The normal cytoarchitectonic pattern of barrels in layer IV of mouse SmI face cortex is altered by early damage to the mystacial vibrissae (Van der Loos and Woolsey, '73). In the present study, the middle row of vibrissae (row-C) on one side of the face in groups of Swiss mice was cauterized on the day of birth (postnatal day [PND] -1)or on PND's - 2, 3, 4, 5, 7, 10, 12, 15, 20 and 30; littermates in each group served as controls. All animals were perfused on PND-60 and the brains sectioned parallel to SmI layer IV. For each specimen, the posteromedial barrel subfields (PMBSF) of the two hemispheres were reconstructed with a camera lucida and the cross-sectional areas of individual barrels measured using a small computer. The findings are: (1) The hemispheres ipsilateral to the vibrissal damage are quantitatively indistinguishable from the littermate controls indicating that the ipsilateral hemispheres in lesioned animals can serve as controls for observations of the type reported in this paper. (2) There are no consistent differences in the cross-sectional areas of the PMBSF's as a whole in the hemispheres ipsi- and contralateral to the peripheral damage, suggesting that there is no net loss of cortex as a result of the lesions. (3) The contralateral row-C barrels are reduced in size. Expressed as a percentage of normal values; row-C is reduced to 17% for animals lesioned on PND-1, 16% on PND-2, 38% on PND-3, 52% on PND-4 and 79% on PND-5; on PND-7 and later the cross-sectional areas of row-C barrels are normal. This implies that the barrel field of the SmI face cortex becomes progressively refractory to the effects of peripheral damage during the first postnatal week and in the period prior to PND-6, an intact periphery is necessary for normal cortical development. (4) In every case, the decreased cross-sectional area of row-C is accompanied by precisely increased cross-sectional areas of the barrels in adjacent rows-B and D. in the case of the restricted peripheral damage which we produced, there is a "compensation" in the contralateral hemisphere, which can be correlated with patterns of the specific thalamocortical projections.
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 is 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 a t 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.The neurons of layer IV of the mouse SmI middle row of vibrissae (row-C) which were face cortex are organized into multicellular made on different postnatal days (PND)) cytoarchitectonic units
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