells form the  presubplate  ( kostovic and rakic , 1990 ; meyer et al . , 2000 ) . the subplate zone proper becomes visible around 14/15  gw as a cell - poor / fiber - rich layer situated between the intermediate zone and the cortical plate . it reaches maximal width and highest cellularity from 2236  gw , when it is four times thicker than the cortical plate . thereafter , the subplate gradually decreases in size and becomes unrecognizable around the sixth postnatal month ( kostovic and rakic , 1990 ) . npy - immunoreactive neurons attributed to the subplate appear around 14  gw in the subplate and decrease in number by the end of gestation ( bayatti et al . a similar time course of subplate development has been described in the monkey ( smart et al . , 2002 ) , showing that the subplate develops differently in nonprimate and primate species . although human subplate neurons are heterogeneous , a useful marker of the glutamatergic component is the putative transcription factor t - brain-1 ( tbr1 ) ( bayatti et al . , 2008 ; hevner et al . , 2001 ; the chronology of tbr1 expression in human fetuses can be traced to the early cortical plate at 10  gw , which is strongly tbr1 + ( figure 1a ) . from 14 to 25  gw , large numbers of tbr1 + neurons are continuously added to the subplate compartment , which increases in width concurrent with the growth of the cortical plate , although the highest density is always at the border between cortical plate and subplate ( figures 1b , c ) ( meyer , 2007 ) . in perinatal brains , tbr1-immunoreactivity changes from a nuclear to a cytoplasmic staining that is widely distributed in neurons in the cortical gray and white matter , and thus no longer useful as a marker molecule of the subplate . in the absence of molecules specific for the human subplate it is difficult to ascertain how many subplate cells survive as in . ( a ) at 10 gestational weeks ( gw ) , highest tbr1 staining is in the early cortical plate ( cp ) . ( b ) at 16  gw , tbr1 + cells are concentrated along the cp / sp border and distributed all over the sp . ( c ) at 25  gw , the sp , here in the temporal lobe , has increased in width and is filled with tbr1 + neurons extending from the intermediate zone ( iz ) to the deep cp border . the growth of the sp despite the expansion of the cortical wall indicates the continuous addition of new tbr1 + neurons . scale bar in ( a ) 100 m , in ( b ) 200 m , in ( c ) 1  mm . altogether , these data show that the in of the human wm are not identical to the early - born subplate neurons described in rodents and cat . rather , the cell populations in the maturing wm seem to be complemented by newly arriving neurons generated at much later stages of corticogenesis . a possible explanation for the discrepancy across species may be the extraordinary increase in cortical connectivity during evolution , which leads to an increase in size and complexity of the wm in the primate brain . in parallel to the increase of the wm compartment , a continuous supply of in may be required during the whole period of corticogenesis . this implies that primate in are not just incidental remnants of early - born neurons , but rather seem to belong to a distinct neuronal system that is intimately connected to the wm and may carry out activities pertinent to this location . further studies are necessary to define the developmental origins and possible functions of in in the adult wm of the primate cortex . the gabaergic interneurons of the cortical gray matter are highly diverse , and many attempts have been undertaken to classify them according to a variety of morphological and neurochemical properties ( ascoli et al . , 2008 ) . a recent inventory of mouse cortical interneurons has led to the identification of 13 cell classes based on the combined expression of the calcium - binding proteins calbindin ( cb ) , calretinin ( cr ) and parvalbumin ( pv ) , and neuropetides , such as vasointestinal polypeptide , npy , cholecystokinin , somatostatin and cholinacetyltransferase ( gonchar et al . , 2008 ) . of the three calcium - binding proteins present in the cortical gray matter , only cb and cr are expressed in in . pv+ cells are the largest group of cortical interneurons which includes basket cells and chandelier cells ( de felipe , 2002 ) . pv is not found in the adult human wm , and the few deep pv+ neurons occasionally found below the gray matter are more likely to represent displaced layer vi neurons . cb+ in have been reported in the wm ( yan et al . , 1996 ) ; they are concentrated in the superficial wm , often aligned along the gray / wm border and most numerous in the apex regions . they are small to medium size , have bipolar or multipolar dendritic trees ( figures 2c , d ) , and are rare in the deep wm . the variety of in in the adult human wm . ( a ) cr+ in are the most abundant cell class in the superficial wm . . they may be the origin of the fiber terminals in layer vi , indicated by arrowheads . ( d ) a multipolar cb+ in in the superficial wm . ( e ) a reelin+ in in the superficial wm . scale bar in ( a ) 200 m , in ( b ) 150 m , in ( c ) 30 m , in ( d ) 20 m , in ( e ) 15 m . cr is abundant in gray - matter interneurons mostly of supragranular layers ( gonchar et al . , 2008 ) , but its presence in the wm has not attracted much attention . this is surprising insofar as cr+ in are the most prominent cell population in the superficial and deep adult human wm ( figures 2a and 3 ) . they have diverse sizes , ranging from small to large somata , and bipolar or multipolar dendritic trees , regardless of their position in the superficial or deep wm . small cr+ in occur also in the internal capsule , and even in the periventricular wm of the temporal horn of the lateral ventricle . here they form conspicuous cell clusters that give rise to local plexuses of varicose fibers , which often surround blood vessels or follow the vascular wall ( figures 3a , b ) . the small , dense fiber plexus , mostly restricted to the cluster and with few fibers spreading into the adjacent wm , suggests synaptic contacts between neighboring cells ( figure 3c ) . this staining was not observed with other markers and seems to be unique to cr . ( c ) cluster in the deep wm . the dense local axonal plexus is mostly confined to the cluster , with few fibers extending into the neighboring wm . scale bar in ( a ) 50 m , in ( b ) 100 m , in ( c ) 50 m . the distribution and relative prominence of interneurons expressing calcium - binding proteins are species and area - dependent ( hof and sherwood , 2005 ) . cr+ cells seem to be more prominent in primate than in rodent cortex , not only in the gray matter but also in the wm . in the mouse , cr+ interneurons derive from the caudal ganglionic eminence and migrate tangentially all over the cortex ( xu et al . , 2004 ) . by contrast , primate interneurons have a double origin , with early - born cells migrating from ganglionic eminences , and later - born cells deriving from the subventricular zone ( svz ) of the cortical wall ( letinic et al . , in particular , cr+ cells are very prominent in the svz and deep wm during late human fetal development . characteristic clusters of doublecortin / cr+ neurons in the svz were interpreted as locally born interneurons destined for the superficial cortical layers ( meyer et al . , 2002 ) . the cr+ cell clusters in the adult wm suggest that not all of these cells migrate into the gray matter but may give rise to resident cells of the wm , or stay close to their place of origin . in any case , the cr+ in in the deep wm are not derivatives of an early generated subplate , but rather late additions at a time when the fiber fascicles of the wm mature and may need positional cues . further studies of this cell population may be interesting , particularly with regard to neuropathological alterations . nitric oxide ( no ) is a gaseous messenger molecule synthetized by several isoforms of the enzyme nitric oxide synthase ( nos ) . in the brain , two nos forms activation of nnos and enos requires the influx of calcium ions , usually upon the activation of glutamate nmda - receptors , and the presence of nicotinamide adenine dinucleotide phosphate ( nadph ) as a co - substrate . nitrergic , i.e. no - producing neurons , can be visualized by nadph - diaphorase histochemistry , as well as by immunohistochemistry using anti - nnos antibodies ( bredt et al . , 1991 ; estrada and de felipe , 1998 ; hope et al . , 1991 ; vincent and kimura , 1992 ) . due to its high diffusibility and short half - life , no is associated with many diverse functions , such as cerebrovascular coupling , neurotransmission , neuronal survival and death , memory , and synaptic plasticity ( reviewed by calabrese et al . nadph - d neurons in the cerebral cortex have been studied extensively in a variety of species from rodent to human ( barone and kennedy , 2000 ; estrada and de felipe , 1998 ; garbossa et al . , 2005 ; judas et al . , 1999 there are two main cell classes : type 1 nadph - d neurons are intensely stained in a golgi - like fashion , displaying medium size to large somata and long varicose processes ( figure 4a ) . most type 1 neurons are in the superficial wm , whereas type 2 nadph - d neurons are restricted to the cortical gray matter . type 2 neurons are only lightly stained and have small somata and short processes ( barone and kennedy , 2000 ; sandell , 1986 ) . both types express gaba , and a 4% of type 1 neurons co - express cb ( yan et al . , 1996 ) . type 1 neurons can also express neuropeptide y ( figure 2b ) and somatostatin ( vincent et al . , 1983 ) . although most gabaergic neurons are interneurons with local axons , some nadph - d / nnos+ neurons in the wm of rat , cat and monkey project over long distances to distant , functionally unrelated cortical areas ( higo et al . , 2007 ; meyer et al . , 1991 ; tomioka and rockland , 2007 ) . ( a ) ( b ) magnified view of a type 1 neuron near a microvessel that is contacted by nadph - d+ axonal fibers ( arrowheads ) . scale bar in ( a ) 150 m ; in ( b ) 20 m . one of the most interesting features of the type 1 neurons is their close association with blood vessels . their axonal plexuses form a dense network around microvessels ( figure 4b ) , and their long processes may contact distant arterioles and capillaries ( estrada and de felipe , 1998 ; estrada et al . , 1993 ; 1996 ) . since no is a potent vasodilator , nos - containing neurons are thought to be involved in the coupling of metabolic changes related to neuronal function with local increases in blood flow . due to their strategic location just below the cortical gray matter , nos+ in may be contacted by corticopetal fibers and , in response , act on neighboring microvessels . on the other hand , npy is a powerful vasoconstrictor able to antagonize the vasodilating effect of no ( abounader and hamel , 1997 ; cauli et al . , 2004 ) somatostatin and npy ( figure 3b ) are expressed in in of the superficial wm . they act directly on smooth muscle cells of cortical arterioles , and may thus constrict cortical microvessels in an activity - dependent manner ( cauli et al . , 2004 ) . a possible mechanism of the combined activity of no and npy in the same neuron has been proposed by estrada and de felipe ( 1998 ) : a stimulated type 1 nadph - d+ neuron might release no in a diffuse way in the vicinity of the soma and main processes and increase local flow , whereas axonal branches of the same neurons ramify around more distant microvessels and release npy , producing a spatially restricted vasoconstriction . the nos / npy+ in thus form part of the neural system involved in the coupling of cortical microvessels to neuronal activity . the neurovascular interactions leading to the hemodynamic changes during enhanced or decreased cortical activity are the basis of functional neuroimaging using positron - emission tomography ( pet ) and functional magnetic resonance imaging ( fmri ) . the blood oxygen level - dependent ( bold ) signal reflects the hemodynamic response coupled to neural signalling processes ( for review , see attwell and iadecola , 2002 ; logothetis and wandell , 2004 ; ogawa et al . , the nos+ in in the cortical wm are an important component of the vasoactive pathways which also include subcortical cholinergic and serotoninergic systems . in fact , most nos - expressing in are cholinoceptive , meaning that they receive cholinergic fibers from the nucleus basalis ( kocharyan et al . , 2008 ; smiley et al . , 1998 ) . since the axons of nos+ in may spread over considerable distances into the cortical gray matter , a single in may coordinate local blood flow in neighboring and distant cortical areas in response to corticopetal and corticofugal activation . the subcortical wm and its resident in have been associated with a variety of neurological and psychiatric disorders . alterations of somatostatin , npy and/or nadph - d+ in were observed in alzheimer disease ( e.g. kowall and beal , 1988 ; tao et al . , 1999 ; van de nes et al . , 2002 ) . however , schizophrenia is the disease which seems to show the most dramatic abnormalities of the wm . diffusion tensor imaging revealed disturbances of myelin function and distribution , alterations of connectivity and integrity of fiber tracts such as the cingulate bundle and uncinate fasciculus , with a higher incidence in the frontal lobes , middle temporal structures including hippocampus and amygdala , and superior temporal gyrus , as well as in subcortical centers ( reviewed by kubicki et al . schizophrenia also affects the in in diverse ways . in the frontal lobe of schizophrenic patients , the in density was decreased in the superficial wm , but increased in the deeper wm , with nadph - d+ in showing the same maldistribution as microtubule associated protein 2 ( map2 ) positive cells in general ( akbarian et al . , 1993 , 1996 ) . while some studies reported an increase in in density in inferior parietal and dorsolateral prefrontal areas in deficit syndrome patients ( kirkpatrick et al . , 1999 , 2003 ) , others observed no change neither in superficial nor in deep white matter ( beasley et al . , 2002 ) . the conflicting reports on the changes of in density in schizophrenia were summarized by eastwood and harrison ( 2005 ) , who observed a density increase in the superficial wm and no change in deeper compartments . a special and rather minor subclass of in expresses the extracellular matrix molecule reelin , which is important for brain development and adult neuronal plasticity ( reviewed by herz and chen , 2006 ; tissir and goffinet , 2003 ) . in the adult cortical gray matter , reelin is expressed by a subgroup of gabaergic interneurons ( pesold et al . , 1998 ) ; in the wm , very few scattered reelin+ cells can be visualized using immunohistochemistry ( figure 2e ) . conversely , reelin mrna has been reported to be abundant in in in the superior temporal cortex , and to be significantly reduced in schizophrenic patients ( eastwood and harrison , 2003 ) , in keeping with the finding that alterations of reelin expression are a putative vulnerability factor in schizophrenia and mood disorders ( reviewed by fatemi et al . , 2008 ) . most discussions of in changes in psychoses are based on the view that in are remnants of the early - born subplate population . the early generation of in in rodents and carnivores , and their maldistribution in schizophrenic patients , have led to the hypothesis that a migration defect of the subplate during embryonic or early fetal development underlies the pathogenesis of schizophrenia . as stated above , the developmental history of the subplate is very different in nonprimate mammals and in primates including human ( kostovic and rakic , 1980 ; meyer , 2007 ; meyer et al . , 2000 ; smart et al . , 2002 ) . an important task for future research would be a molecular taxonomy of all neuronal populations in the wm , similar to the work done on interneurons in the gray matter . it would be particularly important to differentiate between early - born components , probably related to transient roles of the subplate , and later - appearing resident cells , which may not be important for development but rather involved in activities proper to the adult wm . the recent discovery of subplate - specific molecules in mice ( hoerder - suabedissen et al . , 2008 ) is a useful step in this direction , and it is hoped that similar work will shed light on the origins , categories and functional roles of human in . the authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest .
the white matter ( wm ) of the adult human neocortex contains the so - called 
 interstitial neurons  . 
 they are most numerous in the superficial wm underlying the cortical gyri , and decrease in density toward the deep wm . 
 they are morphologically heterogeneous . 
 a subgroup of interstitial neurons display pyramidal - cell like morphologies , characterized by a polarized dendritic tree with a dominant apical dendrite , and covered with a variable number of dendritic spines . 
 in addition , a large contingent of interstitial neurons can be classified as interneurons based on their neurochemical profile as well as on morphological criteria . 
 wm- interneurons have multipolar or bipolar shapes and express gaba and a variety of other neuronal markers , such as calbindin and calretinin , the extracellular matrix protein reelin , or neuropeptide y , somatostatin , and nitric oxide synthase . 
 the heterogeneity of interstitial neurons may be relevant for the pathogenesis of alzheimer disease and schizophrenia . 
 interstitial neurons are most prominent in human brain , and only rudimentary in the brain of non - primate mammals . 
 these evolutionary differences have precluded adequate experimental work on this cell population , which is usually considered as a relict of the subplate , a transient compartment proper of development and without a known function in the adult brain . 
 the primate - specific prominence of the subplate in late fetal stages points to an important role in the establishment of interstitial neurons . 
 neurons in the adult wm may be actively involved in coordinating inter - areal connectivity and regulation of blood flow . 
 further studies in primates will be needed to elucidate the developmental history , adult components and activities of this large neuronal system .