Hannah C. Kinney

Sequence of Central Nervous System Myelination in Human Infancy. I. An Autopsy Study of Myelination

This study establishes the sequence of myelination in a population of autopsied infants from birth through the second postnatal year. Myelination was assessed in 62 precisely defined central nervous system (CNS) sites of 162 infants with diverse diseases who were autopsied from 1972 to 1984 at Childrens Hospital, Boston. The degree of myelination was graded on an ordinal scale of 0–4 using the inferior cerebellar peduncle as an internal standard. This grading system is a modification of that used for fetal myelination in the National Collaborative Perinatal Project (NCPP). The data are summarized by 1) median degree of myelination for each age group and site; and 2) Ayer estimates for the age at which at least 10, 50, and 90% of infants reach a particular myelin degree in each site. “Marker” sites in the cerebrum are provided for the pathologist to compare myelination between an individual infant brain and the brains from our autopsy population. These data should be useful in identifying diverse peri- and postnatal conditions affecting myelination in human infancy. They also provide guidelines for the assessment of CNS myelination by sophisticated imaging techniques in living infants.

The Sudden Infant Death Syndrome

Recent years have seen major advances in our understanding of the sudden infant death syndrome (SIDS), including the discovery that the prone sleep position more than triples the risk of SIDS. This finding has led to a worldwide campaign advocating the supine sleep position for infants, which has been associated with a decrease of 50 to 90% in the SIDS rate. This review considers demographic and pathophysiological factors and current concepts about SIDS.

Decreased muscarinic receptor binding in the arcuate nucleus in Sudden Infant Death Syndrome

Muscarinic cholinergic activity in the human arcuate nucleus at the ventral medullary surface is postulated to be involved in cardiopulmonary control. A significant decrease in [3H]quinuclidinyl benzilate binding to muscarinic receptors in the arcuate nucleus is now shown to occur in sudden infant death syndrome (SIDS) infants, compared to infants dying acutely of known causes. In infants with chronic oxygenation abnormalities, binding is low in other nuclei, as well as in the arcuate nucleus. The binding deficit in the arcuate nucleus of SIDS infants might contribute to a failure of responses to cardiopulmonary challenges during sleep.

The Brainstem and Serotonin in the Sudden Infant Death Syndrome

The sudden infant death syndrome (SIDS) is the sudden death of an infant under one year of age that is typically associated with sleep and that remains unexplained after a complete autopsy and death scene investigation. A leading hypothesis about its pathogenesis is that many cases result from defects in brainstem-mediated protective responses to homeostatic stressors occurring during sleep in a critical developmental period. Here we review the evidence for the brainstem hypothesis in SIDS with a focus upon abnormalities related to the neurotransmitter serotonin in the medulla oblongata, as these are the most robust pathologic findings to date. In this context, we synthesize the human autopsy data with genetic, whole-animal, and cellular data concerning the function and development of the medullary serotonergic system. These emerging data suggest an important underlying mechanism in SIDS that may help lead to identification of infants at risk and specific interventions to prevent death.

Brainstem serotonergic deficiency in sudden infant death syndrome.

CONTEXT Sudden infant death syndrome (SIDS) is postulated to result from abnormalities in brainstem control of autonomic function and breathing during a critical developmental period. Abnormalities of serotonin (5-hydroxytryptamine [5-HT]) receptor binding in regions of the medulla oblongata involved in this control have been reported in infants dying from SIDS. OBJECTIVE To test the hypothesis that 5-HT receptor abnormalities in infants dying from SIDS are associated with decreased tissue levels of 5-HT, its key biosynthetic enzyme (tryptophan hydroxylase [TPH2]), or both. DESIGN, SETTING, AND PARTICIPANTS Autopsy study conducted to analyze levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA); levels of TPH2; and 5-HT(1A) receptor binding. The data set was accrued between 2004 and 2008 and consisted of 41 infants dying from SIDS (cases), 7 infants with acute death from known causes (controls), and 5 hospitalized infants with chronic hypoxia-ischemia. MAIN OUTCOME MEASURES Serotonin and metabolite tissue levels in the raphé obscurus and paragigantocellularis lateralis (PGCL); TPH2 levels in the raphé obscurus; and 5-HT(1A) binding density in 5 medullary nuclei that contain 5-HT neurons and 5 medullary nuclei that receive 5-HT projections. RESULTS Serotonin levels were 26% lower in SIDS cases (n = 35) compared with age-adjusted controls (n = 5) in the raphé obscurus (55.4 [95% confidence interval {CI}, 47.2-63.6] vs 75.5 [95% CI, 54.2-96.8] pmol/mg protein, P = .05) and the PGCL (31.4 [95% CI, 23.7-39.0] vs 40.0 [95% CI, 20.1-60.0] pmol/mg protein, P = .04). There was no evidence of excessive 5-HT degradation assessed by 5-HIAA levels, 5-HIAA:5-HT ratio, or both. In the raphé obscurus, TPH2 levels were 22% lower in the SIDS cases (n = 34) compared with controls (n = 5) (151.2% of standard [95% CI, 137.5%-165.0%] vs 193.9% [95% CI, 158.6%-229.2%], P = .03). 5-HT(1A) receptor binding was 29% to 55% lower in 3 medullary nuclei that receive 5-HT projections. In 4 nuclei, 3 of which contain 5-HT neurons, there was a decrease with age in 5-HT(1A) receptor binding in the SIDS cases but no change in the controls (age x diagnosis interaction). The profile of 5-HT and TPH2 abnormalities differed significantly between the SIDS and hospitalized groups (5-HT in the raphé obscurus: 55.4 [95% CI, 47.2-63.6] vs 85.6 [95% CI, 61.8-109.4] pmol/mg protein, P = .02; 5-HT in the PGCL: 31.4 [95% CI, 23.7-39.0] vs 71.1 [95% CI, 49.0-93.2] pmol/mg protein, P = .002; TPH2 in the raphé obscurus: 151.2% [95% CI, 137.5%-165.0%] vs 102.6% [95% CI, 58.7%-146.4%], P = .04). CONCLUSION Compared with controls, SIDS was associated with lower 5-HT and TPH2 levels, consistent with a disorder of medullary 5-HT deficiency.

The developing oligodendrocyte: key cellular target in brain injury in the premature infant

Brain injury in the premature infant, a problem of enormous importance, is associated with a high risk of neurodevelopmental disability. The major type of injury involves cerebral white matter and the principal cellular target is the developing oligodendrocyte. The specific phase of the oligodendroglial lineage affected has been defined from study of both human brain and experimental models. This premyelinating cell (pre‐OL) is vulnerable because of a series of maturation‐dependent events. The pathogenesis of pre‐OL injury relates to operation of two upstream mechanisms, hypoxia‐ischemia and systemic infection/inflammation, both of which are common occurrences in premature infants. The focus of this review and of our research over the past 15–20 years has been the cellular and molecular bases for the maturation‐dependent vulnerability of the pre‐OL to the action of the two upstream mechanisms. Three downstream mechanisms have been identified, i.e., microglial activation, excitotoxicity and free radical attack. The work in both experimental models and human brain has identified a remarkable confluence of maturation‐dependent factors that render the pre‐OL so exquisitely vulnerable to these downstream mechanisms. Most importantly, elucidation of these factors has led to delineation of a series of potential therapeutic interventions, which in experimental models show marked protective properties. The critical next step, i.e., clinical trials in the living infant, is now on the horizon.

Myelin Abnormalities without Oligodendrocyte Loss in Periventricular Leukomalacia

The cellular basis of myelin deficits detected by neuroimaging in long‐term survivors of periventricular leukomalacia (PVL) is poorly understood. We tested the hypothesis that oligodendrocyte lineage (OL) cell density is reduced in PVL, thereby contributing to subsequent myelin deficits. Using computer‐based methods, we determined OL cell density in sections from 18 PVL and 18 age‐adjusted control cases, immunostained with the OL‐lineage marker Olig2. Myelination was assessed with myelin basic protein (MBP) immunostaining. We found no significant difference between PVL and control cases in Olig2 cell density in the periventricular or intragyral white matter. We did find, however, a significant increase in Olig2 cell density at the necrotic foci, compared with distant areas. Although no significant difference was found in the degree of MBP immunostaining, we observed qualitative abnormalities of MBP immunostaining in both the diffuse and necrotic components of PVL. Abnormal MBP immunostaining in PVL despite preserved Olig2 cell density may be secondary to arrested OL maturation, damage to OL processes, and/or impaired axonal‐OL signaling. OL migration toward the “core” of injury may occur to replenish OL cell number. This study provides new insight into the cellular basis of the myelin deficits observed in survivors of PVL.

Neuropathological findings in the brain of Karen Ann Quinlan. The role of the thalamus in the persistent vegetative state

BACKGROUND Karen Ann Quinlan had a cardiopulmonary arrest in 1975 and died 10 years later, having never regained consciousness. Her story prompted a national debate about the appropriateness of life-sustaining treatment in patients who are in a persistent vegetative state and led to the development of medicolegal guidelines for the care of such patients. This report describes the neuropathologic features of Quinlans brain. METHODS The entire brain and spinal cord were systematically sampled for histologic examination. The brain stem and central cerebrum were embedded en bloc and serially sectioned. Three-dimensional computer reconstructions helped visualize the topographic features of the lesions. RESULTS Contrary to expectation, the most severe damage was not in the cerebral cortex but in the thalamus, and the brain stem was relatively intact. The neuropathological findings included extensive bilateral thalamic scarring, bilateral cortical scars primarily in the occipital pole and parasagittal parieto-occipital region, and bilateral damage to cerebellar and focal-basal-ganglia regions. The brain stem and basal forebrain and the hypothalamic components of the ascending arousal systems and brainstem regions critical to cardiac and respiratory control were undamaged. The lesions were consistent with hypoxia-ischemia after the cardiopulmonary arrest. CONCLUSIONS Although the neuropathological findings in the case of Karen Ann Quinlan were complex, the disproportionately severe damage in the thalamus as compared with the cerebral cortex supports the hypothesis that the thalamus is critical for cognition and awareness and may be less essential for arousal.

Axonal development in the cerebral white matter of the human fetus and infant

After completion of neuronal migration to form the cerebral cortex, axons undergo rapid elongation to their intra‐ and subcortical targets, from midgestation through infancy. We define axonal development in the human parietal white matter in this critical period. Immunocytochemistry and Western blot analysis were performed on 46 normative cases from 20–183 postconceptional (PC) weeks. Anti‐SMI 312, a pan‐marker of neurofilaments, stained axons as early as 23 weeks. Anti‐SMI 32, a marker for nonphosphorylated neurofilament high molecular weight (NFH), primarily stained neuronal cell bodies (cortical, subcortical, and Cajal‐Retzius). Anti‐SMI 31, which stains phosphorylated NFH, was used as a marker of axonal maturity, and showed relatively low levels of staining (approximately one‐fourth of adult levels) from 24–34 PC weeks. GAP‐43, a marker of axonal growth and elongation, showed high levels of expression in the white matter from 21–64 PC weeks and lower, adult‐like levels beyond 17 postnatal months. The onset of myelination, as seen by myelin basic protein expression, was ∼54 weeks, with progression to “adult‐like” staining by 72–92 PC weeks. This study provides major insight into axonal maturation during a critical period of growth, over an age range not previously examined and one coinciding with the peak period of periventricular leukomalacia (PVL), the major disorder underlying cerebral palsy in premature infants. These data suggest that immature axons are susceptible to damage in PVL and that the timing of axonal maturation must be considered toward establishing its pathology relative to the oligodendrocyte/myelin/axonal unit. J. Comp. Neurol. 484:156–167, 2005.

Oxidative and nitrative injury in periventricular leukomalacia: a review.

Periventricular leukomalacia (PVL) is the major substrate of cerebral palsy in survivors of prematurity. Its pathogenesis is complex and likely involves ischemia/reperfusion in the critically ill premature infant, with impaired regulation of cerebral blood flow, as well as inflammatory mechanisms associated with maternal and/or fetal infection. During the peak period of vulnerability for PVL, developing oligodendrocytes (OLs) predominate in the white matter. We hypothesize that free radical injury to the developing OLs underlies, in part, the pathogenesis of PVL and the hypomyelination seen in long‐term survivors. In human PVL, free radical injury is supported by evidence of oxidative and nitrative stress with markers to lipid peroxidation and nitrotyrosine, respectively. Evidence in normal human cerebral white matter suggests an underlying vulnerability of the premature infant to free radical injury resulting from a developmental mismatch of antioxidant enzymes (AOE) and subsequent imbalance in oxidant metabolism. In vitro studies using rodent OLs suggest that maturational susceptibility to reactive oxygen species is dependent, not only on levels of individual AOE, but also on specific interactions between these enzymes. Rodent in vitro data further suggest 2 mechanisms of nitric oxide damage: one involving the direct effect of nitric oxide on OL mitochondrial integrity and function, and the other involving an activation of microglia and subsequent release of reactive nitrogen species. The latter mechanism, while important in rodent studies, remains to be determined in the pathogenesis of human PVL. These observations together expand our knowledge of the role that free radical injury plays in the pathogenesis of PVL, and may contribute to the eventual development of therapeutic strategies to alleviate the burden of oxidative and nitrative injury in the premature infant at risk for PVL.