Does Perinatal Intermittent Hypoxia Affect Cerebrovascular Network Development?

Abstract Perinatal hypoxia is an inadequate delivery of oxygen to the fetus in the period immediately before, during, or after the birth process. The most frequent form of hypoxia occurring in human development is chronic intermittent hypoxia (CIH) due to sleep-disordered breathing (apnea) or bradycardia events. CIH incidence is particularly high with premature infants. During CIH, repetitive cycles of hypoxia and reoxygenation initiate oxidative stress and inflammatory cascades in the brain. A dense microvascular network of arterioles, capillaries, and venules is required to support the constant metabolic demands of the adult brain. The development and refinement of this microvasculature is orchestrated throughout gestation and in the initial weeks after birth, at a critical juncture when CIH can occur. There is little knowledge on how CIH affects the development of the cerebrovasculature. However, since CIH (and its treatments) can cause profound abnormalities in tissue oxygen content and neural activity, there is reason to believe that it can induce lasting abnormalities in vascular structure and function at the microvascular level contributing to neurodevelopmental disorders. This mini-review discusses the hypothesis that CIH induces a positive feedback loop to perpetuate metabolic insufficiency through derailment of normal cerebrovascular development, leading to long-term deficiencies in cerebrovascular function.


Introduction
Repetitive hypoxic episodes, or intermittent hypoxia, during the first week of life is a major contributor to impairment of central nervous system function [1][2][3] and can lead to neurobehavioral disorders and cognitive deficits [4,5].Chronic intermittent hypoxia (CIH) is more common than continuous hypoxia [6] during early postnatal life due to an instability of cardiovascular or respiratory control and often results from sleep-disordered breathing (apnea) or bradycardia events [7].CIH is a frequent cause of death and disability in neonates [8], experienced by both preterm and term infants, although it is more frequent in extremely low-birth weight infants [9,10].The pathway by which CIH initiates a pathological cascade is incompletely understood.However, both clinical and preclinical studies suggest that CIH induces brain injury in part through increased neuroinflammation [11] and oxidative stress [12], which decreases gray and white matter integrity [11].It has been associated with impairment of oligodendrocyte maturation, myelination, axon development, and synapse formation, leading to poor cognitive outcomes [13].
The brain consumes a large amount of oxygen and is extremely sensitive to oxygen and blood flow fluctuations.Oxygen delivery to the brain depends on tightly regulated cerebral hemodynamics and appropriate concentrations of blood oxygenation.The unceasing metabolic demands of the brain are supported by an intricate 3-dimensional cerebrovascular network, designed to efficiently distribute blood to all brain cells [14].In rodent models, the development and refinement of this microvascular network is orchestrated throughout gestation and the first weeks after birth [15,16].In humans, it unfolds over a longer time frame encompassing the first few years of postnatal life [17,18].The timing of brain microvascular development varies across brain regions [19].Successful development of both the central nervous system and its vascular supply relies on the significant exchange of maturation cues and neurogliovascular signals.Neuronal and astrocyte-derived angiogenic and maturation signals such as vascular endothelial growth factor A (VEGF-A) stimulate vessels to grow and create a network pattern [20], but also activate signaling pathways such as Wnt7a/b, which are important for acquisition of blood-brain barrier (BBB) properties [21].Further, the brain vasculature serves many roles beyond blood supply, for example, by serving as migratory scaffolds for oligodendrocytes [22] and neurons [23] and regulation of neural stem and progenitor cell behavior [24].For more comprehensive reviews on the mechanisms of cerebrovascular development, we direct the reader to the following articles [15,18,25].
Hypoxia plays an important role in the regulation of vasculogenesis and angiogenesis [26].There are periods in which tissues are metabolically active and consuming oxygen, yet vascular networks have not been sufficiently built to distribute blood and sustain energetic demands.The pattern of hypoxic tissues stimulates a regulated angiogenesis, involving induction of gradients in VEGF-A expression [27] that leads to organized vascular network generation.However, CIH episodes are likely to disrupt this process by elevating the levels of hypoxia and altering its spatial distribution, and thus the cues for vascular growth, in the developing brain.The consequence of this effect on cerebrovascular development in the short and long term remains poorly understood.In this article, we present the hypothesis that perinatal CIH leads to lasting alterations in cerebrovascular structure and function based on review of current evidence.

Origin and Maturation of the Brain Vasculature
Blood vessels serve as highways to deliver oxygen and nutrients, inflammatory and progenitor cells, as well as the removal of waste products to and from the brain.Vascular networks are organized into arteries/arterioles that carry blood into the brain, capillaries where most nutrients and wastes are exchanged, and venules/veins responsible for drainage of the blood away from the brain [18].The composition and arrangement of cells that make up the cerebrovasculature is often called the neurovascular unit (NVU).The neural elements of the NVU consist of astrocytes, microglia, and neurons, while the vascular components include endothelial cells, vascular smooth muscle cells (VSMCs), pericytes, and perivascular fibroblasts.The NVU is also composed of acellular components of the vascular basal lamina, including matrix molecules such as laminin, fibronectin, and type IV collagen.Together, these cellular and acellular elements form the NVU responsible for maintaining the homeostasis and function of the brain microenvironment by synchronizing the energetic demand with vasodynamic changes.The NVU composition differs depending upon location within the cerebrovascular network [28,29].
The chief engineers of the brain's vascular network are endothelial cells that cooperate with other neural and non-neural cells to expand the complexity of the capillary system.Additionally, their concerted interaction results in BBB refinement and maintenance.The cerebrovasculature originates by vasculogenesis.In humans, the de novo formation of a primitive vascular network from a leptomeningeal plexus occurs at about 24 days of gestation [30].These neovessels grow and differentiate from the surface toward the midbrain [31].At gestational weeks 13-15, intracortical vessels emerge mainly in the ventricular zone [32].As early as 14 gestational weeks, vasculature is present throughout the pallium (which will become the cortex) and subpallium [33].There is increased concentration of vasculature in the ventricular zone in the second trimester followed by increased vascularization in the cortical neuronal layers after 25 weeks of gestation [32].This regional shift of vasculogenesis from the ventricular zone to the cortex might reflect the Cerebrovascular Development and Intermittent Hypoxia shifts in metabolic demand to neural stem/progenitor cell proliferation in cortical layer development.The brain capillary network arborizes quickly to facilitate perfusion of the developing cortex via angiogenesis [34], the growth and branching of existing blood vessels to expand the complexity of vascular networks [35].Studies in human brain samples demonstrate that vessels in the deeper cerebral white matter develop earlier than in the cortex and superficial white matter [33,36], with exception for germinal matrices where angiogenesis remains active until week 34 [37].By full-term pregnancy, the capillary density in the telencephalic white matter is largely established [17,38,39].In contrast, the capillary beds of the cortical gray matter continue to expand throughout the first 3 or 4 years of postnatal life [17,38].
The maturation of the brain primary vascular network involves the recruitment of mural cells (VSMCs and pericytes), establishment of the BBB properties (barriergenesis), the generation of the extracellular matrix, and enwrapping by astrocytic endfeet.Cerebral arteries undergo refinement until the third postnatal month [40] as studies have shown that arteriolar VSMC expands and matures during this period and becomes contractile to better control cerebral blood flow [41].Astrocytic recruitment and differentiation occurs during the late embryonic and early postnatal periods [42].Since astrocyte coverage is delayed after most developmental angiogenesis, astrocytes may seem to serve a role in vascular maturation and barriergenesis and maintenance of the BBB during adulthood [43].

Hypoxia: The Breath of Life
In the 20th century, Barcroft [44] coined the term "Everest in utero," used in modern day to describe the hypoxic conditions in which the fetus normally develops.He hypothesized that the womb was the most hypoxic condition that we are ever likely to experience, with few exceptions, including severe pathophysiology illness and environmental hypobaric hypoxia at high altitude.Years later, a study confirmed [45] that the arterial partial pressure of oxygen (PaO 2 ) in adult humans recorded at the summit of Mount Everest (PaO 2 of 2.55 kPa−19.1 mm Hg) is near those measured within the fetus (extrapolations from umbilical blood measurement suggest that PaO 2 is between 2.5-3.5 kPa and 18.0-26.3mm Hg) [46].Transition from the uterine to an atmospheric environment involves a shift in several mechanisms to adapt to a sudden increase in oxygen content increasing from average of 3.3 kPa (25-30 mm Hg) in the fetus to 10.5 kPa (75-85 mm Hg) only 5 min after cord blood circulation ceases [47].At birth, cerebral blood flow and metabolic rate are both low but increase gradually through the first years of postnatal life [48], reaching a maximum at about age six in humans, after which there is a gradual decline throughout adult life.This phenomenon seems to be conserved in other mammalian species [49,50].Additionally, fetal hemoglobin is gradually replaced with adult hemoglobin after birth, which has less affinity for oxygen [51].We are all victorious climbers of Barcroft's Everest in utero and the footprint of that journey might impact how we react to brain injuries or disease conditions in adult life.

Hypoxia and Angiogenesis
Hypoxia is the key physiological factor influencing cerebrovascular development due its role on angiogenesis.Hypoxia-inducible factors induce angiogenesis by regulation of the transcription of VEGF, which promotes endothelial cell migration toward hypoxic tissues.Hypoxia triggers the angiogenic process through transcriptional activation of VEGF via binding of hypoxia-inducible factor-1α (HIF-1α) to the VEGF gene promoter [52,53].Secreted VEGF binds to its receptor tyrosine kinases (VEGFR1 and VEGFR2) located on the surface of endothelial cells triggering a cascade of intracellular signaling pathways to initiate angiogenesis [54].With angiogenesis switched on, metalloproteinases degrade the extracellular matrix and VEGF molecules activate ECs.Within these ECs, endothelial tip cells respond to VEGF and form characteristic protruding and actin-rich filopodia.Although tip cells do not proliferate, they migrate toward the source of VEGF, giving directionality to an angiogenic sprout.Endothelial stalk cells proliferate behind the leading tip cells, allowing the elongation of the angiogenic sprouts and the formation of the vessel lumen.Then, these nascent vessels merge with existing and patent blood vessels that ultimately supply the area with oxygenated blood.In both mice [50] and humans [33], this process initiates from the cerebral veins and venules, which carry less oxygen content and are probably more conducive to vessel sprouting given lower blood pressure and thinner vascular walls.Once established, the new vessels undergo maturation steps and signaling transduction that involve the acquisition of specialized features, including the BBB and formation of a basement membrane.Recent live animal imaging studies showed that endothelial cells and pericytes migrate in parallel during vasculogenesis [55] and angiogenesis processes [50] showing there is no window of time that endothelium is not covered by pericytes, as previously theorized [56], at least with cortical angiogenesis.

Clinical Studies
While chronic fetal hypoxia is well documented [57] and is known to increase the risk of cardiac and endothelial dysfunction in later life [58], the effects of postnatal CIH remain poorly understood.As described above, a hypoxic environment is required for proper fetal development, but fluctuations in oxygen availability in the postnatal phase can also present significant challenges for cerebrovascular development.During our literature search, we only found a few studies that evaluated brain oxygenation and cerebrovascular function after CIH in human newborns.The majority of these studies were conducted in preterm babies since CIH occurs more frequently in premature infants (<32 weeks) who have apnea of prematurity.In the preterm brain, angiogenesis is especially active in the germinal matrix, making a more fragile vasculature with increased propensity for mechanical rupture and bleeding known as germinal matrixintraventricular hemorrhage [37].Therefore, the first 72 h after premature birth can be a critical phase of hemodynamic instability and subsequent tissue hypoperfusion and hypoxia [59,60].To evaluate this pathophysiology, cardiac output, systemic vascular resistance, blood pressure, regional tissue perfusion, and oxygenation are carefully monitored.In response to CO 2 challenge tests, term infants (39 +/− 2 weeks gestation) exhibited an expected increase in anterior cerebral artery flow, whereas this response was inconsistent and reduced in preterm infants with apnea (29 +/− 2 weeks gestation) [61].Notably, the studies reported a decrease in cerebral blood flow and brain oxygenation during the bradycardia events.The alterations in cerebral tissue oxygen desaturation, measured through multimodal techniques as nearinfrared spectroscopy, transcutaneous Doppler, or pulse oximeter, seem largely dependent on the duration and depth of the arterial hypoxemia [59,62,63].Cerebral oxygenation can significantly decrease during apnea, especially when accompanied by the drop of peripheral oxygen saturation and heart rate, resulting in hypercapnia to compensate by increasing the blood flow [59,62].However, in both neonates <34 weeks gestational age and postmenstrual age <40 weeks with severe intermittent hypoxemia or bradycardia requiring supplemental oxygen and treated with caffeine, cerebral tissue oxygen saturation rarely decreased <60% despite severe peripheral oxygen saturation hypoexmia, suggesting that compensation mechanisms preserve cerebral oxygen supply [62].Nevertheless, no major investigation of CIH impact on the function of the cerebrovasculature was made.Moreover, no correlations have been described about possible sex, age maturity, or weight differences and effects.

Rodent Studies
It is difficult for clinical studies to isolate the consequences of CIH on cerebrovascular function from other comorbid factors such as epilepsy or prematurity.The response to hypoxia will also differ with developmental stage, adding substantial variability to clinical measurements.Animal models of perinatal CIH can provide insight into how CIH affects cerebrovascular development.While animal models cannot fully recapitulate the complexity of the clinical scenario, CIH models do produce an injury profile similar to that seen in humans, with respect to white matter loss, neurodevelopmental delay, and functional deficits [11,13].Very recently, it has been suggested that postnatal CIH might be a risk factor for developing cardiorespiratory diseases in adulthood as well [64].The models generally involve several daily sessions of intermittent hypoxia over various days [65].Each daily session involves cycling of normoxic and hypoxic conditions, with hypoxic periods lasting several seconds at a time.
Of the few studies that examined cerebrovascular architecture and function by histology, CIH protocols in rodents caused a transient increase in the capillary density.In one study on CD1 mice, pups were exposed to CIH with cycling between normoxia and hypoxia (11% oxygen) starting at postnatal day (P)2 and over a period of 2-4 weeks.A time-dependent increase in capillary density between ~20-30% was observed in the cerebral cortex and hippocampus.Interestingly, this work also examined chronic continuous hypoxia (CCH) and reported that in mice returned to normoxia for an additional 4 weeks after CCH, capillary density returned to relatively normal levels [66].In contrast to capillary changes, the demyelination that occurred with CCH worsened upon reoxygenation.Reoxygenation following CIH was not examined.In a separate study, a marked ~40% increase in capillary density was measured in the hippocampus after only 5 days of intermittent hypoxia in mice [67].This coincided regionally with neurogenesis also observed with this treatment.In a study on rat pups, hyperoxia (50% oxygen) was interleaved with hypoxia (12% oxygen) in the CIH protocol lasting between P0 and P14 [68].The basis for this modification was to better mimic CIH treatment with hyperoxia, where nonphysiological increases in oxygen may in itself lead to long-term developmental issues.In that study, capillary densities also increased by ~20% in cerebral cortex, but examination of Cerebrovascular Development and Intermittent Hypoxia other brain regions revealed that the periventricular white matter did not mount angiogenic responses.Although the capillary density also returned to normal with normoxia for two additional weeks, hyperoxichypoxic cycling caused lasting alterations in BBB permeability and excessive motor impulsivity on rotarod [68].
Together, these rodent studies are consistent in demonstrating angiogenesis induced by CIH.However, there is regional specificity in the angiogenic response, and poorly vascularized tissues such as white matter may be less equipped to mount responses.Capillary densities are reported to return to normal levels with reoxygenation.But there may be persistent vascular abnormalities that impair BBB properties and render the vasculature vulnerable to secondary insults.

Effects of Reoxygenation and Hyperoxia
Oxygen radicals are second messengers under physiological conditions and potent regulators of arteriole tone [69,70].At pathological levels, oxygen radicals cause damage to all cellular components via increased oxidative stress.In CIH, the cycles of hypoxia and reoxygenation are akin to reperfusion injury in transient stroke, where a sudden influx of oxygen overwhelms the mitochondrial electron transport chain leading to singlet oxygen production.Relative hyperoxia immediately after birth or even supplemental oxygen as a treatment can lead to oxidative stress.Immature antioxidant systems and decreased capacity to scavenge reactive oxygen species (ROS) in neonates [71] can further add to increased oxidative stress [72].Indeed, astroglia are central to management of oxidative stress in the brain [73], and maturation of astrocytes occurs in postnatal stages [42].
Vascular pathology in the brain is regulated to a large extent by hypoxia-induced neovascularization [83,84].During reoxygenation following a hypoxic episode, elevations in VEGF [85] and production of ROS causes vessels to become more permeable [86].Moreover, ROS production seems to be essential for endothelial migration and angiogenesis [87,88].CIH creates a feed-forward loop causing more pronounced damage in the immature brain [89], shown in Figure 1.The severity of CIH injury on the brain vasculature is likely to be influenced not only by the duration of CIH but also by the fragility of the developing cerebrovascular system due to ongoing angiogenesis and immaturity in vasoregulation [90].

Cerebrovascular Hypothesis of Altered
Neurodevelopmental Outcomes following CIH CIH in the neonatal brain is associated with neurodevelopmental impairment during early infancy [91,92] that might have long-lasting neurocognitive effects [93].Not all patterns of intermittent hypoxia are deleterious, and some may even improve neurodevelopmental outcomes like mild CIH and when associated with other pathologies or injuries [67].However, the behavior observed in adolescence and adulthood after perinatal CIH deviates from the norm, such as altered locomotor activity [94] and language and cognitive delays [95].Many of these alterations have been attributed to neuronal loss [96], synaptogenesis alterations [97], or neuronal development [13] as well as glial response [98].Our hypothesis is that CIH induces a positive feedback loop to perpetuate metabolic insufficiency through derailment of normal cerebrovascular development and that this leads to long-term deficiency in cerebrovascular function.This idea is supported by preclinical studies showing CIH induces a transient increase in microvascular density [68] (shown in Fig. 1), which is a deviation from the carefully timed process of vascular patterning shaped by the metabolic needs of brain activity during perinatal development.While increased capillary density can aid oxygen delivery, there is the cost of creating a disorganized vascular architecture.For example, nascent capillaries created during CIH may differ in their diameter and tortuosity compared to normally developed capillaries, making them less equipped to support and regulate blood flow.Further, misplaced or incompletely formed vessels can lead to unbalanced distribution of capillary networks through introduction of flow shunts and capillary "dead ends".Oxidative stress can cause vasoconstriction, BBB damage, and leukocyte recruitment that all impede cerebral blood flow.Stronger hypoxic signals may induce sprouting from arteries and arterioles, which normally do not support angiogenesis and may create arteriovenous shunts that circumvent flow through capillaries.Collectively, these factors increase capillary flow heterogeneity, which reduces the efficiency of blood oxygen extraction [99].We envision that these changes would slow the development of neuronal connections and glial proliferation and therefore disrupt the release of neural and glial cues needed to promote normal vascular growth.This creates a feed-forward cycle of neurovascular disruption.Fortunately, the brain exhibits some resilience to these hypoxia-induced changes after a period of reoxygenation as vascular regression then helps to reestablish a normal density of microvasculature.Although the capillary density increase is transient, it was associated with lasting vascular and behavioral alterations [68].This begs the question of whether normal appearing capillaries truly support normal vascular function and perfusion.In other words, remodeling of poorly designed plumbing may not solve all the issues in the long run.The fine scale of the microvasculature has not been examined in sufficient detail to understand whether there are lasting impairments of the cerebrovasculature after disruption of normal earlylife development.

Knowledge Gaps and Next Steps
Understanding the short-and long-term effects of CIH on the cerebrovasculature requires deeper investigation of several topics.that subsequently regulate HIF-1, yet HIF-1 is required for intermittent hypoxia-induced ROS.This positive feedback loop amplifies the hypoxia that might lead to detrimental angiogenesis.Compared to the regularly patterned and functioning vasculature after normal refinement and maturation (upper middle panel), we hypothesized after neonatal CIH exposure the vascular system suffers structural changes that lead to functional impairment (lower middle panel).The CIH-exposed cerebrovascular network shows increased vascular density, tortuosity, enlarged or irregular vessels, vessels with dead ends, and abnormal flow.Moreover, there is a loss of hierarchy, increased vascular permeability and the mural cells can be structurally defective.These factors result in the development of chronic hypoxia by creating deficiency in blood perfusion.After the CIH is resolved and returned to a normoxia condition, we envision that a certain amount of the abnormal vasculature will be pruned although the vascular function remains compromised (lower right panel).Furthermore, we picture that CIH causes the development of a dysfunctional NVU with distorted crosstalk among vascular and neuronal cells, which can result in poor neurovascular coupling and neurodevelopmental outcomes.

Cerebrovascular Development and Intermittent Hypoxia
Deep Analyses of Cerebrovascular Structure and Function Histological assessment of capillary density in thin brain slices has been the primary technique to date.However, future studies will need to assess 3-dimensional vascular structure using volumetric imaging and analysis approaches, such as optical clearing of brain tissues and light-sheet or multiphoton imaging [100,101].These data could be used to quantify numerous aspects of microvascular structure, such as the length of arteriole-capillaryvenous paths, tortuosity of microvessels, prevalence of capillary "dead ends", and other features that are difficult to assess in thin tissue sections.This knowledge could be coupled with in silico approaches to model microvascular perfusion and predict effects on cerebral perfusion [102].In vivo physiological studies are also needed to understand the quality of blood perfusion through vascular networks, both in the state of heightened capillary density and in "normalized" networks following reoxygenation.This is where in vivo high-resolution imaging of vascular networks in mouse pups can shed light on blood flow [103][104][105].The flow of blood cells can be measured at the single capillary level and neurovascular coupling responses to external sensory stimuli can be assessed.
Evaluation of the NVU Since cerebrovascular patterning is a process orchestrated by all cells of the NVU, it will be important to understand if there are abnormalities or developmental delays in the assembly of the NVU.For example, is mural cell coverage and contractility intact in blood vessels after CIH and reoxygenation?Have astrocytes populated the perivascular niche and are their endfeet properly aligned with the vessel wall and able to support their roles in BBB function [106] and neurovascular coupling [107,108]?Are the various zones within the cerebrovasculature (arterioles, capillaries, venules, and intervening transitional segments) properly formed and correct in their length proportions?Recent studies have demonstrated that different zones of the microvascular network exhibit differing dynamics and likely serve different aspects of cerebral blood flow regulation [109].Further, CIH also unveils brain region-specific vulnerabilities in the microvasculature.The lower vascularity of white matter and apparent inability to mount an angiogenic response may explain why white matter loss is a key component in the CIH pathogenesis.However, the response of neurovascular cell types in white matter remains poorly characterized.By leveraging the growing array of mouse models with fluorescent labeling of cell types within the NVU [103,108], it will be possible to examine these questions both in histology and with in vivo imaging.

Effect of Hyperoxia Treatment
Studies have reported that supplementing oxygen in humans [110] and rodents [111] can lead to cerebrovascular alterations and poor neurodevelopmental outcome.No study has assessed yet the safety and effect of different oxygen exposures after return of spontaneous circulation in term asphyxiated infants.While oxygen supplementation is lifesaving, it can change the landscape of brain tissue oxygen content, which is an important factor in cerebrovascular development.It can also enhance oxidative stress at a time when antioxidant defense mechanisms remain immature.Therefore, the oxygen supplement thresholds that cause vascular dysregulation at different points in the developmental process should be studied in greater detail.Future investigations are also needed to dissect which pathophysiologic effects of perinatal CIH are associated with hypoxic induced injury versus those caused by reoxygenation events and supplemental oxygen.Factors Influencing Outcome More clinical studies are required to understand how sex differences, body weight, and the developmental stage at which CIH occurs influences the outcome of CIH.More preclinical studies are also needed to understand the timing in development of different vascular beds in the brain, and how different brain regions may be more susceptible or resilient to CIH.Ideally, similar animal models and CIH protocols should be used across research groups to improve our ability to compare findings, though it is recognized that additional studies are needed to better align rodent models with the human condition.
Improved Recording and Data Collection of CIH Events in Humans Near-infrared spectroscopy enables continuous recordings of oxygen saturation, and recent studies using this approach have revealed a much higher frequency of CIH events than previously thought [112].For a more efficient diagnosis, there is a need to improve noninvasive imaging technology for real-time monitoring of cerebral blood flow, brain oxygenation, and levels of brain metabolites for possible injury biomarkers.Detailed recording of these data can allow future studies to adequately relate CIH incidence/severity with acute and long-term outcomes.How does the severity, duration, and number of CIH relate to neonates that have mild versus severe manifestations?Does CIH-related changes in the cerebrovasculature predispose to injuries caused by secondary insults later in life?

Conclusion
Our understanding of how the cerebrovasculature responds to CIH is still evolving.Conditions that lead to oxygen and cerebral blood flow insufficiency in newborns can disrupt a carefully orchestrated process of vascular development in the perinatal period.This review identifies many unknowns regarding potential acute and long-term changes that might take place in the brain microvasculature of a newborn exposed to CIH.Deeper investigations at both preclinical and clinical levels can help to elucidate the pathophysiology effects of CIH and identify access points for therapeutic intervention in pediatric patients.

Fig. 1 .
Fig. 1.Schematic illustration of our proposal hypothesis of cerebrovascular network development under chronic intermittent hypoxia (CIH).Under normoxia conditions, sprouting endothelial angiogenesis is related to hypoxic signaling.During development (upper left panel), hypoxia upregulates hypoxiainducible factor-1α (HIF-1α) and targets gene expression and releases angiogenic factors resulting in unstable vessels and activation of vascular endothelial growth factor receptor 2 (VEGFR2)-expressing endothelial tip cells.During CIH (lower left panel), there is a pathological activation of angiogenic factors (i.e., VEGF) and overproduction of reactive oxygen species (ROS) that subsequently regulate HIF-1, yet HIF-1 is required for intermittent hypoxia-induced ROS.This positive feedback loop amplifies the hypoxia that might lead to detrimental angiogenesis.Compared to the regularly patterned and functioning vasculature after normal refinement and maturation (upper