Effects of early vibrissae damage on neurons in the ventrobasal (VB) thalamus of the mouse

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Abstract
We have examined the effects of vibrissal damage on the structure of the VB of the murine thalamus. The findings are: (1) The barreloid (Van der Loos, '76) pattern can be recognized in the normal adult mouse thalamus. On the basis of size, the population of VB neurons in bimodal, the larger neurons–presumed to be thalamocortical relay cells (TCR's)–outnumbering the smaller neurons–presumed to be thalamic interneurons–by 3:1. (2) Damage to vibrissa Row‐C on PND‐1 (postnatal day‐1) results in an altered cytoarchitectonic pattern in the barreloids in the contralateral VB, which is qualitatively similar to the altered pattern of barrels in the cortex. The middle row of large barreloids is affected, which is anatomical evidence for the pattern of somatotopic organization in mouse VB. (3) The time course of alterations in thalamic architecture is different than that for the barrels in the cortex. The barreloids show progressively less severe alteration with increasing age at the time the peripheral lesions are made, like the barrels in the cortex. However, the thalamus of animals lesioned on PND‐4 or later appears normal, while the cortex of animals lesioned on PND‐4 and PND‐5 still can be altered. Thus, in the thalamus, as in the cortex, there is a “critical period” during which vibrissal damage alters cytoarchitectonic patterns, but the thalamic “critical period” ends about two days before that in the cortex. (4) Measurements of TCR's in the barreloids did not reveal any statistically significant size differences between neurons in different sectors of the experimental thalami. In particular, we could find no evidence for increases of TCR size in barreloids bordering affected Row‐C barreloid zones or for reductions in TCR size in barreloids in the C‐zones. (5) In Timm's stained normal material there is a parallel development of Timm's positive staining in the thalamus and in the cortex. Barreloid related Timm's positive staining appears by PND‐4, while Timm's staining in the cortical barrel hollows appears later by PND‐8. Of particular interest is the difference in the appearance of the barrels with the Timm's stain in the PND‐8 animals and in adults. In the former, the barrel hollows are filled with granular precipitate, while in the latter they are practically devoid of Timm's precipitate, suggesting a major changeover in the staining pattern (and possibly the functional attributes) of the terminal fields of the thalamocortical afferents in postnatal development.