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

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 22 Gy alone, or a dose of 2, 8 or 22 Gy 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 22 Gy 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 22 Gy 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 8Gy dose remained Leu-7 positive, and no GFAP positive apoptotic cells were observed.

CONCLUSIONS

(1) There is cell proliferation following radiation-induced apoptosis in the adult rat spinal cord. (2) Dose- and 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.