Formation of myelinated tracts in central nervous system (CNS) regions such as the optic nerve seems to depend on two glial cell types, both of which derive from a common progenitor cell. This oligodendrocyte--type-2 astrocyte (O-2A) progenitor cell gives rise to oligodendrocytes, which produce internodal myelin sheaths, and to type-2 astrocytes, which extend fine processes in the region of the nodal axolemma. The optic nerve also contains a third glial cell, the type-1 astrocyte, which derives from a separate precursor. These three glial cells develop in a fixed sequence over a two-week period: type-1 astrocytes appear at embryonic day 16 (E16), oligodendrocytes at the day of birth (E21 or postnatal day P0), and type-2 astrocytes between P8 and P10. Type-1 astrocytes secrete a potent mitogen which causes expansion of the O-2A progenitor cell population in vitro. Here, we report that dividing O-2A progenitor cells are highly motile and seem to migrate from the brain into the optic nerve, beginning at its chiasmal end. Our results indicate that long-distance migration along the neural axis is characteristic only of progenitors of the O-2A lineage and may serve to distribute these cells to regions of the CNS that will become myelinated. These results also suggest that the intrinsic neuroepithelial cells of the optic stalk may be even more restricted than previously thought, giving rise only to type-1 astrocytes.
1H NMR spectroscopy of human brain in vivo can be used to detect a number of cerebral metabolites including N-acetylaspartate, creatine + phosphocreatine and choline-containing compounds. We have used 1H NMR spectroscopy to analyse these signals in (i) biopsy material from both normal human brain and astrocytomas, and (ii) primary astrocyte cultures. On the basis of this analysis, we conclude that in vivo 1H NMR spectroscopy could play an important clinical role in the non-invasive assessment of neuronal degeneration and proliferation of non-neuronal cells.
1H NMR spectroscopy of brain extracts was used to investigate the metabolic changes that take place during development of the neonatal rat brain. Data were obtained over the range 1-21 days. The concentration of N-acetylaspartate rose by a factor of 9 during this period, the most rapid rise occurring after day 9. The total creatine concentration rose from days 1-21, with a large increase between days 1 and 5. Taurine concentration rose until day 5, then fell from days 5-21. The concentration of choline-containing compounds fell during the 21 day period. The results are discussed in relation to brain development and conventional biochemical data. A major conclusion in relation to spectroscopy of children is that interpretation of changes seen in disease will require adequate data from age-matched controls.
The plasmalemma of mature and growing olfactory axons of the bullfrog has been studied by freeze-fracture . Intramembrane particles (IMPs) of mature olfactory axons are found to be uniformly distributed along the shaft . However, during growth, a decreasing gradient of IMP density is evident along the somatofugal axis. The size histograms of axolemmal IMPs from different segments of growing nerve reveal regional differences in the particle composition . The distribution of each individual size class of particles along the growing nerve forms a decreasing gradient in the somatofugal direction ; the slope of these gradients varies directly with particle diameter. These size-dependent density gradients are consistent with a process of lateral diffusion of membrane components that are inserted proximally into the plasma membrane. The membrane composition of the growth cone, however, appears to be independent of these diffusion gradients ; it displays a mosaic pattern of discrete domains of high and low particle densities . The relative IMP profiles of these growth cone regions are similar to one another but contain higher densities of large IMPs than the neighboring axonal shaft. The shifting distributions of intramembrane particles that characterize the sprouting neuron give new insights into cellular processes that may underlie the establishment of the functional polarity of the neuron and into the dynamics of axolemmal maturation .
Intramembrane particles (IMPS) of the plasmalemma of mature, synapsing neurons are evenly distributed along the axon shaft. In contrast, IMPs of growing olfactory axons form density gradients : IMP density decreases with increasing distance from the perikarya, with a slope that depends upon IMP size , /. Cell Biol., 98: 1422-1433 . These IMP density gradients resemble Gaussian tails, but they are much more accurately described by the equations formulated for diffusion in a system with a moving boundary (a Stefan Problem), using constants that are dependent upon IMP size. The resulting model predicts a shallow, nearly linear IMP density profile at early stages of growth. Later, this profile becomes gradually transformed into a steep nonlinear gradient as axon elongation proceeds. This prediction is borne out by the experimental evidence. The diffusion coefficients calculated from this model range from 0.5 to 1 .8 x 10-7 Cm2/s for IMPs between 14.8 and 3 .6 nm, respectively . These diffusion coefficients are linearly dependent upon the inverse IMP diameter in accordance with the Stokes-Einstein relationship. The measured viscosity is approximately 7 centipoise . Our findings indicate (a) that most IMPS in growing axons reach distal locations by lateral diffusion in the plasma membrane, (b) that IMPS-or complexes of integral membrane proteins-can diffuse at considerably higher rates than previously reported for iso-concentration systems, and (c) that the laws of diffusion determined for macroscopic systems are applicable to the submicroscopic membrane system .The extension of an axon by the neuron involves rapid, vectorial expansion of the plasma membrane. This is largely achieved by the addition of packets of plasmalemmal precursor in the form of vesicles at the distal tip of the advancing axon (e.g., 29, 31). This precursor membrane contains few intramembrane particles (IMPs; 46), which are believed to be complexes of integral membrane proteins (e.g., 5, 25, 45). In the growing axon's plasma membrane, the profile of components as seen by freeze-fracture changes as a function of distance from the perikaryon (see our companion paper, reference 46). A similar distribution is also seen for saxitoxin binding sites, putative Na+-channels (see our companion paper, reference 49). These observations suggest that IMPs-or complexes of integral membrane proteins-are inserted into the plasmalemma proximally, at the perikaryon, and diffuse laterally within the plane of the membrane to reach the distal neuritic shaft. In this paper, we describe an analysis of the IMP density data in terms of diffusion in a system that expands with time. The resulting model matches the measured values with a high degree ofaccuracy and, therefore, strongly supports the diffusion concept. Thus, the investigation of IMP distribution in the growing axon gives insight into diffusion processes in the plasma membrane over extended time periods and long distances and, in particular, in a chemical gradient, a nonequilibrium system. Thus, the gr...
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