Tissue repair in the embryonic rat spinal cord following exposure to N-ethyl-N-nitrosourea

Mechanisms of neurotoxicity induced in the developing brain of mice and rats by DNA-damaging chemicals

It is not widely known how the developing brain responds to extrinsic damage, although the developing brain is considered to be sensitive to diverse environmental factors including DNA-damaging agents. This paper reviews the mechanisms of neurotoxicity induced in the developing brain of mice and rats by six chemicals (ethylnitrosourea, hydroxyurea, 5-azacytidine, cytosine arabinoside, 6-mercaptopurine and etoposide), which cause DNA damage in different ways, especially from the viewpoints of apoptosis and cell cycle arrest in neural progenitor cells. In addition, this paper also reviews the repair process following damage in the developing brain.

Repair process of fetal brain after 5-azacytidine-induced damage

The fetal brain is susceptible to many extrinsic stresses. Some of these stresses induce excessive cell death in the prenatal stage, leading to anomalies in the neonatal brain. However, it is unclear how the developing brain responds to and repairs the prenatal tissue damage. We treated pregnant rats on day 13 of gestation with 5-azacytidine, one of the compounds that induces excessive cell death and inhibits proliferation in neural progenitor cells, to damage the fetal brain, and investigated the repair process up to 60 h after treatment. Histological analysis showed that 5-azacytidine induced strong apoptosis of neural cells. By 60 h, apoptotic cells disappeared and the tissue was repaired, although the telencephalic wall remained thinner than in controls. Flow cytometry analysis showed that the cell cycle distribution also returned to control levels at 60 h, suggesting that the repair process was completed around 60 h. During the repair period, amoeboid microglia infiltrated the brain and ingested the apoptotic cells. These microglial cells were positive for the multiple microglial markers, and mRNAs for the microglia-related cytokines tumor necrosis factor alpha, interleukin 1beta and macrophage colony stimulating factor (M-CSF) were up-regulated. DNA microarray analysis showed the up-regulation of genes relevant to glial cells, inflammation, the extracellular matrix, glycolysis, proliferation and neural development. We show here that the developing brain has the capacity to respond to the damage induced by extrinsic chemical stresses, including changing the expression of numerous genes and the induction of microglia to aid the repair process.

Distinctive expression of midkine in the repair period of rat brain during neurogenesis: immunohistochemical and immunoelectron microscopic observations

Distinctive expression of midkine (MK) was observed during the repair period of fetal brain neuroepithelium. MK is a heparin-binding growth factor that occurs as a product of a retinoic acid-inducible gene, and has a molecular mass of 13 kDa. MK expression was examined immunohistochemically and by immunoelectron microscopy during a period of repair in developing rat brain at the neurogenesis stage. Injury was induced in rat fetuses by transplacental administration of ethylnitrosourea (ENU) on embryonic Day (E) 16, and histological changes were examined up to 48 hr thereafter (i.e., up to E 18). In normal rat fetuses, MK immunostaining was observed in the cytoplasm and radial and horizontal processes of all cells in the neuroepithelium (NE), subventricular zone (SV), and intermediate zone (IMZ). In ENU-administered brains, cells in the NE, SV, and IMZ were damaged severely, especially 16-24 hr after ENU administration. The remaining neuroepithelial cells, with the exception of those in M-phase and the tips of processes at the ventricular surface, were negative for MK immunohistochemistry 16-24 hr after the administration of ENU. Forty-eight hours after the administration, the cytoplasm and processes of cells in the NE, SV, and IMZ were MK immunopositive. Our previous data reported that the cell cycle of most NE cells is synchronized to the S-phase 16 hr after ENU administration and to the M-phase at 24 hr, and many NE cells were recovered 48 hr after ENU administration. The previous results taken together with the present results indicate that: (1) MK expression does not increase during the repair period of the NE, being different from adults; (2) MK expression is likely to be suppressed at S-phase according to the condition of the NE; and (3) MK expression is not essential for every cell cycle phase of NE cells; but (4) is necessary to maintain the M-phase of NE cells.

Process of repair in the neuroepithelium of developing rat brain during neurogenesis: chronological and quantitative observation of DNA-replicating cells

The neuroepithelium (NE) of the cerebrum of the developing brain has been reported to 'regenerate' within a certain period after being injured. To clarify the process of repair of the NE during neurogenesis, we chronologically examined the number of cells in the neocortex, and the rate of DNA-replicating cells and the mitotic figure ratio in the NE of rats after injury. The injury was induced by transplacental administration of ethylnitrosourea (ENU), which is cytotoxic immediately after administration, to the pregnant rats on embryonic day 16. The number of living and pyknotic cells in a 220-micron width of the neocortex was evaluated in each of three groups, such as NE, subventricular zone (SVZ) + intermediate zone (IMZ) + subplate (SP), and cortical plate (CP). The 5-bromo-2-deoxyuridine (BrdU)-labeling index and the mitotic index were examined 4, 8, 16, 24, 36, and 48 h after ENU administration. Up to 16 h after ENU administration, the number of living cells per 220 microns width in the NE and SVZ + IMZ + SP decreased, but increased in CP. From 16 to 24 h, the living cell number per 220 microns width in the NE and CP was unchanged, but increased in the SVZ + IMZ + SP. From 24 to 36 h, the living cell number per 220 microns width increased in all the groups, NE, SVZ + IMZ + SP and CP. From 36 to 48 h, the living cell number per 220 microns width in the NE was unchanged, but increased in the SVZ + IMZ + SP and CP. The BrdU-labeling index reached its nadir at 8 h, but markedly increased by 16 h, and then decreased to the control level by 36 h. No mitotic figures were observed at 16 h after administration, but a significant increase in mitotic index was noted at 24 h, and after which it decreased to almost the control value by 36 h. These findings indicate: (i) that temporary arrest of neuroepithelial cell cycle occurs in the G1-phase from 4 to 8 h after ENU administration, (ii) that the cell synchronizes in the S-phase at 16 h, (iii) that a proportion of neuroepithelial cells of rat fetal neocortex at the neurogenesis stage return to neuroepithelial cell proliferation stage, to repair the NE, and (iv) that regulation of the cell kinetics of neuroepithelial cells depends on the number of cells in a certain width of NE during regeneration.

Differential target dependence in the developing brain: implications for mental retardation

During development of the brain, many neurons exhibit a dependence on other neuronal populations for their survival and differentiation (target dependence). Evidence suggests that some neural pathways are much more dependent on single target neuronal populations for their survival than are others (differential target dependence). This phenomenon has important implications both for animal models of congenital human brain damage and for ideas concerning the aetiology of behavioural abnormalities associated with human mental retardation. Predictions of the neuronal deficits likely to arise from exposure to cytotoxic agents (e.g. ionizing radiation, hyperthermia, viral infection) at a particular time must take differential target dependence into account. It is known that target dependence affects corticopetal pathways involved with the discriminative senses (e.g. vision), more than monoaminergic and cholinergic corticopetal pathways which are believed to be involved with arousal, selective sensory attention, sleep, memory and cortical vasomotor function. Following prenatal damage to superficial layers of the cerebral cortex, this effect of differential target dependence leads not only to a relative hyperinnervation of the cortex with monoaminergic and cholinergic projections, but a specific deficit in visual pathways. The implications of this combined deficit for the behaviour and rehabilitation of the mentally retarded are considered.