Introduction Irradiation induces stem cell apoptosis and exposure to DNA label (eg, tritiated thymidine; [³H]dT) when new stem cells are being generated during recovery, leads to retention of the label in cells located above the Paneth cell region in crypts of small intestine (SI). Persistence of label retaining cells (LRC), despite many rounds of cell division, occurs because of asymmetric segregation of DNA strands in which the old/template strands are selectively retained in daughter cells that are destined to remain stem cells (immortal strand hypothesis). The aims of our studies were to (i) determine whether asymmetric DNA strand segregation occurs in stem cells (located at crypt base) of colon (CO) and (ii) assess radiosensitivity of newly generated stem cells in SI.

Methods (A) Following 12 Gy irradiation, [³H]dT was administered to BDF1 mice over 5 days. SI and CO were collected at 40 min and 4, 7 and 15 days after the last dose of [³H]dT (chase period). (B) As in (A) above, and additionally, second dose of radiation (1 Gy) after 7-day chase, followed by tissue collection after 4.5 h to assess for apoptotic epithelial cells that had taken up [³H]dT. [³H]dT-labelled (autoradiography) and apototic (morphology) cells were assigned a cell position (cp) along the crypt (cell number increasing sequentially from cp 1 at the centre of crypt base). Percentage of [³H]dT-labelled cells at a particular cell position is expressed as the labelling index (LI). Data are expressed as mean (SEM).

Results (A). In contrast to CO, peaks of LRC were consistently seen in SI at cp 4–9 at chase periods of 4, 7 and 15 days (Abstract 040). After 15-day chase, only occasional LRC were seen in different regions of crypts of CO. (B). In SI, LRC undergoing apoptosis were seen predominantly at cp 4–6 (LI 2.2(0.4)) and cp 7–9 (1.0(0.3)).

Conclusion Following irradiation, LRC are predominant in the stem cell region above Paneth cells in SI crypts and these newly generated stem cells are sensitive to radiation-induced apoptosis. LRC are not consistently seen in the stem cell region of colonic crypts. This implies lack of asymmetric segregation of DNA strands in colonic stem cells, which may explain the increased risk of cancer in the colon (compared to SI).