Joseph Altman

Are New Neurons Formed in the Brains of Adult Mammals

In an autoradiographic investigation, the production of brain lesions in rats was combined with intracranial injection of thymidine-H3. Nuclei of numerous glia cells were found labeled in brain regions associated with the traumatized areas. In addition, some neurons and neuroblasts showed labeling, suggesting the possibility of proliferation of neurons in adult rats.

Proliferation and migration of undifferentiated precursor cells in the rat during postnatal gliogenesis

Rats aged 13 days were injected with thymidine-H3 and were killed 6 hours, 1, 3, 6, 12, 20 and 60 days afterwards. Incorporation of this labeled precursor of chromosomal DNA was studied autoradiographically with reference to the origin of neuroglia cells in the neocortex. A large proportion of the subependymal cells of the lateral ventricle were labeled after brief survival, indicating rapid regional proliferation. In animals with longer survival there was considerable increase in the proportion of labeled cells, combined with label dilution. In spite of this evidence of rapid and continuous cell multiplication, the concentration of subependymal cells declined considerably with age, indicating that the bulk of new cells do not stay here. The rate of regional proliferation was lower in the white matter and corpus callosum, though there was a considerable increase in the proportion of labeled cells with prolonged survival. Significantly, the addition of this large number of new cells did not lead to a net increase in the total cell population. These facts imply that many of the cells found in these fibrous regions are migratory ones, and as new cells arrive here older ones depart. Finally, in the gray matter of the cortex regional cell proliferation was low at this age, but there was a steady gain in the proportion of labeled cells with increased survival time. On the basis of these findings it is concluded that the cells formed around the lateral ventricle, which are destined to reach the cortex, utilize the white matter and corpus callosum as a migratory pathway.

Autoradiographic study of degenerative and regenerative proliferation of neuroglia cells with tritiated thymidine

Abstract Proliferation of neuroglia cells was provoked in rats by bilateral electrolytic lesions in the lateral geniculate body and this was combined with unilateral intracranial injection of thymidine-H 3 . The animals were then sacrificed 1 day, 1 week, 2 weeks, 1 month, and 2 months after the operation. Using fine-resolution autoradiography, reduced silver grains were concentrated over nuclei of neuroglia and microglia cells in and around the lesion area, and in various visual structures not directly affected by the lesion, such as the optic radiation, lower layers of the striate cortex, the pretectal region, and the brachium of the superior colliculus. Labeled neuroglia cells were also seen in structures which do not obtain fibers directly from the lateral geniculate body, such as the corpus callosum and hippocampus. Labeled neuroglia cells in all these structures were present on both the injected and noninjected side. The concentration of grains over neuroglia cells was highest in animals with 1-day survival period, and then declined, while the number of labeled cells was lowest after 1 day of survival and then increased up to 1 month—suggesting continued division of the already labeled cells. The experiment suggested that not only astrocytes and microglia cells, but also oligodendrocytes proliferate. Astrocytes and microglia cells predominated in the directly affected regions, while oligodendrocytes were most numerous along the pathways and gray matter which are directly or indirectly connected with the lateral geniculate body.

Differential radiosensitivity of stationary and migratory primitive cells in the brains of infant rats

The heads of infant rats were irradiated with 200 r X-ray. In one group the animals were injected with thymidine-3H immediately after irradiation, in the other, 24 hours before irradiation. All were killed at 3 days of age, 6 hours after irradiation and 6 or 30 hours after injection. In autoradiograms from the brains of unirradiated animals killed 6 hours after injection, the proportion of labeled cells, reflecting regional rate of cell poliferation, was determined at several brain sites. The relative reduction in the rate of cell proliferation in irradiated animals was also established. In matched stained sections the proportion of pyknotic cells, reflecting rate of cell death, was determined in the same brain regions. Finally, pyknosis index/labeling index ratios were calculated to relate radiation-induced cell death to rate of cell proliferation at different loci. It was established that migrating cells and prospective migratory cells, whether mitotic or postmitotic, are extremely radiosensitive, with a large proportion of them killed by exposure to 200 r. Relatively few of the stationary proliferating cells were killed with this dose. Stationary differentiating cells, and mature cells were not visibly affected. These results suggest that damage to the cellular mechanisms that enable primitive cells to locomote, rather than damage to DNA, is the principal cause of cell death in the nervous system exposed to low-level radiation. Moreover, the selective effect of this procedure could provide a technique for the identification of migratory cells at different brain sites during different stages of development.

Selective destruction of precursors of microneurons of the cerebellar cortex with fractionated low-dose x-rays.

Abstract The study was undertaken to determine the amount of irradiation needed to selectively destroy certain postnatally-forming neural elements of the brain. The heads of kittens during their first 2 weeks of life were irradiated unilaterally with five repeated doses of x-ray, single doses ranging from 50 to 400r; the animals were killed on the sixteenth day. The effects of radiation were evaluated quantitatively on the irradiated and control sides in the ansiform lobule of the cerebellar cortex. Repeated doses of 50 to 100r reduced the migratory cell population of the external granular layer; 150r produced a subtotal, and 200r a total destruction of the layer. With 150r and higher doses the total area and the cell population of the internal granular layer of the ansiform lobule were appreciably reduced; with 200r and higher doses the cell packing density of the granular layer was also affected. Irradiation with 150r or higher doses reduced the ratio of granule cells to Purkinje cells. Although x-ray irradiation up to 200r had no effect on the control side, effects were produced with 300 and 400r. The Purkinje cells were adversely affected only with 400r. We concluded that 200r is the optimal dose in kittens for selective destruction of the precursors of the postnatally-forming granule cells in circumscribed parts of the cerebellar cortex.

Gross morphological consequences of irradiation of the cerebellum in infant rats with repeated doses of low-level X-ray

Abstract The cerebellum of rats was irradiated with daily doses of 50–200 r hard X-ray, on successive days following birth, total number of exposures ranging from 1 to 10. The purpose was to design a method of selectively eliminating a specifiable proportion of the postnatally proliferating and migrating precursor cells of cerebellar microneurons. In this study the gross morphological consequences of different irradiation schedules were determined. Mortality rate was not raised by localized cerebellar irradiation, except in rats that received ten successive daily doses, and the animals that survived through infancy, including those that received 10 × 200 r, showed normal body growth. Total brain weight was markedly reduced in the animals receiving more than 2 × 100–200 r; the highest reduction was seen in animals that received 10 × 200 r. The bulk of reduction in total brain weight could be attributed to weight loss in the intended area of irradiation. Areal measurements showed that cerebellar irradiation did not affect appreciably the growth with age in the width and length of the cerebrum, but increase in the length and height of the cerebellum was markedly retarded. With single or multiple doses of 200 r cerebellar length was greatly reduced by day 10. Reduction at 30 and 90 days was not evident with 1–2 × 200 r, but with 4–5 × 200 r cerebellar length was reduced to the level of control animals 10 days of age, and with 10 × 200 r to the level of neonates. The cytological bases of the drastic effects produced by radiation remain to be determined.

An electrophysiological study of the superior colliculus and visual cortex

Abstract In cats anesthetized with Nembutal, simultaneous recording of evoked potentials from superior colliculus and visual cortex to single light flashes gave mean onset latencies of 36.5 msec in the former and 27 msec in the latter. Onset latency to electric stimulation of optic nerve was 8 msec in colliculus and over 1 msec in cortex. Since similar latency differences were obtained from colliculus and cortex.to both retinal (photic) and postretinal (electric) stimulation, latency differences were attributed to slower conduction in the retinocollicular pathway. The thresholds were higher in superior colliculus than visual cortex to light flashes of short duration or low intensity, or electric pulses applied to the optic nerves. Simultaneously recorded collicular and cortical evoked potentials differed from each other in shape, duration, and several other respects. With microelectrodes, three functionally differentiated layers of the superior colliculus were distinguished: a dorsal layer in which evoked potentials and single units responding to optic stimuli were obtained; an intermediate layer in which evoked potentials of unchanged amplitude, but no driven units, were recorded; and a ventral layer in which evoked potentials were greatly reduced, and where only “spontaneously” discharging units were found. The optically driven units were classified as those firing with short latency, with long latency, to light-on only, to light-on and light-off, and those inhibited by photic or optic nerve stimuli.

Behavioral manipulations and protein metabolism of the brain: Effects of motor exercise on the utilization of leucine-H3

Abstract The utilization of systemically injected leucine-H 3 by nerve cells was investigated autoradiographically in exercised and unexercised rats. Handled rats reared in an enriched environment were run daily for a long period in a motor-driven exercise wheel to adapt them to forced exercise. On the day of administration of the radiochemical the “exercised” animals were forced to run for 1 hr prior and 1 hr after the injection, whereas the “unexercised” animals rested before and after injection. Microdensitometric evaluation of autoradiographic grain density over single nerve cells in several brain regions indicated that there were no significant differences in the utilization of the radiochemical by proteins of the brain in the two groups of animals, though there was a slight trend of increased incorporation of leucine-H 3 in the exercised group of animals. The results suggest that the presumed functional activation of the brain by this sensori-motor task need not lead to enhanced protein metabolism if the stress produced by forced exercise is reduced or eliminated by pre-adaptation to the task.

Increased utilization of an amino acid and cellular proliferation demonstrated autoradiographically in the optic pathways of pigeons.

Utilization of a radioactively labeled amino acid (glycine-H3) was investigated by autoradiography in the visual pathways of pigeons, one eye of which had been removed or blindfolded 1 month previously. The radiochemical was injected systemically and the animals were killed after an exchange period of 2 hours. In the enucleated animals, uptake of the labeled glycine was considerably greater in the severed than in the normal retinotectal pathway, indicating increased protein metabolism in the “degenerating” fiber tract. The differential incorporation was evident also in the stratum opticum of the optic lobes but ambiguous in the deeper layers. The increased uptake in the severed optic pathway was associated with an increase in the total number of neuroglia cells and the appearance of cells not observed in the undamaged pathway. No differential uptake was observed in any other parts of the central nervous system, the forebrain included. In the unilaterally blindfolded animal, no difference was observed in uptake of labeled glycine between the normal and “functionally deafferented” optic pathways. The increased protein metabolism in the distal stumps of the severed optic pathway, combined with proliferation of cells in this region, may reflect a regenerative attempt at this phase of Wallerian degeneration.

Behavioral manipulations and protein metabolism of the brain: Effects of visual training on the utilization of leucine-H3

Abstract The utilization of systemically injected leucine-H 3 by the brain was compared in visually-deprived rats and rats receiving prolonged training in a maze on a series of visual pattern-discrimination tasks. Microdensitometric evaluation of autoradiographic grain density over single nerve cells in a variety of visual and nonvisual brain regions indicated that there was no significant difference in the utilization of the radiochemical by proteins of the brain in the two groups of animals; though there was a trend of increased uptake in several visual and nonvisual structures in the visually-deprived animals. This result supports our previous conclusion that the presumed functional activation of the brain by behavioral engagement need not lead to increased protein metabolism, as measured by our technique. The slight increase in the utilization of leucine-H 3 in the visually-deprived animals can be attributed to the greater stress produced by the injection procedure in these unhandled animals.