https://www.cell.com/cell/pdf/S0092-8674(00)80783-7.pdf

Subventricular Zone Astrocytes Are Neural Stem Cells in the Adult Mammalian Brain

Microglia sculpt postnatal neural circuits in an activity and complement-dependent manner.

During the development of the mammalian brain, neuronal precursors migrate to their final destination from their site of birth in the ventricular and subventricular zones (VZ and SVZ, respectively). SVZ cells in the walls of the lateral ventricle continue to proliferate in the brain of adult mice and can generate neurons in vitro, but their fate in vivo is unknown. Here SVZ cells from adult mice that carry a neuronal-specific transgene were grafted into the brain of adult recipients. In addition, the fate of endogenous SVZ cells was examined by microinjection of tritiated thymidine or a vital dye that labeled a discrete population of SVZ cells. Grafted and endogenous SVZ cells in the lateral ventricle of adult mice migrate long distances and differentiate into neurons in the olfactory bulb.

https://science.sciencemag.org/content/sci/264/5162/1145.full.pdf

Long-distance neuronal migration in the adult mammalian brain

Neuronal plasticity is a central theme of modern neurobiology, from cellular and molecular mechanisms of synapse formation in Drosophila to behavioural recovery from strokes in elderly humans. Although the methods used to measure plastic responses differ, the stimuli required to elicit plasticity are thought to be activity-dependent. In this article, we focus on the neuronal changes that occur in response to complex stimulation by an enriched environment. We emphasize the behavioural and neurobiological consequences of specific elements of enrichment, especially exercise and learning.

Neural consequences of environmental enrichment.

Over the past 5 yr, we have examined some of the sharpest edges of the pathology of aging. We have studied the capacity of aged organisms to respond appropriately to stress and the capacity of stre...

The Neuroendocrinology of Stress and Aging: The Glucocorticoid Cascade Hypothesis

In recent electron microscope studies of the corpus callosum in 60–80 gm male rats, some of the cells were identified as microglia by staining with the weak silver carbonate method of del Rio-Hortega, and others as astrocytes by staining with the gold chloride sublimate method of Ramon y Cajal. In the present work, the cells which did not belong in one of these two groups were examined in the light microscope using semithin sections stained with toluidine blue and in the electron microscope using thin sections stained with uranyl-lead.



These cells make up a large, somewhat heterogeneous group, but they have a few common features: regular nucleus, cytoplasm rich in ribosomes and microtubules, and a variable number of narrow, non-branching fine processes of uniform diameter. Because of these features, the cells are all taken to be oligodendrocytes.



Three classes may be described within this group of cells: (a) light oligodendrocytes, which are large cells with pale nucleus, a large nucleolus, a cytoplasm containing rather small organelles as well as many free ribosomes, and giving off numerous fine processes; they make up about 6% of the glial cells in the corpus callosum; and they undergo mitosis at a rapid rate; (b) medium-shade oligodendrocytes, which are somewhat smaller cells with moderately dense nucleus and cytoplasm, containing well developed organelles, and giving off a fair number of fine processes; they make up about a quarter of the glial cells; and they undergo mitosis at a moderate rate; (c) dark oligodendrocytes, which are even smaller cells with very dense nucleus and cytoplasm, containing a prominent Golgi and structures referred to as lamellar bodies, and giving off very few fine processes; these cells make up about 40% of the glial cells; they do not undergo mitosis and probably arise from divisions of medium oligodendrocytes.

Electron microscopic identification of three classes of oligodendrocytes and a preliminary study of their proliferative activity in the corpus callosum of young rats.

The staining of half-micron thick Epon sections with toluidine blue provides a reliable method for the identification of glial cells. The diagnostic features of these cells observed in the corpus callosum of one-month-old rats are as follows: (1) Astrocytes have a very pale nucleus and cytoplasm; the nuclear envelope is sharply outlined by a thin chromatin lining with occasional chromatin beads; (2) Microglia have a small nucleus in which large, dark chromatin masses contrast with the nucleoplasm; the nucleus is round or elongated, and may be somewhat angular. The pericytes, which have a similar but usually crescentic nucleus, may be related to microglia. (3) Oligodendrocytes vary from pale to very dense and may be arbitrarily classified into three subgroups: The light oligodendrocytes are large pale cells with round to ovoid nucleus containing a prominent nucleolus and little or no condensed chromatin; the cytoplasm is extensive and appears pale, although less so than the nucleus. The medium shade oligodendrocytes are smaller cells that have an ovoid nucleus carrying small chromatin clumps and appearing moderately basophilic throughout; the cytoplasm is less extensive and somewhat darker than in the “light” subgroup. The dark oligodendrocytes are usually smaller than medium shade cells and often have an indented or angular nucleus with large chromatin masses; both nucleoplasm and cytoplasm are densely stained; the cytoplasm is often scanty and accumulated on one side of the nucleus.



Semithin sections of the cerebral cortex show glia with similar features, with the exception of the astrocytes, which have a smaller but more basophilic cytoplasm than in corpus callosum.



The cells of the subependymal layer of the lateral ventricle, when examined in semithin sections in the neighborhood of the corpus callosum, display a patchy, irregular nucleus and a scanty cytoplasm. Similar cells with a more regular nucleus are found outside the layer and in the corpus callosum; they are designated free subependymal cells.

Investigation of glial cells in semithin sections. I. Identification of glial cells in the brain of young rats.

The \"axonal flow\" theory holds that the nerve cell body is continuously forming new cytoplasm which enters the axon and travels along its length. This theory was formulated by Weiss and collaborators (Weiss and Hiscoe, '48) to account for the accumulation of material in front of a nerve constriction and for the high concentration of enzymes appearing in the proximal stump of a sectioned nerve (Sawyer, '46; Hebb and Waites, '56; Friede, '59). For instance, since the so-called neurosecretory substance of the hypothalamus accumulates in the central stump of the severed pituitary stalk, this substance is believed to migrate down into the posterior pituitary (Scharrer and Scharrer, '54). Techniques that do not damage the nerves have been used in a search for direct evidence in support of the axonal flow theory. Thus, radioactive substances were given, which it was hoped would be taken up into the nerve cell body and flow down the axon: phosphate-PJ2 (Samuels et al., '51; Ochs and Burger, '58; Ochs et al., '62), glucose-Ci4 (Waelsch, quoted by Weiss, '60) , and labeled amino acids (Waelsch, '58; Koenig, '58b; Schultze and Oehlert, '58; Verne and Droz, '60). However, the radioactive substances used were taken up not only by nerve cell bodies, but also by Schwann cells. Hence the difference in radioactivity in successive segments of nerve trunks was small and Weiss ('60) commented that, while the results obtained \"were expected to lend deeper insight into the processes involved, this expectation has thus far remained disappointingly unfulfilled.'' In fact, several authors hold the view that there is no axonal migration at all (Schultze and Oehlert, '58; Koenig and Koelle, '60; Miani, '60). At the present time, an experimental approach is needed which will either prove or disprove the axonal flow theory. It was thought that the attempts at tracing amino acids into the proteins of neurons might be repeated, but in such a way that the radioactivity of individual axons could be distinguished from that of glial cells. This aim might be achieved with tritium labeled amino acids, since a high radioautographic resolution is obtained with tritium. The first step was to make sure of the sites of protein synthesis. That such synthesis takes place continuously in the central nervous system was shown biochemically by the incorporation of labeled amino acids into the protein fraction (Friedberg et al., '48; Gaitonde and Richter, '56; Palladin, '57; Vladimirov, '57; Lajtha et al., '57). Radioautography after injection of a labeled amino acid indicated that the protein synthesis takes place in the body of nerve cells (Cohn et al., '54; Fisher et al., '56; Flanigan et al., '57; Leblond et al., '57; Koenig, '58a; Oehlert et al., '58; Droz and Verne, '59). However, these experiments were carried out using amino acids labeled with isotopes which do not allow good radioautographic resolution (such as and the animals were sacrificed several hours after injection, that is, too late to be sure that the newly-formed proteins had not migrated out of their sites of synthesis (Warshawsky et al., '63). It was, therefore, decided to reinvestigate the problem using mainly tritium labeled amino acids and sacrificing the animals very soon after injection, so that newlyformed proteins would still be at their site of synthesis. Once the sites of synthesis of protein were known with certainty, it was possible

Axonal migration of proteins in the central nervous system and peripheral nerves as shown by radioautography.

The corpus callosum of 60–80 gm male rats was processed by the weak silver carbonate method of del Rio-Hortega for the detection of microglia in the light microscope. Because of the opacity of the metal stain taken up by these cells, they could be recognized in the electron microscope. The characteristic features of microglia were thus found to be a small nucleus in which dark chromatin contrasts with light nucleoplasm, and a cytoplasm containing dense bodies (presumed to be lysosomes) and poor in endoplasmic reticulum. After gaining experience with microglia in the electron microscope, it became possible to recognize their features in 0.5–1 μ thick toluidine blue stained Epon sections examined in the light microscope.



In the electron microscope, some microglia are found within expansions of the basement membrane of capillaries (pericytal microglia); they usually have little cytoplasm, few dense bodies and few processes. There is evidence that these cells can break out of their basement membrane enclosure and enter the brain tissue proper. There, they scatter singly between other cells and fibers. These cells (interstitial microglia) send of long processes. They may contain numerous dense bodies and be surrounded by irregular spaces (suggesting the existence of surface activity). These observations support the classical view of microglia as brain macrophages.



Microglia constitute 5.5% of the glial population of the corpus callosum. After injection of 3H-thymidine and radioautography in the light or electron microscope, none showed labeling. Hence, it is presumed that microglia do not proliferate under normal conditions.

C. P. Leblond

Papers

Electron microscopic identification of three classes of oligodendrocytes and a preliminary study of their proliferative activity in the corpus callosum of young rats.

Investigation of glial cells in semithin sections. I. Identification of glial cells in the brain of young rats.

Axonal migration of proteins in the central nervous system and peripheral nerves as shown by radioautography.

Identification of microglia in light and electron microscopy

Evidence for division and transformations of neuroglia cells in the mouse brain, as derived from radioautography after injection of thymidine-H3