Segmental identity and vulnerability in cerebral arteries.

Clinical experience shows that certain diseases involve specific areas of the vascular tree and remarkably spare others. Topographic differences in the vascular environment already suggest a regional specificity of the vascular anatomy. The biological grounds of such regional differences, although unknown, can account for the specificity of biological responses to stimuli. Such segmental specificities are beyond morphological analysis. They create an invisible discontinuity in an apparently homogenous anatomical, histological and haemodynamic system.We call this property segmental identity and thus vulnerability. Most of this identity is established during development and is preserved throughout life; its expression, however,may vary over time according to various stresses and create various clinical phenotypes. The memory of the evolutionary steps and their chronology is imprinted on the arterial anatomy and thus potentially readable. One can postulate that since the age of each arterial segment is different, its resistance to time and stimuli is most likely variable. The vulnerability of these segments cannot be permanent both in a qualitative and quantitative way. Some genetic functions only seem to be active during a short period of time: during vasculogenesis for example. Therefore either the trigger is always active and the target vulnerability window of the cells time-limited, or the target is permanently exposed and the trigger agent can either be exogenous and rare, or most of the time inactive or inactivated.