Apical dendritic clusters

Pyramidal Cell Modules Anatomically Defined Units Based on Apical Dendrite Clustering... 

Fig. 1 Rat visual cortex sectioned in the vertical plane and labeled with an antibody to MAP2 to show the apical dendritic clusters arrows). Note the densely stained layer 1 beneath the pia, and the pale staining of the white matter. Scale bar = 100 (rm...

Apical Dendritic Clusters in the Neocortices of Other Animals... 

Fig. 4 (continued) Monkey primary visual cortex. Vertical sections labeled with an antibody to MAP 2. The illustration shows the apical dendritic clusters (arrows) arising from layer 5 pyramids and the bundles of apical dendrites (arrowheads) arising from layer 6a pyramidal cells. The locations of layers 4, 5, and 6 are indicated. From Peters and Sethares (1991). Scale bar = 100 p.m... 

The Spatial Arrangement and Connections of Apical Dendritic Clusters... 

Obviously, the question of the geometric distribution of dendritic clusters needs to be examined further, and any modular theory of cortical function based on the concept that the apical dendritic clusters are the axes of functional cortical modules, or minicolumns, has to take into account that there is biological variation in the composition of the modules, as shown by studies like those of Lev and White (1997) and Vercelli et al. (2004). 

In addition to vertical bundles of myelinated axons, the cerebral cortex of monkeys (e.g., DeFelipe et al., 1990) and of humans (e.g., del Rio and DeFelipe, 1995) also contains vertically oriented bundles of unmyelinated axons that are referred to as horsetails. These horsetails are the axonal plexuses of the inhibitory double bouquet cells and can be demonstrated in monkey neocortex by immunolabeling with antibodies to calbindin and tachykinin. As shown by DeFelipe et al. (1990), in the monkey these axonal bundles are widespread and form a regular columnar system descending from layer 2 to layers 3-5. The bundles are most evident in tangential sections taken at the level of layer 3, where they can be seen to have a center-to-center spacing of 15-30 fim. In a later study of the calbindin labeled double bouquet cells in monkey striate cortex, Peters and Sethares (1997) showed that there is one double bouquet cell, and therefore one vertically oriented double bouquet cell axonal plexus, or horsetail, per pyramidal cell module (Fig. 7). Within layer 2/3 the double bouquet axons run alongside the apical dendritic clusters, while in layer 4C they are closely associated with the vertical myelinated axonal bundles. DeFelipe et al. (1989 1990) proposed that the axon terminals of the double bouquet cell synapse with the shafts and spines of basal dendrites and oblique shafts of apical dendrites of pyramidal cells, but the exact role of these vertical bundles of inhibitory axons is not known. It is likely that they constitute a vertical inhibitory system that acts upon pyramidal cells within the minicolumns. 

In a recent review article on the anatomy of autism Amaral et al. (2008) point out that in these studies by Casanova and his colleagues, only 14 cases of autism, 9 of which had seizures and at least 10 with mental retardation, have been examined for minicolumn pathology. Consequently, more studies using a greater number of autistic brains with fewer other complications need to be carried out before any definite conclusions can be reached about changes that can only be attributed to autism. It would also be appropriate to examine brains in which the apical dendritic clusters and myelinated axon bundles have been stained to confirm the sizes of the minicolumns as detected in digitized images from autistic brains. 

Fig. 2 Transverse section of rat visual cortex at the level of layer 4. The section is labeled with MAP2 antibody to show the darkly stained apical dendrites within the clusters (arrows). Scale bar = 50 p.m...

As first shown by Escobar et al. (1986) in the neocortex of the mouse, the apical dendrites of the pyramidal cells in layer 6a do not join the clusters formed by the apical dendrites of the pyramidal cells in layers 5-2. Instead they form their own independent groupings, which we have referred to as bundles. Since the pyramidal cells of layer 6a are small, their apical dendrites are thinner than those of the pyramidal cells in upper layers and they form numerous bundles that ascend as far as layer 4, where they terminate and form their apical tufts (see Figs. 3 and 4). 

Another interesting variation in the clustering of apical dendrites occurs in the cat sensorimotor cortex. There it has been shown by Fleischhauer (1974) that in the posterior sigmoid gyrus the apical dendrites of the layer 5 pyramidal cells frequently bifurcate soon after they emerge from the large cell bodies. The secondary dendritic branches then run obliquely for a short distance and at the level of layer 3 they join secondary branches from other apical dendrites to form dendritic clusters... 

Fig. 5 (Continued) Note that in area 17 the apical dendrites of layer 5 pyramids pass into layer 2/3 where they are joined by the apical dendrites pyramidal cells in that layer, but the bundles of apical dendrites of the layer 6A pyramidal only extend as far as layer 4C. In area 18, the apical dendrites of the layer 5 and layer 6A pyramidal cells both extend only up to layer 3A. Nearly all of the apical dendrites in the clusters in the upper layers of the cortex arise for the layer 2 and 3 pyramidal cells. From Peters et al. (1997)...

It is evident that the modules of pyramidal cells whose apical dendrites form clusters and the vertical bundles of myelinated axons are facets of the same basic, modular organization of neurons into vertical units that we can refer to as minicolumns. And... 

Lev, DL, White EL (1997) Organization of pyramidal cell apical dendrites and composition of dendritic clusters in the mouse. Emphasis on primary motor cortex. Europ J Neurosci 9 280-290. 

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