Patterns of vascular sprouting in the postnatal development of the cerebral cortex of the rat

Light microscopic histochemistry for alkaline phosphatase was employed in a study of the development of vascular sprouting, with respect to time and distribution, in the rat cerebral cortex. Sprouts were counted in the full thickness of the cerebral cortex at each day from birth to 21 days of age. Several distinct bursts of sprouting activity were observed at specific times and levels of cortex. From birth to 4 days of age, sprouting was intense in the superficial third of the cortex. At 7 to 8 days, a burst of sprouting was found which was greatest in the middle third. Additional bursts of sprouting appeared at 10 and 14 days. Developing vessels with characteristics of arteries, capillaries, or sprouts were alkaline-phosphatase positive, while veins were not. It is concluded that alkaline phosphatase is a useful marker for identification of both mature and immature vasculature, as it reveals patent and nonpatent vessels, and the sprouts which are precursors of the mature vascular bed. New vessels developing in the cortex arise mainly from blind sprouts of capillaries, evidently in response to the metabolic demands imposed by the maturational process. At birth, the majority of intracortical vessels are capillaries. By 10 days of age, most perforating vessels from the surface have taken on arterial or venous characteristics. The findings are discussed in connection with morphological and biochemical differentiation and the pattern of vascularization in the mature cerebral cortex.     

Corneal neovascularization as studied by scanning electron microscopy of vascular casts

Rat corneas were cauterized chemically, and the induced neovascularization was studied by scanning electron microscopy of vascular casts. Casts were prepared by filling the pericorneal and intracorneal vessels with an acrylic monomer by an intracardiac injection. The initial response to injury was a vasodilation of pericorneal vessels and the appearance of impressions in the walls of veins and venules consistent with those of marginating leukocytes. The first new vascular buds emerged from the pericorneal venules and capillaries at 27 hours after injury. These sprouts lengthened and multiplied by 72 hours to produce a rich anastomosing plexus. New vessels were not seen arising from arteries and arterioles. By 7 days, numerous channels reached the cautery site and, by 13 days, many of the redundant intracorneal vessels had regressed leaving large looping channels connecting either with a pericorneal artery or a vein. The casts of those continuous with the artery had surface features suggesting arterial or arteriolar differentiation, whereas the smooth surfaces of the remaining channels were consistent with those of veins or capillaries. Transmission electron microscopy confirmed this differentiation by documenting intracorneal vessels with the smooth muscle and prominent endothelial cell nuclei which characterize arteries and arterioles. Vascular casts have several advantages in the study of neovascularization. They depict the three-dimensional characteristics of new vessel formation and reflect the vascular and cellular events in the accompanying acute inflammatory response; define more readily than histologic sections the time that the first new buds appear; emphasize capillaries and venules as the predominant source of new vessels; and suggest that certain new intracorneal vessels assume morphologic features of arteries or arterioles, whereas others retain capillary or venous characteristics.     

Methods for histochemical demonstration of vascular structures at the muscle-bone interface from cryostate sections of demineralized tissue

In tissue decalcified with MgNa2EDTA at a neutral pH activity for ATPase can used be for demonstration of the vascular structures at the muscle-bone interface. The GOMORI method for alkaline phosphatase is only of value, when fresh unfixed tissue is to be examined. The azo-dye method for alkaline phosphatase failed to give satisfactory results, and so did the alpha-amylase PAS method. 5'-nucleotidase activity is present in both capillaries and in cells lining the surfaces of bones, while larger blood vessels are poorly stained.     

Rearrangement of the metaphyseal vasculature of the rat growth plate in rickets and rachitic reversal: a model of vascular arrest and angiogenesis renewed.

The morphology of the metaphyseal microvasculature at the epiphysis was examined at both the light and electron microscopic level in rickets and rachitic reversal. The animals studied were normal, rachitic, and rachitic reversed at 8, 24, and 96 hours post-vitamin D administration. The overall architecture of the metaphyseal vessels was significantly altered throughout the intervals examined. In the rachitic animal, arterioles, venules, and capillaries were found adjacent to the growth plate, either directly apposed to the hypertrophic chondrocytes or separated from them by bone-forming cells. These vessels are in many ways similar to the larger arterioles and venules that normally supply the metaphyseal capillary sprouts, but in the normal growing animal are usually located 350-500 microns from the epiphyseal cartilage. The rachitic capillaries appear relatively well differentiated with a partial basement membrane and a perivascular cell lining. In early rachitic reversal, small vascular projections are induced to grow from the large diameter venules that border upon the hypertrophic chondrocytes. These vascular sprouts that invade the epiphyseal cartilage are quite undifferentiated, with no basement membrane or pericyte lining at the sprout apex and occasional abluminal endothelial cell projections. Within 96 hours, the metaphyseal microvasculature has returned to an apparently normal state with only capillaries at the cartilage-vascular interface and larger vessels (arterioles and venules) located several hundred microns deeper into the metaphysis. The sequential processes of differentiation and cessation of capillary growth followed by dedifferentiation and reinitiation of microvascular growth make the rachitic system a unique one in which to study angiogenesis.     

Rearrangement of the metaphyseal vasculature of the rat growth plate in rickets and rachitic reversal: A model of vascular arrest and angiogenesis renewed

The morphology of the metaphyseal microvasculature at the epiphysis was examined at both the light and electron microscopic level in rickets and rachitic reversal. The animals studied were normal, rachitic, and rachitic reversed at 8, 24, and 96 hours post-vitamin D administration. The overall architecture of the metaphyseal vessels was significantly altered throughout the intervals examined. In the rachitic animal, arterioles, venules, and capillaries were found adjacent to the growth plate, either directly apposed to the hypertrophic chondrocytes or separated from them by bone-forming cells. These vessels are in many ways similar to the larger arterioles and venules that normally supply the metaphyseal capillary sprouts, but in the normal growing animal are usually located 350–500 μm from the epiphyseal cartilage. The rachitic capillaries appear relatively well differentiated with a partial basement membrane and a perivascular cell lining. In early rachitic reversal, small vascular projections are induced to grow from the large diameter venules that border upon the hypertrophic chondrocytes. These vascular sprouts that invade the epiphyseal cartilage are quite undifferentiated, with no basement membrane or pericyte lining at the sprout apex and occasional abluminal endothelial cell projections. Within 96 hours, the metaphyseal microvasculature has returned to an apparently normal state with only capillaries at the cartilage-vascular interface and larger vessels (arterioles and venules) located several hundred microns deeper into the metaphysis. The sequential processes of differentiation and cessation of capillary growth followed by dedifferentiation and reinitiation of microvascular growth make the rachitic system a unique one in which to study angiogenesis.     

Demonstration of the intralobular lymphatics in the guinea pig pancreas by an enzyme-histochemical method

Intralobular lymphatics in the guinea pig pancreas were demonstrated enzyme-histochemically showing the extent, distribution and fine structure by combined light and transmission electron microscopy. 5'-nucleotidase(5'-Nase)-positive lymphatic vessels were present throughout the pancreas. Intralobular lymphatics among the acini were comparatively rare and generally independent of the blood capillaries, pancreatic ducts and acini. These lymphatics revealed the usual structural features, such as typical intercellular junctions and very tenuous vascular walls without continuous basal laminae. Fine precipitates of the cerium-based reaction product for 5'-Nase activity were found to be associated with cell membranes of the lymphatic endothelium and pinocytotic vesicles. Lymphatics were not closely related to the endocrine islets, although alkaline phosphatase(ALPase)-positive blood capillaries were well developed. Collecting lymphatic vessels with valves with weaker 5'-Nase activity were also detected in the interlobular connective tissue. ALPase activity, absent in the lymphatics, was positive in the blood capillaries, suggesting that it is also a useful way of demonstrating, histochemically, the blood capillaries in the guinea pig pancreas.     

Tumor vessel development and maturation impose limits on the effectiveness of anti-vascular therapy

The effect of anti-vascular agents on the growth of experimental tumors is well studied. Their impact on tumor vasculature, the primary therapeutic target of these agents, is not as well characterized, even though this primarily determines treatment outcome. Hypothesizing that the response of vessels to therapy is influenced by their stage of maturation, we studied vascular development and the vascular effects of therapy in several transplanted murine tumor models. Based on size, perfusion, endothelial cell (EC) proliferation, and the presence of pericytes, tumor vessels segregated into three categories. Least mature were highly proliferative, nonperfused EC sprouts emanating from functional vessels. Intermediate were small, perfused vessels which, like the angiogenic sprouts, were not covered by pericytes. Most mature were larger vessels, which were predominantly pericyte-covered with quiescent ECs and few associated sprouts. Thus, a developmental order, similar to that described during physiological neovascularization, was evident among vessels in growing tumors. This order markedly influenced tumor vessel response to anti-vascular therapy with recombinant interleukin-12. Therapy reduced tumor vessel density, which was attributable to a decrease in angiogenic sprouts and induction of EC apoptosis in pericyte-negative vessels. Although the great majority of vessels in growing tumors lacked pericyte coverage, selective loss of less mature vessels with therapy significantly increased the fraction of pericyte-positive vessels after therapy. These data indicate that the therapeutic susceptibility of tumor vasculature to recombinant murine IL-12 and, potentially, other anti-vascular agents is limited by its level of maturation. An implication is that tumor susceptibility is similarly limited, making pericyte coverage of tumor vasculature a potential indicator of tumor responsiveness.     

Development of the high endothelial venule in rat lymph node autografts

Vascular reconstruction during rat lymph node regeneration was investigated in autotransplanted mesenteric lymph node fragments, which had been implanted in the renal parenchyma. In addition to light microscopy, vascular casting and transmission electron microscopy were used. From day 3 onwards capillaries grew into the autografts together with lymphatic vessels. The capillaries showed obvious signs of proliferation by day 5. The surviving interstitial cell at the outer border of the transplant produced extracellular substance. High endothelial venules (HEV) differentiated from capillaries from about day 7. A first sign of their development was a vessel with a narrow, branching luminal space and with endothelial cells containing rich cytoplasm and small Golgi complexes. As the Golgi complexes grew and the cisternae and vesicles increased, the lumen dilated, the cell coat on the luminal surface became prominent, and, finally, lymphocytes emigrated through these venules from around day 10. The typical lymph node structure was complete by day 28. These results suggest that the interaction among the remaining interstitial cells, invading capilaries, and lymphatic penetration results in differentiation and maturation of HEV in lymph node regeneration. The development of Golgi complexes is strongly associated with lymphocyte emigration from the blood. © 1994 Wiley-Liss, Inc.     

Pulmonary vascular involvement in sarcoidosis: a report of 40 autopsy cases.

We examined pulmonary vascular involvement in 40 autopsy cases of sarcoidosis. In these cases granulomatous involvement was observed at all levels from large elastic pulmonary arteries to venules, and venous involvement was more prominent than arterial involvement. The extent of granulomatous vascular involvement was related to that of parenchymal granuloma. No significant difference was found between upper and lower lobes in the incidence of granulomatous vascular involvement. The distribution of granulomata in the blood vessels was segmental and adventitial, and medial involvement was prominent in the larger vessels. Healed lesions of granulomatous vascular involvement also were observed at various levels in blood vessels. Prominent granulomatous involvement was found in the lymphatic capillaries and collecting lymphatic vessels in lungs with sarcoidosis. Serial sections of the lungs demonstrated interstitial granuloma directly connecting the lymphatic capillaries around small blood vessels. Granulomatous involvement in vasa vasorum and lymphatic capillaries is likely to be an important factor in the pathogenesis of granulomatous vascular involvement in lungs with sarcoidosis. The present study suggests that granulomatous vascular involvement and its sequelae may contribute to the development of pulmonary sarcoidosis.     

Tissue engineering of recellularized small-diameter vascular grafts

A tissue-engineered small-diameter arterial graft would be of benefit to patients requiring vascular reconstructive procedures. Our objective was to produce a tissue-engineered vascular graft with a high patency rate that could withstand arterial pressures. Rat arteries were acellularized with a series of detergent solutions, recellularized by incubation with a primary culture of endothelial cells, and implanted as interposition grafts in the common femoral artery. Acellular grafts that had not been recellularized were implanted in a separate group of control animals. No systemic anticoagulants were administered. Grafts were explanted at 4 weeks for definitive patency evaluation and histologic examination; 89% of the recellularized grafts and 29% of the control grafts remained patent. Elastin staining demonstrated the preservation of elastic fibers within the media of the acellular grafts before implantation. Immunohistochemical staining of explanted grafts demonstrated a complete layer of endothelial cells on the lumenal surface in grafts that remained patent. Smooth muscle cells were observed to have repopulated the vessel walls. The mechanical properties of the matrix were comparable to native vessels. Such a strategy may present an alternative to autologous harvest of small vessels for use in vascular bypass procedures.     

Transient functional expression of alphaVbeta 3 on vascular cells during wound repair.

During early granulation tissue formation of wound repair, new capillaries invade the fibrin clot, a process that undoubtedly requires an interaction of vascular cells with the wound provisional matrix composed mainly of fibrin, fibronectin, and vitronectin. Integrin alphaVbeta3 is the vascular cell receptor for these wound-associated adhesive proteins. Therefore, we investigated the expression of this receptor on new capillaries of healing full-thickness cutaneous porcine wounds. During granulation tissue formation, alphaVbeta3 was expressed specifically on capillary sprouts invading the central fibrin clot whereas the closely related integrin alphaVbeta5 failed to localize to these cells. Cyclic peptides or antibody antagonists of alphaVbeta3 specifically inhibited granulation tissue formation in a transient manner during the period of invasive angiogenesis. Immunolocalization studies revealed that alphaVbeta3 became aggregated and lost from sprouting vessels after treatment with a peptide antagonist. In contrast, beta 1 integrins were not modulated by this treatment. Once granulation tissue filled the wound and invasive angiogenesis terminated, the alphaVbeta3 showed little or no expression in the granulation tissue microvasculature. These data demonstrate that integrin alphaVbeta3 plays a fundamental, but transient, role during invasive angiogenesis and granulation tissue formation in a healing wound.     

Postnatal development of the vascular pattern in the rat telencephalic pia-arachnoid

Summary The postnatal changes in arrangement of the vascular system of the pia-arachnoid of rats are described based on scanning electron microscopy of microcorrosion casts and transmission electron microscopy. At birth, the distal arteries and veins are embedded in a dense plexiform network of immature capillaries. Arteries and veins are interconnected by many small capillary anastomoses. The trunks are located above the pial plexus. The underlying plexiform vessels provide the matrix for the formation of additional collateral and precortical segments during further development. During the first postnatal week, the distal pial arteries and veins become visible as separate channels and emerge from the subjacent capillary plexus. The pattern of anastomosing arterial rings is now clearly visible. The pial arterial tree can be subdivided into conductive, collateral, and precortical distributive segments, according to Jokelainen et al. (1982). Subsequently, passive expansion of the vascular system takes place during the period of rapid brain growth. In young adults the majority of the formerly closed arterial rings are interrupted, possibly by regression of single collateral arterial segments (Fig. 6). The dense venous capillary plexus of the pia is maintained during the first eight days in spite of marked brain growth. The process of reduction of this capillary plexus starts at the arterial side and proceeds from proximal to distal segments of the veins during the second and third week. The capillary segments, which provide anastomosis between arterial and venous vessels, disappear at the same time as the regression of the dense venous capillary network. Two processes may biodify the architecture of the pial vasculature during development: brain growth and maturation on the one side and cytodifferentiation of vascular walls on other.This work was presented in part at the Third World Congress for Microcirculation, Oxford, 9–14 September 1984, and was published in abstract form (1984, Int J Microcircul Clin Exp 3:308)     

Development of the intrarenal vascular system of the puppy kidney

This study investigates the development of the vascular system of the puppy kidney (1-21 days after birth) after preparing casts of the renal vessels. At two days, the intrarenal vascular system distal to the afferent arteriole is strikingly different than that of the adult. The glomeruli of the outer cortex consist of a single dilated vessel while those of the mid and inner cortex possess an increasingly larger number of capillary loops. The efferent arterioles vary greatly in appearance from outer to inner cortex. Those in the nephrogenic zone are characteristically short and narrow and join a larger venous vessel termined a sinusoidal capillary. An efferent system somewhat similar to that of the adult is seen in the mid and inner cortex. One of the most obvious differences noted between the puppy and adult kidney is the relative lack of peritubular capillary networks throughout the cortex of the puppy kidney. The puppy possesses large, irregular vessels termed sinusoidal capillaries. The most rudimentary sinusoids are found in the outer cortex with more mature vessels in the inner cortex. The vascular arrangement of the efferent arteriole and sinusoidal capillary appears as a post-glomerular shunt. Functionally, the shunt would direct blood flow away from the proximal tubule and thus could result in a low extraction ratio and Tm for secreted solutes.     

Development of the intrarenal vascular system of the puppy kidney.

This study investigates the development of the vascular system of the puppy kidney (1-21 days after birth) after preparing casts of the renal vessels. At two days, the intrarenal vascular system distal to the afferent arteriole is strikingly different than that of the adult. The glomeruli of the outer cortex consist of a single dilated vessel while those of the mid and inner cortex posses an increasingly larger number of capillary loops. The efferent arterioles vary greatly in appearance from outer to inner cortex. Those in the nephrogenic zone are characteristically short and narrow and join a larger venous vessel termined a sinusoidal capillary. An efferent system somewhat similar to that of the adult is seen in the mid and inner cortex. One of the most obvious differences noted between the puppy and adult kidney is the relative lack of peritubular capillary networks throughout the cortex of the puppy kidney. The puppy possesses large, irregular vessels termed sinusoidal capillaries. The most rudimentary sinusoids are found in the outer cortex with more mature vessels in the inner cortex. The vascular arrangement of the efferent arteriole and sinusoidal capillary appears as a post-glomerular shunt. Functionally, the shunt would direct blood flow away from the proximal tubule and thus could result in a low extraction ratio and Tm for secreted solutes.     

Development of anti-atherosclerotic tissue-engineered blood vessel by A20-regulated endothelial progenitor cells seeding decellularized vascular matrix

To investigate whether decellularized vascular tissues and A20-regulated endothelial progenitor cells can be used for constructing a transgenic tissue-engineered blood vessel with anti-atherosclerotic vascular stenotic properties. A20 gene-transfected endothelial progenitor cells differentiated endothelial cells and smooth muscle cells attached to and migrated into the decellularized porcine vascular scaffolding in a bioreactor. The histology of the conduits revealed viable and layered tissue. Scanning electron microscopy showed confluent, homogeneous tissue surfaces. The mechanical strength of the pulsed constructs was similar to that of the human artery. In vivo, the A20 gene-transfected tissue-engineered blood vessels were transplanted into the carotid artery of a rat for 6 months. Blood vessel xenotransplantation caused hyperacute rejection; all transplanted control blood vessels were completely rejected, but A20-transfected tissue-engineered blood vessels demonstrated good flow on implantation, and remained open for 6 months postoperatively, as assessed by Doppler. The HE stain demonstrated that the vessels were patent, without evidence of stenosis or dilatation after 6 months. These results demonstrate that transgenic tissue-engineered blood vessels have long-term patency and unique anti-stenotic properties.     

人大脑内部微血管构筑——组织化学显示法研究

本文应用碱性磷酸酶组织化学对血管内皮的染色方法,光镜观察了3例人大脑内部的微血管构型。结果,皮质短动脉进入皮质内的分枝去向有:1.皮质返枝;2.水平枝;3.下降支。皮质长动脉末端的分枝类型分为:1.血管栅栏样分枝型;2.烛台样或小锚样分枝型;3.树根样分枝型。看到了从微动脉、毛细血管到微静脉的连续性通路。论述了皮质动脉与静脉之间在形态学方面的差别及皮质内血管吻合的几种形式,为更好地理解大脑皮质内微循环类型提供了形态学依据。     

Embryonic development of coronary vasculature in rats: corrosion casting studies

The aim of this study was to analyze the development of coronary vessels at different stages of embryonic life in rats using corrosion casts and scanning electron microscopy (SEM). We studied morphologic details of vessel maturation, expansion, and pattern formation from the stage of development when the coronary system forms patent connections with the aorta and the right atrium (embryonic day 16 (ED16)) to full-term fetus (ED21). The internal surface morphologies of the arterial and venous vessel walls were different and were dependent on the distance from the orifice and the capillary system. They also depended on the maturation state of a given vessel. In various branches of the coronary system we demonstrated round, fusiform or polygonal, endothelial cell imprints. The capillary network was dense, however, at the early stages of development, it formed a thin layer over the myocardium. By ED21 capillaries assumed an orientation parallel to the long axes of the cardiac myocytes. During all stages of development, different forms of angiogenesis by intussusceptive growth were observed. Splitting of the vessel wall occurred in two or three points along the vessel, forming two- or three-link chains. Certain areas of vessels resembled doughnuts, from which several sister vessels originated. The coronary arteries were situated deep within the myocardial wall. The major coronary veins were mostly located on the surface of the capillary plexuses of the myocardial wall. In conclusion, this method of vessel casting enables the detection of angiogenesis by intussusceptive growth, and the visualization of a capillary's position to the myocardial wall, thickness of the capillary plexuses, and the internal surface morphology of major vessels.     

Morphometric characteristics of microcirculatory bed of olfactory bulbs in the desympathized albino rat

To detect the effect of postnatal chemical desympathization on morphometric characteristics of the vessels of olfactory bulbs in albino rat early postnatal ontogenesis, the study was performed using the histochemical method for demonstration of alkaline phosphatase activity, which is a marker of vascular endothelium, and vessel diameter and relative density were measured during the first three months of life. Desympathization was induced in rats by daily intraperitoneal injection of 60-70 mg/kg of guanethidine during postnatal days 1 to 30. The vessel diameter was found to increase after day 30 simultaneously with the changes of the vascular wall of vessels (irregular contour and non-uniform vasodilatation, non-uniform decrease in alkaline phosphatase activity). Relative density of the vessels was reduced one month after desympathization. After two months this parameter remained depressed in the peripheral zone of the olfactory bulbs, while in the central zone it was not different from that one in control group.     

Scanning electron microscopic studies of the vascular smooth muscle cells and pericytes in the rat heart

The cytoarchitecture of smooth muscle cells and pericytes in the rat cardiac vessels was studied by scanning electron microscopy after the removal of connective tissue matrices using a modified KOH-collagenase digestion method. The initial stem of the coronary arteries had groups of smooth muscle cells which ran in various directions on the outermost layer of the media. Although smooth muscle cells in coronary arteries of more than 100 microm in the outer diameter were arranged in a rough circle around the vessel axis, oblique and/or longitudinal muscle bundles were often present in the medio-adventitial border of the vessels. The presence of irregularly oriented muscular bundles is probably connected with resistance against the stretching force induced by the beating of the heart. As the vessel size decreased toward the periphery, almost all of the smooth muscle cells became spindle-shaped with several tiny processes and ran circularly or helicaly to the vessel axis. In the precapillary arterioles (6-12 microm), smooth muscle cells acquired various cytoplasmic processes which helicaly surrounded endothelial cells. Unmyelinated nerves were often associated with arterioles. Blood capillaries were morphologically divided into three segments: arterial capillaries which had pericytes with wide and circularly oriented processes, true capillaries whose pericytes extended long and thin primary processes bilaterally along the vessel axis, and venous capillaries surrounded irregularly and loosely by wide pericytic processes. The stellate pericytes in the postcapillary venules (10-30 microm) gradually changed into flat tape-like smooth muscle cells, which ran circularly in the collecting venules and veins (30-200 microm). The large collecting veins were finally overwhelmed by superficial thin layer of the myocardium, their own smooth muscle cells being very sparse. This suggests that large veins have poor ability to contract by themselves but are influenced by the surrounding myocardial cells.     

Observations of vascularization in the spinal cord of mouse embryos, with special reference to development of boundary membranes and perivascular spaces

The early stages of vascularizations of the spinal cord of the mouse were studied by graphic reconstruction techniques and electron microscopy. Vascular sprouts arise from the perineural vascular plexus (PNVP) to invade the cord of 10-day embryos. These enter the cord most frequently via the lateral surface between the dorsal root and the ventral root; less frequently, they enter via the ventral and/or dorsal surfaces and anastomose with sprouts that have entered via the lateral surface. During the development of intramedullary blood vessels there are essential changes both in the basal laminae covering the neural parenchyma of the cord and in the relationship between the neural tissue and vascular walls. The basal laminae of the developing spinal cord were classified into three categories. The first is the perineural, external, or primary neural, basal lamina (PNBL), which is the earliest of the three in formation and covers the entire external surface of the cord. The second one is the internal, or secondary neural, basal lamina (INBL), which invests the internal surface of the neural tissue facing the walls of invading blood vessels. The third type is the perivascular basal lamina (PVBL), which surrounds the vascular wall. Blood vessels enter the spinal cord by penetrating the PNBL. Since the PVBL and INBL are absent or incomplete in early stages of vascularization, the neural tissue is in direct contact with intramedullary blood vessels. However, following their development, boundary membranes are formed, separating the neural tissue from neighboring vessels, a situation characteristic of capillaries in the mature CNS. Perivascular spaces are seen along the course of developing vessels and secondarily become continuous with the extramedullary connective tissue space. They are neither artifact nor intramedullary extensions of extramedullary connective tissue space along invading sprouts. The boundary membranes are formed by connection of membrane plaques or by fusion of the INBL and PVBL.