The human brain intracerebral microvascular system: development and structure

Abstract

The capillary from the meningeal inner pial lamella play a crucial role in the development and structural organization of the cerebral cortex extrinsic and intrinsic microvascular compartments. Only pial capillaries are capable of perforating through the cortex external glial limiting membrane (EGLM) to enter into the nervous tissue, although incapable of perforating the membrane to exit the brain. Circulatory dynamics and functional demands determine which capillaries become arterial and which capillaries become venous. The perforation of the cortex EGLM by pial capillaries is a complex process characterized by three fundamental stages: (1) pial capillary contact with the EGLM with fusion of vascular and glial basal laminae at the contact site, (2) endothelial cell filopodium penetration through the fussed laminae with the formation of a funnel between them that accompanies it into the nervous tissue while remaining open to the meningeal interstitium and, (3) penetration of the whole capillary carrying the open funnel with it and establishing an extravascular Virchow-Robin Compartment (V-RC) that maintains the perforating vessel extrinsic (outside) the nervous tissue through its entire length. The V-RC is walled internally by the vascular basal lamina and externally by the basal lamina of joined glial cells endfeet. The VRC outer glial wall appear as an extension of the cortex superficial EGLM. All the perforating vessels within the V-RCs constitute the cerebral cortex extrinsic microvascular compartment. These perforating vessels are the only one capable of responding to inflammatory insults. The V-RC remains open (for life) to the meningeal interstitium permitting the exchanges of fluid and of cells between brain and meninges. The V-RC function as the brain sole drainage (prelymphatic) system in both physiological as well as pathological situations. During cortical development, capillaries emerge from the perforating vessels, by endothelial cells growing sprouts analogous to their angiogenesis, entering into their corresponding V-RCs. These new capillaries to enter into the nervous tissue must perforate through the V-RC outer glial wall, a process analogous to the original perforation of the cortex EGLM by pial capillaries. These emerging capillaries are incapable of reentering the V-RCs and/or perforating vessels. As the new capillary enters into the nervous tissue, it becomes surrounded by glial endfeet and carries a single basal lamina (possibly glial). Capillaries emerging from contiguous perforators establish an anastomotic plexus between them, by mechanisms still poorly understood. The capillaries of this anastomotic plexus constitute the cerebral cortex intrinsic microvascular compartment and together constitute the so-called blood-brain-barrier. The intrinsic capillaries are changing and readapting continuously, by both active angiogenesis and reabsorption, to the gray matter neurons developmental and functional needs. The brain intrinsic capillaries are among the most active microvessels of the human body. Unresolved developmental and functional aspects concerning the cerebral cortex intrinsic capillary plexus need to be further investigated.

Keywords: EGLM; endothelial cell filopodium; human brain; intracerebral microvascular system; meningeal inner pial lamella.

Figure 1

Various developmental aspects of the human brain embryonic vascular system. (A) Streeter drawing of a 50-day-old human embryo (Streeter, 1918) showing the meningeal compartment main vessels. The cerebral cortex (neocortex), not yet vascularized, is covered by arachnoidal vessels and its surface appears smooth and without a pial capillary plexus. (B) Schematic representation of a 50-day-old human embryo's meninges, showing the vessels of its dural, arachnoidal, and pial lamellae. Also a pial capillary anastomotic plexus covers the cortex surface in close proximity to its external glial limiting membrane (EGLM), composed of glial endfeet covered by basal lamina material. The filopodia of one of the capillaries (lower right on ELGM layer) are perforating and entering into the brain nervous tissue. (C) Schematic representation of the three developmental stages of pial capillaries entering into the cortex: (a) pial capillary contact with cortex EGLM with fusion of both vascular and glial basal laminae, (b) endothelial cell filopodia perforation through the fussed basal laminae and entrance into the nervous tissue, and (c) capillary perforation and entrance into the nervous tissue with formation of the extravascular Virchow-Robin Compartment (V-RC) around the vessel. At this stage, some meningeal cells (MC) enter into the V-RC accompanying the vessel (P) and become the source of its smooth muscles. By the continued incorporation of additional glial endfeet around the perforating vessel, the surface EGLM seems to be extending into the brain accompanying it. The V-RC accompanies the perforating vessels through its entire length while remaining open to the meningeal interstitium, functioning as the brain's sole drainage (prelymphatic) system. Perforating vessels remain outside of the nervous tissue and together they represent the brain's extrinsic microvascular compartment. A Glial fibrillary acidic protein-stained brain section of a 5-year-old child with brain damage, showing the V-RC with its central vessel (D), the perivascular spaces (V-RC) with several stained macrophages (M), and its glial stained outer wall. Some macrophages seem to be entering into the V-RC extravascular space. The stained macrophages seem to have phagocytized glial products from the damaged site. Key: G, glial endfeet; V-RC, Virchow-Robin Compartment; M, Macrophages; CGS, perforating capillary growing tip; MC, meningeal cell; P, pericytes.

Figure 2

Hematoxylin and eosin stained sections of the cerebral cortex of (A) a 43-day-old embryo (tubal pregnancy) and of (B) a 50-day-old embryo (hysterectomy) showing the rich pial anastomotic capillary plexus that covers their still-unvascularized cerebral cortex. The pial capillaries are numerous, very small, and invisible to the naked eye. At this early embryonic age, they still contain nucleated red cells. The embryos cortical developments are at the marginal zone (43-d-o) and the primordial plexiform (50-d-o) stages. A variety of neurons (n) are recognized above the matrix (M) zone, including a horizontal Cajal-Retzius cell (C-R). Key: V, ventricle. Scale = 10 mm.

Figure 3

Various electron photomicrographs, from the brains of 12-day-old hamster embryos, showing various aspects of the perforation of the cerebral cortex external glial limiting membrane by pial capillaries. (A) Photomicrograph (4 μm-scale) showing that the cerebral cortex delimited by the external glial limiting membrane (EGLM) composed of glial endfeet (G) covered by basal lamina material a several pial capillaries above it. The capillaries (*) are composed of endothelial cells separated by junctions (arrows) and their size varies, including very small (possibly growing) capillaries. One of the capillary has established direct contact with the cortex EGLM, with fusion of both vascular and glial basal laminae, and some endothelial cell filopodia (PF, long fine arrows) that already have perforated through the EGLM and penetrate into the nervous tissues. This pial capillary is a growing one with numerous internal and external filopodia as well as a sliding endothelial cells over its wall. (B) Detail of endothelial cell filopodia establishing early contacts with the cortex EGLM. (C) Detail of the entrance of an endothelial cell filopodium (PF) into the nervous tissue, through the fused vascular and glial basal laminae, with the formation of a funnel (white arrow) that remains open to the meningeal interstitium. (D) Low-power view showing a pial capillary (*) entrance into the nervous tissue through the EGLM (arrows) with the formation of the Virchow-Robin Compartment (V-RC) around it, and the penetration of a meningeal cell (P) into the extravascular space. The growing sprout of the perforating vessels is composed of several advancing endothelial cells. (E) High-powered view (4 μm scale) of a pial perforating capillary (*) showing its entrance into the cortex (curved arrows) and the formation of the extravascular V-RC (V-RC) around it. The continuous incorporation of additional glial endfeet (G) to the V-RC outer wall seems as an extension the surface EGLM that keep the vessels extrinsic (outside) of the nervous tissue. The entrance of a pericytes (P) from the meningeal space into the extravascular V-RC is also shown. The V-RC remains open to the meningeal interstitium and function as the brain's sole drainage (prelymphatic) system.

Figure 4

Composite of a photomicrograph and two camera lucida drawings, from rapid Golgi preparations, illustrating various aspects of the developing cerebral cortex (motor region) of a 15-week-old human embryo. At this age, the thickness of the cerebral cortex, at the motor region, is around 2 mm. (A) Montage of photomicrographs (50 μm scale) showing, at least 7 different pyramidal cell strata with neurons raging in size from 40 μm for the upper, smaller, and last to reach the Cajal-Retzius cells of the first lamina, to 275 μm for the lower, larger, and first to reach the Cajal-Retzius cells. At this age, the deepest and older pyramidal neurons (P1) have started to develop short basal dendrites (arrow) and a few apical dendritic spines (small arrow heads) indicating the beginning of their ascending functional maturation. The pyramidal neurons of the upper strata are still immature with smooth and spineless apical dendrites and descending axon that start to reach the white matter. Cajal-Retzius horizontal axonic fibers (small arrows) are also recognized within the first lamina. The formation of the pyramidal cell plate that started around the 8th week of gestation is nearly complete at this age. (B) Composite of camera lucida drawings (100 μm scale) from rapid Golgi preparations comparing the size, location, interrelations, and organization of the neuronal, and fibrillar elements of the first lamina (I), the pyramidal cell plate (PCP) and the subplate zone (SP). Only the deepest and older pyramidal neurons have started their ascending functional maturation (P1) by developing basal dendrites and their descending axons have entered the white matter. At this age, horizontally migrating neurons (labeled “?”) are first recognized through the cortex lower pyramidal cell strata; we now know these migrating cells are the precursors of the cortex inhibitory neurons. These neurons become incorporated into the deepest, older and maturing pyramidal cell (P1) stratum and will be its future inhibitory (basket, bi-tufted, and chandelier cells) neurons. Also at this age, the subplate zone (SP) deep primordial neurons (pyramidal-like and Martinotti cells) start to lose their original attachment to the Cajal-Retzius cells of the first lamina. (C) Montage of camera lucida drawings (100 μm scale) reproducing the entire thickness of the human motor cortex (at this gestational age), illustrating the size, morphology, distribution, and organization of its basic neuronal, fibrillar, microvascular, and glial elements. Intrinsic capillary anastomotic plexuses, between adjacent perforators, are recognized through the ependymal (E), paraventricular (PV), white matter (WM), subplate (SP), and lower gray matter (GM) zones (arrows). Some perforating vessels reach the paraventricular zone, few reach the white matter and many more reach the gray matter. Fibrous (white matter) and early protoplasmic (gray matter) astrocytes are recognized around their capillaries. At this age, the cortex gray matter starts to develop its first intrinsic microvascularization (VP) through its deepest, older and maturing pyramidal cell (P1) stratum. The remaining gray matter (GM) pyramidal cell strata are still immature at this age. Also, at this age, the deep primordial neurons of the subplate zone start to lose their original functional attachment to first lamina Cajal-Retzius cells. The white matter is crossed by bundles of corticipetal and corticofugal axonic fibers and by numerous ascending glial and neuronal precursors. Glial cell precursors of fibrous astrocytes and oligodendrocytes are recognized accompanying the white matter fibers in both directions. Also illustrated are radial glial cells (RG) attached to the ependyma, with ascending filaments that reach the cortex surface with endfeet incorporated into the EGLM as well as small glial cells precursors still attached to ependyma.

Figure 5

Photomicrographs of rapid Golgi preparations from the motor cortex of newborn infants, illustrating lower (A) and higher (B) power views of the intracerebral extrinsic (E) and intrinsic (I) microvascular compartments as well as their close structural and functional interrelationships. (A) Photomicrograph of the newborn motor cortex gray (GM) and white (WM) matter intracerebral extrinsic and intrinsic microvascular compartments, illustrating the abundance of capillaries, their three-dimensional organization, as well as the nearly constant intervascular distance between the extrinsic perforating vessels (E) and the similar dimension of the intrinsic (I) microvascular compartments established between them. Both essentially similar measurements are believed to represent physiological constants needed for the normal neuronal functional activity of the mammalian brain. The white matter (WM) compared with the gray matter (GM) has fewer capillaries and larger intercapillary spaces. A few horizontal axonic fibers (at) from Cajal-Retzius are recognized running through the first lamina. (B) High-powered view of the gray matter extrinsic and intrinsic microvascular compartments between a central venule (V) and two adjacent arterioles (A), separated by 400–600 μm. This intervascular distance also delineates the dimension of the intrinsic microvascular compartment between the perforating vessels. The intrinsic microvascular compartment is an anastomotic capillary plexus with small intercapillary spaces where neuron (N) resides; its capillaries have a single basal lamina and together they represent the blood brain barrier. These capillaries may be incapable of responding to brain inflammatory insults and/or participate in inflammatory processes (See also Figure 7A).

Figure 6

Photomicrographs comparing the intracerebral extrinsic and intrinsic microvascular compartments of a newborn (A) and adult (B) human brains. The reproduction in (A) is from a rapid Golgi preparation of the motor cortex of a newborn infant and that in (B) is from an intravascular casting of an adult human brain, from the work of Duvernoy et al. (1981). Despite the significant differences in brain size weight (newborn ca. 410 g and adult ca. 1,350 g) the overall dimension, vascular composition and structural organization of their intracerebral extrinsic and intrinsic microvascular compartments are remarkable similar. These structural and organizational similarities mirror the similar developmental and physiological constrains that endure through the prenatal and postnatal functional maturations of cortical neurons Marín-Padilla (2011). In both brains (A, B), there are more intrinsic capillaries with smaller intercapillary spaces in the gray matter than in the white matter. The abundant of intrinsic capillaries through the cortex gray matter protect the functional activity of its neurons, in both normal and abnormal conditions. Key (A): I, first lamina; GM, gray matter; WM, white matter; A and V, arterial and venous vessels; and in (B) 6, pial vein; 5, venule; 1, arteriole; 3, deep arteriole; 2, recurrent arteriole.

Figure 7

Photomicrographs from rapid Golgi preparations of a newborn infant (A) motor cortex and (B) from the brain of 12- and 14-day-old hamster embryos, (A) a high-powered, three-dimensional view of the complex and rich intrinsic capillary anastomotic plexus of a newborn motor cortex showing the abundance of capillaries, their small intercapillary spaces and a stellate basket cell (BC) in one of them. This basket cells has numerous horizontal, oblique, and vertical axonic terminals (at) and a recognizable (b) pericellular basket. Together, these intrinsic capillaries represent the brain's blood-brain barrier and, physiologically, they are the responsible for the normal functional activity of cortical neurons; and, (B) Golgi preparations, from 12- and 14-day-old hamster embryos brains showing various examples (A–F) of growing capillaries (angiogenesis) showing the polyp-like ending sprouts of advancing endothelial cells with numerous terminal searching filopodia. Also illustrated are: a growing capillary loop (G) between adjacent vessels and a growing vessel (H) in the center of the unstained optic nerve of the eye of a 12-day-old hamster embryo, with terminal searching filopodia. Also illustrated are the eye's fovea and pigmented retina.

Figure 8

Electron photomicrograph of a small (3 × 4 μm) and complex capillary from the cerebral cortex of a 12-day-old hamster embryo showing a main vessel surrounded by several attached and non-attached filopodia (F). The vessel has a main lumen (white star) and several non-communicating additional smaller lumens (*) filled with basal lamina-like material. At least four tight junctions (arrows) are recognized among the endothelial cells. Two of the tight junctions are between the main vessel endothelial cells and the other two junctions are between endothelial cells that appear to be outside the vessel. One endothelial cell of the main vessel has a recognizable nucleus (N). This complex vessel is interpreted as representing the confluence of advancing filopodia and lamelopodia from approaching capillaries that are in the process of establishing intercommunications between them. In the ensuing capillary, the additional lumina will coalesce with the main lumina establishing a common one and the peripheral filopodia will be incorporated into the new capillary wall.