Double bouquet cells

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. 

Yanez et al. (2005) have carried out a survey of the distribution of double bouquet cells in the cortices of various mammalian species. There are no double bouquet cells in the neocortices of rodents and rabbits, and compared to primates there are relatively few double bouquet cells in the cortices of carnivores such as cats, dogs, lions, and cheetahs. Consequently there is great variation in the occurrence of double bouquet cells with horsetail axons, and Yanez et al. (2005) conclude that although double bouquet cells are an important neuronal element in the organization of minicolumns in primate neocortex, this is less true in other mammalian species. 

Fig. 7 Diagram of the microcolumn in monkey visual cortex to show that there is one double bouquet cell horsetail black) per pyramidal cell module. The other colors correspond to those in Fig. 5, which explains the composition of the pyramidal cell modules in monkey area 17. From Peters and Sethares (1997)...

DeFeUpe J, Hendry SHC, Hashikawa T, MoUnari M, Jones EG (1990) A microcolumnar structure of cerebral cortex revealed by immunocytochemical studies of double bouquet cell axons. Neuroscience 37 655-673. 

Del Rio MR, DeFelipe J (1995) A light and electron microscopic study of calbindin D-28 k immunoreactive double bouquet cells in the human temporal cortex. Brain Res 690 133-140. 

Peters A, Sethares C (1997) The organization of double bouquet cells in monkey striate cortex. J Neurocytol 26 779-797. 

Yanez IB, Munoz A, Contreras J, Gonzalez J, Rodriguez-Veiga E, DeFelipe, J (2005) Double bouquet cells in the human cerebral cortex and a comparison with other mammals. J Comp... 

DeFelipe J, BaUesteros-Yanez I, Inda MC et al (2006) Double-bouquet cells in the monkey and human cerebral cortex with special reference to areas 17 and 18. Prog Brain Res 154 15-32... 

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