Anatomical and ultrastructural sex differences in mean diameter and thickness of myelinated axons in adult rat corpus callosum / Diferencias sexuales anatómicas y ultraestructurales del diámetro medio y grosor de los axones mielinizados en el cuerpo calloso de ratas adultas

Sexual dimorphism exists at all levels of the nervous system. These sex differences could underlie genderrelated differences in behavior and neuropsychological function, as well as the gender differences in the prevalence of various mental disorders such as autism, attention deficit disorders, and schizophrenia. Myelination, on the other hand, is a unique cellular process that can have a dramatic impact on the structure and physiology of an axon and its surrounding tissue. The corpus callosum (CC) is the largest of the brain commissures, which connects the cerebral cortices of the two hemispheres, and provides interhemispheric connectivity for information transfer and processing between cortical regions. Variation in the axonal properties of CC will alter the interhemispheric connectivity. The CC consists of myelinated and unmyelinated axons, glial cells and blood vessels. Several functional studies have reported that the function of CC is associated with its axons density and myelination properties. The sexual dimorphism in the axonal content of the CC has always been controversial; hence, the aim of this study was to analyze the differences in axons' diameter and myelin sheath thickness of the CC between male and female rats. For this purpose, five pairs of adult male and female rats were perfused and the CC were removed and sectioned. Four sections from different subregions of the corpus callosum that represent the genu, anterior body, posterior body, and splenium of the CC were stained and electron microscopic images were captured using stereological guidelines. Later, the axons diameter and myelin sheath thickness for each subregion were calculated and compared between males and females. Our preliminary findings of the present study indicated region specific differences in the myelinated axon thickness and diameter in the CC between male and female rats.

El dimorfismo sexual existe en todos los niveles del sistema nervioso. Estas diferencias de sexo podrían ser la base de las diferencias de comportamiento y función neuropsicológica relacionadas con el sexo, así como las diferencias en la prevalencia de diversos trastornos mentales, como el autismo, los trastornos por déficit de atención y la esquizofrenia. La mielinización, por otro lado, es un proceso celular único que puede tener un impacto dramático en la estructura y fisiología de un axón y su tejido circundante. El cuerpo calloso (CC) es la mayor comisura cerebral, que conecta las cortezas cerebrales de ambos hemisferios, y proporciona la conectividad interhemisférica para la transferencia y el procesamiento de información entre regiones corticales. La variación en las propiedades axonales de CC alterará la conectividad interhemisférica. El CC consiste en axones mielinizados y no mielinizados, células gliales y vasos sanguíneos. Varios estudios funcionales han informado que la función de CC está asociada con la densidad de axones y las propiedades de mielinización. El dimorfismo sexual en el contenido axonal del CC siempre ha sido controvertido; por lo tanto, el objetivo de este estudio fue analizar las diferencias en el diámetro de los axones y el grosor de la vaina de mielina del CC entre ratas macho y hembra. Para este propósito, se perfundieron cinco pares de ratas macho y hembra adultas y se extrajeron y seccionaron las CC. Se tiñeron cuatro secciones de diferentes subregiones del cuerpo calloso que representan el genu, el cuerpo anterior, el cuerpo posterior y el esplenio y se capturaron imágenes de microscopía electrónicas utilizando referencias estereológicas. Posteriormente se calculó el diámetro de los axones y el grosor de la vaina de mielina para cada subregión y se compararon entre machos y hembras. Nuestros hallazgos preliminares del presente estudio indicaron diferencias específicas en el grosor y diámetro del axón mielinizado en el CC entre ratas macho y hembra.

Regional density of glial cells in the rat corpus callosum

Axons and glial cells are the main components of white matter. The corpus callosum (CC) is the largest white matter tract in mammals; in rodents, 99% of the cells correspond to glia after postnatal day 5 (P5). The area of the CC varies through life and regional differences related to the number of axons have been previously described. Whether glial cell density varies accordingly is unknown; thus the aim of this study was to estimate glial cell density for the genu, body and splenium -the three main regions of CC-, of P6 and P30 rats. Here we report that the density of CC glial cells reduced by ~10% from P6 to P30. Even so, the density of astrocytes showed a slight increase (+6%), probably due to differentiation of glioblasts. Interestingly, glial cell density decreased for the genu (-21%) and the body (-13%), while for the splenium a minor increase (+5%) was observed. The astrocyte/glia ratio increased (from P6 to P30) for the genu (+27%), body (+17%) and splenium (+4%). Together, our results showed regional differences in glial cell density of the CC. Whether this pattern is modified in some neuropathologies remains to be explored.

Topography and axon arbor architecture in the visual callosal pathway: effects of deafferentation and blockade of TV-methyl-D-aspartate receptors

Visual callosal fibers link cortical loci in opposite hemispheres that represent the same visual field but whose locations are not mirror-symmetric with respect to the brain midline. Presence of the eyes from postnatal day 4 (P4) to P6 is required for this map to be specified. We tested the hypothesis that specification of the callosal map requires the activation of A'-methyl-D-aspartate receptors (NMDARs). Our results show that blockade of NMDARs with MK-801 during this critical period did not induce obvious abnormalities in callosal connectivity patterns, suggesting that retinal influences do not operate through NMDAR-mediated processes to specify normal callosal topography. In contrast, we found that interfering with NMDAR function either through MK801-induced blockade of NMDARs starting at P6 or neonatal enucleation significantly increases the length of axon branches and total length of arbors, without major effects on the number of branch tips. Our results further suggest that NMDARs act by altering the initial elaboration of arbors rather than by inhibiting a later-occurring remodeling process. Since the callosal map is present by P6, just as axonal branches of simple architecture grow into gray matter, we suggest that regulation of arbor development by NMDAR-mediated processes is important for maintaining the precision of this map.

The development of callosal and corticocorcal innervation in the neocortex of the hamster

The development of cortical afferentation by callosal and ipsilateral corticocortical fibers was studied in hamsters by transport of wheat germ agglutinin conjugated to horseradish peroxidase. Elongation of callosal axons (and possibly also of corticocortical fibers) started a couple of days before birth and extended through the first postnatal days. After a "waiting" period of a few days, axons were seen innervating restricted target sectors of the cortex. The zones of origin of these projections were initially exuberant, but they were subsequently trimmed down to overlap with the corresponding terminal fields