Successive depth variations in microvascular distribution of rat somatosensory cortex

Although hemodynamic-based functional brain imaging techniques are powerful tools to explore the brain functions noninvasively, hemodynamic-based signal is strongly affected by spatial configuration of microvessels. Understanding the quantitative relations between microvascular structure and functional activity is therefore significant to make a valid signal interpretation for the imaging techniques. In the present study, we evaluated depth profiles of microvascular distributions in rat somatosensory subfields (barrel field, forelimb region, trunk region and hindlimb region) and characterized depth variations in microvascular structures, such as locations, lengths and directions of microvessels, throughout the cortical layers (I-VI). To obtain the accurate microvascular structure, we made a customized casting method by using confocal laser scanning microscope. We observed that microvascular distribution successively varied throughout the cortical layers (I-VI) and that the maximum number density of microvessels was consistently found in middle layers (III-V). In addition, superficial layers had relatively long microvessels, almost perpendicular to the cortical surface, whereas middle layers had short microvessels propagating in all directions. These regional differences in microvascular structures were closely related to the somatosensory subfields, e.g., barrel field was the greatest number density of microvessels among the investigated subfields. Based on these observations, we compared microvascular profiles with previously reported distribution patterns of tissue partial pressure of oxygen (pO₂). The results showed that tissue pO₂ was correlated with microvascular distribution in some but not all of the subfields. This finding shows that detailed microvascular profiles are helpful to investigate causal relationships between microvascular structure and functional activities in cerebral cortex.