Quantitative cytoarchitecture and distribution of ihibitory neurons in the posterior orbitofrontal cortex of the human brain
Bautista Alvarez, Julied Fernanda
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Damage to the orbitofrontal cortex (OFC) is often accompanied by disorders in personality, mood and social behavior. The purpose of this thesis was to study the cellular composition and architecture of this very complex and functionally important area. The posterior part of the OFC (pOFC) has the most multimodal circuits of the OFC and robust connections with the amygdala, a key center of the brain for emotions. Moreover, because cortico-cortical connections can be predicted on the basis of architectonic features, analyzing the cytoarchitecture of this area is important in order to understand its connections and functions. Previous descriptions show that the architecture throughout the OFC varies along two major gradients of laminar differentiation. These gradients show that the size of layer IV decreases medially and posteriorly and that the most medial and posterior areas lack layer IV and are agranular. In this study we analyzed the transitional dysgranular cortex, which has a narrow layer IV, and lies between the anterior granular cortices, with well-developed layer IV, and the posterior agranular areas. For that purpose, three regions of interest (ROIs) along the mediolateral axis of the transitional pOFC were delineated: lateral, central, and medial. We used unbiased systematic sampling in our ROIs to estimate the densities of neurons, astrocytes, oligodendrocytes and microglia. We also estimated the densities of three classes of inhibitory neurons that are identified by the expression of calcium-binding proteins (calretinin, parvalbumin, and calbindin). We also found that neurons labeled for calretinin are the most common inhibitory neuron class. The density of calretinin labeled neurons in the transitional dysgranular part of pOFC also increases from medial to lateral at a comparable rate to the entire population of neurons. Parvalbumin and calbindin neuron densities also increase from medial to lateral, but the difference is less pronounced. Our findings show that, despite the density gradient from medial to lateral, the proportion of CR, PV, and CB neurons is comparable across the three ROIs. This shows that there is a balance of excitation and inhibition along the transitional dysgranular part of the pOFC, which is functionally important because it has been shown to be affected in disorders like autism.