A comparative study of prenatal development in the olfactory bulb, neocortex and hippocampal region of the precocial mouse Acomys cahirinus and rat

Unlike the remainder of the rodent subfamily Muridae, Acomys cahirinus (the 'spiny' mouse) is born in a precocial state after a long (39 day) gestation. In this paper, the development of the olfactory bulb, neocortex and hippocampal formation of Acomys from prenatal days 14-34 was examined and the rate of maturation compared with that of its cousin, the laboratory rat (Rattus norvegicus). At the earliest stages examined, Acomys was approximately 2 days less mature than the same post-conception aged rat. The difference between the two species increased: Acomys at 28 days postconception resembled the 22-day rat. By the end of gestation, Acomys and the rat were in a relatively similar developmental state. Therefore, Acomys exhibits a quite different timetable of early maturation which includes a protracted period of relatively slow growth during mid-gestation. As such, it offers many benefits as a subject for studies of both early ontogenesis and the mechanisms which result in species differences.     

A comparative study of mossy fiber distribution in the brain of the precocial Acomys cahirinus and of the altricial Rattus norvegicus: neuroanatomical bases and behavioral correlates

Acomys cahirinus, the spiny mouse, is the only precocial murid. Given the relatively advanced state of neural maturation at birth, this species is therefore of interest for comparative studies of brain and behavioural development. Previous work on this species has indicated that (i) hippocampal thickness (relative to body weight) is greater in adult Acomys than in rats and mice; (ii) the hippocampus appears laminated at birth, unlike that of related altricial species; (iii) the degree of olfactory bulb maturation at birth is greater in Acomys than in gerbils or rats. In the study reported, the distribution of hippocampal mossy fibers of Acomys was compared with that of Rattus norvegicus, in order to extend our earlier findings. In addition, the implied involvement of hippocampal mossy fibers in olfaction suggests a relation to our studies of exploratory behaviour in this species.     

Hippocampal maturation in the precocial murid rodent Acomys cahirinus

Growth rates in the dentate gyrus and hippocampus proper of the precocial murid Acomys cahirinus were examined by measuring age-related changes in laminar volume in 0-, 10-, 20-, and 60-day postpartum animals. A biphasic course of maturation was observed in both structures, with rapid expansion occurring in the first 10 postnatal days and more gradual growth thereafter. This pattern of growth resembles those reported for newborn altricial murids such as the laboratory mouse and rat. However, due to the large differences in the length of gestation between Acomys and the rat and mouse, quite different patterns emerge when the species are compared on the basis of postconception age, indicating that rates of neural maturation are not the same in altricial and precocial murids .     

Development of myelination in the human fetal and infant cerebrum: a myelin basic protein immunohistochemical study.

The early development of myelination was studied by means of myelin basic protein (MBP) and luxol fast blue (LFB) stainings of large sections of the cerebral hemispheres. Myelination first occurs in the globus pallidus, pallidothalamic fibers of the posterior internal capsule and the thalamus at 25 weeks, which may be related to the cellular maturation in the globus pallidus and thalamus. Then myelination is observed in the striatum, and precentral and postcentral gyri at 35 weeks, and the anterior internal capsule and optic radiation at 37 weeks. Immunoreactivity with MBP is observed earlier and more strongly in the early myelination period than that with LFB. Thus, MBP may play an important role in myelination and its delay. The macroscopic positivity as to MBP as well as LFB staining may be related to the development of high signal intensity observed in a T1-weighted magnetic resonance imaging, which was observed 1 to 3 months after the first microscopic appearance of myelin.     

A correlative light and electron microscopic study of postnatal myelination in the murine corpus callosum

Oligodendroglial cells differ in their ultrastructural appearance depending on their myelin producing and maintaining activity. To better understand the relationship between light and electron microscopic features of myelination, myelin formation in the corpus callosum was studied in young postnatal mice. Immunostaining for myelin basic protein (MBP), which has an important role in myelin compaction, was compared with conventional Luxol Fast Blue myelin staining and with electron microscopic images of unlabeled tissue. MBP-immunostaining labeled a few oligodendroglial cells at postnatal day (P)3, and a few axons at P7 in the corpus callosum, below the fronto-parietal somatosensory cortex. By P10 there were more myelinated axons below the somatosensory cortex and the first MBP-immunoreaction appeared in the cingulum: labeling appeared even later in the remaining areas of corpus callosum. Electron microscopy revealed numerous medium oligodendroglial cells at P7 in the corpus callosum, below the somatosensory cortex with the first sign of myelination at P10. By P14, there were numerous myelin sheaths with loosely built structure, and the number of myelin sheaths increased continuously thereafter. However, even as late as P28, the presence of both thick, compact and thin, loosely structured myelin sheaths in the same section suggested ongoing myelination. With Luxol Fast Blue myelin staining was first observed in the corpus callosum relatively late, at P14. Areal differences in myelination of the corpus callosum, seen with MBP-immunohistochemistry, indicate that myelin formation follows cortical maturation rather than the rostro-caudal developmental growth of the corpus callosum. Myelination of the afferent and efferent fibers within the cortical areas seems to follow the inside-out maturational pattern of cortical neurons, with the first myelinated axons always appearing in layers V-VI. In addition to the known neuronal and astroglial factors that regulate myelin formation by oligodendroglial cells, we suggest that these cells and their myelin covering may also influence axonal maturation. Light microscopic data obtained with MBP-immunohistochemistry correlates well with electron microscopic observations but not with Luxol Fast Blue staining which reveals myelinated axons only relatively late in development. Therefore, both MBP-immunostaining and electron microscopy are useful, alone or in combination, for the detection of myelination, demyelination as well as remyelination processes in animal models and also in humans.     

Sequence of central nervous system myelination in human infancy. II. Patterns of myelination in autopsied infants

The timing and synchronization of postnatal myelination in the human central nervous system (CNS) are complex. We found eight time-related patterns of CNS myelination during the first two postnatal years in autopsied infants. The intensity of myelination was graded in 162 infants with diverse diseases on an ordinal scale of degrees 0-4. The Ayer method for maximum likelihood estimates for censored data was utilized to generate curves of the temporal changes in the percent of infants with degrees 0 through 4 of myelin in 62 white matter sites. These sites fall into eight subgroups determined by the presence or absence of microscopic myelin (degree 1) at birth and the median age at which mature myelin (degree 3) is reached. There is variability in the timing of myelination within and across axonal systems, and early onset of myelination is not always followed by early myelin maturation. We reexamined general rules governing the timing of myelination proposed by previous investigators, and found that those rules are neither complete nor inviolate, and that there is a complex interplay among them. This study specifies distinct periods of maturation in which myelinating pathways are potentially vulnerable to insult during the first two postnatal years.     

The development of the transferrin-transferrin receptor system in relation to astrocytes, MBP and galactocerebroside in normal and myelin-deficient rat optic nerves

The factor(s) which control the onset of myelination are unknown. It is now accepted that transferrin (Tf), the major iron transport protein in vertebrates, is found in oligodendrocytes in the adult brain. Because of the importance of iron in basic cell metabolism we have hypothesized that iron (mobilized by Tf) may be a permissive agent in the process of myelination. The present study was designed to determine with immunohistochemistry the relationship of Tf receptor expression, Tf accumulation, and the expression of myelin components myelin basic protein (MBP) and galactocerebroside (GAlC)) in the developing rat optic nerve. In addition to Tf and its receptor, the developmental pattern for GalC reported in this study has not been examined in the rat optic nerve. Furthermore, a myelin mutant strain of rats was used to determine if a lack of myelin production affects the Tf-Tf receptor system. Our study found that Tf receptor was expressed from birth on blood vessels and was first seen in the parenchyma of the nerve at 8 days of age. The expression of the Tf receptor preceded that of Tf, MBP or GalC. The accumulation of Tf by oligodendrocytes occurred about the same time as the intracellular appearance of MBP and GalC which was shortly after the onset of myelination. Tf-positive cells as well as MBP- and GalC-positive cells increased in number and staining intensity with age whereas the expression of the Tf receptor declined after reaching a peak at 15 days of age. In the optic nerves of myelin-deficient rats, the Tf receptor expression and Tf accumulation was confined to the vasculature. The results of this study suggest that the expression of the Tf receptor is an early event in oligodendrocytic maturation and is followed by the intracellular accumulation of myelin components and Tf. The temporal association of Tf and myelin production suggests that further study is warranted regarding the possibility that the Tf-iron system supports or perhaps even permits the initiation of the process of myelination.     

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 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.     

Delayed central nervous system myelination in the sudden infant death syndrome

This study was designed to assess whether development of the central nervous system (CNS) is delayed in victims of the sudden infant death syndrome (SIDS). We selected the parameter of myelination because it is a continuously changing and readily accessible marker of CNS development in the SIDS age-range. We assessed myelination blindly in 61 SIDS and 89 autopsy controls. In 62 sites the degree of myelination was visually graded in myelin-stained histological sections on an ordinal scale of 0-4 using the inferior cerebellar peduncle as an internal standard of degree 3. Cases were stratified by postconceptional age at death and SIDS and controls were compared with respect to myelin degree at each site. Significantly delayed myelination (p less than 0.05) occurred in the SIDS group in 25 of the 62 sites examined. Hypomyelination affected fiber systems in which myelination is initiated before or after birth and which myelinate with different tempos and preferentially affect pyramidal and cerebellar (somatomotor) and prefrontal-temporal-limbic (visceromotor) systems. Hypomyelination was not associated with individual clinicopathologic variables in the SIDS group. Somatic growth and brain weight were significantly greater in SIDS than controls. Therefore, we suggest that SIDS is associated with a developmental CNS disorder. Although delayed CNS myelination most likely shares a common antecedent with sudden death and is not its cause, the role of somato- and viscero-motor systems in central cardiorespiratory control and arousal warrants further analysis in SIDS.     

Wallerian degeneration in the optic nerve of the wabbler-lethal (wl/wl) mouse.

Optic nerve pathology was studied in C57BL/6J wabbler-lethal (wl/wl) and control (+/+) mice at postnatal age of 4 weeks (P28). Qualitative light and ultrastructural pathology in wl/wl animals conformed to the criteria of primary axonal (Wallerian) degeneration. Most optic nerve axons in mutant animals appeared normal, as did oligodendroglia, the degree of myelination, the integrity and maturity of vascular elements, astroglia, and most myelin. Still, degenerating axons surrounded by somewhat normal myelin and axons with thickened myelin sheaths were prevalent in wl/wl mice. Dysmyelination or hypomyelination was not evident. At P28, pathology appeared more prominent in large diameter fibers. In the optic nerve of wl/wl mice, axonal degeneration preceded myelin disruption, adding this nerve to other previously reported systems undergoing Wallerian degeneration in this mutant.     

Does very preterm birth impair myelination of the central nervous system?

Myelination of the central nervous system (CNS) can be demonstrated with magnetic resonance (MR) imaging. Myelin formation may be reduced in conditions of neonatal "undernutrition". Very preterm infants have a reduced postnatal growth rate when compared with intrauterine fetuses of the same gestational age. Using MR imaging, we studied qualitative myelination patterns in healthy preterm infants of less than 30 weeks gestation with an optimal nutritional intake and in term infants at 44 weeks postmenstrual age (PMA). At that age, preterm infants had a significantly lower mean body weight than term infants, but mean head circumference did not differ significantly. All preterm and term infants had reached myelination stage M3 (myelin in brainstem, internal capsule, and corona radiata) and M4 (myelin in brainstem, internal capsule, corona radiata, and centrum semiovale). There was no significant difference in myelination stage between the preterm and term infants. We conclude that adequate nutrition in the neonatal period leads to qualitatively adequate myelination of the CNS in very preterm infants.     

Inhibition of Schwann cell myelination in vitro by antibody to the L1 adhesion molecule

The specific axonal and Schwann cell surface molecules that mediate the initiation of myelination have not been identified. We have used cocultures of purified rat dorsal root ganglion neurons and Schwann cells and purified polyclonal antibodies to the L1 adhesion molecule to study the role of L1 in myelin formation. Schwann cells were first arrested in a basal-lamina-free premyelination stage (by serum/ascorbate deprivation), then manipulated to allow basal lamina deposition and myelination (by serum/ascorbate addition) in the absence or presence of anti-L1. Using electron microscopy, immunocytochemistry, and myelin sheath quantitation after Sudan-black staining, we determined the effect of anti-L1 on (1) basal lamina formation, (2) the segregation by Schwann cells of axons into a 1:1 relationship, (3) galactocerebroside (Gal-C) expression, (4) laminin deposition, and (5) myelin formation. Anti-L1 strongly blocked myelin formation, Gal-C expression, and axon segregation but did not block basal lamina formation. In controls, elongated Schwann cell processes completely covered the axons and exhibited uniform surface staining for laminin; in anti-L1-treated cultures, shortened Schwann cells, intensely stained for laminin, were observed in clusters separated by unstained lengths of axons. When 50 micrograms/ml exogenous purified laminin was added to the medium, Schwann cell length and laminin staining were similar in control and treated cultures. However, the inhibition of myelination of anti-L1 was not altered by the addition of laminin. Myelination was also inhibited with antigen-binding fragments (Fab) of polyclonal anti-L1, but an antibody to liver membranes did not block myelination. These results indicate that L1 is involved in the linear extension of Schwann cell processes along axons, the engulfment of axons, and the induction of myelin-specific components within the Schwann cell. We conclude that anti-L1 prevents myelination by blocking these events rather than by blocking basal lamina deposition.     

E-PTA stains oligodendroglial surface membranes and microtubules in optic nerves during myelination.

Aldehyde fixed Xenopus tadpole and frog optic nerves were stained en bloc with ethanolic phosphotungstic acid (E-PTA). During rapid myelination, intense staining was observed on cytoplasmic faces of paranodal terminal loops and loosely wrapped oligodendroglial membranes found along inner and outer surfaces of compact myelin sheaths. Oligodendroglial microtubules also were heavily stained. Where stained cytoplasmic faces fused to form a lamella of compact myelin, the intense staining was reduced to a thinner, fainter line. In optic nerves of adult frogs, the staining was less dense but the pattern was similar. The staining distribution and available histochemical evidence indicate that E-PTA stains positively charged proteins non specifically. Since myelin basic protein is found in oligodendroglia during myelination, we suggest that it is being stained by E-PTA while being transported along microtubules to sites where it is inserted into developing myelin lamellae.     

Myelination of the rat retina by transplantation of oligodendrocytes into 4-day-old hosts

Oligodendrocyte transplantation into the retina enables us to investigate the early events in myelin formation in a new in vivo system. The axons of rat retinal ganglion cells are unmyelinated in the eye but should express a myelination initiation signal since they acquire myelin posterior to the globe. The lamina cribrosa may block the migration of oligodendrocytes from the optic nerve into the retina. Animals that lack a lamina cribosa such as the rabbit have myelinated retinas. We have bypassed the lamina cribrosa by using transplantation techniques and inserted freshly isolated syngeneic 3-week-old rat oligodendrocytes into the unmyelinated 4-day-old rat retina during the period of active optic nerve myelination. The animals are sacrificed at 1-week intervals for 8 weeks. The retinas are examined immunocytochemically for myelin with an antibody to myelin basic protein (MBP). MBP-positive cells are seen extending processes at 1 and 2 weeks. Three and four week retinas show the formation of thicker and longer myelin sheaths oriented along the same radial path as the retinal ganglion axons with maximal MBP staining intensity seen by 5 weeks. Transplanted retinas are negative when stained for P0, a Schwann cell antigen, ruling out Schwann cell myelination of our retinas. We have shown that rat cerebral oligodendrocytes survive, mature, and express a myelin-specific protein in the retinal environment in a pattern consistent with myelination of ganglion cell axons. Retinal transplantation provides a new in vivo model to study oligodendrocyte development and axonal-glial interactions, free from the difficulties inherent in culture systems.     

Ultrastructural evidence of a peripheral nervous system pattern of myelination in the avascular retina of the guinea pig

Summary Myelination of axons in the nerve fiber layer (NFL) of the retina occurs as a sporadic abnormality in several mammalian species including man, monkey, cat and rat. All of these species have vascularized retinae and, in the latter three, ultrastructural studies have demonstrated the pattern of medullation to be similar to Schwann cell myelination in the peripheral nervous system. This contrasts with an oligodendrocytic pattern of myelination normally present in the avascular retina of the rabbit. One possible explanation for this difference is that the pattern of myelination is related to the presence or absence of retinal blood vessels. The present investigation provides the first evidence of NFL myelination in another avascular retina, that of the guinea pig. Myelination in the guinea pig retina was observed in a single bundle of axons and involved only large diameter fibers. With several axons, myelin sheaths terminated at hemi-nodes of Ranvier and in all such cases this occurred in association with marked paranodal infolding. Morphological characteristics of the myelination include (1) a one to one relationship between axon and myelinating cell, (2) cytoplasm between myelin sheath and plasma membrane, (3) basal lamina surrounding the myelinating cell, (4) collagen fibers in the adjacent extracellular space and (5) double intraperiod lines. These morphological features are characteristic of peripheral nerve myelination by Schwann cells. Thus, in all species so far described in which retinal medullation is abnormally present, the pattern of myelination has been Schwann cell in nature rather than oligodendrocytic. The reasons for this Schwann cell predominance remain undefined.Supported by the Medical Research Council of Canada     

Myelination of the developing lateral olfactory tract and anterior commissure

Both the lateral olfactory tract (LOT) and anterior limb of the anterior commissure (AC) carry olfactory information. The LOT forms the projection from the olfactory bulb to the ipsilateral olfactory cortices, while the AC carries odor information across the midline to the contralateral olfactory cortex and bulb. The LOT and AC differ on a number of dimensions, including early development and functional onset. The present work, examining their myelination in mice, reveals additional important differences. For example, the LOT initiates myelination 3–4 days earlier than the AC, evidenced by both an earlier increase in myelin basic protein staining seen with immunohistochemistry and an earlier appearance of myelinated fibers using electron microscopy. While both exhibit a period of rapid myelination, it occurs 4–5 days earlier in the LOT than the AC. The tracts also respond differently to early sensory restriction. Unilateral naris occlusion from the day after birth to postnatal day 30 had no consistent effects on the AC but resulted in significantly thinner myelin sheaths relative to axon caliber in the LOT. Finally, the two tracts differ structurally (the LOT contains larger, more densely packed axons with significantly thicker myelin sheaths resulting in a conduction velocity that is more than twice as fast as the AC). The findings indicate that these two large, accessible tracts provide an important means for studying brain maturation due to basic differences in both the timing of their maturation and general organization.     

The Twitcher mouse: Myelinogenesis in organotypic culture

Myelinogenesis was followed in organotypic cultures of the spinal cord of the neurological mutant mouse, the Twitcher. As a clinically, pathologically and biochemically equivalent model of Krabbe disease this mutant is an important tool for investigating the nervous system. Normal initiation and development of myelination was observed. At 35 days in vitro (DIV) the Twitcher cultures exhibited blisters attached to the intact myelin sheath and bubbling of myelin suggestive of myelin breakdown. Myelin degeneration progressed thereafter. The Twitcher spinal cord survived in culture for more than two months, a period much longer than the life span of affected mice. In order to correlate pathological and biochemical changes, the activity of UDP-galactose:ceramide galactosyltransferase was quantitated in normal and Twitcher cultures. In both the Twitcher and the control groups the galactosyltransferase activity rapidly increased up to 20-25 DIV and then declined. The galactosyltransferase activity of the Twitcher tended to be lower than the controls even during the early myelination period. At 35 DIV the activity in the Twitcher was definitely lower than the controls, and at 52 DIV it was nearly negligible. The galactosyltransferase activity therefore correlated well with the morphologically observed early normal myelination and subsequent myelin degeneration.     

Myelination of cortical-hippocampal relays during late adolescence

The normal developmental series of brains in the Yakovlev Collection has been examined to explore the possibility that various brain regions implicated in schizophrenia may show changes in myelination during late adolescence, a period coinciding with the appearance of early symptoms of this disorder. The prefrontal, cingulate, and parahippocampal (entorhinal) cortex, as well as the perforant pathway, cingulum bundle, and hippocampus, were closely examined because these regions have recently been found to show various neuropathological differences in schizophrenia. Observation of these specimens has confirmed earlier reports by Yakovlev and Lecours (1967) that primary motor and sensory cortices show robust myelination early in the first decade of life. In contrast, associative cortical areas show increased amounts of myelin staining only by the second decade, although some cortical areas, like the cingulate and basofrontal cortex, remain poorly myelinated throughout life. The most striking finding, however, was the appearance of increased myelination of the subicular and presubicular regions during the late adolescent period. Increased myelination in the subiculum was localized to a discrete region at the surface where fibers of the perforant pathway are known to aggregate as they course toward the area dentata. The comparable region in the adjacent presubicular area that also showed increased myelin staining probably contains distal portions of the cingulum bundle. Support for this latter possibility was obtained from a single case in which a stereotaxically placed lesion causing interruption of the cingulum bundle showed less myelin in the presubicular area of the effectively lesioned side.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glial and Neuronal Protein Tyrosine Phosphatase Alpha (PTPα) Regulate Oligodendrocyte Differentiation and Myelination

CNS myelination defects occur in mice deficient in receptor-like protein tyrosine phosphatase alpha (PTPα). Here, we investigated the role of PTPα in oligodendrocyte differentiation and myelination using cells and tissues from wild-type (WT) and PTPα knockout (KO) mice. PTPα promoted the timely differentiation of neural stem cell-derived oligodendrocyte progenitor cells (OPCs). Compared to WT OPCs, KO OPC cultures had more NG2+ progenitors, fewer myelin basic protein (MBP)+ oligodendrocytes, and reduced morphological complexity. In longer co-cultures with WT neurons, more KO than WT OPCs remained NG2+ and while equivalent MBP+ populations of WT and KO cells formed, the reduced area occupied by the MBP+ KO cells suggested that their morphological maturation was impeded. These defects were associated with reduced myelin formation in KO OPC/WT neuron co-cultures. Myelin formation was also impaired when WT OPCs were co-cultured with KO neurons, revealing a novel role for neuronal PTPα in myelination. Canonical Wnt/β-catenin signaling is an important regulator of OPC differentiation and myelination. Wnt signaling activity was not dysregulated in OPCs lacking PTPα, but suppression of Wnt signaling by the small molecule XAV939 remediated defects in KO oligodendrocyte differentiation and enhanced myelin formation by KO oligodendrocytes. However, the myelin segments that formed were significantly shorter than those produced by WT oligodendrocytes, raising the possibility of a role for glial PTPα in myelin extension distinct from its pro-differentiating actions. Altogether, this study reveals PTPα as a molecular coordinator of oligodendroglial and neuronal signals that controls multiple aspects of oligodendrocyte development and myelination.