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The pathophysiology of early ischemic injury after aneurysmal subarachnoid hemorrhage (SAH) is not understood. This study examined the acute effect of endovascular puncture-induced SAH on parenchymal vessel function in rat, using intravascular fluorescent tracers to assess flow and vascular permeability and immunostaining to assess structural integrity and to visualize platelet aggregates. In sham-operated animals, vessels were well filled with tracer administered 10 seconds before sacrifice, and parenchymal escape of tracer was rare. At ten minutes and 3 hours after hemorrhage, patches of poor vascular filling were distributed throughout the forebrain. Close examination of these regions revealed short segments of narrowed diameter along many profiles. Most vascular profiles with reduced perfusion contained platelet aggregates and in addition showed focal loss of collagen IV, a principal component of basal lamina. In contrast, vessels were well filled at 24 hours post-hemorrhage, indicating that vascular perfusion had recovered. Parenchymal escape of intravascular tracer was detected at 10 minutes post-hemorrhage and later as plumes of fluorescence emanating into parenchyma from restricted microvascular foci. These data demonstrate that parenchymal microvessels are compromised in function by 10 minutes after SAH and identify focal microvascular constriction and local accumulation of luminal platelet aggregates as potential initiators of that compromise.

In infancy, rats were provided handling stimulation and compared at 110 and 215 days of age with non-handled controls. Measurements were made of corpus callosum area, perimeter and length; and width measures were taken at 7 points along the longitudinal axis of the callosum. Callosal size was larger in males than in females, even when adjusted for the larger brain weight of the male. At 110 days handling stimulation increased callosal parameters and resulted in a more regular callosum in males, but this effect was no longer apparent by 215 days. Within the callosum, region-specific effects were found, suggesting that certain callosal fiber populations were involved. Handled males have previously been shown to be more lateralized than non-handled males; thus at least in this experimental system, increased callosal size and regularity is associated with greater hemispheric specialization.

Two myelin sheaths sharing a common outer tongue process are illustrated in a thin section electron micrograph from mouse optic nerve. Although they derive from the same outer tongue process, and therefore from the same oligodendrocyte, the two sheaths are of different thickness (6 and 9 turns). The example demonstrates that myelin sheath thickness is regulated independently for each axon, at a site very near the axon.