Research Description

Highlights of Major Accomplishments

• Demonstration of a deficiency in serotonergic binding sites in SIDS victims in brainstem areas (e.g., caudal raphé and arcuate nucleus) critically involved in control of respiratory and cardiac function,

1. Brainstem research in the sudden infant death syndrome

2. Oligodendrocyte maturation in the developing human brain

1. Brainstem research in the sudden infant death syndrome

We originally proposed that SIDS, or a subset of SIDS, results from a failure of a component of the ventral medulla (i.e., the human arcuate nucleus) in mediating homoeostatic responses to life-threatening challenges (e.g., asphyxia, hypoxia, hypercapnia) during sleep, as the infant passes through a vulnerable developmental period. The ventral medulla is composed of neurons and glia along the ventral and ventrolateral rim of the medulla that are involved in chemoreception, respiratory drive, and blood pressure responses. DiI labeling studies by us in human midgestational fetuses demonstrated connections between the arcuate nucleus and the caudal raphé (nucleus raphé obscurus). Neurotransmitter receptor binding studies involved extensive, hypothesis-driven analyses of the human brainstem across early development and between SIDS and age-matched controls in a database collected between 1985 and 1997. These studies, involving 19 brainstem regions and 6 neurotransmitter systems, demonstrated isolated decreased muscarinic and kainate receptor binding in the arcuate nucleus.

Perhaps most significantly, these studies demonstrated also serotonergic binding deficiencies in SIDS victims in the caudal raphé and in five other functionally and developmentally related components of ventral medulla (including the arcuate nucleus), all regions critically involved in chemoreception, respiratory drive, blood pressure responses, upper airway reflexes, and/or thermoregulation. Four of the six affected regions, including the caudal raphé and arcuate nucleus, are considered derivatives of a common embryonic anlage, the rhombic lip, and five of the six regions contain serotonergic neurons in the developing human brainstem. Taken together, these findings suggest a primary defect in serotonergic precursors in the rhombic lip in at least a subset of SIDS victims, resulting in a failure of cell division, migration, and/or differentiation. These studies have led to an expanded hypothesis concerning the role of the developing ventral medulla in SIDS: SIDS, or a subset of SIDS, is due to a developmental abnormality in a ventral network composed of rhombic-lip derived, serotonergic neurons, and that this abnormality results in a failure of protective responses to life-threatening challenges (e.g., asphyxia, hypoxia, hypercapnia) during sleep.

2. Oligodendrocyte maturation in the developing human brain

Human myelinogenesis

In terms of the cellular sequences of human myelinogenesis, we developed novel immunocytochemical methods that permitted three successive stages in the human OL lineage to be uniquely distinguished relative to axons. Axonal enstheathment was preceded by the longitudinal extension of specialized OL processes, designated "pioneer processes", that initiated contract with axons. Pioneer processes were only associated with O4 and O1 antibody-labeled immature OLs. Unlike rodents, labeling with O1 was not sufficient to commit to an early myelinating phenotype. A quiescent population of immature OLs was detected at least 3 months before the onset of myelination. In fact, individual fiber tracts initially were not myelinated en bloc, but rather myelination was restricted to a small subset of axons. Unexpectedly, there was not an invariant one-to-one relationship between OL processes and axonal enstheathment, and individual OL processes often divided to wrap more than one axon segment. Since the minority of the ensheathed axons initially labeled for myelin basic protein (MBP), this supported the concept of an O4+O1+ premyelin sheath that preceded the incorporation of MBP for myelin compaction. These observations support three sequential phases of fetal myelinogenesis: 1) the initial enstheathment of axons by premyelin tubules generated by immature OLs; 2) the insertion of MBP into transitional early myelin; and 3) the generation of mature, fully compacted myelin.

Human oligodendroglial development

In terms of human OL lineage progression, we developed methods to co-visualize anti-NG2, O4, O1, and anti-MBP antibodies in specific combination that identified three successive OL stages in the parietal white matter of 26 autopsy human brains between 18 and 41 postconceptional weeks. Between 18-27 weeks four distinct morphological subtypes of NG2+O4+ late OL progenitors (preOLs) were distinguished in the white matter and cerebral cortex. PreOLs were the major OL stage present. No NG2+O4- early OL progenitors were detected. Only 9.9+1.5% of total OLs (n=9) were O1+ immature OLs. Approximately 30 weeks marked the onset of a marked expansion in the immature OL population. Between 30-41 weeks there was a three-fold increase in O1+ OLs that comprised 31.6+2.6% of total OLs (n=9). PreOLs still comprised the major OL population in both the white matter and cerebral cortex. The approach of term gestation was accompanied by a progressive increase in MBP+ myelin tubules that were first detected in the deep white matter. Human OL lineage progression in parietal white matter follows a defined developmental time-table, in which a commitment to myelination begins prior to birth.