Cerebral Microvessels

Abstract

It has always been customary to remove the meninges prior to examining the brain. Deprived of meninges, the brain surface appears smooth and without any apparent vascularity, a well-known fact to anyone interested in studying it. An apparently avascular surface is also applicable to the cerebellum, medulla, and spinal cord deprived of meninges (Fig. 1). This is intriguing considering that the first 2.5 mm below the brain surface has one richer cellular and functionally active region of the human body, namely, the cortex gray matter where most neurons reside. Brain oxygen consumption represents roughly 20% of that consumed by the entire body although it only represents a small fraction of its volume (Hossmann 1994). Therefore, the cortex gray matter, where most neurons reside, must be richly vascularized, a fact that conflicts with its apparently avascular surface.

By removing the meninges, the vascular elements connecting the meninges and brain are severed leaving its surface apparently avascular (Fig. 1). Recent studies have demonstrated that these connecting vessels originate in the pial anastomotic capillary plexus, an important vascular compartment of the meninges that has remained essentially ignored (Marín-Padilla 2012). The pial capillaries establish a short-linked anastomotic plexus that covers the cortex’ expanding surface. The capillary size ranges from 3 to 7 μm and is, therefore, invisible to unaided observation. The meninges removal inevitably will carry the pial capillaries that will be lost when it is discarded. Perhaps, the invisibility of pial capillaries as well as the customary meninges castoff could explain why both the connecting capillaries and the pial anastomotic capillary plexus have been seldom recognized. The smooth and apparently avascular surface of the mammalian brain, deprived of meninges, has become a well-known and undisputed fact (Fig. 1).

The simple inspection of the cortex surface (deprived of meninges) with a good magnifying glass (or dissecting microscope) solves the dilemma (Marín-Padilla 2012). Throughout its entire surface, there are small openings with a severed small blood vessel on each one. The orifices represent the Virchow-Robin compartments, clearly distinguishable from the severed blood vessel inside them. Both the orifices and their inside vessels are invisible to unaided (naked eye) observation. Anyone with a good magnifying glass could corroborate these observations, which are also applicable to any mammalian brain deprived of meninges.