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Contents  Summary 647 Introduction 648 Overview of higher plant plasma membrane  anion channels 648 Anion channels in higher plant roots 655 Conclusions and prospects 661  Acknowledgements 662  References 662 Summary Recent years have seen considerable progress in identifying anion channel activities in higher plant cells. This review outlines the functional properties of plasma membrane anion channels in plant cells and discusses their likely roles in root function. Plant anion channels can be grouped according to their voltage dependence and kinetics: (1) depolarization‐activated anion channels which mediate either anion efflux (R and S types) or anion influx (outwardly rectifying type); (2) hyperpolarization‐activated anion channels which mediate anion efflux, and (3) anion channels activated by light or membrane stretch. These types of anion channel are apparent in root cells where they may function in anion homeostasis, membrane stabilization, osmoregulation, boron tolerance and regulation of passive salt loading into the xylem vessels. In addition, roots possess anion channels exhibiting unique properties which are consistent with them having specialized functions in root physiology. Most notable are the organic anion selective channels, which are regulated by extracellular Al3+ or the phosphate status of the plant. Finally, although the molecular identities of plant anion channels remain elusive, the diverse electrophysiological properties of plant anion channels suggest that large and diverse multigene families probably encode these channels.

The results suggest that different mechanisms of chromosomal radiosensitivity operate in G2 and G0 cells and that, in general, each chromosomally radiosensitive patient is defective in only one such mechanism, possibly via mutation (or polymorphism) of a single gene. Such mutations may confer cancer predisposition, of low penetrance, in a substantial proportion of patients.

In vivo bromodeoxyuridine (BrdUrd) labelling of the human large bowel was performed and a detailed histochemical localisation of label in' sections of crypts was undertaken using a monoclonal antibody to BrdUrd containing DNA. Flow cytometric studies on extracted nuclei were also performed (data presented elsewhere). The average crypt in the human large bowel (excluding the rectum) was 82 cells in height and 41 cells in circumference, with a total of about 2000 cells (assuming a topographical correction factor of 0.6). Ten per cent ofthe cells were replicating their DNA -that is, were in the S phase of the cell cycleand 0-4% were in mitosis. The median position for the labelling index versus cell position frequency plot is at the 20th cell position -at a quarter of the crypt height. The lower and upper limits of the cell proliferation are given by the 5th and 95th percentiles at cell positions 4 and 43 respectively. The peak labelling index is about 30% and it occurs at cell position 15. The labelling index at the crypt base, the probable stem cell zone, is about 14%, suggesting that these cells have a longer cell cycle.Taking a value of 8*6 hours for the duration of the S phase (deduced from the flow cytometric data) and assuming a growth fraction of 1 0 for the mid-crypt, these data provide an estimate of about 30 hours for the cell cycle time. The rectal crypts are about the same size but contain about 30% fewer S phase cells. The data also yielded a per cent BrdUrd labelled mitosis curve.

In this study we examined the possibility that regular or circadian fluctuations occur in the frequency of spontaneous spermatogonial apoptosis. Apoptosis of A2, A3 and A4 type spermatogonia occurring spontaneously in the normal rat testis was studied by light and electron microscopy. Normal and apoptotic A3 spermatogonia were quantified in 36 animals killed at two-hourly intervals over a 24 h period. Three sequential phases of spermatogonial apoptosis were defined and quantified separately: (i) an early phase in which cells showed margination of nuclear chromatin, (ii) an intermediate phase in which phagocytosed apoptotic bodies were partly degraded and (iii) a late phase in which only debris of degraded apoptotic bodies was evident. Groups of spermatogonia linked by intercellular bridges underwent apoptosis synchronously. Normal and apoptotic A3 spermatogonia occurred at a mean frequency of 33.4 and 9.6 per 10 seminiferous tubule profiles respectively; there was a large variation in these frequencies between animals, but no peaks or circadian periodicity were detected. Progressive degradation of apoptotic bodies was evident, the average ratios of intermediate and late bodies to early bodies being 1.5 and 3.5, respectively. Absence of a circadian rhythm in these data does not exclude the possibility that initiation of apoptosis in susceptible spermatogonial clones is synchronous, and that affected clones undergo lag periods of differing duration before expressing morphological apoptosis.

3Pathology and 4Surgery, Christie and Withington Hospitals, Manchester M20 9BX, UK.Summary The aim of this study was to determine the proliferative activity within the epithelial cells of the normal human breast in 122 patients (6 reduction mammoplasties and 116 fibroadenoma excisions) in relation to age and the phase of the menstrual cycle. Thirty three of the patients were on oral contraceptives and 33 were parous.Thin tissue slices were incubated with tritiated thymidine and processed for autoradiography. Other samples were fixed directly and prepared for histology. The labelling, mitotic and apoptotic indices (LI, MI and Al) were determined and all illustrated considerable variability. The labelling indices are significantly (P<0.05) influenced by both patient age and stage during the menstrual cycle and ranged from 0-11.5%. Maximum LI values were obtained on the 20.8th day of the cycle. A square root transformation of the data was used to reduce the skewness of the data to a more normal distribution. The square root of the LI declined by 0.22 per decade. The mitotic data showed similar significant (P<0.05) correlations against age and day of cycle with a peak on the 21.5th day of the cycle, a decline by 0.072 per decade and a range from 0-0.6%. The data for apoptotic cells were less clearly influenced by the stage of the menstrual cycle but showed a significant (P<0.5) decline with age. The Al in parous patients was significantly higher than that in non-parous patients. There was no significant effect of oral contraceptives on any of the parameters measured when age and stage of cycle were taken into account. The considerable variability in the data could not be fully accounted for by either technical factors, the age of the patients, or the day of the cycle.We conclude that proliferation is negatively related to age and is influenced by the menstrual cycle but that additional as yet unknown factors must account for a large part of the variability seen in the data.Events which occur early in reproductive life, such as exposure to ionising radiation or an early menarche, are known to be related to the subsequent development of malignant neoplasms of the breast (Land et al., 1980, MacMahon et al., 1973. The breast may be susceptable during this period because of a high rate of cellular proliferation and because carcinogenic agents may act preferentially upon proliferating tissues (Russo et al., , 1987. However there are few data concerning the association between age and proliferative activity of the normal human breast (Anderson et al., 1982, Meyer, 1977Russo et al., 1987) and one aim of this study was to investigate this relationship further.Oestrogen is a major growth promoting hormone of the breast: under some circumstances progesterone acts as an antioestrogen and appears to inhibit proliferation (MauvaisJarvis et al., 1986). On the basis of these observations Korenman (1980) proposed that the risk of breast cancer was related to a defective luteal phase with low or absent progesterone secretion. Women w...

A high dietary intake of soy products (eg, as in Japan and Singapore) has been associated with a reduction in the incidence of breast cancer in premenopausal women. Phytoestrogens present in soybeans inhibit human breast cancer cell proliferation in vitro and breast cancer development in animal models, but no data exist on the effects of phytoestrogens on histologically normal human breasts. This study examines the effects of dietary soy supplementation on the proliferation rate of premenopausal, histologically normal breast epithelium and the expression of progesterone receptor. Women (n = 48) with benign or malignant breast disease were randomly assigned to receive their normal diet either alone or with a 60-g soy supplement (containing 45 mg isoflavones) taken daily for 14 d. Biopsy samples of normal breasts were labeled with [ 3 H]thymidine to detect the number of cells in S phase and were immunocytochemically stained for the proliferation antigen Ki67. The phytoestrogens genistein, daidzein, equol, enterolactone, and enterodiol were measured in serum samples obtained before and after supplementation. Serum concentrations of the isoflavones genistein and daidzein increased in the soy group at 14 d. Results showed a strong correlation between Ki67 and the thymidine labeling index (r = 0.868, P ≤ 0.001). The proliferation rate of breast lobular epithelium significantly increased after 14 d of soy supplementation when both the day of menstrual cycle and the age of patient were accounted for. Progesterone receptor expression increased significantly in the soy group. Short-term dietary soy stimulates breast proliferation; further studies are required to determine whether this is due to estrogen agonist activity and to examine the long-term effects of soy supplementation on the pituitary gland and breast. Am J Clin Nutr 1998;68(suppl):1431S-6S.

Many inherited cancer-prone conditions show an elevated sensitivity to the induction of chromosome damage in cells exposed to ionizing radiation, indicative of defects in the processing of DNA damage. We earlier found that 40% of patients with breast cancer and 5%-10% of controls showed evidence of enhanced chromosomal radiosensitivity and that this sensitivity was not age related. We suggested that this could be a marker of cancer-predisposing genes of low penetrance. To further test this hypothesis, we have studied the heritability of radiosensitivity in families of patients with breast cancer. Of 37 first-degree relatives of 16 sensitive patients, 23 (62%) were themselves sensitive, compared with 1 (7%) of 15 first-degree relatives of four patients with normal responses. The distribution of radiosensitivities among the family members showed a trimodal distribution, suggesting the presence of a limited number of major genes determining radiosensitivity. Segregation analysis of 95 family members showed clear evidence of heritability of radiosensitivity, with a single major gene accounting for 82% of the variance between family members. The two alleles combine in an additive (codominant) manner, giving complete heterozygote expression. A better fit was obtained to a model that includes a second, rarer gene with a similar, additive effect on radiosensitivity, but the data are clearly consistent with a range of models. Novel genes involved in predisposition to breast cancer can now be sought through linkage studies using this quantitative trait.