Cerebral Capillary Network Research Articles

Autocorrelation analysis-based OCT velocimetry for axial blood flow velocity imaging of the cerebral capillary network.

The accurate measurement of blood flow velocity in the capillary network is challenging due to the small size of the vessels and the slow flow of red blood cells (RBCs) within the vessel. Here, we introduce an autocorrelation analysis-based optical coherence tomography (OCT) method that takes less acquisition time to measure the axial blood flow velocity in the capillary network. The axial blood flow velocity was obtained from the phase change in the decorrelation period of the first-order field autocorrelation function (g1) of the OCT field data, which was acquired with M-mode acquisition (repeated A-scans). The rotation center of g1 in the complex plane was first re-centralized to the origin, then the phase change due to the movement of RBCs was extracted in the g1 decorrelation period which is usually 0.2-0.5 ms. In phantom experiments, the results suggest that the proposed method could accurately measure the axial speed with a wide range of 0.5-15 mm/s. We further tested the method on living animals. Compared with the phase-resolved Doppler optical coherence tomography (pr-DOCT), the proposed method can obtain robust axial velocity measurements with more than five times shorter acquisition time.

Modeling the pressure distribution in a spatially averaged cerebral capillary network

<p style='text-indent:20px;'>A boundary value problem for the Poisson's equation with unknown intensities of sources is studied in context of mathematical modeling the pressure distribution in cerebral capillary networks. The problem is formulated as an inverse problem with finite-dimensional overdetermination. The unique solvability of the problem is proven. A numerical algorithm is proposed and implemented.</p>

Brain Capillary Networks Across Species: A few Simple Organizational Requirements Are Sufficient to Reproduce Both Structure and Function.

Despite the key role of the capillaries in neurovascular function, a thorough characterization of cerebral capillary network properties is currently lacking. Here, we define a range of metrics (geometrical, topological, flow, mass transfer, and robustness) for quantification of structural differences between brain areas, organs, species, or patient populations and, in parallel, digitally generate synthetic networks that replicate the key organizational features of anatomical networks (isotropy, connectedness, space-filling nature, convexity of tissue domains, characteristic size). To reach these objectives, we first construct a database of the defined metrics for healthy capillary networks obtained from imaging of mouse and human brains. Results show that anatomical networks are topologically equivalent between the two species and that geometrical metrics only differ in scaling. Based on these results, we then devise a method which employs constrained Voronoi diagrams to generate 3D model synthetic cerebral capillary networks that are locally randomized but homogeneous at the network-scale. With appropriate choice of scaling, these networks have equivalent properties to the anatomical data, demonstrated by comparison of the defined metrics. The ability to synthetically replicate cerebral capillary networks opens a broad range of applications, ranging from systematic computational studies of structure-function relationships in healthy capillary networks to detailed analysis of pathological structural degeneration, or even to the development of templates for fabrication of 3D biomimetic vascular networks embedded in tissue-engineered constructs.

Effect of p-tyrosol on hemorheological parameters and cerebral capillary network in young spontaneously hypertensive rats

BackgroundMany pathological mechanisms are involved in the development of arterial hypertension; disturbance of the rheological properties of blood and microvascular rarefaction are among those mechanisms. ObjectiveThe effect of p-tyrosol (Tyr) on hemorheological parameters and microvascularization in the cerebral cortex of spontaneously hypertensive rats (SHRs) at the stage of blood pressure rising (5–11 weeks) was studied. MethodsBlood viscosity (BV), plasma viscosity (PV), hematocrit, erythrocyte aggregation and deformability, the oxygen transport capacity index (OTCI), and the capillary network in the cerebral cortex after the course of treatment of Tyr (50 mg/kg daily i.g. for 6 weeks) were studied. Control normotensive Wistar-Kyoto (WKY) rats and control SHRs received an equivalent amount of 1% starch mucilage. ResultsIn comparison with WKY rats, disturbances of rheological blood parameters and a decrease in OTCI were revealed in control SHRs at the 11 weeks of life. By the end of the experiment, brain microvascular rarefaction was observed in the control SHRs (the average density of the capillary bed was reduced due to a decrease in the number of capillaries with a diameter of 3–7 μm). In SHRs rats treated with Tyr, BV and PV, the indices of erythrocyte aggregation were lower, and OTCI was higher in comparison with control SHRs. The density of the capillary network and the number of capillaries of 3–7 μm in the cerebral cortex of SHRs rats receiving Tyr were significantly higher than the corresponding values in control SHRs. ConclusionWhen Tyr is administered to young SHRs during the development of hypertension, it limits the development of hyperviscosity syndrome, improves the oxygen transport capacity and eliminates microvascular rarefaction in the cerebral cortex.

A zinc fixative for 3D visualization of cerebral capillaries and pericytes

BackgroundA large volume of data indicates that disturbances in the morphology and function of the capillary wall may play a causal role in several types of neurodegenerative disorders. We present a highly reproducible staining method for investigating the cerebral capillary network and the pericyte cells within the basement membrane in mice – a specie specific challenging task when uniform staining in thick sections was needed for confocal microscopy or a quantitative analysis, e.g. stereological investigation using 3D probes. New methodWe perfused C57BL6/Jbom mice and immersion fixated the brains with an aldehyde free zinc fixative, which is normally used for paraffin embedded tissues, and stained for CD31 and Collagen Type IV positive capillaries in 100μm thick sections. ResultsUsing the milder zinc fixative allowed complete immunohistochemical visualization of the cerebral capillary network in 100μm thick sections using CD31 or Collagen Type IV antibodies. Moreover CD31 or Collagen Type IV staining revealed the presence of pericytes, which was confirmed by a fluorescent co-localization with the NG2 pericyte marker. Comparison with existing methodsCompared with conventional aldehyde-based fixative, this method resulted in a homogeneous staining through the entire depth of thick sections with very limited background staining and well-preserved morphology. ConclusionsThis method is suitable for 3D stereological analysis of capillary networks and pericytes within thick brain sections using CD31 or Collagen Type IV antibodies.

Quantitative analysis of the capillary network of aged APPswe/PS1dE9 transgenic mice

A combination of immunohistochemical and stereological techniques were used to investigate the capillary network in the cerebral cortex of 18-month-old APPswe/PS1dE9 transgenic (Tg) mice and control littermates. Data regarding total capillary length, segment number, diffusion radius, and pericyte number are presented. The total length was 60 meters and there was a one-to-one relationship between the number of capillary segments and pericytes in both groups. Significant differences were not observed in the Tg and wild-type controls indicating that the Alzheimer's-like amyloidosis produced in this Tg mouse has a minimal affect on the structural integrity of the cerebral capillary network.

Влияние ноопепта на морфофункциональное состояние капиллярной системы коры большого мозга крыс

В экспериментальных исследованиях на крысах изучено влияние ноопепта (этилового эфира N-фенилацетил-L-пролилглицина) на капиллярную сеть коры большого мозга крыс при нарушении мозгового кровообращения, вызванного перманентной окклюзией правой сонной артерии (ОПСА). Состояние микроциркуляторного русла оценивалось после ОПСА и на фоне введения препарата через 30 мин после оперативного вмешательства, безинъекционным кальций-аденозинтрифосфатным методом выявления внутриорганного сосудистого русла. Срезы получали из коры большого мозга крыс, декапитированных через 60 мин после внутрибрюшинного введения ноопепта в дозе 1 мг/кг. Животным контрольной группы через указанный промежуток времени после ОПСА вводили 0,9 % раствор хлорида натрия. Для оценки состояния капиллярной сети рассчитывали средний диаметр и количество функционирующих капилляров, а также распределение сосудов по диаметру. Результаты проведенных исследований показали, что ноопепт достоверно (p < 0,05) предотвращает наблюдаемое на фоне ОПСА уменьшение количества функционирующих капилляров в обоих полушариях (на 24,8 % в правом и на 25,8 % в левом). Под влиянием препарата отмечалось также статистически значимое (p < 0,05) увеличение диаметра функционирующих капилляров, как в ипсилатеральном на 7,9 %, так и контралатеральном на 7,6 %, полушариях, по сравнению с показателями контрольной группы. Полученные данные указывают на то, что в проявлении нейропротекторных свойств ноопепта задействована его способность стимулировать микроциркуляцию коры большого мозга при ишемических поражениях.

Two-photon microscopy with double-circle trajectories for in vivo cerebral blood flow measurements

Scanning microscopes normally use trajectories which produce full-frame images of an object at a low frame rate. Time-resolved measurements are possible if scans along a single line are repeated at a high rate. In conjunction with fluorescence labeling techniques, in vivo recording of blood flow in single capillaries is possible. The present work investigates scanning with double-circle trajectories to measure blood flow simultaneously in several vessels of a capillary network. With the trajectory centered near a bifurcation, a double circle crosses each vessel twice, creating a sensing gate for passing dark red blood cells in fluorescently labeled plasma. From the stack of scans repeated at 1,300 Hz, the time-resolved velocity is retrieved using an image correlation approach. Single bifurcation events can be identified from a few fluorescently labeled red blood cells. The applicability of the method for in vivo measurements is illustrated on the basis of two-photon laser scanning microscopy of the cerebral capillary network of mice. Its performance is assessed with synthetic data generated from a two-phase model for the perfusion in a capillary network. The calculation of velocities is found to be sufficiently robust for a wide range of conditions. The achievable limits depend significantly on the experimental conditions and are estimated to be in the 1 μm/s (velocity) and 0.1 s (time resolution) ranges, respectively. Some manual fine-tuning is required for optimal performance in terms of accuracy and time resolution. Further work may lead to improved reliability with which bifurcation events are identified in the algorithm and to include red blood cell flux and hematocrit measurements. With the capability for time-resolved measurements in all vessels of a bifurcation, double-circle scanning trajectories allow a detailed study of the dynamics in vascular networks.

A computational model of hemodynamic parameters in cortical capillary networks

The analysis of hemodynamic parameters and functional reactivity of cerebral capillaries is still controversial. To assess the hemodynamic parameters in the cortical capillary network, a generic model was created using 2D voronoi tessellation in which each edge represents a capillary segment. This method is capable of creating an appropriate generic model of cerebral capillary network relating to each part of the brain cortex because the geometric model is able to vary the capillary density. The modeling presented here is based on morphometric parameters extracted from physiological data of the human cortex. The pertinent hemodynamic parameters were obtained by numerical simulation based on effective blood viscosity as a function of hematocrit and microvessel diameter, phase separation and plasma skimming effects. The hemodynamic parameters of capillary networks with two different densities (consistent with the variation of the morphometric data in the human cortical capillary network) were analyzed. The results show pertinent hemodynamic parameters for each model. The heterogeneity (coefficient variation) and the mean value of hematocrits, flow rates and velocities of the both network models were specified. The distributions of blood flow throughout the both models seem to confirm the hypothesis in which all capillaries in a cortical network are recruited at rest (normal condition). The results also demonstrate a discrepancy of the network resistance between two models, which are derived from the difference in the number density of capillary segments between the models.

The cerebral microcirculation in ischemia and hypoxemia. The Arisztid G. B. Kovách Memorial Lecture.

The cerebral capillary circulation exhibits heterogenous perfusion and undergoes characteristic changes in the distribution of RBC flow in response to systemic physiological stimuli. Hypoxemia, hypercapnia and hypotension increase the homogeneity of capillary perfusion, which is thought to preserve or enhance transcapillary exchange. Redistribution of capillary RBC flow between nutritive capillaries and preferential channels may contribute to this response. Selective changes in capillary flow may be brought about by non-smooth muscle-based contractile or blood-borne mechanisms. Isovolemic hemodilution anemia increases RBC velocity and supply rate with no decrease in capillary hematocrit. The effect of cerebral ischemia on microvascular patency depends on the severity and time course of the insult and whether the injury is global or focal. Capillary plugging is not observed following transient forebrain ischemia in the rat cerebral cortex but may contribute to tissue injury prior to reperfusion and during prolonged and severe ischemia. In the future, a better understanding of the functional architecture of the cerebral capillary network and its significance in the adaptation to altered circulatory conditions will continue to be an important goal of research. More work will have to be done to (i) substantiate the postulated physiological regulation of cerebral capillary flow, (ii) determine the cellular mechanism of integration of flow-dependent and neuronal activity-dependent signals, and (iii) identify the principal mediators, their cellular sources and molecular targets. The final answer to these questions will in a large part depend on our ability to directly, i.e. microscopically, visualize microvascular, neuronal and molecular phenomena as they occur in the brain in a spatially and temporally distributed manner.

Cortical electrical stimulation alters erythrocyte perfusion pattern in the cerebral capillary network of the rat

The effect of direct cortical electrical stimulation on the pattern of erythrocyte perfusion in the capillary network of the rat cerebral cortex was studied by fluorescence intravital video-microscopy. The movement of fluorescently labeled red blood cells (FRBCs) in individual capillaries 50–70 μm subsurface in the dorsal somatosensory cortex was visualized using a closed cranial window. Cortical stimulation electrodes were placed on opposite sides of the window. FRBC velocity (mm/s) and supply rate (cells/s) were measured in 51 capillaries from six rats before and during electrical stimulation of increasing intensities (15-s trains of 3-Hz, 3-ms, 0.5–5.0-mA, square pulses). FRBC velocity, supply rate, and the instantaneous capillary erythrocyte content (lineal cell density, LCD, cells/mm) increased with the stimulation current and reached maxima of 110, 160 and 33% above control, respectively. Capillaries with low resting velocity showed a greater response than those with high resting velocity. The fraction of capillaries in which FRBC velocity increased was not constant, but increased with the stimulation current, as did the magnitude of the velocity change in these capillaries. A few capillaries showed a negative FRBC velocity response at stimulations <4 mA. These results suggest that a robust rise in the fraction of responding (engaged) capillaries and a smaller rise in the capillary LCD contribute to neuronal activation-induced cortical hyperemia. Thus, capillary engagement and erythrocyte recruitment appear to represent important components of the cortical functional hyperemic response. These results provide insight into some of the specific hemodynamic changes associated with functional hyperemia occurring at the capillary level.

Hypoxemia Alters Erythrocyte Perfusion Pattern in the Cerebral Capillary Network

The effect of acute hypoxemia on erythrocyte perfusion rates in individual capillaries of the rat cerebral cortex was studied by intravital video microscopy. The motion of erythrocytes in subsurface capillaries of the frontoparietal cortex was visualized through a closed cranial window using fluorescently labeled red blood cells (FRBC) as markers of flow. FRBC velocity and FRBC supply rate were measured in each capillary at rest, moderate hypoxemia (PaO2 = 40 mm Hg), and severe hypoxemia (PaO2 = 26 mm Hg). Lineal density of FRBC in the capillaries was calculated as the ratio of supply rate and velocity. Hypoxemia increased erythrocyte perfusion in virtually all capillaries. Average FRBC supply rate increased by 104% in moderate hypoxemia and by 281% in severe hypoxemia. Average FRBC velocity increased by 66 and 173%, respectively. During severe hypoxemia, FRBC supply rate increased significantly more in capillaries with low resting supply rate compared to those with high resting supply rate. Changes in FRBC velocity exhibited a similar pattern. Lineal density of FRBC increased by 28% in moderate hypoxemia and by 48% in severe hypoxemia. The results suggest that acute hypoxemia promotes perfusion homogeneity and recruitment of erythrocytes in the cerebral capillary network.

Effect of hemodilution on RBC velocity, supply rate, and hematocrit in the cerebral capillary network.

The effect of isovolemic hemodilution on the circulation of red blood cells (RBCs) in the cerebrocortical capillary network was studied by intravital videomicroscopy with use of a closed-cranial-window technique in the rat. Velocity and supply rate of RBCs were measured by tracking the movement and counting the number of fluorescently labeled cells. Arterial blood was withdrawn in increments of 2 ml and replaced by serum albumin. Arterial blood pressure was maintained constant with an infusion of methoxamine. Both velocity and supply rate of RBCs increased, by approximately equal amounts, as arterial hematocrit was reduced from 44 to 15%. The maximum increase in RBC velocity was 4.6 and in RBC supply rate was 5.2 times the baseline value. Calculated lineal density of RBC, an index of capillary hematocrit, did not change with hemodilution. The results suggest that RBC flow and oxygen supply in the cerebral capillary network are maintained during isovolemic hemodilution. The "optimal hematocrit" is as low as 15%.

Blood flow in the cerebral capillary network: a review emphasizing observations with intravital microscopy.

Capillary perfusion in the brain is characterized by an essentially continuous flow of erythrocytes and plasma in almost all capillaries. Rapid fluctuations and spatial heterogeneity or red blood cell (RBC) velocity (0.5-1.8 mm/s) within the capillary network are present. In addition, low-frequency (4-8 cpm) synchronous oscillations in RBC velocity in the capillary network emerge when perfusion to cerebral tissue is challenged. Despite the tortuous, three-dimensional architecture of microvessels, functional intercapillary anastomoses are absent. At rest, red cells travel through the capillary network in 100-300 ms along 150- to 500-micron-long paths. Physiological challenges elicit sizable changes in RBC velocity with a minor role for capillary recruitment, change in capillary diameter, or flow shunting. During acute hypoxia, RBC velocity increases in all capillaries; the corresponding response to hypereapnia is more complex and involves redistribution of capillary flow toward more homogeneous perfusion. The response of capillary flow to decreased perfusion pressure reflects autoregulation of cerebral blood flow but also involves intranetwork redistribution of RBC flow between two populations of capillaries, postulated as thoroughfare channels and exchange capillaries. Flow reserve may be provided by the thoroughfare channels and may help maintain flow velocity and capillary exchange and protect the microcirculation from perfusion failure. Isovolemic hemodilution increases RBC velocity three- to fourfold and increases RBC flux to a moderate degree with a relatively small decrease in capillary hematocrit, under normal and compromised arterial blood supply. In cerebral ischemia, leukocyte adhesion is enhanced and appears reversible when the ischemia is moderate but may be progressive when the injury is severe. The observed flow behavior suggests the presence of a physiological regulatory mechanism of cerebral capillary flow that may involve communication among various microvascular and parenchymal cells and utilize locally acting endothelial and parenchymal mediators such as endothelium-derived relaxing factor or nitric oxide.

Synchronous Oscillations in Cerebrocortical Capillary Red Blood Cell Velocity after Nitric Oxide Synthase Inhibition

Low-frequency (4–12 cpm) spontaneous oscillations in cerebral blood flow are well known and their augmentation after nitric oxide synthase inhibition has recently been described. However, the presence of these oscillations in blood flow velocity in the capillary network of the brain has not been demonstrated. In this paper, low-frequency oscillations in red blood cell flow velocity in cortical capillaries using intravital video microscopy were studied before and after infusion of the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME). Fluorescein isothiocyanate-labeled red blood cells were injected intravenously and served as markers of capillary flow. Red cell velocity was measured by off-line image tracking. After infusion ofL-NAME (30 mg/kg), red cell velocity decreased from an average of 1.0 ± 0.1 mm/sec to 0.53 ± 0.1 mm/sec. Simultaneously, low-frequency oscillations in velocity emerged as indicated by an 81 ± 17% increase in standard deviation of the 4- to 8-cpm frequency components. There was a significant temporal correlation (r= 0.58,P< 0.01) in red cell velocity between neighboring capillaries afterL-NAME. Principal component analysis suggested that the high temporal correlation was a consequence of low-frequency oscillations rather than phase. These results are consistent with a model in which low-frequency spontaneous oscillations in flow velocity are brought about by precapillary vasomotion. This study provides for the first time direct evidence for low-frequency synchronous oscillations of red cell flow velocity in the cerebral capillary network.

Microanatomical changes of the cerebral capillary network in aged rats: influence of treatment with a dihydropyridine-type calcium antagonist

A variety of age-dependent changes affect cerebral vasculature. Morphological changes of the brain microvasculature characterized by microvascular fibrosis , vascular convolutions , thickening or membranous changes of the capillary basal membrane and gliofibrillary proliferation have been reported with aging. These changes may be related with age-dependent impairment in behavioral performance. Increasing evidence suggests that Ca+2 antagonists of the dihydropyridine family, which are used primarily in the therapy of cardiovascular disorders may be useful for treating neurologic diseases including cerebral ischemia, age-related neurodegenerative disorders, senile dementia and epilepsy. This in view of the capability of these compounds to decrease Ca+2 overload which can result in increased cell death.Recent studies have shown that the dihydropyridine Ca2+ antagonists delayed the expression of agerelated microvascular changes in the rat forebrain and sciatic nerve without reducing systolic blood pressure. The present study was designed to assess the influence of long term treatment with the dihydropyridine derivative darodipine [diethyl 4-2,1,3- (benzoxadiazal-4-yl)-l,4-dihydro-2,6-dimethylpyridine- 3,5-dicarboxylate, PY 108-068] on age-related microvascular changes occurring in the rat cerebral cortex and hippocampus. The brain capillary network was investigated using alkaline phosphatase histochemistry associated with image analysis.

Effect of long term treatment with the dihydropyridine-type calcium channel blocker darodipine (PY 108-068) on the cerebral capillary network in aged rats

The effects of treatment with the dihydropyridine Ca +2 antagonist darodipine (PY 108-068) on age-related changes in the cerebral capillary network was studied using alkaline phosphatase histochemistry with quantitative image analysis. The investigation was performed on male Wistar rats of 12 months (adult reference group) and 27 months. The 27-month-old rats consisted of two groups, the first of control untreated animals and the second of rats receiving an oral dose of 5 mg/kg/day of darodipine from the 21st to the 27th month. The cerebral areas examined included the frontal cortex, the occipital cortex, Ammon's horn of the hippocampus, and the dentate gyrus. The number and the average length of alkaline phosphatase-positive capillaries were decreased in old rats, when compared with adult rats. The intercapillary distance, which is considered as a sensitive parameter for capillary density was increased in aged rats in comparison to adult rats. The capillary diameter was increased slightly or unchanged in old rats. The Ammon's horn and the frontal cortex were the cerebral areas most affected by age-dependent changes of the capillary network. Treatment with darodipine increased the number and the average length of alkaline phosphatase-reactive capillaries and reduced the intercapillary distance and the diameter of cerebral capillaries in old rats. The pericapillary microvenvironment of the Ammon's horn was the most sensitive to treatment with darodipine. The above results showed that treatment with darodipine is capable of counteracting some microvascular changes occurring in the brain of aged rats. This suggests that the blockade of dihydropyridine-type Ca 2+ channels has a positive effect on the brain microvascular system and may counteract the impairment of pericapillary microenvironment occurring with aging.

Effects of aging and vincamine derivatives on pericapillary microenvironment: stereological characterization of the cerebral capillary network

Changes in the pericapillary microenvironment of adult (18-month-old) and senescent (27 1 2 -month-old) Fischer-344 rats treated for 6 weeks with daily IP injections of brovincamine or apovincamine (0, 2.5, 5, 10 mg/kg) were correlated with spontaneous locomotor activity and [ 14C]-2-deoxyglucose uptake of the brain. The animals were tested for spontaneous locomotor activity in a tunnel maze. Twenty-four hr after behavioral testing and subsequently after a [ 14C]-2-deoxyglucose injection, brains were removed and capillaries stained with alkaline phosphatase reaction, being later measured with an optical-electronic image analysis technique. Results revealed an increase in intercapillary distance, as a sensitive parameter for capillary density, in the hippocampus (CA1) and in the parietal cortex (area 39) in association with aging. Capillary diameter in the parietal cortex was found to be increased age dependently. A similar age-related increase was also observed in the CA1 field but this age trend was not significant. Chronic treatment with the vincamines produced a dose-dependent reduction in intercapillary distance in senescent animals which approached the level of untreated adult control rats. Significant negative correlations were found between maze locomotion and intercapillary distance among senescent rats. Furthermore, intercapillary distance and local relative 2-deoxyglucose uptake tended to be negatively correlated in both age groups. These findings provide evidence for the working hypothesis that mean intercapillary distance can be considered as an indicator of neuronal activity in the pericapillary microenvironment.

Cellular enzymatic differentiation in brain vessels of normal and protein-deprived rats. Biochemical studies.

Brain capillary development was studied in normal and protein-deprived rats using the specific activity of alkaline phosphatase (EC 3.1.3.1) and gamma-glutamyl transpeptidase (EC 2.3.2.1) in whole brain homogenates and microvessels obtained by gradient centrifugation according to Orlowski et al. (1974). Pre- and postnatal protein deprivation was induced by a 50% reduction in the dietary protein content. The density of microvessel fragments changed during development. Most of the early developmental increase in the specific activity of both enzymes in whole brain homogenates of normal rats can probably be explained by a rapid formation of new capillary segments. The increase in specific activity of gamma-glutamyl transpeptidase in microvessels was interpreted as a sign of cellular differentiation. Protein deprivation resulted in reduced specific activity of both enzymes in whole brain homogenates of 30-day-old rats, probably as a result of the decreased length per volume of the cerebral capillary network at this age (Conradi et al. 1979a). Signs of impaired endothelial growth were also present in the protein-deprived rats since the distribution of microvessel fragments in the 30-day-old protein-deprived rats was similar to that in 3-week-old normal rats. The specific activity of alkaline phosphatase was decreased in the microvessel fractions of 30- and 96-day-old protein-deprived rats, apparently signifying an effect of the protein deprivation on the endothelial cells. These effects of protein deprivation on the brain capillary endothelial transport system may have negative consequences for growth and function in the brain.