title: Apoptosis and its relationship with cell proliferation in the irradiated rat spinal cord.
abstract: OBJECTIVE
  To assess the relationship of oligodendroglial apoptosis with cell proliferation after irradiation.
  
  
  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.
authors.affiliation: Division of Experimental Therapeutics, Ontario Cancer Institute and Princess Margaret Hospital, University of Toronto, Canada.
authors.forename: Y Q
authors.initials: YQ
authors.lastname: Li
authors.forename: C S
authors.initials: CS
authors.lastname: Wong
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000528:radiation effects*; D001973:Bromodeoxyuridine / Q000378:metabolism; D002455:Cell Division / Q000528:radiation effects*; D003513:Cycloheximide / Q000494:pharmacology; D005260:Female; D007150:Immunohistochemistry; D009457:Neuroglia / Q000528:radiation effects; D051381:Rats; D011916:Rats, Inbred F344; D013116:Spinal Cord / Q000528:radiation effects*; D014965:X-Rays / Q000009:adverse effects
chemical_list: D003513:Cycloheximide; D001973:Bromodeoxyuridine