Axonal bundles

The ontogenesis of the AOS then, is closely bound up with the formation of its principal connection site within the mature brain. The sequence of events in mammals is revealed as a process which involves (1) early specialisation of presumptive GnRH cells (2) their attachment to and movement along specific (and transient) axonal bundles of the VN and N. terminalis tracts, and (3) coalescence of the neurocrine cells in the hypothalamus, where they complete differentiation as multi-axonal neurocrine cells. 

Lohmann and Koppen (1995) have also shown that the dendritic clusters and the vertical axonal bundles in rat visual cortex originate from the same neurons, and that the axonal bundles and the dendritic clusters have similar center-to-center spacing. Recently Casanova et al. (2008) have concluded that in the human cortex the pyramidal cell arrays and the vertical bundles of myelinated axons have similar spacing. 

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. 

Lohmann H, Kbppen HJ (1995) Postnatal development of pyramidal cell modules and axonal bundles in the visual cortex of the rat. J Hirnforsch 36 101-111. 

Chan-Palay V (1973c) Cytology and organization in the nucleus lateralis of the cerebellum The projections of neurons and their processes into afferent axon bundles. Z. Aruit. Entwickl-Gesch., 141, 151-159. 

Studies in the grasshopper embryo have provided some of the most conclusive evidence of this type. Bastiani and Goodman (1986) showed that the axon of the pCC neuron in the central nervous system (CNS) of this insect embryo grows along a specific axon bundle or fascicle which is pioneered by the axons of the dMP2 and MPl neurons (Fig. 

Mechanical Conditioning. Tissue regeneration has been shown to benefit from external mechanical stimuli. One study demonstrated that constant mechanical tension elongated the axon bundles of synapsed primary embryonic rat cortical neurons in vitro (Smith et al., 2(X)1). Moreover, certain... 

Three-dimensional (3D) visualization of fiber tracts in axonal bundles in the brain and muscle fiber bundles in heart and skeletal muscles can now be done interactively. The images are no longer composed of scalar" (single) values in the voxels, but a complete diffusion tensor" (a 3 x 3 symmetric matrix" ) is measured in each voxel. 

CARS microscopy was used for the study of axonal myelin under physiological conditions in spinal cord, using white matter strips isolated from the spinal cord of guinea pigs and kept alive in oxygen-bubbled Krebs solution. Both forward-and epi-detected CARS were used to probe the axon bundles and resolve detailed structures such as nodes of Ranvier. In addition, simultaneous CARS microscopy of myelin and TEPF imaging of intra- and extra-axonal calcium was demonstrated [58]. 

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