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.

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 approximately 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.

Immature oligodendrocytes (OLs) derive from a large pool of late OL progenitors that populate human cerebral white matter throughout the latter half of gestation. We recently reported that a minor population of immature OLs are present in human cerebral white matter for at least 3 months before these cells commit to myelinogenesis around 30 wk postconceptional age. Since this finding supports dissociation between the events that regulate human immature OL maturation and their commitment to myelinogenesis, we characterized here the cellular sequence of events that characterize immature OLs during the transition from a premyelinating to a myelinating state. Commitment of immature OLs to myelinogenesis in human cerebral white matter correlated with the longitudinal extension of specialized processes, designated "pioneer processes," that made multiple types of apparent contacts with axons. This event coincided with the appearance of 3 distinct populations of sheaths that varied in their labeling for myelin basic protein (MBP). Since few axons initially labeled for MBP, this supported the occurrence in vivo of O4-negative, O1-positive premyelin sheaths that precede MBP-positive compacted myelin. These observations identify 3 sequential stages of early myelinogenesis: 1) the initial ensheathment of axons by premyelin sheaths generated by immature OLs; 2) the initial insertion of MBP into transitional sheaths; and 3) the generation of MBP-rich mature myelin.