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Cerebral Cortex January 2007;17:130--137
doi:10.1093/cercor/bhj134
Advance Access publication February 1, 2006
A Temporal Continuity to the Vertical
Organization of the Human Neocortex
Manuel F. Casanova, Juan Trippe II and Andrew Switala
Radial translaminar arrays of pyramidal cells—minicolumns—are
a pervasive structural motif of placental mammalian neocortex,
which are anticipated in the earliest stages of cortical development
by the formation of ontogenetic cell columns comprising radial glial
cells and associated radially migrating neurons. In the present study
we examine the temporal continuity in these structures throughout
development and aging. Computerized image analysis of micrograph
Nissl-stained postmortem tissue produced estimates of the median
free path through neuropil in the radial direction (parallel to pyramidal
cell arrays) and in the tangential direction (parallel to the cortical
surface). These data were modeled as a biphasic power law with
respect to in utero development and postnatal age, multiplied by
a decay factor. No significant change in the ratio of radial to
tangential neuropil space was demonstrated in either the prenatal
or postnatal sample population. Neuropil development follows a prenatal phase of cubic volumetric growth with a postnatal phase of
linear volumetric growth. The data suggest the continuity of columnar
structures from early in gestation through postnatal maturation.
Minicolumnar anatomy has been characterized principally in
terms of 4 radially oriented components: pyramidal cell arrays,
apical dendritic bundles, myelinated axon bundles of projection
neurons, and double-bouquet cell axon bundles of inhibitory
interneurons. Each of these features has been shown to exhibit
minicolumnar-scale periodicity in various species and cortical
areas (von Bonin and Mehler 1971; Seldon 1981; DeFelipe and
others 1990; Ong and Garey 1990; Viebahn 1990; Peters
and Sethares 1991, 1996; Ferrer and others 1992; del Rı́o and
DeFelipe 1997; Peters and others 1997). Peters and Sethares
(1996) were the first to characterize the spatial relation of
pyramidal cells in layers III and V with the radially oriented
translaminar dendritic bundles arising from them, calling these
assemblages ‘‘pyramidal cell modules.’’ Lohmann and Köppen
(1995) further demonstrated in rat V1 cortex that apical
dendritic and myelinated axon bundles project in register
with each other at minicolumnar-scale intervals (52.6 and 50.1
lm, respectively), consistent with other studies (Buxhoeveden
and Casanova 2002). Double-bouquet axons in peripheral
neuropil similarly were found to align with myelinated axon
bundles. This pattern of alignment and of double-bouquet
synaptic contacts with pyramidal cells was found to be similar
in human temporal and macaque visual cortex (del Rı́o and
DeFelipe 1997). This suggests that they are part of a general
organizing motif in primates. Thus, linkages demonstrated
among these principal components show that they provide
redundant measures of minicolumnar center-to-center spacing
and suggest that inferences might be made from measurements related to any component/compartment and overall
minicolumnar dimensions. Several studies have endeavored to
develop quantitative methods, biased (Schlaug and others 1995;
Buldyrev and others 2000; Buxhoeveden and others 2000) or
unbiased (Skoglund and others 2004; Vercelli and others 2004)
in the stereological sense, for assessing minicolumnar morphometry in terms of parameters such as verticality, columnar
width, mean cell spacing, compactness, gray-level index, and
others.
The developmental relation between ontogenetic cell columns and the mature cortical minicolumn has been heretofore
relatively unexplored. Optical imaging of physiologic activity in
fetal rat brain slices revealed columnar structure, which the
investigators identified with both radial units and mature
cortical columnar units (Yuste and others 1992). The diameter
of these structures (50--120 lm in neonatal rat) is greater than
that typical of minicolumns in mature rat cortex. Further, the
linking of cells within them by gap junctions suggests that these
structures may be aggregations of cell columns coordinating
activity in larger modular units (Weissman and others 2004).
Keywords: brain, development, minicolumns, neocortex, neuropil
Introduction
The minicolumn is an elemental modular unit of the cortex
(Mountcastle 1997; Buxhoeveden and Casanova 2002) serving
as an architectonic template according to which are arranged
representative cortical cellular elements and their connections.
A chain of excitatory neurons with their axonal and dendritic
bundles constitutes its core, extending radially in layers II
through VI (Peters and Sethares 1991, 1996). Around the
periphery of this core is a neuropil containing synaptic elements integrating translaminar connections. Peripheral neuropil also contains inhibitory interneurons (prominently radially
oriented GABAergic double-bouquet cells) providing a ‘‘sheath’’
of inhibitory activity (del Rı́o and DeFelipe 1997). A given
minicolumn is characterized functionally by a common set
of physiological response properties of its constituent neurons. The basic minicolumnar circuit is distributed iteratively
throughout neocortex, incorporated into a nested hierarchy of
modular units. Cortical microstructure exhibits variability intraand interareally across hemispheres within a given brain, as well
as among individuals within and across species with regard to
cell size, density, number, and size of axonal and dendritic
bundles. Nevertheless, the basic organization of microcircuitry
is maintained throughout the cortex in all individuals and
species (Douglas and Martin 2004; Buxhoeveden and Casanova
2005). Similarly, with changes in connections and synaptic
content, temporal as well as spatial contiguity is maintained in
minicolumnar organization.
Department of Psychiatry and Behavioral Sciences,
University of Louisville, Louisville, KY 40292, USA
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Krmpotić-Nemanić and others (1984) identified vertical cell
columns in a limited developmental series derived from human
fetal and neonatal auditory cortex, reporting that they could
trace the developmental transformation from ontogenetic cell
columns into mature minicolumns. A later study of human
fetal cortical development identified lamina-specific differences in emergence of minicolumnar morphological features
(Buxhoeveden and others 1996). McKinstry and others (2002)
conducted an in vivo imaging study of human cortex in subjects aged 26--40 gestational weeks (GW). They employed diffusion tensor imaging to determine patterns of water diffusion
anisotropy in developing cortex. Water diffusion patterns are
determined by tissue microstructure and reflect alignment of
lipophilic cell membranes and myelin sheaths. Radially oriented
anisotropy at 26 GW indicates vertical orientation of cortical
structures. At 36 GW radial anisotropy decreases, reflecting the
superimposition of tangentially oriented collateral elements
upon the primary vertically oriented cortical structure. None
of these previous studies, however, employed quantitative
methods to determine contiguity of radial structures during
development.
The purpose of our study was to provide an unbiased
quantitative analysis of change over time in the morphometry
of minicolumnar peripheral neuropil. We derived a model to
describe rate of change in median neuropil space (MNS) or
average radial or tangential distance from a given point in
neuropil space to a threshold boundary delimiting a cellular
compartment. A ratio of radial to tangential MNS greater than
unity is consistent with vertical orientation of peripheral
neuropil space. Lack of significant change in the ratio of radial
to tangential MNS during gestation and postnatal life is consistent with a stable vertical columnar organization. This
negative finding would support the assertion that there is
a continuity of structural identity between ontogenetic cell
columns appearing in the cortical plate during the second
trimester and the cortical cell minicolumn identified in the
mature brain.
Methods
Materials
Postmortem materials were obtained from the Yakovlev-Haleem collection of the Armed Forces Institute of Pathology, Washington, DC. Our
sample consisted of sections obtained from the brains of 13 males and
6 females, from 4 months to 98 years of age at death (mean 38.5 years,
standard deviation 33.6 years). All postnatal individuals in our sample
died from nonneurological causes. Postmortem tissue from these
individuals revealed no sign of neuropathology. Tissue samples were
celloidin embedded, cut into 35-lm-thick serial coronal sections, and
were Nissl-stained. Prenatal materials were obtained from 47 individuals
(22 male and 25 female) either stillborn or deceased within a few days of
birth, with gestational ages ranging from 10 to 42 GW. These cases were
fixed in formalin, embedded in celloidin, and cut into coronal sections
that were 15- to 30-lm thick depending on gestational age.
Image Acquisition and Preprocessing
Measurements were obtained in the left hemisphere of the temporal and
parietal lobes in 67 individuals. One coronal section was selected, per
individual, at approximately the middle posterior--anterior axis. Four
micrographs were taken of each section from the postnatal cases,
sampling Brodmann areas 4, 21, 22, and 40. Three micrographs were
taken of each section of fetal tissue, where cytoarchitectural areas are
undeveloped, at locations approximately 30°, 90°, and 150° off the
inferior--superior axis. The region of interest in each image was outlined
manually to include cortical laminas II through VI. The images were
adaptively thresholded using Otsu’s method in each 256-by-256 pixel
block. Threshold values so calculated were applied to the center of the
block; the threshold value of each pixel in the image is obtained using
bilinear interpolation through the 4 block centers nearest the pixel. The
binary images were morphologically smoothed using a circular structuring element of radius 1 pixel.
Image Analysis
Let I be an image of Nissl-stained cerebral cortex, with Fourier transform J. Pyramidal cell arrays give rise to a striped appearance, or texture,
whereas the individual cell somata produce a blotchy texture. Superposition of these textures is reflected in J, where minicolumnar
structure stands out as a strongly directional signal against an isotropic
background. The dominant orientation h of the signal is found using the
formula (Pourdeyhimi and others 1994)
h=
2
1 – 1 +k j Jk j sin 2hk
tan
;
2
+k j Jk j2 cos 2hk
where j Jkj is the amplitude of component k of the discrete Fourier
transform, and the sum is over all 2-dimensional frequencies fk = ( fk,gk).
The angle hk is not the phase of Jk but the direction of the signal giving
rise to the Fourier component at that frequency, that is tan hk = gk =fk :
The directionality, or strength of the directed texture, is given by
Pourdeyhimi and others (1994) as
D2 =
+k j Jk j2 cos 2ðhk – hÞ
;
2
+k j Jk j
which is normalized to lie between zero and one.
A local estimate of r9 is produced by computing the above quantities
in small (about 0.20 mm2) regions of an image:
r9region = ðcos hregion ; sin hregion Þ:
Two modifications to this basic method ensure that the estimates of r9
do not vary unrealistically across the image: neighboring regions are
made to overlap substantially so that the respective estimates therein
are not independent. In addition, the final result is smoothed by a 3-by-3
weighted median filter. Local directionalities D2 are used for weights,
and the median filter is applied to the components of r9 individually.
The neuropil distribution is derived from the empirical ‘‘linear contact
distribution’’ (Fig. 1), the distribution of distances from pixels in the
region of interest to the nearest object, measured along a line with fixed
orientation u. In the event that the pixel of origin is itself part of an
object, the nearest neighbor distances are zero. Let Lðr ; uÞ be the
cumulative distance function of the linear contact distances, and let
L̃ðr ; uÞ be the cumulative distribution function conditioning on the
event that the pixel of origin lies in the neuropil
L̃ðr ; uÞ =
Lðr ; uÞ – Lð0Þ
:
1 – Lð0Þ
Then the median radial neuropil space is sR = L̃ –1 ð0:5; hÞ; and the median
tangential neuropil space is sT = L̃ –1 ð0:5; h + ðp=2ÞÞ:
Results
MNS estimates were binned for purposes of statistical analysis.
The mean of sR (respectively, sT) over the 3 or 4 micrographs
of each brain was taken as the estimate of sR (respectively, sT)
for the temporal/parietal region of the cerebral cortex. Postnatal neuropil development was modeled as a power law,
representing early growth, and multiplied by an exponential
curve, representing the effects of aging. Prenatal development
was modeled to follow a power law with a different growth
rate. Although the growth rate may change at birth, the neuropil space must change continuously with age: prenatal and
postnatal developmental function curves are contiguous. These
considerations lead to the functional form s = b0 g b1 ðg + aÞb91 e–b2 a ,
Cerebral Cortex January 2007, V 17 N 1 131
Figure 1. Estimation of the linear contact distributions. The local radial direction r9 in each panel is 30° anticlockwise from vertical. The free path from a point in the neuropil (center
of red circle) is the distance to the nearest object in either direction along a straight-line path with fixed orientation. Typically, the radial free path is greater than that in the tangential
direction (A), although the opposite also obtains (B). The free path is identically zero for points outside of the neuropil (C). The free path from points near the boundary of the field of
view (D) may be less than the distance to the nearest visible object; the data are censored from above. Such observations are incorporated using a Kaplan-Meier--type estimator for
the linear contact distribution.
where g is gestational age divided by 40 weeks and a is postnatal
survival, also divided by 40 weeks. The dependent variable s is
the MNS (radial or tangential) for a given individual.
The model was fit (Table 1) using ordinary least squares after
taking the logarithm of both sides. Parameters b1, b91 ; and b2
were functions of direction (tangential or radial), and b0 was
modeled as a function of the sex of the individual and direction.
Each of the 4 parameters differed significantly (P < 0.05) from
zero with Student t scores 65.2, 8.8, 12.8, and 5.1 for b0, b1, b91 ;
and b2, respectively, with 59 degrees of freedom. Section
thickness (see Materials) was not accounted for, so neuropil
in the youngest prenatal cases may have been overestimated.
The effect on the results, if any, would be a bias toward zero
in b1. Directional dependence was significant only for the
parameter b0 (t = 3.02). Sex dependence in b0 was statistically
insignificant (t = –0.16; P = 0.87). Significance tests on t scores
were 2 sided.
Statistically significant directional dependency of the neuropil space, with the median distance measured radially exceeding
the median distance measured vertically, is consistent with
a columnar arrangement. Here, the bulk of the neuropil lies in
the periphery of minicolumns, where the mean free path is
greater in the radial direction, staying within the periphery, than
in the tangential direction, oriented toward a pyramidal cell
132 Minicolumns
d
Casanova and others
Table 1
Parameters of the fitted model
Parameter
b0
b1
b91
b2
rad
tan
MLE
CI
7.69 lm
6.43 lm
0.62
0.33
0.0064
(7.08 lm, 8.36 lm)
(5.92 lm, 6.98 lm)
(0.48, 0.76)
(0.28, 0.38)
(0.0039, 0.0088)
Note: Insignificant effects are averaged, so that values of b0 are the average for male
and female, and values of the other parameters are the average over both directions.
MLE 5 maximum likelihood estimate; CI 5 95% confidence interval; rad 5 radial direction;
tan 5 tangential direction.
array. There is no evidence that the ratio of median radial
neuropil space to median tangential neuropil space changes
over time (Fig. 2). The absence of significant directional
dependency in the rates of change in neuropil space with aging
suggests that the basic columnar structure is preserved
throughout pre- and postnatal development.
The estimated rate of prenatal neuropil growth is b1 + b91 1:
Because MNS s has dimensions of length, the total volume of
cortical neuropil increases ‘‘cubically’’ with time in utero.
Shortly after birth, the estimated growth rate drops to
Figure 2. Median free path from an arbitrary point in the neuropil. Solid lines are the
expected values of the fitted model function for the radial (red) and tangential (blue)
data.
b91 1=3; corresponding to cortical neuropil volume increasing
‘‘linearly’’ with age. The effective rate of postnatal neuropil
growth decreases due to the decay term b2 and becomes zero at
about 40 years of age and negative thereafter.
The model was subsequently fit to the postnatal series alone
in order to examine possible differences between cortical areas
(Fig. 3). Parameter b1 was omitted because g is assumed equal to
unity for the postnatal series. Parameters b0, b91 ; and b2 were
permitted to vary with cortical area. No statistically significant
difference in b0 was found between areas (F3,142 = 0.564, P =
0.64). Regional variability in b91 and b2 were even less significant.
Discussion
Our study is the first to employ unbiased quantitative morphometric techniques in the analysis of the development of radial
structure in cortex. Our principal finding provides strong
support for the hypothesis that values of structural parameters
of neuropil space exhibit stability over the course of development. Our findings suggest that continuity is maintained in the
morphometry of radial minicolumns in human neocortex
throughout the life span, from emergence of the laminated
cortical plate until late maturity. Additionally, our data were
derived from postmortem tissue exhibiting no sign of neuropathology and obtained from individuals dying from nonneurological causes. Our results thus provide normative data as a basis
of comparison for analysis of minicolumnar morphometric
changes potentially associated with various neurological or
psychiatric pathologies.
The minicolumnar organization of cortex is anticipated in the
earliest stages of telencephalic development. The ventricular
neurepithelium comprises a mosaic of radial units. These are
spatially discrete clonal aggregates consisting of neuronal
precursors and transient radial glial (RG) cells that serve as
progenitors for their radial units. Neurons thus migrate radially
outward into the cortical plate along the processes of RG cells.
Glial processes provide a lattice for maintaining the linear
spatial organization of daughter neurons, which in later stages
migrate hundreds of microns into an expanding cortical plate
that is starting to form convolutions and convexities (Rakic
1988; Kriegstein and Noctor 2004). These ontogenetic cell
columns are the first vertically oriented structures appreciable
in the fetal cortex and provide the matrix for its further
development. Subsequently, neurites grow radially from the
soma of ontogenetic columns and develop first into apical
dendrites and then other processes (Hirst and others 1991).
Ontogenetic cell columns have been identified in a wide range
of mammalian species, suggesting that they are a well-conserved
feature of cortical development (Gressens and Evrard 1993).
Selective influences operating during development and on
functional circuits in the mature cortex combine to maintain
continuity of radial morphology throughout the life span.
Phylogenetic expansion of the mammalian cortical sheet,
particularly in primates, required adaptations of radial units of
glia and their related neuronal progenitors. Increased gyrification in larger primate cortices resulted in longer, curvilinear
radial migratory pathways (Rakic 2003). Associated with this
trend, RG and neuronal progenitor populations in ventricular
zone exhibit specialized molecular and structural phenotypes
(Hartfuss and others 2001; Rakic 2003; Weissman and others
2003). These features are identified at the earliest stages of
corticogenesis and in primate radial glia include early glial
fibrillary acidic (GFAP) immunoreactivity and lamellate expansions (Schmechel and Rakic 1979; Levitt and Rakic 1980; Levitt
and others 1981, 1983). This neocortical expansion provided the
substrate for functional specialization of minicolumns derived
from these radial units, both locally within macrocolumnar
aggregates, as well as among various cortical areas. Regional
differences in proliferative kinetics, both of symmetrically dividing clonal founding progenitors as well as progenitors within
each clonal unit could result in regional variation in minicolumnar numbers and size (Kornack and Rakic 1998). Hypothetically,
each of a diverse array of specialized minicolumns could provide
a structurally stable template upon which plastic and specific
activity-driven circuits could later be imposed.
A notable feature of the enlarged primate cortex and its
regional and columnar specializations is the large proportion of
inhibitory interneurons derived from dorsal telencephalic proliferative zones (Letinic and others 2002) in contrast to rodent
cortex in which most interneurons originate in the subpallium
(Marı́n and Rubenstein 2003). The primate brain contains
a greater proportion and diversity of interneurons (DeFelipe
and others 2002). Radially migrating lineages of inhibitory
interneurons on the basis of common intrinsic mechanisms
and signaling environments may be better coordinated with
colocalized pyramidal cells in the formation of minicolumns
exhibiting greater numbers and types of interneurons. Regionaland laminar-specific production of inhibitory and excitatory
neurons might thus be closely matched. This is particularly
relevant to the development of layer IV, as differences in total
number of neurons in columnar volume between various
cortical areas are due largely to differences in numbers of layer
IV cells (Beaulieu and Colonnier 1989).
This is also relevant to the greater elaboration and plasticity of
inhibitory synaptic networks in layer IV. In rats, layer IV had 62%
more GABAergic synapses than other layers due to increased
number of contacts per bouton, rather than higher number of
boutons, with a comparable total area of synaptic coverage
(Beaulieu and others 1994). Remodeling of synaptic elements
occurs within a framework of axonal and dendritic arbors and
boutons that maintains morphometric stability. An enriched
environment for rats results in consolidation of the number of
Cerebral Cortex January 2007, V 17 N 1 133
Figure 3. Data from Brodmann areas 4, 40, 21, and 22 postnatally are very similar. No statistically significant differences were found (note that x axis and y axis scales are linear,
not logarithmic as in Fig. 2).
synaptic contacts in visual cortex layer IV but does not affect
the ratio of boutons per neuron (Beaulieu and Colonnier 1988).
Stable laminar-specific neuronal architecture allows for compartmentalization of function within each layer. Mean densities
as well as the ratio of symmetric to asymmetric synaptic
contacts in macaque V1 are not affected by early enucleation.
Ratio of spine-to-shaft contacts in layer IV does not undergo
comparable shift seen in animals with normal vision. Thus,
experimental animals exhibit layer-specific (to thalamoreceptive sublayers) alteration of fine-scale contacts within minicolumns whose overall morphometry is preserved (Bourgeois and
Rakic 1996). As well as increased numbers of inhibitory
interneuron elements within primate striate cortex layer IV,
the developing primate cortical mantle also exhibits an expanded subplate (Kostović and Rakic 1990). Transient radial
circuits between subplate and layer IV subserve the establishment and organization of thalamocortical projections to layer IV
and subsequent synaptic remodeling of those projections
(Allendoerfer and Shatz 1994; Kanold 2004). Ablation of
subplate affects organization of both ocular dominance columns
(Ghosh and Shatz 1992) as well as minicolumnar-scale orientation columns (Kanold and others 2003).
Minicolumnar compartmentalization may also be affected by
geometric constraints, which serve to maintain overall morphometry. Layer II neuron counts are comparable across cat
visual areas. Whereas the monocular region of area 17, area 18,
and posteromedial suprasylvian have comparable cell counts
under a specified area (Rockel and others 1980), the laminar
distribution of neurons varies. Thus the relative decrease in cell
numbers in layer IV of the latter 2 areas is compensated for by
increases in other layers, primarily layer III (Beaulieu and
Colonnier 1985). This finding suggests that the total cell count
of the minicolumnar laminae and corresponding minicolumnar
134 Minicolumns
d
Casanova and others
dimensions might be constrained by packing limits imposed by
their associated radially oriented processes. Thus, greater width
of axonal and apical dendritic bundles and of vertically oriented
inhibitory interneurons required to modulate them would
increase the tangential distance between minicolumns requiring ever greater expansion of the cortical sheet. This subsequently would impose escalating wiring, metabolic, and
signal-timing costs (Striedter 2005). Neocortical minicolumns
therefore serve as a modular template or tabula rasa within
which a stable matrix of dendritic and axonal arbors provides an
array of potential synaptic connection sites (Kalisman and
others 2005). These sites develop into functional synaptic
connections presumably via coordinated pre- and postsynaptic
activity. Multiple functional microcircuits exhibiting both finescale specificity and plasticity can be overlain within this
common matrix of potential connections (Stepanyants and
Chklovskii 2005).
The maintenance of constant ratios of radial to tangential
MNS over the course of development and the scale of these
dimensions on the order of 10 lm suggests that these
morphometric relations reflect an intrinsic circuit organization
present from the earliest stages of cortical plate formation. This
fundamental organization of neuropil may serve as a template
upon which later epigenetic influences operate. Ben-Ari and
others (2004) reviewed evidence obtained in hippocampus
indicating that GABAergic interneuron networks play a formative role in the organization of cortical microcircuits. They
suggest that early—initially excitatory—GABAergic synapses
among phenotypically diverse interneurons in peripheral neuropil establish patterns of activity by which pyramidal neurons,
quiescent at this stage, may later be coordinated in the
formation of circuits. As robust radially oriented morphometry
implies stereotypical synaptic organization, so conversely,
stereotyped arrangements of cells and their connections into
radially oriented circuits implies an origin in early ontogeny for
radial orientation of cortical structure. Budd and Kisvárday
(2001) and Kozloski and others (2001) showed that local
connections are highly specific, stereotypic, and determined,
tending to fall within minicolumn dimensions. Kozloski and
others (2001) found precisely determined cortical microcircuits in mouse layer V cortex, showing that local connectivity is
not probabilistic or random. The position of the targeted
neurons appeared ‘‘remarkably precise, indicating robust developmental control of circuit formation.’’ Stability of minicolumnar architecture throughout the life span supports recent
findings that neuronal generation in the mature brain is limited
to specialized niches (Kornack and Rakic 2001). Introduction of
newborn neurons in the adult cortex might result in heterotopic structures potentially disrupting the organization of
established specific circuits.
This high degree of developmental continuity in the radial
alignment of peripheral neuropil spatial features may in part
reflect an optimization of the density and spatial arrangement of
processes and synaptic elements in relation to neuronal cell
bodies. This optimization may encompass constraints of signal
timing, metabolism, and degree of dendritic arborization required to maintain given synaptic densities, all in turn determining cell body packing arrangements (Rakic and others
1994; Chklovskii and others 2002; DeFelipe and others 2002). In
support of this view, a number of investigators as cited by
DeFelipe and others (2002) have reported a degree of uniformity in synaptic densities among different cortical areas within
and across species (O’Kusky and Colonnier 1982; Beaulieu and
Colonnier 1989). This uniformity is anticipated in a common
time course and rate of production/regression of synapses
observed throughout primate cortex (Rakic and others 1986).
Subsequently, other studies using more refined quantitative
methods revealed significant differences in neuronal and
synaptic density across layers within a given area in intraspecies
and cross-species comparisons of visual and somatosensory
cortex in mouse, rat, monkey, and human (Beaulieu and others
1992, 1994; DeFelipe and others 2002). These differences may
reflect a functional laminar compartmentalization of minicolumnar architecture whereby synaptic density is regulated by
the interaction of cell-intrinsic determinants with thalamocortical and monoaminergic modulatory inputs. For example,
dopamine D1 and D2 receptors have a layer- and region-specific
distribution (Lidow and others 1991) and modulate synapses of
projection neurons (Yang and Seamans 1996). This activity
serves to gate corticocortical and corticothalamic feedback
activity underlying working memory (Sawaguchi and GoldmanRakic 1991) and is reflected in regulation of synaptic densities
in prefrontal cortex (Sugahara and Shiraishi 1999).
A number of investigators have asserted a general inverse
relation between neuronal density and number of synapses per
neuron (Cragg 1967; O’Kusky and Colonnier 1982; DeFelipe
and others 2002). In analyses of human, rat, and mouse cortex
this relation was confirmed in comparing all layers of human
cortex with those of rat and mouse, yet the relation did not hold
in interlaminar comparisons in human and mouse (DeFelipe and
others 2002), nor was there a common pattern found in
determining ratios of asymmetric to symmetric synapses.
Notably, this group found that thickness of layers II and III in
the human was greater than twice that of the rat and 3 times
that of the mouse, whereas the neuronal density was less than
one-half and one-quarter of that of the rat and mouse, respectively. This plainly suggests the presence of nonneuronal
components and a spatial organization specific to supragranular
layers of human cortex. Human cortex contains a greatly
expanded network of astrocytes in comparison with nonprimate species and, in particular, palisades of vellate astrocytes
with superficially situated cell bodies, and processes extending
inward through supragranular layers may represent a significant
nonneuronal component of neuropil (Reisin and Colombo
2002; Colombo and Reisin 2004). It will thus be profitable for
future studies to conduct similar morphometric analyses of
verticality in neuropil using immunocytochemical labeling for
GFAP or other nonneuronal markers.
In summary, the results of this study are notable for the fact
that no significant change in the ratio of radial-to-tangential
neuropil space was demonstrated in 67 individuals representing
a full range of prenatal and postnatal stages of cortical development. This lends strong support to the hypothesis that
there is a fundamental temporal continuity in the vertical
organization of cortex, preeminent to formation of laminas
and other higher-order organization, for example, synapse
formation. This vertical organization provides the fundamental
structural motif within which processes mediating cortical
plasticity such as synaptic remodeling and apoptosis might
occur during the course of development.
Notes
This article is based upon work supported by the Stanley Medical
Research Foundation, the National Alliance for Autism Research, and
National Institute of Mental Health grants MH61606, MH62654, and
MH69991. The authors express their gratitude for their helpful advice
and facilities to Mr Archibald Fobbs (Armed Forces Institute of
Pathology) and Elizabeth Lockett (National Museum of Health and
Medicine).
Funding to pay the Open Access publication charges for this article
was provided by NIH grant MH062654.
Address correspondence to Manuel F. Casanova, MD, Gottfried and
Gisela Endowed Chair in Psychiatry, Department of Psychiatry, University of Louisville, 500 South Preston Street, Building A, Room number
217, Louisville, KY 40292, USA. Email: [email protected].
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