Quantitative analysis of cerebral vessels in the newborn puppy: the structure of germinal matrix vessels may predispose to hemorrhage

Accounting for arterial and capillary blood gases for calculation of cerebral blood flow in preterm infants

One of the most feared neurological complications of premature birth is intraventricular hemorrhage, frequently triggered by fluctuations in cerebral blood flow (CBF). Although several techniques for CBF measurement have been developed, they are not part of clinical routine in neonatal intensive care. A promising tool for monitoring of CBF is its numerical assessment using standard clinical parameters such as mean arterial pressure, carbon dioxide partial pressure (pCO₂) and oxygen partial pressure (pO₂). A standard blood gas analysis is performed on arterial blood. In neonates, capillary blood is widely used for analysis of blood gas parameters. The purpose of this study was the assessment of differences between arterial and capillary analysis of blood gases and adjustment of the mathematical model for CBF calculation to capillary values. The statistical analysis of pCO₂ and pO₂ values collected from 254 preterm infants with a gestational age of 23-30 weeks revealed no significant differences between arterial and capillary pCO₂ and significantly lower values for capillary pO₂. The estimated mean differences between arterial and capillary pO₂ of 15.15 mmHg (2.02 kPa) resulted in a significantly higher CBF calculated for capillary pO₂ compared to CBF calculated for arterial pO₂. Two methods for correction of capillary pO₂ were proposed and compared, one based on the mean difference and another one based on a regression model.

CONCLUSION

Capillary blood gas analysis with correction for pO₂ as proposed in the present work is an acceptable alternative to arterial sampling for the assessment of CBF.

WHAT IS KNOWN

• Arterial blood analysis is the gold standard in clinical practice. However, capillary blood is widely used for estimating blood gas parameters. • There is no significant difference between the arterial and capillary pCO₂ values, but the capillary pO₂ differs significantly from the arterial one.

WHAT IS NEW

• The lower capillary pO₂ values yield significantly higher values of calculated CBF compared to CBF computed from arterial pO₂ measurements. • Two correction methods for the adjustment of capillary pO₂ to arterial pO₂ that made the difference in the calculated CBF insignificant have been proposed.

Assessing haemorrhage-critical values of cerebral blood flow by modelling biomechanical stresses on capillaries in the immature brain

The development of intraventricular haemorrhages (IVH) in preterm newborns is triggered by a disruption of the vessels responsible for cerebral microcirculation. Analysis of the stresses exerted on vessel walls enables the identification of the critical values of cerebral blood flow (CBF) associated with the development of IVH in preterm infants. The purpose of the present study is the estimation of these critical CBF values using the biomechanical stresses obtained by the finite element modelling of immature brain capillaries. The properties of the endothelial cells and basement membranes employed were selected on the basis of published nanoindentation measurements using atomic force microscopes. The forces acting on individual capillaries were derived with a mathematical model that accounts for the peculiarities of microvascularity in the immature brain. Calculations were based on clinical measurements obtained from 254 preterm infants with the gestational age ranging from 23 to 30 weeks, with and without diagnosis of IVH. No distinction between the affected and control groups with the gestational age of 23 to 26 weeks was possible. For infants with the gestational age of 27 to 30 weeks, the CBF value of 17.03 ml/100 g/min was determined as the critical upper value, above which the likelihood of IVH increases.

Modeling Cerebral Blood Flow Dependence on Carbon Dioxide and Mean Arterial Blood Pressure in the Immature Brain With Accounting for the Germinal Matrix

Intraventricular hemorrhage (IVH) is one of the most critical complications in the development of preterm infants. The likelihood of IVH is strongly associated with disturbances in cerebral blood flow (CBF) and with microvascular fragility in the germinal matrix (GM). The CBF value and its reactivity to changes in arterial carbon dioxide pressure (pCO₂) and mean arterial blood pressure (MABP) are relevant indicators in the clinical assessment of preterm infants. The objective of the present study is mathematical modeling of the influence of pCO₂ and MABP on CBF in immature brain, based on clinical data collected from 265 preterm infants with 23–30 gestational weeks. The model was adapted to the peculiarities of immature brain by taking into account the morphological characteristics of the GM capillary network and vascular reactivity, according to gestational and postnatal age. An analysis of model based values of CBF and its reactivity to changes in MABP and pCO₂ was performed separately for each gestational week and for the first two days of life both for preterm infants with and without IVH. The developed model for the estimation of CBF was validated against equivalent experimental measurements taken from the literature. A good agreement between the estimated values of CBF, as well as its reaction on changes in MABP and pCO₂ and the equivalent values obtained in experimental studies was shown.

Neonatal Hypoxia Ischaemia: Mechanisms, Models, and Therapeutic Challenges

Neonatal hypoxia-ischaemia (HI) is the most common cause of death and disability in human neonates, and is often associated with persistent motor, sensory, and cognitive impairment. Improved intensive care technology has increased survival without preventing neurological disorder, increasing morbidity throughout the adult population. Early preventative or neuroprotective interventions have the potential to rescue brain development in neonates, yet only one therapeutic intervention is currently licensed for use in developed countries. Recent investigations of the transient cortical layer known as subplate, especially regarding subplate's secretory role, opens up a novel set of potential molecular modulators of neonatal HI injury. This review examines the biological mechanisms of human neonatal HI, discusses evidence for the relevance of subplate-secreted molecules to this condition, and evaluates available animal models. Neuroserpin, a neuronally released neuroprotective factor, is discussed as a case study for developing new potential pharmacological interventions for use post-ischaemic injury.

Pharmacological Preventions of Brain Injury Following Experimental Germinal Matrix Hemorrhage: an Up-to-Date Review

Germinal matrix hemorrhage (GMH) is defined as the rupture of immature blood vessels in the subependymal zone of premature infants with significant mortality and morbidity. Considering the notable social and ecological stress brought by GMH-induced brain injury and sequelae, safe and efficient pharmacological preventions are badly needed. Currently, several appropriate animal models are available to mimic the clinical outcomes of GMH in human patients. In the long run, hemorrhagic strokes are the research target. Previously, we found that minocycline was efficient to alleviate GMH-induced brain edema and posthemorrhagic hydrocephalus (PHH) in rats, which may be closely related to the activation of cannabinoid receptor 2 (CB2R). However, how the two molecules correlate and the underlined molecular pathway remain unknown. To extensively understand current experimental GMH treatment, this literature review critically evaluates existing therapeutic strategies, potential treatments, and potentially involved molecular mechanisms. Each strategy has its own advantages and disadvantages. Some of the mechanisms are still controversial, requiring an increasing number of animal experiments before the therapeutic strategy would be widely accepted.

Effect of oxygen inhalation on cerebral blood flow velocity in premature neonates

BACKGROUND

The study tested the hypothesis that hyperoxemia and hypoxemia differentially alter cerebral blood flow velocity (CBFV) in a gestational age-dependent manner.

METHODS

Cases comprised 98 neonates with mild respiratory distress, receiving oxygen for >24 h in first 48 h of life. Ninety-eight age- and-weight-matched healthy neonates served as controls. Infants with perinatal asphyxia, shock, sepsis, malformations, acidosis/alkalosis, and hypo/hypercarbia were excluded. Resistance index (RI), pulsatility index (PI), peak systolic flow velocity (PSV), and vascular diameter were measured in internal carotid, vertebral, and middle cerebral arteries by transcranial doppler ultrasonography between 24 and 48 h of life with immediate postdoppler arterial blood gas analysis. For subgroup analysis, neonates were divided by gestational age and PaO2.

RESULTS

An overall decrease in RI/PI and increase in PSV and vasodilation was observed in cases. Hyperoxemia (PaO2 >90 mm Hg) was more common in premature neonates. Neonates <32 wk showed an increase in CBFV (decreased RI/PI and increased PSV/diameter) in association with hyperoxemia. An opposite response was observed in neonates ≥ 32 wk, where CBFV increased in response to hypoxemia (PaO2 <50 mm Hg) and decreased in hyperoxemia. Increased CBFV showed high predictive accuracy for immediate mortality and intracranial hemorrhage.

CONCLUSION

Depending on gestational maturity, hyperoxemia or hypoxemia produce differential effects in CBFV.

Multifocal haemorrhagic brain damage following hypoxia and blood pressure lability: case report and rat model

AIMS

Haemorrhagic brain damage is frequently encountered as a complication of premature birth. Much less frequently, multifocal petechial haemorrhage is identified in asphyxiated term newborns. Our goal was to develop an experimental rat model to reproduce this pattern of brain damage.

METHODS

Neonatal rat pups were exposed to a 24-h period of 10% or 8% hypoxia followed by a single dose of phenylephrine. Acute and subacute changes, as well as long-term outcomes, were investigated by histology, brain magnetic resonance imaging and behavioural assessment. Immunostaining for vascular endothelial growth factor and caveolin-1 was performed in the rat brains as well as in a 17-day human case.

RESULTS

Small foci of haemorrhage were identified in almost all regions of the rat brain subjected to hypoxia plus phenylephrine, but not hypoxia alone. Exposure to 8% hypoxia was associated with more haemorrhagic foci than 10% hypoxia. With rare exceptions, the blood deposits were too small to be detected by magnetic resonance imaging. Altered immunohistochemical detection of vascular endothelial growth factor and caveolin-1 in the child and the rat model suggests a role for blood-brain barrier compromise. There were no clear behavioural changes and no residual morphological abnormalities in the 78-day follow-up of the rats.

CONCLUSIONS

We conclude that transient hypoxia, in a dose-dependent manner, can weaken the vasculature and predispose to brain haemorrhage in the situation of labile blood pressure. Persistent hypoxia is likely to be important in the genesis of permanent severe brain damage.

Vulnerability of the developing brain to hypoxic-ischemic damage: contribution of the cerebral vasculature to injury and repair?

As clinicians attempt to understand the underlying reasons for the vulnerability of different regions of the developing brain to injury, it is apparent that little is known as to how hypoxia-ischemia may affect the cerebrovasculature in the developing infant. Most of the research investigating the pathogenesis of perinatal brain injury following hypoxia-ischemia has focused on excitotoxicity, oxidative stress and an inflammatory response, with the response of the developing cerebrovasculature receiving less attention. This is surprising as the presentation of devastating and permanent injury such as germinal matrix-intraventricular haemorrhage (GM-IVH) and perinatal stroke are of vascular origin, and the origin of periventricular leukomalacia (PVL) may also arise from poor perfusion of the white matter. This highlights that cerebrovasculature injury following hypoxia could primarily be responsible for the injury seen in the brain of many infants diagnosed with hypoxic-ischemic encephalopathy (HIE). Interestingly the highly dynamic nature of the cerebral blood vessels in the fetus, and the fluctuations of cerebral blood flow and metabolic demand that occur following hypoxia suggest that the response of blood vessels could explain both regional protection and vulnerability in the developing brain. However, research into how blood vessels respond following hypoxia-ischemia have mostly been conducted in adult models of ischemia or stroke, further highlighting the need to investigate how the developing cerebrovasculature responds and the possible contribution to perinatal brain injury following hypoxia. This review discusses the current concepts on the pathogenesis of perinatal brain injury, the development of the fetal cerebrovasculature and the blood brain barrier (BBB), and key mediators involved with the response of cerebral blood vessels to hypoxia.

Morphological evaluation of the cerebral blood vessels in the late gestation fetal sheep following hypoxia in utero

Hypoxia can significantly contribute to the development of permanent brain injury in the term neonate; however the response of cerebral blood vessels is not well understood. This study aimed to quantitatively measure vascular density and morphology using laminin immunohistochemistry as a marker of blood vessels, and determine the effects of a single, severe bout of hypoxia (umbilical cord occlusion, UCO) late in gestation on the developing cerebrovasculature in fetal sheep. At 124-126 days gestation singleton fetal sheep underwent surgery for implantation of catheters and placement of an inflatable cuff around the umbilical cord. A 10 min UCO or sham UCO (n=5) occurred at 132 days gestation. Fetal brains were collected at 24 h (n=5) or 48 h (n=4) after UCO for vascular density and morphology analysis of laminin immunohistochemistry. 48 h following a single, brief bout of severe hypoxia late in gestation decreased vascular density was seen in the caudate nucleus and no changes in vascular morphology occurred. However closer analysis revealed a significant shift in the frequency of smaller (≤10 μm) to larger (≤100 μm) perimeter blood vessels in periventricular and subcortical white matter. Close examination of the frequency distribution of vascular perimeter highlights that alterations in vascular morphology persist in the near term fetal brain for up to 48 h following a brief (10 min) hypoxia in white but not gray matter. These findings suggest that the near term brain may still be vulnerable to white matter injury following in utero hypoxia.

Animal models of germinal matrix hemorrhage

Germinal matrix hemorrhage refers to bleeding that arises from the subependymal (or periventricular) germinal region of the immature brain. Clinical studies have shown that infants who experience germinal matrix hemorrhage can develop hydrocephalus or suffer from long-term neurologic dysfunction, including cerebral palsy, seizures, and learning disabilities. Understanding the causative factors and the pathogenesis of subsequent brain damage is important if germinal matrix hemorrhage is to be prevented or treated. Appropriate animal models are necessary to achieve this understanding. A number of animal species, including mice, rats, rabbits, sheep, pigs, dogs, cats, and primates, have been used to model germinal matrix hemorrhage. This literature review critically evaluates the animal models of germinal matrix hemorrhage. Each model has its own advantages and disadvantages; no single model is suitable for the study of all aspects of brain damage.

Morphometric assessment of collagen accumulation in germinal matrix vessels of premature human neonates

Germinal matrix haemorrhage in premature neonates is commonly attributed to vascular immaturity, possibly related to an abbreviated process of angiogenesis. Terminal steps in the progression of angiogenesis are the formation of a subendothelial basal lamina containing collagen IV and an extracellular matrix containing collagens I and III. Immature vessels would predictably be deficient in these collagen subtypes. We analysed germinal matrix (GM), cortical, and white matter (WM) vessels with antibodies specific for collagens I, III and IV to test the hypothesis that GM vessels are immature. Brains were collected during post-mortem from prematurely born human neonates ranging in age from 17 weeks to 36 weeks postconception. All GM vessels were immunoreactive for collagen subtypes I, III and IV. Using digital image analysis, collagen IV immunoperoxidase-labelling was measured in vessels in GM, cortex and WM. Intensity values in GM and WM were normalized relative to cortical intensity within the same subject. At week 17 of gestation, GM vessels exhibited a higher concentration of collagen IV than did WM or cortical vessels. Regression analysis demonstrated that collagen intensity in GM was greater than that in cortex and WM at all stages. We conclude that GM vessels in even the youngest, prematurely born, viable neonates do not exhibit evidence of structural immaturity. The high incidence of GM haemorrhage in premature neonates may be related to factors other than a deficiency in accumulated collagen.

Anatomic analysis of blood vessels in germinal matrix, cerebral cortex, and white matter in developing infants

The germinal matrix (GM) located in the thick subependymal cell layer of the thalamostriate groove is a major site of cerebral hemorrhage in premature infants. Comparing the morphology of vasculature among GM, gray and white matter of the brain may help in understanding the pathogenesis of GM hemorrhage and also of periventricular leukomalacia. The objective of the present study was to determine the morphology of blood vessels in the GM, gray matter, and white matter and to examine maturational changes in the morphology of these vessels as a function of gestational age. We measured vessel density, percentage of blood vessel area, mean surface area, length, breadth, perimeter, radius, and shape of blood vessels in coronal sections of the GM, gray matter, and white matter in postmortem human brain samples for 17 fetuses and premature infants of gestational age 16-40 wk and 2 adults. We performed immunohistochemical staining using anti-laminin primary antibody, confocal microscopy to acquire images, and analysis using Metamorph version 6.1. Vessel density and the percentage of blood vessel area increased as a function of gestational age in the GM, gray matter, and white matter (p < 0.001 each). The blood vessel density and the percentage of blood vessel area were largest in the GM followed by gray matter and then white matter in all of the gestational age categories (p < 0.001 for all comparisons). Increased vascularity of the GM compared with gray and white matter may play a role in GM hemorrhage, whereas a relatively low vascularity of white matter may increase the propensity for the occurrence of periventricular leukomalacia in premature infants

Subependymal veins in premature neonates: implications for hemorrhage

The germinal matrix contains a concentrated network of blood vessels. The unusual structural qualities of these vessels are implicated as a factor underlying the high incidence of hemorrhage that occurs in the germinal matrix of prematurely born neonates. The present study is a histologic analysis of an postmortem examination series of brains collected from neonates born between 23 weeks gestation and term and is designed to determine if subependymal veins can be recognized in neonates born at the limits of viability, approximately 23 weeks gestation. Alkaline phosphatase histochemistry is used to differentiate cerebral afferent from efferent vessels. The results demonstrate that precursors of the subependymal veins can be recognized as early as the twenty-third gestational week. These veins increase progressively in diameter from 23 weeks to term, but the wall of the veins, which at early stages consists of endothelial cells only, does not thicken until after postconception week 36. Thus in all premature neonates, including the youngest capable of independent existence, the subependymal veins are present and appear vulnerable to rupture. These data support our suggestion that the structural immaturity of these veins in premature neonates is causally related to the high incidence of germinal matrix hemorrhage in these patients.

Neonatal venous cerebral hemorrhage

Intracranial pathological changes can occur as a result of impaired craniocervical venous return. Thrombosis of central venous access catheters was demonstrated in two neonates born at 38 and 27 weeks' gestation. Neither infant developed hemorrhage of prematurity as confirmed on cranial ultrasonography. Clinical evidence of vena cava thrombosis and associated spontaneous intraventricular hemorrhage developed on Day 24 and 36, respectively, and these findings were confirmed on imaging studies. In one infant the hemorrhage was accompanied by communicating hydrocephalus.

The cause of the intracranial disease was attributable to the retrograde cerebral venous congestion. This, together with the primitive venous bed developing in the periventricular region, was associated with the spontaneous hemorrhage in the region of the foramen of Monro.

To the authors' knowledge, this is the first report in the English-language literature of spontaneous neonatal intra-cerebral hemorrhage, due to thrombosis of the superior or inferior vena cava.

The natural history of this condition is resolution without sequelae after appropriate therapeutic intervention for the vena cava thrombosis.

Temporal expression pattern of cerebrovascular endothelial cell alkaline phosphatase during human gestation

In premature human neonates, immaturity of cerebral vessels can contribute to clinical problems such as germinal matrix hemorrhage and white matter damage. Afferent cerebral vessels in the brain of term babies express alkaline phosphatase (AP), an ectoenzyme located on the surface of endothelial cells. Using AP enzyme histochemistry we have examined the cerebrovasculature of premature live-born human neonates to determine when cerebral afferent vessels begin to express AP. Brains were collected at autopsy and processed for histological examination. AP-stained vessel density in the periventricular white matter was quantified using digital imaging and automated morphometry. Babies born prior to 28 wk gestation display few AP-positive vessels in the periventricular white matter, whereas, babies born after 28 wk gestation exhibit an AP-positive vascular pattern that resembles the adult pattern. In contrast, immunostaining for collagen revealed an extensive vascular network in both early and late gestation infants. Our measurements indicate that neonates born prior to 28 wk gestation are characterized by immature cerebral white matter afferent vessels and raise the possibility that the immaturity compromises vascular function.

An in vitro three-dimensional coculture model of cerebral microvascular angiogenesis and differentiation

The microvasculature of the developing brain is plastic and responds differently to the many insults associated with preterm birth. We developed three-dimensional in vitro culture models for the study of the responses of the developing cerebral microvasculature. Beagle brain microvascular endothelial cells (BBMEC) were isolated by differential centrifugation from newborn beagle pups on postnatal Day 1 and placed in three-dimensional culture dispersed in a collagen gel. Alternatively, BBMEC were placed in a three-dimensional coculture with neonatal rat forebrain astrocytes. Cultures were analyzed for extracellular matrix components at 1 and 6 d, and total RNA was extracted for Northern analyses. Urokinase plasminogen activator activity was assayed in both mono- and cocultures of the two cell types. Studies of three-dimensional BBMEC/astrocyte cocultures demonstrated progressive tube formation with only low levels of endothelial proliferation. By 6 d in three-dimensional coculture, the BBMEC formed capillarylike tubes with a wrapping of glial processes, and basement membrane protein synthesis was noted. Urokinase plasminogen zymography suggested intercellular signaling by the two cell types. These data suggest that the three-dimensional beagle brain germinal matrix microvascular endothelial cell/neonatal rat astrocyte coculture provides a good model for the investigation of microvascular responses in the developing brain.

The role of prostaglandin receptors in regulating cerebral blood flow in the perinatal period

Prostaglandins exert significant effects on the range of cerebral blood flow autoregulation. However, the newborn exhibits a narrow cerebral blood flow autoregulatory range compared to the adult, and this apparently contributes to the susceptibility of the newborn to major perinatal complications such as intraventricular cerebral haemorrhage. Reduced vasoconstriction in response to prostaglandins due to the fewer prostaglandin receptors, especially for PGE2 (EP) and PGF2 alpha (FP), seems to contribute in part to the narrower range of cerebral blood flow autoregulation in the newborn. Evidence suggests that high levels of prostaglandins in the perinatal period are responsible for the down-regulation of neurovascular EP and FP receptors. We review the pharmacology of prostaglandin receptors, in particular PGE2 and PGF2 alpha receptors, their ontogeny on the neural vasculature, the perinatal regulation of their expression, and how these changes relate to the control of neural blood flow autoregulation.

Intraventricular hemorrhage in the preterm infant

Intraventricular hemorrhage (IVH) is a common neonatal morbidity among premature infants which is diagnosed by cranial ultrasound in the newborn special care unit. Although very premature infants are more likely to experience the highest grades of hemorrhage, a number of perinatal and postnatal events have been shown to be associated with its occurrence. Factors such as vaginal delivery, labor, and intrapartum asphyxia have been associated with early onset of hemorrhage, whereas antenatal exposure to steroids may be protective. Respiratory Distress Syndrome has also been associated with hemorrhage. Since infants with a history of IVH have increased mortality rates and are at increased risk of seizures, periventricular leukomalacia, hydrocephalus, and neurodevelopmental handicap, many investigators have studied management techniques and pharmacologic interventions to decrease the incidence of IVH, including muscle paralysis, phenobarbital, Vitamin E, indomethacin, ethamsylate and surfactant. Our investigations have shown that low dose indomethacin (0.1 mg/kg i.v.) at 6-12 postnatal hours and every 24 h for two more doses decreases the incidence of all grades of IVH within the first 5 days of life. Although tremendous progress has been made in the understanding of the pathogenesis and prevention of IVH, innovative animal and human studies are needed to further reduce the incidence of this important neonatal morbidity.

Regional differences of cerebral blood flow in the preterm infant

The purpose of our study was to evaluate the regional distribution of the resting cerebral blood flow (CBF) pattern in preterm neonates. Sixty-eight preterm babies with a gestational age of less than 34 weeks and a birth weight of less than 1500 g were enrolled into the study. The CBF was measured by the noninvasive intravenous 133Xenon method at three different times. Depending on the age we classified our measurements into three groups. Group 1: measurement between 2-36 h (n = 46). Group 2: measurement between 36-108 h (n = 39). Group 3: measurement between 108-240 h (n = 41). In all three groups CBF was significantly lower in the occipital region than in the frontal and parietal regions (group 1: frontal region 12.8 +/- 3.5 ml/100 g/min, parietal region 12.8 +/- 3.9 ml/100 mg/min, and occipital region 11.6 +/- 3.18 ml/100 g/min; group 2: frontal region 15.4 +/- 4.2 ml/100 g/min, parietal region 15.3 +/- 4.1 ml/100 g/min, and occipital region 13.4 +/- 3.5 ml/100 g/min; group 3: frontal region 14.6 +/- 3.6 ml/100 g/min, parietal region 14.6 +/- 3.2 ml/100 g/min, and occipital region 12.8 +/- 2.7 ml/100 g/min.). CBF did not differ between the left and the right hemispheres in either of the three measured regions. No gradient was found in infants between 108 h and 240 h of age with periventricular leukomalacia and periventricular haemorrhage. CONCLUSION. In preterm neonates the antero-posterior gradient of CBF is already present. Periventricular leukomalacia as well as periventricular haemorrhage may affect the regional regulation of CBF.

Germinal matrix microvascular maturation correlates inversely with the risk period for neonatal intraventricular hemorrhage

The risk period for intraventricular hemorrhage (IVH) of the preterm neonate is the first 3-4 postnatal days. For infants of < 34 weeks' gestation, this risk period is independent of gestational age. We hypothesized that this risk period is attributable to the perinatal induction of maturation of the germinal matrix microvasculature and tested this hypothesis by examining changes in the classical ultrastructural features of the blood-brain barrier over the first ten postnatal days in the newborn beagle model for neonatal IVH. Newborn beagle pups (n = 6) were anesthetized and systemically perfused and the brains were removed and prepared for electron microscopic examination. Examination of electron micrographs from the germinal matrix of animals on the first, fourth and tenth postnatal days demonstrated no difference in perimeter lengths and capillary and endothelial cell areas; in contrast, luminal areas significantly decreased across postnatal age (P = 0.04). Significant increases were found in basement membrane area between days 1 and 4 (P = 0.01) and tight junction length (day 1 vs. day 10, P = 0.02). In addition, on day 1, 19% of germinal matrix capillary perimeter was determined not to be covered by supporting cell processes, while by day 10, only 5% was bare. In contrast, the microvessels of the white matter exhibited no changes in these parameters during these three time points. These studies are consistent with the concept that basal lamina deposition and organization precede increases in endothelial cell tight junction formation and coverage by supporting cells.

Cerebral blood flow in preterm infants affected by sex, mechanical ventilation, and intrauterine growth

Sex differences in cerebral blood flow (CBF) values have been demonstrated in adults but not in newborns. This study evaluated the influence of sex, intrauterine growth, and need of mechanical ventilation on resting cerebral blood flow in preterm neonates. Sixty-eight preterm infants with gestational ages of less than 34 weeks and birth weights of less than 1,500 gm were enrolled into the study. Cerebral blood flow was measured by the noninvasive intravenous xenon 133 method 3 times. Measurements were classified into 3 groups: group 1: measurement at 2-36 hours (n = 46); group 2: measurement at 36-108 hours (n = 39); and group 3: measurement at 108-240 hours (n = 41). In all 3 groups, the CBF in girls was significantly lower than in boys (group 1: 11.5 +/- 2.8 ml/100 gm/min vs 14.0 +/- 4.1 ml/100 gm/min; group 2: 13.4 +/- 2.9 ml/100 gm/min vs 16.3 +/- 4.3 ml/100 gm/min; group 3: 12.9 +/- 3.2 ml/100 gm/min vs 15.3 +/- 3.1 ml/100 gm/min). In group 1, the CBF in neonates requiring mechanical ventilation was significantly lower (P < .05) than in patients who were spontaneously breathing (11.5 +/- 3.7 ml/100 gm/min vs 14.2 +/- 3.1 ml/100 gm/min), and the CBF in neonates who were too small for gestational age was significantly higher (P < .005) than in children with appropriate intrauterine growth (16.1 +/- 4.1 ml/100 gm/min vs 11.5 +/- 2.6 ml/100 gm/min). It is concluded that in preterm neonates CBF is substantially affected by sex, intrauterine growth retardation, and the need of mechanical ventilation.

Intraventricular Hemorrhage: An Update

Although the incidence of intraventricular hemorrhage (IVH) has decreased in recent years, the increasing survival rates for the smallest premature infants indicate that the lesion will continue to be a major problem in neonatal intensive care facilities. We review prenatal, perinatal, and postnatal variables that have been associated with enhanced risk of IVH and address some of the methodological limitations of previously reported studies. The neuropa-thology is characterized by bleeding into the subependymal germinal matrix, with subsequent rupture into the lateral ventricle. The pathogenesis of IVH relates to intravascular, vascular, and extravascular factors. Intravas-cular factors are those that relate primarily to control of blood flow and pressure in the microcirculation of the germinal matrix. Vascular factors relate to the microcirculation of the matrix, the site of initial bleeding. A maturation-dependent alteration of vascular integrity and a vulnerability of matrix vessels to injury appear important. Extravascular factors include those relevant to mesenchy-mal and glial support for matrix vessels and to local fibrino-lytic activity in the germinal matrix. Prognosis in the setting of IVH relates to the mechanisms of brain injury, the most important of which is pcriventricular hemorrhagic infarction, often inappropriately called grade IV IVH and often associated with subsequent motor and cognitive deficits. Prevention of IVH remains the most important goal. Prenatal interventions include prevention of premature birth, transportation of premature infants to a tertiary facility in utero rather than after birth, possibly prenatal administration of phenobarbital or vitamin K, and optimal management of labor and deliver. Postnatal interventions include careful newborn resuscitation, correction or prevention of major hemodynamic disturbances, and correction of abnormalities of coagulation. Postnatal pharmacological interventions that have been studied in detail include phenobarbital, indomethacin, ethamsylate, and vitamin E. No single agent among this group has been shown consistently to lead to a decrease in incidence and severity of IVH.

Radial glial interaction with cerebral germinal matrix capillaries in the fetal baboon

The occurrence of intraventricular hemorrhage (IVH) is a frequent problem in premature infants delivered between 24 (60%) and 32 weeks (80%) of gestation, a time during which the germinal matrix (GM) is prominent over the head of the caudate nucleus. Most IVH arises from the GM and it has been proposed that an important factor in the pathogenesis of IVH is a weakness of GM capillary walls due to deficient support by surrounding immature glial cells. The purpose of this study was to examine the glial-capillary interaction in the GM of fetal baboons sacrificed at 100 days (54%) gestation, a stage of GM development comparable to that during which human neonatal IVH occurs. Brains from a later gestational stage (162 days, 88%), after GM involution, were also examined. At 100 days of gestational age, the GM was prominent over the head of the caudate and contained vimentin positive, but not glial fibrillary acidic protein positive, radial glial cells which formed endfeet on capillaries in the region. Ultrastructurally, all the GM capillaries examined from this gestational time had complete, continuous endothelia marked by few pinocytotic vesicles and prominent tight junctions. The endothelial cells rested upon uninterrupted basement membranes which were contacted by clearly identifiable glial endfeet. These data show that GM capillaries have morphologies typical of CNS capillaries and suggest that the capillary immaturity within the GM is not a major contributing factor to IVH.

CO2 reactivity and autoregulation in fetal brain

Intraventricular hemorrhage (IVH) in preterm infants is well known to be associated with the high morbidity and mortality of this group. Previous studies have suggested altered cerebral blood flow (CBF) as an important pathologic factor. We measured the CBF in near-term rabbit fetuses using the hydrogen clearance technique. The local CBF of the rabbit fetuses was significantly low compared with that of the maternal rabbits. The response of CBF to changes in PaCO2 was observed in rabbit fetuses. The CO2 reactivity index of the fetal rabbit was lower than that of the maternal rabbit. This low CO2 reactivity might reflect the immaturity of the fetal brain and its low CBF. We were unable to monitor the fetal blood pressure, but the fetal CBF remained stable when the maternal blood pressure was altered. It is well known that IVH in preterm infants originates from the subependymal germinal matrix and that this has many fragile vessels. Our observation suggests that even a small increase of CBF during hypercapnia might have a large effect towards producing hemorrhage.

Beagle pup germinal matrix maturation studies

Intraventricular hemorrhage, or hemorrhage into the germinal matrix tissues of the developing brain, remains a common problem of preterm infants. The "risk period" for this insult is the first 3-4 postnatal days. We hypothesized that this risk period for hemorrhage is related to rapid perinatal maturation of the germinal matrix vasculature and employed the newborn beagle pup model for the study of this maturation. Newborn beagle pups (n = 30) were anesthetized and systemically perfused with buffered formalin; the brains were removed and prepared for immunohistochemical study. Sections stained with Bandeiraea lectin demonstrated that there was no difference in germinal matrix vessel density between postnatal days 1 and 4. Germinal matrix sections were also stained for antibodies to alpha-smooth muscle actin, collagen IV, collagen V, desmin, factor VIII-related antigen, fibronectin, glial fibrillary acidic protein, laminin, transferrin, and vimentin. Vasculature staining by alpha-smooth muscle actin was not noted until postnatal day 10, and differential staining was detected for antibodies to laminin and collagen V. Quantification of staining intensity by confocal microscopy demonstrated a significant increase in both extracellular matrix components at postnatal day 4 compared with day 1 (p less than 0.05 for both). These basement membrane proteins may add sufficient structural integrity to germinal matrix vessels to prevent capillary rupture and thus intraventricular hemorrhage.

Cerebrovascular regulation and neonatal brain injury

Neuropathology occurring as a result of hemodynamic injury occurs in up to 25% of preterm newborns of less than 1,500 gm birth weight and in a much smaller, but nonetheless meaningful, proportion of more mature infants. Abnormalities in cerebrovascular regulation have been proposed as major contributing factors to both ischemic and hemorrhagic injuries in the newborn brain. In this review we explore several factors that play a role in cerebrovascular regulation in the immature brain and relate them to what is known about vascular regulation in the mature brain and to the types of pathology that occur in the newborn brain. One goal in this "decade of the brain" should be to increase our basic and clinical knowledge about the cerebrovasculature of the newborn in order to enhance our ability to predict and prevent perinatal brain injury.

Changes in cerebrovascular prostaglandins and thromboxane as a function of systemic blood pressure. Cerebral blood flow autoregulation of the newborn

Cerebrovascular concentrations of prostaglandin E (PGE), prostaglandin F2 alpha (PGF2 alpha), 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha), and thromboxane B2 (TXB2) were determined over a blood pressure range of 17-117 mm Hg (induced by inflation of balloon-tipped catheters placed in the thoracic descending aorta and at the aortic root) in eight newborn piglets to access the role of prostanoids in cerebral blood flow (CBF; measured using radioactive microspheres) autoregulation. Basal systemic blood pressure, heart rate, blood gases, total CBF, and prostanoid concentrations were stable. CBF was constant between 50 and 90 mm Hg, but beyond this range CBF varied directly with blood pressure (tau = 0.48; p less than 0.05). Sagittal sinus concentrations of PGE, PGF2 alpha, and 6-keto-PGF1 alpha varied with blood pressure according to a quadratic function (R2 = 0.92 to 0.96; p less than 0.0001), exhibiting lowest values between mean blood pressures of 60 and 90 mm Hg. During hypotension (17-49 mm Hg), there was a greater relative increase in sagittal sinus concentrations of TXB2 than of PGE, PGF2 alpha, and 6-keto-PGF1 alpha; at the lowest blood pressures, TXB2 increased by 658 +/- 44%, and prostaglandins increased on the average by 331 +/- 49% (p less than 0.01) from their values during normotension (50-90 mm Hg). During hypertension (91-117 mm Hg), cerebrovascular production and concentrations of prostaglandins increased by 142 +/- 31% and 45 +/- 10%, respectively, but did not change for TXB2.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoconstriction in telencephalic microvessels: a response to one model for intraventricular hemorrhage in beagle pups

Telencephalic microvessels were morphologically and morphometrically evaluated in beagle pups at 24, 48, and 72 hours of age following an experimental insult consisting of exposure to acute hypovolemic hypotension followed by rapid corrective blood reinfusion. Animals were anesthetized throughout the procedure and maintained for 75 minutes prior to sacrifice. Microvessels from the germinal matrix, where intraventricular hemorrhage (IVH) most commonly occurs, were compared to those of the adjacent cerebral cortex and choroid plexus. Perfusion, fixation and processing of the tissue followed our previously published protocol (Leuschen et al.: Anatomical Record 208:435-443, 1984). Both germinal matrix and cortical microvessels had collapsed lumens and normal endothelial cell morphology; mitochondria contained distinct cristae, junctional complexes were present and a basement lamina completely surrounded most vessels. Morphometric data on matrical and cortical microvessels supported vasoconstriction. Microvessels from choroid plexus were not vasoconstricted; tight junctions were infrequently seen and microvesicular elements were present. An increased cerebral blood flow to the germinal matrix and the adjacent cerebral cortex has been reported during and immediately following hypovolemic hypotensive insult. By 75 minutes following insult, our ultrastructural studies support a rebound phenomena including vasoconstriction of microvessels. The resulting vasoconstriction and associated cerebral ischemia may play an important role in subsequent tissue damage to the matrical area and ultimately to the etiology of IVH.

Intraventricular hemorrhage in the premature infant--current concepts. Part I

Although the incidence of periventricular-intraventricular hemorrhage (IVH) has decreased in recent years, the increasing survival rates for the smallest premature infants indicate that the lesion will continue to be a major problem in neonatal intensive care facilities. The neuropathology is characterized by an elemental lesion, bleeding into the subependymal germinal matrix, with subsequent rupture into the lateral ventricle. Important neuropathological consequences are germinal matrix destruction, posthemorrhagic hydrocephalus, and periventricular hemorrhagic infarction. The last of these appears to be a venous infarction and is a critical determinant of neurological outcome. Neuropathological accompaniments, not caused by the IVH, include periventricular leukomalacia and pontine neuronal necrosis. The pathogenesis of IVHs is related to intravascular, vascular, and extravascular factors. Intravascular factors involve primarily control of blood flow and pressure in the microcirculation of the germinal matrix. Particular pathogenetic importance can be attached to fluctuations in cerebral blood flow, abrupt increases in flow, decreases in flow with injury to matrix vessels, increases in cerebral venous pressure, and, in selected infants, disturbances of platelet function and coagulation. Vascular factors relate to the microcirculation of the matrix, the site of the initial bleeding. A maturation-dependent alteration in vascular integrity and a vulnerability of matrix vessels to ischemic injury appear important. Extravascular factors include those relevant to mesenchymal and glial support for matrix vessels and to local fibrinolytic activity in the germinal matrix. The latter may be a manifestation of the proteolytic activity now recognized to be of general importance in developmental remodeling of the mammalian central nervous system.

Morphometry of blood vessels in the cortex and germinal plate of premature neonates

Germinal plate hemorrhage with intraventricular rupture has been a leading cause of morbidity and mortality among premature neonates. Because germinal plate hemorrhage decreases with increasing gestational age, germinal plate vessel size and/or structure may be an important etiologic factor. Electron micrographs of the cortical and germinal plate blood vessels from 13 premature infants ranging in age from 19-37 weeks gestation were studied; vessel, lumena, and endothelial areas were measured using a digitizing tablet. In infants between 25-32 weeks gestation, the germinal plate vessel and lumenal areas were significantly greater than the areas of cortical vessel and lumena. This finding suggests that one of the factors involved in germinal plate hemorrhage is described by LaPlace's law which states that the larger the vessel diameter, the greater the pressure on the vessel wall.