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In spite of the widespread use of rats in gastrointestinal research, there is a lack of information on the qualitative and quantitative histological characteristics. Therefore, a study was performed in 69 male Wistar rats with ages ranging from one day to one year old. The features studied included: height and number of villi in the duodenum, jejunum and ileum, and depth and number of crypts in the duodenum, jejunum, ileum, colon and rectum. Morphometric observations were expressed in a mathematical logarithmic curve that showed a normal pattern of intestinal growth for each intestinal level. The number of villi in the small intestine decreased from 1 to 3S days of age, whereas the other intestinal parameters all increased during the same period. After 3S days the rates of increase or decrease were lower. The quantification of these intestinal changes provides a new complementary pattern as a reference for research as indicators of normality or malfunction in the rat intestine. Keywords Intestine; histology; postnatal development; villi; cryptsThe intestine undergoes an array of adaptative changes during postnatal development, similar to other organs. In the intestinal mucosa, these changes occur largely in structures related to the exchange and absorption processes, such as villi, crypts, enterocytes and microvilli. The morphological and biochemical changes that occur in the human intestinal mucosa under different physiological and pathological conditions, have been studied using the rat as a model system. These include colitis IRachmilewitz et al. 1993), ulcers (Matsumoto et al. 1994),' enteritis (Pothoulakis et al. 1993

Knowledge and lessons from past accidental exposures in radiotherapy are very helpful in finding safety provisions to prevent recurrence. Disseminating lessons is necessary but not sufficient. There may be additional latent risks for other accidental exposures, which have not been reported or have not occurred, but are possible and may occur in the future if not identified, analyzed, and prevented by safety provisions. Proactive methods are available for anticipating and quantifying risk from potential event sequences. In this work, proactive methods, successfully used in industry, have been adapted and used in radiotherapy. Risk matrix is a tool that can be used in individual hospitals to classify event sequences in levels of risk. As with any anticipative method, the risk matrix involves a systematic search for potential risks; that is, any situation that can cause an accidental exposure. The method contributes new insights: The application of the risk matrix approach has identified that another group of less catastrophic but still severe single-patient events may have a higher probability, resulting in higher risk. The use of the risk matrix approach for safety assessment in individual hospitals would provide an opportunity for self-evaluation and managing the safety measures that are most suitable to the hospital's own conditions.

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Combined with embryo morphology evaluation on Days 2 and 3, a scoring system based on pronuclear morphology seems to provide a good criterion when selecting embryos for transfer.