CORTICAL NEUROPATHOLOGY AND AUTISM
Manuel F. Casanova
Professor, Department of Psychiatry and Behavioral Sciences
University of Louisville, Louisville, Kentucky
The following report summarizes research into the neuropathology of autism,
including some of my own work. The opinions in this report are based on knowledge I
have gained through my education, training and professional experience as a medical
doctor and researcher, and are expressed to reasonable degree of scientific probability.
I am a Professor in the Department of Psychiatry and Behavioral Sciences at the
University of Louisville, where I hold the Gottfried and Gisela Kolb Endowed Chair in
Psychiatry and Associate Chair for Research. I received my M.D. degree at the
University of Puerto Rico School of Medicine in 1979, and subsequently went on to
become Chief Resident in Neurology at the University District Hospital in Rio Piedras,
Puerto Rico. I received further training as a Clinical Fellow in the Neuropathology Clinic
at The Johns Hopkins University School of Medicine in Baltimore, Maryland. I was a
Major in the Medical Corps for the U.S. Army Reserves from 1984 through 1990, and
served as the Director of the Brain Bank Unit in the Neuropathology Laboratory of the
Clinical Brain Disorders Branch at the National Institutes of Health from 1987 to 1991.
During that same time, I held several positions consulting, lecturing and practicing in the
Washington, D.C. area. In 1991, I became a faculty member at the Medical College of
Georgia. I was at that institution until 2003, when I moved to the University of
Louisville. I am board certified in neurology, and am a reviewer for numerous scientific
journals. I am also a member of several professional societies, and have served on
committees including the National Alliance for Autism Research Scientific Advisory
Board, the Center for Scientific Review Special Emphasis Panel for the National Institute
of Health, and the Tissue Advisory Board of the Autism Tissue Program. My
qualifications are further detailed in my curriculum vitae.
Autism is a neurodevelopmental condition defined by clinical criteria in three
primary behavioral domains: communication skills, social interaction and restricted
interests and activities (APA 2000). Diagnostic criteria require that symptoms appear
within the first three years of life. While some patients receive a diagnosis of autism as
early as 18 months of age, many others are not formally diagnosed until age five (Filipek
et al. 1999). The broader term “autism spectrum disorder” (ASD) encompasses autism
and two other conditions that share core autistic symptoms: Asperger disorder and
Pervasive Developmental Disorders-not otherwise specified (PDD-NOS) (CDC 2007).
Throughout this report I will interchangeably use the terms “autism” and “ASD” when
discussing findings and hypotheses about these three conditions.
Autism often occurs in the presence of other medical conditions such as seizures,
mental retardation and chromosomal abnormalities, such as tuberous sclerosis complex
(TSC), fragile X, Down, William-Beuren, Angleman/Prader-Willi, velocardiofacial and
Möbius syndromes (Filipek 2005). None of these comorbidities invariably accompanies
autism, so the presence or absence of any these conditions will not require or exclude a
diagnosis of autism. Nevertheless, as I will discuss throughout my report, our knowledge
of how these concomitant conditions develop, as well as our understanding of autistic
neuropathology, indicates that autism originates early in the first trimester of gestation.
Three Components in the Development of Autism
The current clinical consensus views autism as a multifactorial condition
involving primarily one organ, the brain. Research suggests that three elements play roles
of varying importance in the development of autism. First, and most importantly, several
lines of research, including my own, indicate that autism manifests after a developmental
insult or error that occurs during a specific critical period for brain development,
primarily the first trimester. Second, genetic factors are known to play a significant role
in the development of autism. Lastly, exogenous stressors (i.e., anything other than
genetics) also may play a role in causing autism. In this regard, studies have investigated
the possible role, for example, of in utero exposures to valproic acid and thalidomide at
very specific times early in gestation as a cause of autism (Strömland & Miller 1993;
Strömland et al. 1994; Ingram et al. 2000; Moore et al. 2000; Williams et al. 2001;
Rodier, 2002; Schneider & Przewłocki 2005). In addition, the first trimester is the time of
development for multiple congenital anomalies that may produce autistic manifestations,
for example Möbius syndrome, which is possibly caused by genetic abnormalities,
vascular deficits, maternal trauma or the use of certain drugs during pregnancy (Towfighi
et al. 1979). Thus, both the genetic and prenatal environmental components associated
with autism point to the first trimester as the period during which autism originates.
Research into autism has uncovered differing histopathological manifestations
(i.e., patterns of tissue damage) in autistic brains and varied clinical expression in autistic
patients. These variations suggest that the timing of the underlying insult, which appears
to be a constant against the background of different etiologies, may tell us as much as, if
not more than, the anatomical loci affected. In other words, when considering the
developing brain and its orchestrated sequential maturation of neurons, synapses and
cortical maps, “when is as important as what” (Ben-Ari 2006). While findings by others
have implicated a possible vulnerability period during the second and third trimesters of
gestation, there is a growing body of evidence for vulnerability in the early stages of
gestation (Coleman & Betancur 2005).
The Cortex and Mini-columnar Pathology in Autism
Neurologists traditionally view autism as a disorder of the cortex due to evidence
of seizures in a significant proportion of cases and the absence of either spasticity or
vision loss. Clinically, the dysfunction of higher cognitive functions (e.g., social behavior
and expressive language) pinpoints the putative deficit to the isocortex. More
specifically, autism appears to arise from a defect in the modular organization of the
cortex that provides for the emergence of cognitive properties.
Figure 1. Developing cerebral cortex at 12 weeks of
gestation. Young cells migrating from the germinal zone
(A) form ontogenetic columns (B), thought to be the
precursor to the minicolumn. Scalebar: 200 μm.
The cortex consists of a
highly complex network of neural
connections in which the activity
of single neurons or small groups
of neurons contributes to larger
patterns of activity throughout the
cortical network. Maintaining a
constant degree of
number of connections per neuron
within a network—means that the number of connections must grow geometrically
relative to growth in number of neurons. This growth, as well as selective constraints to
minimize the consumption of energy and space, leads to a “small world” network in
which neurons maintain short connection length within clusters, which in turn are linked
by longer-range projections (Watts & Strogatz 1998; Chklovskii et al. 2002). These
constraints influence neuronal networks in the developing isocortex to assemble in the
form of the ontogenetic cell column, a radially oriented linear array of pyramidal neurons
which extends though multiple layers of the cortical plate () (Rakic, 1988;
Kriegstein & Noctor 2004). Hypothetically, the migration and development of neurons
along a radially oriented framework should maximize the efficient use of brain space and
facilitate close-range connections among many neurons.
After 30 weeks gestation in humans, the pervasive columnar organization of the
cortical plate is obscured to varying degrees by the migration of glia and interneurons and
the growth of dendritic and axonal branches. However, the underlying radial organization
of these pyramidal cell columns remain intact (). Imaging studies of post-
mortem cortical tissue from individuals of various ages have demonstrated the continuity
of columnar morphometry during fetal and postnatal development and throughout the
lifespan (Casanova et al. 2006b). As these fundamental radial circuits within the
pyramidal cell column core mature and develop their synaptic connections, mini-columns
emerge within the original columns to provide further structure to the isocortex.
Four principal cellular features have been studied to assess mini-columnar
morphometry and morphology: the core pyramidal cell column, the apical dendritic and
vertical myelinated axon bundles arising from the pyramidal cells, and radially oriented
translaminar axon bundles of double-bouquet inhibitory cells situated in peripheral
neuropil (Von Bonin & Mehler 1971; Seldon, 1981; DeFelipe et al. 1990; Ong & Garey
1990; Viebahn, 1990; Peters & Sethares 1991, 1996, 1997; Ferrer et al. 1992; Del Río &
DeFelipe 1997). Morphometric linkages between these four components suggest that they
Figure 2. The minicolumn is the first structural motif observed within the cortex. The four panels
illustrate the development/maturation of minicolumns in four individuals at different stages of
development: A) 26 weeks of gestation, B) 32 weeks of gestation, C) 48 days old, and D) 9 years old.
provide complementary information from which the general structure of mini-columns
can be derived (Casanova et al. 2006b).
Figure 3. Minicolumns in Brodmann area 4, lamina III, in
an autistic patient (bottom) and an age-matched control
case (top). Insets highlight the cores of minicolumn
fragments identified by our software, illustrating the
reduction in minicolumnar width (CW). Scale bars
measure 200 μm on left and 50 μm on right
To date, no pathological
entity has been conclusively and
systematically identified with
autism at the cellular level.
However, recent post-mortem
studies performed by me and
others have shown area-specific
changes in the mini-columns of
autistic individuals. Specifically, in
brains of autistic individuals, mini-
columnar width has been found to
be significantly narrower, with
most of that decrease attributable
to reduction of peripheral neuropil
space (). At the same
time, mini-columns appeared to be more numerous within the images of brain tissue that
we examined (Casanova, 2006; Casanova et al. 2002a, b, 2006a, b).
What are the functional implications of increased numbers of narrower mini-
columns containing smaller projection neurons? These modular microcircuit assemblies
are interconnected by thousands of collateral projections within larger networks. Each
mini-column is linked to local networks through myelinated bundles in superficial, or
radiate, white matter, and to more distant cortical areas via deeper white matter tracts.
Additive increase in mini-column numbers would entail a geometric increase in short-
and long-distance projection fibers in order to maintain a constant degree of transcortical
connectivity among modules (Hofman, 2001). Longer white matter fibers occupy more
space, require disproportionately larger soma to support increased metabolic costs and
result in signal processing delays. Selection pressure would therefore be expected to have
given rise to modules internally linked by radially oriented processes and integrated into
local networks by short collaterals. Proportionately less white matter would be devoted to
longer-range connections, encouraging regional functional specialization.
Neuropathological descriptions of decreased cell size and narrow mini-columns
(Casanova et al. 2006a), studies revealing increased superficial white matter (Herbert et
al. 2004), and functional imaging studies revealing decreases in activity linking prefrontal
and posterior areas (Just et al. 2004) support this view. On a functional level, a putative
increase in local interconnectivity and reduced long-distance connections in areas
subserving cognitive flexibility and prioritizing and emotional and social cognition, such
as the prefrontal cortex, is consistent with the clinical picture of stereotypy, rigidity and
interpersonal deficits characterizing autism.
The number of mini-columns is established within the first 40 days of gestation,
indicating that any environmental influence that might have the capacity to interfere with
mini-columnar development would have to act within that timeframe. The findings, as
described above, are not consistent with mercury’s toxic affects in the brain. An
increased number of minicolumns can explain the larger than normal brains of autistic
individuals. In contrast, mercury intoxication causes brains to be smaller (atrophic) with
corresponding cell loss in certain areas of vulnerability, e.g., granule cell layer, depths of
sulci, and calcarine cortex (Graham & Montine 2002; Nelson & Bauman 2003). In autism
we have not found this selective vulnerability (Casanova et al. 2006b) and instead of cell
death we have reported increased neuronal density (Casanova et at 2006a).
Manuel F. Casanova, M.D.
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