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There are a number of images on the Web, most beautifully Visible Body's artwork, which show the caudate nucleus and putamen linked by regularly spaced connections across the barrier of the internal capsule. Since vertebrate forebrains are treated as having only a few segments (see Michael Carstens, 2017), which seem tough to relate to the C-shape of deep cerebral structures, there is something about such figures that piques curiosity. In a paper like Heide Hörtnagl et al., 2020 the situation seems less clear to me, but they weren't really focusing on this issue. Is there a regular/regulated spacing between the connecting bundles, and does it correlate with any other observable anatomical regularity?

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  • $\begingroup$ "vertebrate forebrains are treated as having only a few segments" - I think you may be overextrapolating from early developmental models (especially those making cross-species/evolutionary comparisons). In any event, that structure wouldn't have any relationship to the pattern I think you're asking about. $\endgroup$
    – Bryan Krause
    Jul 1 '21 at 18:37
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These seem to be referred to in literature as "transcapsular" or "caudolenticular" (gray) bridges. My initial thought was that the picture you shared is just some weird artistic rendering but upon seeing histological slides these are quite prominent and familiar to me as part of the basic "look" of these structures in histology. You can see them in any coronal slice through this area, this atlas, "Cross-Sectional Atlas of the Human Head" by JS Park has them labeled though often they are not labeled.

caudate putamen and bridges

(left image from JS Park; annotations on the right image are mine. There are many more "bridges" than I've drawn but I highlighted the most prominent to compare on the left side. Probably those deeper in the section relative to the camera are showing up a bit faint or blurred. This is also just one histological section, the bridges are found throughout the length of the area where caudate and putamen are located, at least a couple cm)

There may be other terminology, as well, but I'm having trouble finding it.

I see no reason or foundation on which to connect these to any sort of segmentation of the developing vertebrate forebrain. Since resources are sparse it seems difficult to find a reference, but my thought is that this is just an emergent pattern you get as different fiber pathways cross one another, like how your fingers look when you fold your hands together.

Some other references:

Mendoza, J. E., & Foundas, A. L. (2008). The Basal Ganglia. In Clinical Neuroanatomy: A Neurobehavioral Approach (pp. 153-193). Springer, New York, NY.

Nieuwenhuys, R., Voogd, J., & Van Huijzen, C. (2008). Brain slices. The Human Central Nervous System, 137-173.

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  • $\begingroup$ Thank you for a great answer! Though perhaps I should have asked a better question... I don't have ready access to these resources - can you tell from the sections you see whether the number and positioning of the bridges is consistent between individuals, as you'd expect from a regulated structure, or does the number and position vary more randomly as you'd expect from something emergent but unregulated? Thanks again. $\endgroup$ Jul 2 '21 at 14:29
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    $\begingroup$ @MikeSerfas In the limited experience I have with anatomy of individual human brains (albeit focused on cortex) I have to say I'm amazed by just how much variability there is. I can't see these structures as having consistent structure but don't have a source to prove that either, only a strong suspicion that if they did have a regulated structure it would be remarkable and highly discussed. I would say though that the artist picture you show vastly underrepresents the number which may be why they appear so orderly. $\endgroup$
    – Bryan Krause
    Jul 2 '21 at 14:38
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    $\begingroup$ @MikeSerfas I added a picture of one of the slides I referenced. I think you'll appreciate that they are quite diffuse and scattered, not nearly as orderly as that cartoon suggests, nor are they found in a plane like it appears there. $\endgroup$
    – Bryan Krause
    Jul 2 '21 at 15:43
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    $\begingroup$ Oh! I have seen those in images, but based on the artist's drawing I was expecting larger, rarer connections. Now I see what you mean. $\endgroup$ Jul 2 '21 at 19:45
  • $\begingroup$ @MikeSerfas Yeah that was exactly my thought too - went from something I hadn't heard of to seeing a picture and realizing I recognized that from any coronal section of the the caudate I'm quite familiar with the look. I don't know for sure but I'm guessing it's that those fibers are unmyelinated/less myelinated than the rest of the fibers. $\endgroup$
    – Bryan Krause
    Jul 2 '21 at 19:50

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