Download Development of the human cerebral cortex: Boulder Committee

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Development of the human cerebral
cortex: Boulder Committee revisited
Irina Bystron*||, Colin Blakemore* and Pasko Rakic§
Abstract | In 1970 the Boulder Committee described the basic principles of the development
of the CNS, derived from observations on the human embryonic cerebrum. Since then,
numerous studies have significantly advanced our knowledge of the timing, sequence and
complexity of developmental events, and revealed important inter-species differences. We
review current data on the development of the human cerebral cortex and update the
classical model of how the structure that makes us human is formed.
Neocortex
The evolutionarily newest
portion of the cerebral cortex.
It is particularly enlarged in
primates and underpins higher
mental functions for humans.
Neuroepithelium
A layer of proliferating
neuroepithelial cells that
makes up the neural plate
and neural tube.
*Department of Physiology,
Anatomy & Genetics,
University of Oxford, Oxford,
OX1 3PT, UK. ||Department of
Morphology, Institute of
Experimental Medicine, St
Petersburg, 197376, Russia.
§
Department of Neurobiology
and Kavli Institute of
Neuroscience, Yale University
Medical School, New Haven,
Connecticut 208001, USA.
Correspondence to P.R. & I.B.
e‑mails:
[email protected]; irina.
[email protected]
doi:10.1038/nrn2252
The mammalian cerebral cortex is a complex laminated
structure that contains a bewildering diversity of neurons
and has rich local and extrinsic connectivity. Regional
variations in cytoarchitecture are superimposed on a
common plan of layers, cell types and connections1.
The explosion in the size of the cerebral hemispheres
during mammalian evolution is correlated with increasing behavioural and cognitive capacity. The enormous
human cortex underpins our perception, memories,
thoughts and language.
In all mammals, the neocortex forms at the outer surface
of the embryonic cerebral vesicle, at the rostral end of the
neural tube, through the migration of neurons from proliferative regions near the cerebral ventricle2,3. The arrival
of migrating neurons establishes laminar compartments,
some of which change or even disappear during development. The sequence of events that take place during cortical development, as it was then understood, was described
more than 35 years ago by the Boulder Committee4. This
committee, meeting in Boulder, Colorado, was convened by the American Association of Anatomists, the
main forum for the growing fields of neuroanatomy and
embryology. The committee’s remit was to standardize
the heterogeneous and confusing nomenclature for the
developing vertebrate CNS that had emerged from
the blossoming of knowledge. Richard Sidman presented
diagrams of embryonic cortical development (FIG. 1A), based
on the observations of P.R., then an assistant professor
in his department. The committee’s report stated:
The layers and cells of the early developing
central nervous system lack direct counterparts
in the adult and must be designated by a special
terminology. The inconsistent and inaccurate
language now in use leads to misunderstanding
and a revision is proposed in which the four
110 | february 2008 | volume 9
fundamental zones are termed the ventricular,
subventricular, intermediate, and marginal zones.
Each is defined according to the form, behaviour,
and fate of its constituent cells. All neurons and
macroglia of the central nervous system can be
derived from these developmental zones.
Thus, the committee recommended names for each
of the transient embryonic cellular compartments (or
‘zones’) and gave their interpretation of the major developmental events. This model has been widely adopted
as a generic description of the development of the entire
vertebrate CNS.
In the past three decades, experimental studies on
normal and genetically altered rodents have driven the
analysis of cortical development. This has given us new
insights into the regulation of cell division and programmed cell death, which determine neuron number5,6,
and the mechanisms of neuronal migration3,7,8. Patterns
of genetic expression are being elucidated, including the
expression of transcription factors that are thought to
influence regional differentiation and regulate broad
aspects of mitotic activity, fate determination and differentiation9–12. Recent studies have revealed new types
of transient neurons and proliferative cells outside the
classical neuroepithelium, new routes of cellular migration
and additional cellular compartments13–18. Furthermore,
we know much more about intrinsic connectivity and
the formation of axonal projections into and out of the
cortex19–21. Thus, our understanding of the timing and
sequence of developmental events in the cerebral wall
has changed radically since the work of the Boulder
Committee.
Despite the march of knowledge, the Boulder
Committee’s nomenclature continues to be widely used.
Their summary diagram (FIG. 1A) has been reproduced,
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M
CP
M
CP
I
M
I
I
M
V
V
Aa
V
Ab
S
S
V
V
Ac
Ad
Ae
(SG)
MZ
CP
MZ
SP
CP
IZ
IZ/SP
PP
SVZ
VZ
Ba
PP
SVZ
SVZ
VZ
VZ
VZ
Bb
Bc
VZ
Bd
Be
Figure 1 | The Boulder Committee’s 1970 schematic model of human neocortical development, and a proposed
Neuroscience
revision. A | The Boulder Committee’s original summary diagram of neocortical development.Nature
B | OurReviews
revised| version.
Comparison of these two illustrations summarizes our redefinition of the sequence of events and the formation of transient
compartments, including the preplate (PP) and the intermediate and subplate zones (IZ and SP). The panels in part B
correspond to the following approximate ages (for the lateral part of the dorsal telencephalon): a: embryonic day (E) 30;
b: E31–E32; c: E45; d: E55; e: gestational week 14. CP, cortical plate; I & IZ, intermediate zone; M & MZ, marginal zone;
S & SVZ, subventricular zone; (SG), subpial granular layer (part of the MZ); V & VZ, ventricular zone. Part A reproduced,
with permission, from REF. 4  (1970) Wiley.
in its original form or modified, in virtually every monograph on neuroembryology, as well as in textbooks and
research papers on the development of the CNS in a
wide variety of mammals22–26. Attempts have been made
to integrate more recent observations into the Boulder
Commitee’s scheme24,27–29, but we believe that it is time
for their description and nomenclature to be updated to
reflect new developments in the field.
Here we summarize the major advances in our
understanding of the development of the cerebral cortex
and incorporate new cell types and cellular zones into a
revised version of the Boulder model. We recommend
a nomenclature that preserves the essential features
of the Boulder scheme but which would, if generally
nature reviews | neuroscience
adopted, avoid some of the difficulties that have emerged
as researchers have tried to set recent discoveries into the
traditional Boulder framework.
Recent research has revealed potentially important
differences between rodent and human development
that have exacerbated the confusion and diversity in
the use of Boulder terminology. Unless the species is
specifically mentioned, the updated model of corticogenesis that we present here is based on observations
of material derived from humans, whose protracted
development allows more precise sequencing of cellular
events. However, we have attempted to provide basic
nomenclature and definitions that are widely applicable
to other species.
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a Bis/H3
E30
b Bis/TU20
E32
c Bis/TU20
<
PP
<
<
<
VZ
<
VZ
<
d Bis/H3
E40–E41
<
<
PP
25 µm
e Bis/H3
E50–E51
<
VZ
Mes
Mes
25 µm
E33
Mes
25 µm
f Bis/H3
E56
<
SVZ
MZ
SVZ
PP
IZ/SP
MZ
<
>
Mes
VZ
CP
VZ
VZ
<
IZ/SP
CP
*
50 µm
<
*
100 µm
>
>
<
<
100 µm
<
>
>
<
>
<
>
<
<
Figure 2 | Early formation of layers in the embryonic human ventrolateral cerebral wall. The ventricular zone (VZ)
is seen in all sections. In a–d arrowheads demarcate the pial surface, with the mesenchyme (Mes)
outside
the |surface.
Nature
Reviews
Neuroscience
Bisbenzimide (Bis) staining of cell nuclei is shown in blue. Phosphohistone‑H3 staining of mitotic cells is shown in pink in a
and d–f. TU20 immunoreactivity, which labels postmitotic neurons, is shown in yellow in b and c. a | Initially the cerebral
wall consists entirely of neuroepithelial cells. b | Predecessor neurons, migrating into the cortical primordium from the
subpallium, initiate formation of the preplate (PP) at embryonic day (E) 31–E32. c | From E33, postmitotic neurons,
generated in the VZ, migrate radially to join the predecessor neurons. d | As shown in ref. 14, and illustrated in another
example here, at E40–E41 mitotic cells (indicated by the arrows) start to accumulate at the basal border of the VZ,
initiating formation of the subventricular zone (SVZ) before the cortical plate (CP) begins to form. e | As shown in ref. 64,
at E50–E51 radially migrating neurons initiate formation of the CP. The emerging CP splits the preplate into the marginal
zone (MZ) and a compartment that contains heterogeneous cell populations. At this stage the cell density in the
compartment between the SVZ and the CP is nearly homogeneous, and the boundary between the intermediate zone
and the incipient subplate is unclear. Thus, this compartment is called the intermediate/subplate zone (IZ/SP). f | By E56, at
the end of the human embryonic period, the lower part of the IZ/SP is seen as a cell-sparse area (demarcated with
arrowheads). Extrinsic axons invade this area in the most differentiated part of the ventrolateral cortex (see FIG. 4e–g).
Asterisks in e and f mark the striatocortical boundary. Parts a–c reproduced, with permission, from REF. 13  (2006)
Macmillan Publishers Ltd. Parts e and f reproduced from REF. 108.
In humans there is a clear gradient of maturation
across the hemisphere that is more prominent and prolonged than in species with smaller forebrains. Estimates
in this article of the timing of events are based primarily
on observations of the ventrolateral part of the human
dorsal telencephalon.
Neural plate
The neuroectodermal
epithelium before the neural
groove and neural tube form.
Interkinetic nuclear
movement
The apical-basal-apical
migration of the neuroepithelial
cell nucleus during the cell
cycle.
The proliferative zones: factories of the cortex
In humans the neural tube closes at embryonic day (E)
30 (ref. 30). Initially the telencephalic primordium is
composed entirely of dividing neuroepithelial cells (often
called neural stem cells), the direct descendants of cells of
the neural plate (FIG. 2a). These proliferative cells form the
‘matrix’, ‘germinal epithelium’ or ‘primitive ependyma’
described by classical embryologists, which was termed
the ventricular zone (VZ) by the Boulder Committee4.
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Before the onset of neurogenesis, the proliferative cells
of the VZ constitute a homogeneous pseudo-stratified
epithelium. They have radial processes31 and divide
symmetrically. The nucleus moves through the apical
process towards the ventricular surface shortly before
metaphase and cytokinesis, and migrates towards the
pial surface during DNA synthesis (a process termed
interkinetic nuclear movement)32,33.These cells have highly
polarized features, most notably a localized concentration of various transmembrane proteins (including
prominin‑1; also known as CD133), tight junctions
and adherens junctions at the apical pole (BOX 1), and
receptors for constituents of the basal lamina (including
integrin-α6) at the basal pole34,35.
Early proliferation increases the surface area and thickness of the VZ. This is particularly dramatic in the human
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Box 1 | What is up and what is down?
In naming the subventricular zone (SVZ) the Boulder Committee followed the
convention of classical embryology: the ventricular surface was considered the top of
the proliferative zone and the layers were described downwards from the ventricular
surface. Similarly, the term ‘apical’ traditionally signifies orientation towards, or
proximity to, the ventricular surface, whereas ‘basal’ refers to features further from the
ventricle.
To the contemporary observer, focused on the eventual functions of the cerebral
cortex, it is logical to view the pial surface as the top and therefore to think of
structures or layers that are closer to the pia as being superficial, outer or apical. This is
the convention for numbering the mature layers of the cortex. Layers closer to the pia
than layer 4 — the granular layer — are called supragranular; deeper layers are
infragranular. Equally, the major dendrites of pyramidal neurons, which are directed
towards the pia, are called ‘apical’ dendrites, whereas those at the lower pole of the cell
are ‘basal’ dendrites — this nomenclature is directly opposite in orientation to that of
the apical and basal poles of early neuroepithelial cells.
It might be logical to take the pial surface consistently as the reference for all
terminology. In this case the SVZ would be the ‘supraventricular zone’. However, the
term SVZ has been universally accepted and will undoubtedly be retained.
(Consistency, though, would demand that when the VZ disappears in the adult
cerebrum and is replaced by the ependyma, the SVZ ought to be called the
‘subependymal zone’, as it was originally in the neuropathological literature!)
We can see no way in which the inconsistency in defining what is up and what is down
can be eliminated, but it should be acknowledged and embraced. Current terminology
is fairly uniform in describing the orientation of features in the proliferative layers in
relation to the ventricular surface, whereas features in the developing layers of postmigratory cells (preplate, plate and subplate) are defined with respect to the pial
surface. We suggest that this convention be explicitly adopted.
embryo, in which the forebrain primordium is much
larger and the VZ much thicker than in rodents13. At a
certain point, neuroepithelial stem cells begin to switch
to an asymmetrical mode of cell division36–38, marking
the onset of neurogenesis — this occurs at approximately
E33 in the lateral part of the cortical wall in humans13 and
at approximately E10 in mice39. One daughter remains a
progenitor, the other is a postmitotic cell that is destined
to become a neuron or a glial cell40–42.
At the start of neurogenesis neuroepithelial stem cells
downregulate their epithelial characteristics, including
the presence of tight junctions and the apical polarity of some of their plasma membrane proteins34. The
morphological and molecular differentiation of the
subsequent proliferative cells is currently a subject of
intensive investigation, but one type — radial glial cells
— is particularly prominent and distinctive (BOX 2).
Neuroepithelial cells are induced to generate radial
glia by a set of genes that includes forkhead box G1
(FOXG1), LIM homeobox 2 (LHX2), paired box 6
(PAX6) and empty spiracles homologue 2 (EMX2)10.
Radial glia share some molecular characteristics with
earlier neuroepithelial cells, including the expression of
nestin and antigens that are recognized by the RC1 and
RC2 antibodies43. Radial glia differ in aspects of their
gene expression from region to region in the brain, a
factor that might influence regional differences in their
postmitotic progeny. Cortical radial glia express PAX6,
which is required for their normal development44.
Until recently there was thought to be much greater
precursor diversity in primates than in lower vertebrates45–47. Even shortly after the start of neurogenesis not
nature reviews | neuroscience
all mitotic cells in the primate VZ have extensive radial
processes, and distinct neuronal and glial progenitors
have been described in both the monkey and human
VZ48,49. Recent evidence has shown that the VZ of rodents
also contains a variety of progenitors, which become progressively restricted in their differentiation potential as
development progresses6,10,50.
The use of specific markers and electron microscopy
has revealed a distinct population of short neural precursors. Like radial glia, these cells have radial processes.
However, whereas the apical process contacts the ventricular surface, the basal process is of variable length
and is retracted during mitotic division47. Short neural
precursors are distinguished by their expression of the
tubulin-α1 promoter.
At some point after the onset of neurogenesis, dividing cells start to appear at the basal border of the VZ4,51.
Accumulation of these intermediate or ‘basal’ progenitors creates a distinct new compartment above the VZ,
which the Boulder Commitee named the subventricular
zone (SVZ)4 (BOX 1). These cells are not attached to the
ventricular surface and do not undergo interkinetic
nuclear movement41. Studies in rodents have shown
that intermediate progenitors are generated by the asymmetrical division of radial glia, and they subsequently
migrate into the SVZ52,53. Originally the SVZ was thought
to generate mainly glia41,54, but it is now clear that early
SVZ progenitors are largely neurogenic: many cortical
neurons originate from the SVZ in mice, monkeys and
humans16,55–57.
Most early SVZ progenitors are thought to undergo
terminal symmetrical division to create pairs of neurons 58. Unlike radial glia, intermediate precursors
express the transcription factors TBR2 , neurogenin 2
(NGN2), cut-like homeobox 1 (CUX1) and CUX2, but
not GLAST or PAX6 (Ref. 35). The progressive attenuation of PAX6 expression and the initiation of TBR2
expression characterize the transition to intermediate
progenitors59. Recently, rodent radial glia and early
intermediate precursors have been shown to differ in
their downstream signalling from the Notch receptor60.
Signalling through the classical C‑promoter-binding
factor 1 (CBF1) pathway, which inhibits neurogenesis,
is attenuated in early intermediate precursors. Hence,
radial glia are more multipotent than intermediate progenitors: transplantation experiments suggest that radial
glia produce similar proportions of neurons, astrocytes
and oligodendrocytes, whereas early intermediate
precursors produce mainly neurons60.
The degree of SVZ-progenitor diversity and the precise relationship of these cells to cells of the VZ are still
largely unknown. Unlike in rodents, cells isolated from
the human VZ and SVZ, even at early stages of cortico­
genesis, generate separate neuron-restricted and gliarestricted precursors in vitro61. Furthermore, cell-lineage
analysis in human embryonic slices has demonstrated
that VZ and SVZ progenitors undergo multiple divisions
before they begin radial migration to the neocortex56.
Subdivision of the neuroepithelium into the VZ
and the SVZ is particularly distinctive in humans62
(FIG. 2e,f). The Boulder Committee (FIG. 1Ad) and many
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Box 2 | What are radial glial cells?
Early silver-impregnation methods revealed a population of elongated, non-neuronal cells in the fetal human brain; these
cells were initially called epithelial cells or fetal glia57. However, the Boulder Committee did not refer to such cells
specifically in their model. Later electron-microscopic and immunohistochemical studies in humans and non-human
primates revealed the presence of glial fibrillary acidic protein (GFAP) in these cells, confirming their glial nature141,142.
The basal end feet of these bipolar cells form the pial surface of the fetal cerebrum57,142. P.R. proposed that these ‘radial
glial cells’, as he called them143, might act as a guidance scaffold for neuronal migration. However, it has since become
clear that radial glia are also true precursors that can divide to produce postmitotic cells and other progenitors144. A
subset of GFAP-positive radial glial cells stop dividing in mid-gestation of the primate, presumably to serve as a migratory
scaffold, but they subsequently resume mitosis and generate astrocytes57.
Although GFAP is not expressed in rodent VZ cells until the completion of corticoneurogenesis57, it is widely accepted
that there are comparable cells in rodents. Indeed, it has been suggested that the neocortical VZ is composed primarily
of multipotential radial glia, and that these are the sole type of VZ precursor144,145. However, it is generally agreed that the
VZ in murine dorsal telencephalon contains multiple types of neuronal precursor, as is the case in humans and nonhuman primates47. Radial glial cells in rodents, as in primates, are derived from neuroepithelial cells and do not appear
until after the start of neurogenesis. Although GFAP expression is delayed, the radial glia can be identified by their
astroglial characteristics, such as the presence of glycogen granules and their expression of vimentin, brain-lipid-binding
protein and the astrocyte-specific glutamate transporter, GLAST34,57.
Initially it was assumed that proliferation of radial glial cells produces only additional radial glia and glioblasts48.
However, it is now recognized that radial glial cells can be neurogenic and produce neuronal precursors as well as
postmitotic neurons40,46,146. Some authors refer to radial glial cells as neural stem cells60, whereas others use this term for
true neuroepithelial cells147. Until a consensus emerges regarding the definition of neural stem cells, it is important to
retain the accepted names of distinct cell types.
Predecessor cell
A type of neuron that arrives
beneath the pial surface of the
human embryonic forebrain
before the neural tube has
even finished closing.
Predecessor cells appear in the
developing cortex before any
other post-mitotic cells.
subsequent authors2,4,16,63 assumed that the SVZ appears
only after the cortical plate (CP; see below) has begun
to form. However, recent reports indicate that it arises
earlier14,29,64,65. Our data, which were obtained using antiphosphohistone‑H3 labelling in the human cerebral vesicle, show mitotic cells accumulating above the VZ nearly
a week before the CP begins to form14 (FIG. 2d). By the end
of the embryonic period the SVZ is of similar thickness
to the VZ in the ventrolateral cortex64 (FIG. 2f ).
The SVZ has increased in size and complexity during
evolution and its cellular organization in primates is significantly different from that in other species15,16,29. Later
in development — after the twentieth gestational week
(GW) in humans and after approximately E72 in monkeys — a band of tangentially orientated fibres divides
the SVZ into inner and outer sublayers in some parts
of the cortex. The inner part of the SVZ (the part closest
to the VZ) in humans shows higher expression of PAX6,
TBR2 and Ki67 (Ref. 66). PAX6 is thought to suppress cell
cycle exit and the expression of postmitotic markers in
progenitor cells67. Therefore PAX6 expression, which is
much higher in the human SVZ than in the mouse SVZ,
indicates that there is a large reservoir of progenitor cells
in the human SVZ.
By GW25–GW27, when the SVZ is still proliferating,
the human VZ has reduced in size to a one-cell-thick
ependymal layer16. Just as in the macaque monkey15,
this occurs before the cessation of the major phase of
neuron production, indicating that the SVZ becomes
the principal source of cortical neurons in the expanded
cerebrum68.
The Boulder Committee model assumed that mitosis
is restricted to the VZ and SVZ4. However, dividing cells
have been described outside the classical proliferative
zones in the rodent and human cortex 14,62,69. In our
human material, occasional ‘abventricular’ mitotic cells
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are seen directly under the pial surface, even before the
formation of the preplate13, but it is unclear what types
of postmitotic cells they produce.
Preplate: a community of transient neurons
The Boulder Committee suggested that before any postmitotic cells appear in the prospective cerebral cortex
the sub-pial processes of ventricular cells constitute a
cell-sparse layer, which they termed the marginal zone
(MZ) (FIG. 1Ab). They assumed that all neurons arise in
the local neuroepithelium and migrate radially4, and they
proposed that the first such neurons migrate through
the intermediate zone (IZ), which lies below the MZ
(FIG. 1Ac; see below). Since then, many investigators and
most textbooks have used the term MZ70 or early MZ
(Refs 71,72) to refer to a cell-sparse layer under the pial
surface before the CP forms.
Our own analysis of the embryonic human telencephalon shows cell bodies of neuroepithelial cells
extending right up to the pial surface until the first neurons (predecessor cells; see below) invade tangentially,
directly below the surface, to initiate formation of the
preplate13 (FIG. 2a,b). At no earlier stage is there a distinct
cell-free sub-pial layer. We therefore propose that the
term marginal zone should not be used before the CP
forms.
In the past two decades, data from various mammalian species have revealed a complex sequence of early
events that take place above the proliferative neuro­
epithelium. Neurons with various molecular signatures,
some of which migrate from extra-cortical sites of origin,
occupy a thickening layer under the pial surface (BOX 3).
In recognition of its cellular complexity and the fact that
it precedes the formation of the CP, investigators have
introduced the terms primordial plexiform layer and
preplate to describe this compartment73–76. We suggest
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Box 3 | Neurons of the preplate
In 1893 Retzius described unusual horizontal bipolar neurons located beneath the surface of the fetal human forebrain at
approximately mid-gestation148. He acknowledged that Cajal had found similar cells in other species149, and they are
thought to exist in all amniotes. The cells have radial processes that contact the pial surface and a plexus of horizontal
axons that synapse with the apical dendrites of later-arriving pyramidal neurons (FIG. 5a). These Cajal–Retzius cells,
which disappear during corticogenesis, produce developmentally important proteins — in particular reelin, which
regulates the formation of cortical layers150,151. Cajal–Retzius cells have been reported to have diverse origins in rodents.
Some cells arise in the neocortical neuroepithelium6,152; others migrate tangentially from the medial cortical hem39 and
other extra-cortical sites17,153. It was widely thought that these cells are the first to occupy the cortical preplate84.
However, a population of locally generated pioneer neurons that arise before the Cajal–Retzius cells has been described
in rats154.
In humans, we recently identified an even earlier population of neurons, called predecessor cells, which have not yet
been reported in any other species13 (FIG. 2b,c). These cells arrive under the pial surface of the ventrolateral cerebral wall
at approximately embryonic day (E) 31, before the neural tube completely closes and neurogenesis begins in the local
VZ. They are bipolar and express the neuronal marker TU20, but they lack axons. They have long horizontal processes,
through which the soma translocates dorsally from the basal telencephalon. They form an extensive network over the
forebrain that might act as a guidance scaffold for subsequent cellular or axonal navigation. Predecessor neurons
co-express TBR1, which was thought to be specific to the cells of the dorsal telencephalon in rodents13.
In humans the first reelin-expressing cells appear in the rostral telencephalon at approximately E40: many of these
are generated in the local VZ and migrate radially13,72 (FIG. 5b,c). Others migrate tangentially from diverse origins91,151.
Neurons labelled with antibodies to MAP2, golli and calbindin were reported in the early preplate64,91. Cells labelled
with interneuron markers, such as calretinin and GABA (γ-aminobutyric acid), appear in the dorsal telencephalon
from approximately E43 (Refs 72,91). A number of preplate GABAergic cells co-express either NKX2.1 or the DLX
transcription factors that are characteristic of the basal telencephalon91. A few days later glutamic acid
decarboxylase (GAD)-positive interneurons migrate tangentially into the preplate from the basal telencephalon91,
just before the arrival of the first neurons of the cortical plate — at E50–E51 in the rostral ventrolateral cortex30,72.
Thus, in the preplate, various cell types and their processes intermingle with each other and with the processes of
proliferative cells, providing an opportunity for cell–cell interactions.
that ‘preplate’ be used to describe the entire compartment
of heterogeneous post-migratory cells and neuropil that
forms between the proliferative zone and the pial surface
of the dorsal telencephalon before the appearance of the
CP. The preplate is a dynamic, evolving, largely-transient
structure that comprises various cell types, most of
which are destined to die.
Immunocytochemical analysis in humans reveals
subcompartmentalization of the preplate shortly before
the CP forms. Cajal–Retzius cells, which express reelin,
shift into a superficial sub-pial position and delineate the
future MZ13,72. Because reelin terminates the migration
of cortical neurons, the bottom of this subcompartment
sets the upper boundary of the CP13,72 (FIG. 3a,b). The
reelin-negative subcompartment, which contains a heterogeneous cell population, extends down to the SVZ.
After the emergence of the CP, these former preplate
cells remain below the CP and contribute to the subplate
(SP; see below)30.
Golgi staining
A staining technique
introduced by Camillo Golgi in
1873 that selectively stains
neurons with silver nitrate.
Cortical plate: the emerging blueprint
In all mammalian species the arrival of the first neurons
of the future neocortex is a critical developmental event.
These post-migratory cells accumulate to form a progressively thickening layer — the CP. In humans, as in other
species, there is a clear temporal gradient of maturation
across the hemisphere, with the CP appearing first in the
most lateral part of the rostral telencephalic wall, at E50,
and then approximately 1 week later in the dorsocaudal
pole30,64 (FIGS 2e,3a,b).
Early [3H]thymidine autoradiographic studies in
both primates and rodents established that radially
nature reviews | neuroscience
migrating neurons accumulate in the CP in an insideout sequence77,78. The earliest-born neurons are destined
to become the future layer 6, and the last-born neurons
will become layer 2. At every stage the most recently
arrived, closely packed neurons form a band in the uppermost part of the CP, which is sometimes called the ‘dense
cortical plate’. According to the Boulder Committee model,
migrating neurons form the CP between the IZ and the
MZ. Marin-Padilla proposed that CP cells accumulate
within the primordial plexiform layer (the preplate), splitting it into the MZ above and the SP below79,80. Although
this hypothesis was widely accepted, recent observations
indicate that additional cells become incorporated into the
MZ and the SP after the CP forms81,82. Differences across
species in the organization of the region between the CP
and the germinal layer have fuelled uncertainty about the
definition of this region (see below).
Golgi staining and other techniques have provided
information about the differentiation of human cortical
neurons83. Our study of the morphology of individual
cells at GW8–GW9 confirms that early differentiated
neurons of the CP have a fusiform cell body, a descending axon and an apical dendrite (BOX 1) with a dense
terminal tuft in the MZ79 (FIG. 3c,d). In humans, as in
rodents, the length of the apical dendrite of a cortical
neuron reflects its developmental age84.
Several developmental stages are recognized after
the initial formation of the human CP85. Peak migratory
activity is thought to occur between the third and fifth
months of gestation, and migration is completed during
the third trimester2,86,87. The period after GW22 is the
most significant time for areal, laminar and cytological
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a Bis
b Bis/reelin
E52–E55
c Bis/DiI
E52–E55
GW9
CP
PP
PP
*
*
SVZ
**
**
MZ
MZ
SVZ
VZ
SVZ
**
IZ/SP
IZ/SP
IZ/SP
**
**
100µm
MZ
**
CP
50µm
CP
d Bis/DiI
e Bis
GW9
E52–E55
SP
SVZ
VZ
IZ/SP
*
IZ
>
MZ
>
<
<
CP
LE
CP
25µm
200µm
f Bis
g Bis/DiI
VZ
SVZ
IZ/SP
Figure 3 | Subcompartmentalization within the cortical wall. At E52–E55,
the cortical plate (CP; indicated by asterisks in parts a and b) is appearing in the
Nature (e).
Reviews
| Neuroscience
dorsolateral cortex (a, b) but is more mature in the ventral cortex
At this
stage
the boundary between the intermediate zone (IZ) and the incipient subplate (SP) is
unclear30,64,120. The IZ/SP is much thinner in the dorsolateral cortex (a–c) than it is more
ventrally (e–g). a | Bisbenzimide (Bis) staining of cell bodies in the cortical wall. b | The
same image, merged with one showing reelin staining, reveals that reelin-expressing
preplate (PP) neurons form a distinct subcompartment, which becomes the marginal
zone (MZ)13,72. c | A DiI crystal placed ventrally in the cerebral wall at gestational week
(GW) 9 has labelled axons running in the IZ/SP, as well as some cells of the CP and MZ,
and a single radial glial cell (indicated by the arrow). d | A similar DiI crystal placement
has labelled two CP cells with apical dendritic tufts in the MZ, and a SP cell with long
tangential processes. e | In the ventral cortex at E52–E55, the lower part of the IZ/SP is
seen as a cell-sparse area (demarcated with arrowheads). f,g | Higher power views of
the striocortical boundary, indicated by the asterisk in part e. In part g, a DiI crystal
placed in the thalamus reveals axons running through the lower part of the IZ/SP. All
sections show Bis counterstaining. LE, lateral ganglionic eminence; SVZ,
subventricular zone;VZ, ventricular zone. Parts a and b reproduced, with permission,
from REF. 13  (2006) Macmillan Publishers Ltd. Parts c and d reproduced, with
permission, from REF. 108. Parts e–g reproduced from REF. 120.
116 | february 2008 | volume 9
differentiation of the CP63. By the seventh month the
cortex is clearly divided into six layers (including the MZ
(layer 1); see below). It remains to be elucidated whether
neocortical neurons are still being generated in humans
during the last few months of gestation and, if so, which
cortical layers such cells are destined to enter.
Studies of transcription-factor expression in mice
and heterochronic transplantation of cortical precursors in ferrets suggest that the VZ generates the projection
neurons of the deeper layers of the neocortex whereas
the SVZ produces those of the more superficial
layers29,88–90. However, the situation might be more
complex in humans in view of the early appearance
of the SVZ14,64 (FIG. 2d–f). In rodents nearly all cortical GABAergic interneurons originate in, and migrate
tangentially from, the basal telencephalon, whereas in
humans a subpopulation of interneurons is born locally,
in the SVZ of the dorsal telecephalon16,56,91.
In all mammals a common basic pattern of cortical lamination can be recognized. In mice, precursors
in the local neuroepithelium have an intrinsic timing
mechanism that determines the laminar specification
of successive postmitotic cells6. However, lamination is
elaborated differently in different species and in different
cytoarchitectonic areas of the hemisphere2,29. The upper
cortical layers are disproportionately thickened in larger
brains, which have a greater proportion of late-derived
neurons92. Each layer has a distinct cellular appearance
and connectivity that are characteristic of the specific
neuronal types that comprise it. A major challenge for
the future will be to identify the genetic interactions that
underlie the generation of unique cellular phenotypes
and the mechanisms that are involved in the specification
of the laminar identity of cortical neurons.
In many mammals, variation in neuronal activity,
especially in the sensory areas of the cortex, has been
shown to influence the distribution of incoming axons
and the morphology and functional characteristics of
cortical neurons, and hence to contribute to cytoarchitectonic differentiation in cortical areas93,94. The primary
visual cortex provides one of the best-known examples.
Thalamic axons from the right-eye and left-eye laminae
of the lateral geniculate nucleus terminate in segregated,
surface-parallel bands in layer 4 of the cortex, and the
relative width of these bands can be modified by early
postnatal deprivation of vision in one eye95. A recent
study reveals similar effects resulting from early loss of
vision in one eye in humans96. However, little is known
about the extent to which activity influences the functional maturation and differentiation in the rest of the
human cortex.
Intermediate zone: migration and connectivity
His recognized a compartment, which he called the
‘mantle layer’, above the VZ, through which postmitotic
cells migrate into what he saw as a cell-sparse ‘marginal
zone’. Kershman97 first used the term IZ to describe the
compartment that lies between the proliferative layers
and the cortical plate at a later stage in development. The
Boulder Committee tried to conflate these concepts. They
defined the early IZ, which exists before the appearance
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REVIEWS
Projection neuron
A type of glutamatergic
neuron, characterized by a
typical pyramidal morphology,
that extends its axon to distant
intracortical, subcortical or
subcerebral targets. Projection
neurons are born in the VZ,
and their lineage is different
from that of the interneurons.
Interneurons
A heterogeneous group of nonpyramidal, mostly GABAergic
neurons that project locally
and appear to be mainly
inhibitory. In rodents most
interneurons originate in
the ganglionic eminence of the
ventral forebrain and then
migrate tangentially to the
neocortex, whereas in humans
and non-human primates
they originate from both the
ganglionic eminence and
the SVZ of the dorsal
telencephalon. In the human
brain there might be
pronounced differences in the
proportions of neurons from
the two sources.
Corticofugal axon
An axon that originates from a
cortical neuron but projects
ouside the cortex to
subcortical structures.
Internal capsule
A large bundle of axons that
reciprocally connects the
cortex with the subcortical
structures of the brain.
of the CP, as a layer containing radially-migrating postmitotic cells that directly adjoins the VZ, below what
they called the MZ (FIG. 1Ac). For the later stages they
applied the same term, IZ, to the entire area between the
SVZ and the CP. They pointed out that long-range axons
course through this compartment (FIG. 1Ad,e). Thus, the
white matter of the cortex forms within, and eventually
replaces, the IZ. Many subsequent authors have tried to
retain various aspects of the Boulder definition of the IZ
for humans24,63,98 and other species27,99.
Birthdating, cellular labelling and axon-tracing studies in various mammalian species have greatly clarified
the status of the cells and axons that lie directly above the
germinal layer. At early stages, before the appearance
of the CP, early-born cells of the preplate fill this space.
Some of these cells give rise to corticofugal axons, which
in rodents project towards the putative internal capsule,
even before the first CP cells arrive100–103. These axons
are later joined by early ascending axons. Cells born
in the basal telencephalon, which eventually enter the
cortex to become GABAergic inhibitory interneurons,
migrate tangentially above the SVZ 102. In humans
such cells are seen as early as GW6–GW7, before the
appearance of the CP91.
Recognition of the existence of the preplate and
the absence of a distinct, cell-sparse MZ at early stages
demands the modification of the Boulder Committee’s
concept of the early IZ. Radially migrating ‘pioneer’
neurons do contribute to the preplate, but no separate
compartment of such migratory cells is visible at an early
stage72.
Conversely, there is general agreement that a compartment does emerge at later stages between the proliferative layers and the sub-pial post-migratory cells. This
compartment contains both radially and tangentially
migrating neurons, and a thickening band of long-range
axonal projections, both afferent and efferent, forms
within it. However, an examination of the literature
reveals no agreed definition of this IZ, and there is a wide
divergence of opinion on when it first appears and what
its boundaries are.
Some authors state that in rodents the early IZ is
the space between the VZ and the preplate, and that the
lower part of the IZ becomes the SVZ104. According to
this definition the IZ would contribute to cortical neurogenesis99. However, most researchers follow the Boulder
Committee in defining the lower border of the IZ as
the top of the entire germinal compartment. Others27,
attempting to reconcile the Boulder nomenclature with
more recent evidence, represent the early IZ as equivalent to the preplate, extending from the germinal layer
right up to a narrow, cell-sparse MZ.
The recognition of the SP as a distinct and functionally important cellular compartment (see below) has
produced further confusion with regards to how to
define the upper border of the IZ (BOX 1). Some authors
consider the IZ to abut the lower border of the CP99, and
therefore define it as containing most if not all of the
SP27. However, as the SP is largely or at least partly
the lower part of the original preplate (see below), and
as it constitutes a distinct and functionally important
nature reviews | neuroscience
compartment of post-migratory cells, most authors take
the SP to be separate from the IZ28,82,105.
We suggest that the term IZ be used to describe
the heterogeneous compartment that lies between the
proliferative layers and the postmigratory neurons
above. It is defined by the virtual absence of precursors or postmigratory cells. It contains radially and
tangentially migrating cells and a thickening layer of
extrinsic axons. Thus, the lower border of the IZ abuts
the SVZ. At early stages it is difficult to be sure whether
there is a separate zone containing only migrating
cells between the SVZ and the preplate. In rodents, a
distinct fibre layer can sometimes be discerned even
before the appearance of the CP84, and some authors
have called this layer the IZ84,106.
Our axon-tracing studies in humans64 did not reveal
a distinct layer of corticofugal fibres to be present before
the formation of the CP. The arrival of the first CP
cells creates the appearance of an apparently uniform,
loosely filled compartment between the CP and the
SVZ (FIGS 2e,3a,b), and many authors have called this
entire compartment the IZ72,107 (FIG. 2e). Presumably the
lower part of this compartment consists of radially and
tangentially migrating cells, but undoubtedly the upper
part contains former preplate cells (part of the SP). If we
are to be rigorous in our functional definition of the IZ
(that is, if we define it as containing migrating cells and
axons), we should avoid the temptation to describe this
entire compartment as the IZ and simply admit that,
in the absence of markers of post-migratory cells, the
boundary between the IZ and the incipient SP is unclear.
At slightly later stages (approximately GW8–GW9), starting in the ventral cortex, a cell-sparse compartment filled
with extrinsic axons emerges directly above the SVZ64,108
(FIGS 2f,3e–g). It is tempting to describe this fibre layer
as the entire IZ but, again, we cannot be certain that its
upper border coincides with the lowest postmigratory
cells. During the establishment of the CP at GW8–GW9,
the compartment above the SVZ and below the CP has
sometimes been called the intermediate/subplate zone
(IZ/SP)66; we suggest that this term be adopted. It has
been shown that, at GW9–GW10 in humans, the upper
half of this compartment expresses markers that are
characteristic of synaptic connectivity and axonal outgrowth66. Hence, the authors called this the SP and used
IZ for the lower half of the compartment.
At subsequent stages, the invasion of large numbers
of corticocortical fibres and the beginning of myelination2 transform the original IZ into the white matter.
Some cells (which are usually considered to be part of
the SP) terminate their migration in the white matter to
become interstitial cells of the white matter, which marks
the end of the IZ as we have defined it.
Subplate: enigmatic post-Boulder discovery
The Boulder Committee did not recognize a distinct
neuronal compartment between the CP and the IZ
(FIG. 1Ad,e). However, a few years later such a lamina was
described, first in the human109 and then in the monkey
telencephalon110 — this region was called the SP111. The
SP is present in all mammals, but its morphological
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a Bis
GW12 b TU20
GW12 c Bis/TU20
MZ
MZ
MZ
IZ
SVZ
SVZ
CP
IZ
GW12
CP
IZ
SVZ
VZ
SP
VZ
100µm
50µm
VZ
100µm
d Bis
IZ
SP
SP
100µm
GW12 e DiI
GW12 f Bis/DiI
CP
CP
50µm
SP
GW12
CP
50µm
Figure 4 | Human cerebral wall at GW12. The subplate (SP) in humans is a separate
30,64,66
Nature(CP)
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Neuroscience
layer that forms shortly after the emergence of the cortical plate
.|Here
we
provide examples of the complexity of this zone in humans. a,c,d,f | Bisbenzimide (Bis)
staining shows that the cells of the SP are sparsely distributed in radial palisades.
b,c | TU20 staining, which reveals both neurons and their processes, suggests that the
spaces between SP cells are filled with axons. The arrangement of the cells in palisades
gives the impression that the axons are also radially orientated, but DiI staining from a
crystal placed in the ventral cortex (e,f) reveals that the fibres pass obliquely between the
cells. IZ, intermediate zone; MZ, marginal zone; SVZ, subventricular zone; VZ, ventricular
zone. Figure reproduced, with permission, from REF. 156.
characteristics and persistence in adulthood vary
among species.
In rodents (mice and rats) and carnivores (cats and
ferrets), the SP is present as a distinct layer from the
start of the formation of the CP82. Indeed, in these species [3H]thymidine birthdating has shown that nearly
all SP neurons are preplate cells — that is, they are
born before the first cells of the CP27,112,113. A significant
minority of the SP interneurons are produced late in
neurogenesis114. Precocious, mainly transient SP neurons are thought to have important roles in development, giving rise to early descending axons that form
a scaffold over which thalamocortical afferent axons
navigate103,115, as well as forming transient connections
into the developing CP116.
The SP contains heterogeneous cell populations,
including glutamatergic neurons that migrate radially
from the germinal zone, and interneurons, which are
mainly produced from progenitors in the basal telencephalon82. Early-born glutamatergic neurons extend
subcortical pioneer axons and express the transcription
factors TBR1 and EMX1, the transgene golli–lacZ, the
p75 neurotrophin receptor, kynurenine aminotransferase
and other markers82,117. Interneurons in the SP express
transcription factors such as DLX and LHX6, as well as
β‑galactosidase, neuropeptides, calbindin, and other
markers associated with GABAergic interneurons82,105,118.
Many of these neurons appear to be migrating through
the SP82.
118 | february 2008 | volume 9
Although the SP was first described in humans and
monkeys, its origin and early development in primates
are not yet fully understood30,107. The dynamic changes
in the fibre content and cell morphology of the early SP
and the IZ, as well as their complex spatial arrangement,
limit their clear delineation. Although a number of
molecular markers for the SP have been reported, most
are also expressed by other cell populations. A recent
study of the developing SP in mice and humans highlighted the inadequacy of current molecular markers for
distinguishing between the multiple types of neuron in
this compartment82.
There is no consensus on the origin and timing of
formation of the SP in humans. Many consider that a
thin, cell-sparse ‘pre-SP’ appears at GW10, with the SP
proper forming after GW12 (REFS 105,107). Some recognize no component of the SP even as late as GW11
(Ref. 98). Others see at least a thin SP layer as soon as the
CP emerges30,64. As described above, there is no simple
way of defining a boundary between IZ and SP at early
stages (FIGS 2,3). However, there is recent evidence that
cells of the upper half of this IZ/SP compartment express
molecular markers that are characteristic of synaptic
connectivity66. By GW9–GW10, partition into upper
(SP) and lower (IZ) layers becomes obvious14,30,66.
The balance of this evidence suggests that, as in other
mammals, at least some human SP cells are born before
the first cells of the CP. However, unlike in rodents, the
SP of primates (including humans) thickens substantially during corticogenesis, especially under regions
of association cortex. Although there are no neuronal
birthdating data for humans, the general timescale of
appearance of the SP in humans is similar to that in
monkeys, in which the SP expands rapidly between E72
and E100 (REFs 15,107). Between GW9 and GW12 the
human SP thickens considerably and its cell density
decreases14,66 (FIG. 4).
Accumulation of axons and neuropil107,119, causing early-born cells to disperse downwards, certainly
contributes to the expansion of the SP, but the SP is so
thick in some parts of the human cortex that it seems
likely that many late-born cells contribute to it82. On the
other hand, most human SP cells express TBR1, like all
the early-born cells with pioneering axons in the mouse
SP82. A minority of SP cells in humans are GABAergic
interneurons that express DLX. In mice, a fraction of
such cells are born after CP formation, but many appear
to be migrating through the SP82. The extent to which
later-generated neurons enter the primate SP remains
to be elucidated.
On the basis of silver staining it was concluded
that thalamic afferents invade the human dorsal telencephalon at E47–E50, before the CP forms, and that
they are present in the SP at E50–E56 (Ref. 30). However,
our carbocyanine dye tracing data indicate that thalamo­
cortical axons reach the lateral ganglionic eminence
by E47–E50 and invade the cortical wall only after the
formation of the CP64,120. As an example, FIG. 3g shows
thalamocortical axons running tangentially above
the SVZ at approximately E53, but not substantially
invading the upper part of the IZ/SP.
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a
Cajal, 1893
b Reelin-positive cells
c Reelin-positive cells
E42
E52
PP
PP
VZ
25µm
25µm
Figure 5 | Morphology and origin of Cajal–Retzius cells. a | Cajal’s original
drawing155 of Golgi-stained neonatal human visual cortex shows
Naturetransient
Reviews neurons
| Neuroscience
(labelled A, B and C) that were named Cajal–Retzius cells in honour of their
co-discoverers. b,c | Cajal–Retzius cells in the embryonic human cortex at early stages
of development, revealed by their reelin immunoreactivity. At embryonic day (E) 42 (b),
radial columns of reelin-positive cells extend through the ventricular zone (VZ),
indicating that some Cajal–Retzius cells in humans derive from the local neuroepithelium
of the dorsal telencephalon13,151. By E52 (c), reelin expression has increased and these
early-born neurons are restricted to the upper part of the preplate (PP)13,72. Parts b and c
reproduced, with permission, from REF. 13  (2006) Macmillan Publishers Ltd.
At GW12, staining with the neuronal marker TU20
reveals palisades of neurons in the SP, creating the impression that axons might be streaming radially through the
spaces between these columns (FIG. 4c). However, DiI
labelling reveals that extrinsic axons run obliquely from
the lower IZ into the SP. As in other mammalian species, the
human SP appears to serve as a ‘waiting compartment’
for thalamic axons, although in primates the network
formed by these axons is more elaborate than in other
mammalian species (FIG. 4). Much later, presumably when
the CP becomes growth-permissive21, these axons invade
and find their ultimate targets in layer 4 (Ref. 121).
The SP in humans reaches its maximum thickness
roughly two-thirds of the way through gestation122,123,
when it is approximately four times thicker than the
CP63. Cell density then gradually decreases, leaving only
scattered ‘interstitial’ cells in the white matter, which are
still visible during the sixth postnatal month70,107,124.
The marginal zone: from preplate to layer 1
We have proposed that the term MZ should be used to
refer to the part of the former preplate that lies above
the emerging CP and that later becomes layer 1 of the
mature cortex. In rodents most MZ cells are generated
before the CP forms125; however, the MZ is more complex in primates107. Neurons are continuously added to
the monkey MZ by tangential migration during the first
two-thirds of gestation, and this is also likely to be the
case in humans91,126.
nature reviews | neuroscience
At GW11, cells originating in the basal periolfactory
SVZ migrate tangentially to the surface of the ventro­
lateral cortex, giving rise to a sub-pial granular (SG)
layer at the top of the MZ91,127–129. By GW14 this layer,
which contains granular GABAergic interneurons and
Cajal–Retzius cells, covers the entire cortical surface. A
subset of layer 1 neurons express NKX and DLX transcription factors, which are characteristic of cells born
in the basal telencephalon in rodents91. However, in the
human brain the expression of these transcription factors extends dorsally into regions of the cortical VZ. The
transient SG layer, which is non-existent or negligible
in mice, is prominent in the fetal cortex of humans and
non-human primates at later stages. [3H]thymidine
analysis in the macaque monkey revealed an uncommonly thick SG in some regions of the MZ, where it
might supply local circuit neurons to the underlying
cortex, as it does in the area 17 (Ref. 126). The SG layer
progressively disappears after GW28 and is not present
in the newborn brain130.
At GW18–GW28 the MZ is remarkably complex in
neuronal differentiation and laminar composition, and
six sublayers have been distinguished. However, no sublamination is visible in layer 1 of the newborn child’s
brain131. The mature layer 1 contains few neurons and is
largely filled with arborizations of apical dendrites and
instrinsic tangential axons.
Concluding remarks
A huge corpus of data on the development of the cere­
bral cortex has accumulated since 1970, demanding
revision of the Boulder Committee’s classical schematic
model. Much of the new evidence comes from rodents,
but it generally fits remarkably well with what is known
of human corticogenesis, with studies on non-human
primates providing a valuable bridge. Our revised
model and nomenclature emphasize the timing,
sequence, scale and peculiarities of events in the develop­
ment of the human cortex (BOX 4). However, we believe
that they are broadly applicable to all mammals.
Although the known differences in corticogenesis
between rodents and humans are mainly small and often
neglected, they might indicate important evolutionary
steps that have considerable functional significance.
Purely quantitative differences (for example, in the period
of neurogenesis and the duration of the cell cycle) might
relate simply to the expansion of the forebrain. Apparently
unique human characteristics are more intriguing. We
need to learn the functional implications of distinctive
features of the developing human cortex, such as the early
expression of GFAP in radial glia at the onset of neurogenesis45,48,132, the existence of non-dividing radial glia133,
the production of interneurons in the SVZ, the particularly distinctive SG layer, and the existence of entirely new
classes of neurons, such as the fusiform neurons134, precocious predecessor cells13 and late-born SP cells. Do any
of these features relate to the role of the human cortex in
distinctively human aspects of behaviour and cognition?
Might partly heritable developmental cognitive disorders,
including autism and dyslexia, involve genetic anomalies
in the regulation of human-specific features?
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REVIEWS
Box 4 | Summary of the revisions of the Boulder model
•A transient layer with a diverse population of neurons forms between the
neuroepithelium and the pial surface of the dorsal telencephalon before the
appearance of the cortical plate (CP). We suggest that the term preplate, which is
already widely used, should be adopted for this layer.
•The subventricular zone appears as a distinctive proliferative layer before the
emergence of the CP, earlier than previously recognized.
•There is no distinct cell-sparse layer under the pial surface before the CP forms. Thus,
the term marginal zone should be used only after the appearance of the CP, to refer to
the residual superficial part of the preplate, which becomes the layer 1 of the mature
cortex.
•The term intermediate zone (IZ) has been used in various ways. We propose that it
should be reserved for the heterogeneous compartment that lies between the
proliferative layers and the postmigratory cells above. The IZ contains radially and
tangentially migrating cells and a thickening layer of extrinsic axons that eventually
constitutes the white matter.
•The subplate (SP) is a distinct and functionally important transient layer, located directly
below the cortical plate, which was not recognized by the Boulder Committee. In
rodents and carnivores most SP neurons are born before the first CP cells. In humans,
preplate cells also contribute to the SP, but its substantial thickening at later stages
probably involves the addition of later-born neurons.
Perhaps the most distinctive features of the human
cerebral cortex are the expansion of the prefrontal,
parietal and temporal association cortices, and the
1.
2.
3.
4.
5.
6.
7.
8.
9.
Brodmann, K. Beitraege zur histologischen
lokalisation der grosshirnrinde. VI. Mitteilung: die
cortexgliederung des menschen. J. Psychol. Neurol.
(Lzp.), 10, 231–246 (1908).
Sidman, R. L. & Rakic, P. Neuronal migration, with
special reference to developing human brain: a review.
Brain Res. 62, 1–35 (1973).
This was the first review of the modes and patterns
of neuronal migration in the embryonic and fetal
human brain, and it remains the most
comprehensive. It places a special emphasis on the
cerebral and cerebellar cortices.
Marin, O. & Rubenstein, J. L. Cell migration in the
forebrain. Annu. Rev. Neurosci. 26, 441–483 (2003).
This is a comprehensive and highly informative
review on the pattern of neuronal migration to the
cerebral cortex that places a particular emphasis
on the tangential migration of GABAergic
interneurons from the ganglionic eminence of the
ventral telencephalon (see also reference 154).
The Boulder Committee. Embryonic vertebrate central
nervous system: revised terminology. Anat. Rec 166,
257–261 (1970).
This landmark article recommended a clarified
terminology for the transient embryonic zones, as
presented in P.R.'s model based on human Golgistained embryonic cerebral tissue.
Haydar, T. F., Kuan, C. Y., Flavell, R. A. & Rakic, P. The
role of cell death in regulating the size and shape of
the mammalian forebrain. Cereb. Cortex 9, 621–626
(1999).
Shen, Q. et al. The timing of cortical neurogenesis is
encoded within lineages of individual progenitor cells.
Nature Neurosci. 9, 743–751 (2006).
This is a recent review of the multiple lines of
evidence that indicate that the basic neuronal
phenotypes are determined at the time of the last
cell division in the proliferative zones (see also
references 31 and 137).
Hatten, M. E. & Mason, C. A. Mechanisms of glialguided neuronal migration in vitro and in vivo.
Experientia 46, 907–916 (1990).
Nadarajah, B. & Parnavelas, J. G. Modes of neuronal
migration in the developing cerebral cortex. Nature
Rev. Neurosci. 3, 423–432 (2002).
Gleeson, J. G. & Walsh, C. A. Neuronal migration
disorders: from genetic diseases to developmental
mechanisms. Trends Neurosci. 23, 352–359 (2000).
appearance of functional areas that are devoted to
language135. The forerunners of some of these areas,
such as the prefrontal association cortex, can be recognized in the mouse telencephalon by the pattern
of expression of specific transcription factors in the
VZ — modification of these transcription factors can
selectively enlarge the frontal cortex, causing changes
in the remainder of the forebrain136. Such experiments
in mice give insight into possible mechanisms of
primate evolution5,37.
Molecular studies indicate that cortical size, lamination and connectivity are largely genetically regulated137,138. Several brain-related genes, including RNA
genes, evolved much faster in the primate lineage that
led to humans than in the lineage that led to the chimpanzee98. Changes in gene expression, modifications of
sequence and the appearance of entirely novel genes
might all have contributed to the unique features of the
human cerebral cortex92,139,140.
There is little doubt that research on rodents will continue to inform our general understanding of forebrain
development. However, new opportunities and new
technologies for studying the developing human brain
will also be essential if we are to understand the features
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Acknowledgements
We are grateful to J. Rubenstein, R. Hevner, D. D. M. O’Leary,
S. Lindsay and F. Guillemot for discussion about the IZ and
the SP. This work was supported by the Kavli Institute for
Neuroscience at Yale (P.R. & I.B.), the National Institute of
Neurological Disorders and Stroke of the US Public Health
Service (P.R.) and the Hill Foundation (I.B.)
DATABASES
Entrez Gene: http://www.ncbi.nlm.nih.gov/entrez/query.
fcgi?db=gene
CUX1 | CUX2 | EMX1 | EMX2 | FOXG1 | integrin-α6 | Ki67 |
LHX2 | LHX6 | NGN2 | PAX6 | prominin‑1 | reelin | TBR1 | TBR2 |
FURTHER INFORMATION
Pasko Rakic’s homepage: http://rakiclab.med.yale.edu
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