Apoptosis and its relationship with cell proliferation in the irradiated rat spinal cord

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Abstract
Purpose: To assess the relationship of oligodendroglial apoptosis with cell proliferation after irradiation. Materials and methods: The adult rat spinal cord (C2-T2) was irradiated with a single dose of 2, 8 or 22Gy alone, or a dose of 2, 8 or 22Gy followed by a second 8 Gy dose given at 1-63 days after the initial dose. Apoptosis was assessed histologically according to its specific morphological features. Cell proliferation and glial cell identity were assessed immunohistochemically using BrdU, Leu-7 and GFAP as markers for oligodendrocytes and astrocytes respectively. Results: The total apoptotic yield (TAY) per spinal cord section over a 24h period after a single dose of 2, 8 or 22Gy was 2.4, 9.0 and 10.9% respectively. Cycloheximide delayed the onset of apoptosis by about 8 h. Unirradiated spinal cord showed a very low BrdU labelling index (LI) of 0.13% in the glial cells. After a single dose of 8 Gy, the BrdU LI increased by 2 days, peaked at 14 days (1.05%), and returned to control level by 42 days. A smaller increase in the BrdU LI was seen after doses of 2 or 22Gy compared with 8 Gy. Labelled cells at 2 weeks appeared to be Leu-7 positive and GFAP negative. After an initial dose of 2 Gy, a second 8 Gy dose given at 1-63 days gave reduced TAY values of 3.7-6.7% respectively. After an initial 22 Gy dose, little apoptotic response was induced by the second 8 Gy dose regardless of the time interval between the two doses (TAY 1.2-2.6%). For an intermediate initial dose of 8 Gy, TAY from the second 8 Gy dose given at 1 day was reduced to 3.7%, but there was recovery of the apoptotic response with the second dose given at 14 days (TAY 9.8%). A much higher percentage of apoptotic cells were observed in BrdU positive (4.8-21.7%) compared with the negative (0.25-0.54%) glial cells after split-dose irradiation, and 20.0% of apoptotic cells showed immunostaining for BrdU. Apoptotic cells after the second 8 Gy dose remained Leu-7 positive, and no GFAP positive apoptotic cells were observed. Conclusions: (1) There is cell proliferation following radiationinduced apoptosis in the adult rat spinal cord. (2) Doseand time-dependent apoptotic recovery is seen after split-dose irradiation. It is postulated that this recovery is due to the cell proliferation that replenishes the apoptosis-sensitive oligodendrocytes.

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