Circadian rhythms in the incidence of apoptotic cells and number of clonogenic cells in intestinal crypts after radiation using normal and reversed light conditions

Variations in the number of radiation-induced morphologically dead or dying cells (apoptotic cells) in the crypts in the small intestine of the mouse have been studied throughout a 24-h period under a normal light regimen (light on, 07.00-19.00 h; light off, 19.00-07.00 h). A clear circadian rhythm was displayed in the apoptotic incidence 3 or 6 h after irradiation for each gamma-ray dose studied (range 0.14-9.0 Gy). The most prominent circadian rhythm was obtained after 0.5 Gy. The peak time of day for inducing apoptosis was 06.00-09.00 h, and the trough occurred at 18.00-21.00 h. Some mice were also transferred to a room with the light cycle reversed, and were irradiated on different days after the transfer. The apoptosis induced by 0.5 Gy or 9.0 Gy, or the number of surviving crypts (microcolonies) after 11.0 Gy or 13.0 Gy was examined. The transition point for reversal (i.e. the switch time from the normal-light pattern to the reversed-light pattern) of the circadian rhythm in apoptosis (after 0.5 Gy) occurred 7 days after the transfer and the rhythm was reversed by 14 days. The rhythm for crypt survival (i.e. for clonogenic cell radiosensitivity) was disturbed on 1 day and the transition point for reversal occurred 3 days after the transfer. The rhythm became reversed by 7 days. These observations are discussed in relation to the identity of clonogenic cells, (functional) stem cells, proliferating transit cells and the cells sensitive to small doses of radiation (i.e. hypersensitive cells) in the crypt.     

Crypt base columnar cells in ileum of BDF1 male mice--their numbers and some features of their proliferation

Some features of the proliferative cells at the bottom of the ileal crypts in BDF1 mice have been studied in relation to the distribution of Paneth cells (PC) in an attempt to clarify the nature and function of these crypt base columnar cells (BCC) and to elucidate some aspects of the role of the microenvironment created by the PC. Longitudinal sections of crypts have shown that the ratio of PC to the BCC, which are scattered amongst the PC, is 2.7:1 in sections or approximately 29 PC and 9 BCC per whole crypt, i.e., a ratio of 3.2:1. The labelling index of BCC is about 35%, which is comparable to that of mid-crypt columnar cells. Although the BCC do become labeled, it is concluded that they cannot create vertical pairs or runs of several adjacent BCC since this would seriously disturb the distribution of Paneth cells. Only in dividing crypts are such runs (consisting of 3 to 5 cells) observed. The ability of BCC to synthesize DNA is not dependent on their position in the Paneth cell zone. In 95% of the crypts, the highest Paneth cell is below the 7th cell position from the bottom of the crypt, and the positions of the highest PC on either side of a given crypt are similar. The secreted granules or the cytoplasm of PC specifically bind pokeweed lectin, and this can be used for identification. Tracer doses of 3HTdR (37 kBq/gm body weight) result in the histological death of some BCC, and these damaged cells are evenly distributed throughout the Paneth cell zone. These tracer doses are somewhat selectively incorporated into BCC, i.e., the BCC have a higher grain count in autoradiographs, probably because they possess more thymidine kinase enzyme activity. This ability is very sensitive to the withdrawal of food, because 24 hr of fasting abolished the observed gradient in the intensity of labelling, which is very well correlated with the distribution of BCC. Regeneration of the crypts following cytotoxic exposure to Ara-C is initiated at the base of the crypt and hence may involve the BCC with possible help from the Paneth cells. The latter are insensitive to cytotoxic (S phase specific) agents and may help in the regeneration by preserving the architecture of the base of the crypt.     

A dynamic model of proliferation and differentiation in the intestinal crypt based on a hypothetical intraepithelial growth factor

A widely accepted model of the temporal and spatial organization of proliferation and differentiation in intestinal epithelia is based on a cellular pedigree with all cells descending from a few active stem cells and undergoing a sequence of transitory divisions until the non-proliferating maturing cell stages develop. Model simulations have shown that such a pedigree concept can explain a large variety of data. However, so far there is neither a direct experimental proof for the existence of an intrinsic age structure in the transitory proliferative cell stages nor for the distinction between stem and transitory cells. It is our objective to suggest an alternative model which is based on evidence for intercellular communications such as might be mediated through gap junctions. We consider the diffusion of a hypothetical intraepithelial growth factor in a chain of cells which are connected via gap junctions. Individual cells can divide if a critical growth factor concentration is exceeded. Simulation studies show that the model is consistent with many observed features of the small intestinal crypt in steady state and after perturbation.     

Poly(ADP-ribose) polymerase-1 is a survival factor for radiation-exposed intestinal epithelial stem cells in vivo

Poly(ADP-ribose) polymerase-1 (PARP-1) is a key enzyme mediating the cellular response to DNA strand breaks. It plays a critical role in genomic stability and survival of proliferating cells in culture undergoing DNA damage. Intestinal epithelium is the most proliferative tissue in the mammalian body and its stem cells show extreme sensitivity to low-level genotoxic stress. We investigated the role of PARP-1 in the in vivo damage response of intestinal stem cells in crypts of PARP-1^–/– and control mice following whole-body γ-irradiation (1 Gy). In the PARP-1^–/– mice there was a significant delay during the first 6 h in the transient p53 accumulation in stem cells whereas an increased number of cells were positive for p21^CIP1/WAF1. Either no or only marginal differences were noted in MDM2 expression, apoptosis, induction of or recovery from mitotic blockage, or inhibition of DNA synthesis. We further observed a dose-dependent reduction in crypt survival measured at 4 days post-irradiation in control mice, and this crypt-killing effect was significantly potentiated in PARP-1^–/– mice. Our results thus establish that PARP-1 acts as a survival factor for intestinal stem cells in vivo and suggest a functional link with early p53 and p21^CIP1/WAF1 responses.     

Sharpening the focus; the business of epithelial cell biology. An interview with Chris Potten by David Pearton

Dr Chris Potten is a singularly influential figure in the field of epithelial biology. His contributions have been seminal and include the introduction of the epidermal proliferative unit and of the concept of epidermal stem cells. With around 400 scientific papers and reviews to his credit as well as two books, he has certainly made his mark. His contributions have been recognised by the award of the Curie medal and recently the Weiss medal for radiation biology. Dr. Potten graciously agreed to be interviewed for this Special Issue of The International Journal of Developmental Biology. This interview was conducted via e-mail during June -August 2003.