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Connections between cells of the internal capsule, thalamus, and cerebral cortex in embryonic rat
1University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, United Kingdom. zoltan.molnar@ibcm.unil.ch
Insights
Early mammalian pallium development shows initial connections forming via the internal capsule, with dorsal thalamus connections delayed until later embryonic stages. Subplate projections approach the diencephalon but may not enter the dorsal thalamus.
Area of Science:
- Neuroscience
- Developmental Biology
- Neuroanatomy
Background:
- Understanding the precise timing and pathways of early neuronal connections is crucial for deciphering brain development.
- The mammalian pallium's initial wiring, particularly between the cortex and thalamus, remains incompletely understood.
Purpose of the Study:
- To map the emergence and distribution of early cellular and axonal connections in the embryonic rat pallium.
- To investigate the developmental timeline of connections involving the dorsal/ventral thalamus, internal capsule, perirhinal cortex, and dorsal cortex.
Main Methods:
- Utilized carbocyanine dye tracing techniques in fixed embryonic rat brains (E14-E17).
- Employed both anterograde and retrograde tracing to identify neuronal projections and cell bodies.
- Performed crystal placements in specific brain regions for localized tracing.
Main Results:
- First corticofugal projections reached the internal capsule by embryonic day 14.
- Internal capsule established early connections with the perirhinal and ventral cortex, but not the dorsal cortex until E17.
- Axon growth to the dorsal thalamus was delayed, with two distinct phases of impediment identified within the internal capsule and thalamic reticular nucleus.
Conclusions:
- The development of connections between the cortex and dorsal thalamus is a protracted process with specific regional delays.
- Early wiring patterns suggest a sequential establishment of connections, prioritizing ventral and perirhinal regions over dorsal cortex.
- Subplate projections may not ultimately enter the dorsal thalamus despite early proximity.
Abstract:
The aim of our study is to understand the development of the earliest connections in the mammalian pallium by documenting the distribution of cells and fibres labelled from the dorsal and ventral thalamus, internal capsule, perirhinal, and dorsal cortex during the period between embryonic day (E) 14 and 17 by using carbocyanine dye tracing in fixed embryonic rat brains. Dye placed in the thalamus of E14 brains backlabels cells in the thalamic reticular nucleus and within the primitive internal capsule. Both anterograde and retrograde tracing confirmed that the first corticofugal projections reach the internal capsule by E14. At E15-E16, after the first cortical plate cells have migrated into the lateral cortex, some cells of the cortical plate and subplate and marginal zone, are backlabelled from the internal capsule, but still not from the dorsal thalamus, even with very long incubation periods. Crystal placement into the perirhinal cortex at E14-E15 labels numerous cells within the internal capsule, whereas no such cells are revealed from dorsal cerebral cortex until E17, suggesting that internal capsule cells establish early connections with the perirhinal and ventral but not dorsal cortex. We propose that the growth of axons from cortex to dorsal thalamus is delayed in two regions: first from E14-E15 at the lateral entrance of the internal capsule and then, from E16, closer to the thalamus, probably within the thalamic reticular nucleus. Subplate projections reach the proximity of the diencephalon at an early stage, but they might never enter the dorsal thalamus.