Cell population kinetic profile of the mouse thymus, and the changes induced by prednisolone.

The cell population kinetic parameters of the thymus in BALB/c mice have been estimated using stathmokinetic and [3H]TdR techniques in both control animals and animals treated with prednisolone. FLM data were analysed by computer using the Gilbert program. The study showed that prednisolone had an inhibitory effect mainly in the DNA synthesis phase and in G1. Stathmokinetic data also showed a decrease in the cell birth rate and an increase in the apparent cell cycle time (or potential doubling time) after treatment. The labelling index, the mitotic index and the growth fraction were also decreased. The study also shows a good agreement between the data obtained by stathmokinetic and [3H]TdR techniques.     

CELL POPULATION KINETIC PROFILE OF THE MOUSE THYMUS, AND THE CHANGES INDUCED BY PREDNISOLONE

The cell population kinetic parameters of the thymus in BALB/c mice have been estimated using stathmokinetic and [³H]TdR techniques in both control animals and animals treated with prednisolone. FLM data were analysed by computer using the Gilbert program. The study showed that prednisolone had an inhibitory effect mainly in the DNA synthesis phase and in G₁. Stathmokinetic data also showed a decrease in the cell birth rate and an increase in the apparent cell cycle time (or potential doubling time) after treatment. The labelling index, the mitotic index and the growth fraction were also decreased. The study also shows a good agreement between the data obtained by stathmokinetic and [³H]TdR techniques.     

Feasibility and reproducibility of the [3H]-thymidine labelling index in breast cancer

For several malignancies, autoradiographic evaluation of the S phase cell fraction by means of [3H]-thymidine ([3H]dT) has proved a valid prognostic tool which should be included in a 'risk factor profile system' to estimate prognosis in individual patients. However, its clinical implementation requires a methodological standardization, a high feasibility and an assessment of the reproducibility of results, within and between different laboratories. The recent availability of a kit for in vitro incubation with [3H]dT and histological fixation of solid tumour specimens has contributed to the methodological simplification and standardization of the technical procedure, increasing its feasibility in several institutions. For breast cancer, preliminary results from a quality control study promoted by the Italian Society of Basic and Applied Cell Kinetics (SICCAB) showed a high inter-observer reproducibility within (r = 0.96) and between different laboratories (r = 0.93). Moreover, on a series of 20 autoradiographic samples sent to 15 different institutions, only 2% of tumours shifted from one of the cell kinetic subgroups, defined using the median [3H]dT labelling index (LI) of 2.8% as a cutoff, to the other. In addition, a marked consistency of results was observed among the different Italian institutions involved in cell kinetic studies on breast cancers, both in terms of median [3H]dT LI values and basic correlations. Periodic assessment of the reproducibility of [3H]dT LI evaluation among different laboratories is needed, especially for institutions involved in multicentre clinical protocols based on cell kinetic characteristics.     

Cell proliferation in the subependymal layer of the adult mouse in vivo and in vitro

Pulse labelling experiments with [³H] thymidine (dT) and double labelling experiments with [³H]dT and bromodeoxyuridine (BrdUrd) were carried out on cells of the subependymal layer in the brain of adult normal mice in vivo, in vivo/in vitro and in vitro. The results should (i) lead to information about cell cycle parameters of these cells in the brain of adult mice, since these cells have been studied mostly in the rat brain up to now and (ii) answer the question whether results concerning cell proliferation obtained in vivo correspond with those from brain slices incubated in vitro with or without prelabelling in vivo. In vivo an LI of 20.2 ± 2.7% (x?± SEM) and T_s= 7.2 ± 0.7h were found. Furthermore, grain count halving experiments led to a surprisingly short cycle time (T_c) of 11.2–14.2 h. The longer T_c values (18–20 h) reported in the literature for subependymal cells in the rat brain seem to be due to evaluations of different areas around the lateral ventricle without considering the migrating behaviour of these cells which is quite different regionally. The in vitro studies (with or without prelabelling in vivo) showed a significantly reduced LI due to the fact that about 20% of the S phase cells, possibly lying in the middle of S, stopped further DNA synthesis after transfer to culture. This was shown by comparing the cell fluxes at the G₁/S and S/G₂ borders of in vivo vs. in vitro studies.     

Cell cycle status of stromal cells in long-term haematopoietic cultures

This study was performed to further define the mechanism by which the stromal micro-environment regulates haematopoiesis. In long-term marrow cultures the interactions between stromal cells and haematopoietic cells can be investigated at the cellular level. Long-term marrow cultures from hamsters do not require repopulation or addition of hydrocortisone and are suitable for investigation of cell kinetics. The cellular kinetics of haematopoietic and stromal cells, as studied by tritiated thymidine ([3H]dT) incorporation, revealed that DNA synthesis occurred in both the non-adherent and the adherent cells. In established cultures the adherent stromal cells were predominantly in a quiescent non-cycling state: less than 2% adherent cells incorporated [3H]dT within 5 h. Removal of the supernatant cells did not affect the labelling index of adherent cells, since the labelling indices at the 50-75 h time point were 14.3% and 12.5% in the presence and absence of supernatant cells respectively. An apparent stimulus for stromal cells to incorporate [3H]dT was attachment or adhesion. Following replating of supernatant cells of long-term marrow cultures, 23.3% of the reformed adherent layer cells were labelled compared with 12-14% in cultures with previously formed unmobilized adherent cells (P less than 0.01). The data indicate that adherent cells are not required to synthesize DNA for maintenance of haematopoiesis in established long-term marrow cultures, and that recruitment into the cell cycle has an independent mechanism that is not influenced by feed-back from the supernatant cells.     

Cell proliferation of carcinoma in Bilharzial bladder: an autoradiographic study

The rate of cell production in thirty-five cases of carcinoma in Bilharzial bladder was evaluated from the labelling index after in vitro incubation with [3H]TdR. Squamous cell carcinoma was the most frequent histological type in this series and had a median LI of 8.0% which corresponds to a potential doubling time of 5.9 days. In squamous cell tumours the LI increased with the histological grade. Transitional cell tumours had a somewhat greater LI. In all histological types the LI was significantly greater in the deep infiltrating parts of the tumour than in the superficial parts. The discrepancy between the estimated potential doubling time and the growth rate normally attributed to such tumours suggests the existence of an extensive cell loss factor. Areas of focal or diffuse mucosal hyperplasia were associated with increased LI.     

Turnover of polyadenylated messenger RNA in fission yeast. Evidence for the control of protein synthesis at the translational level.

Polyadenylated RNA was isolated from fission yeast (Schizosaccharomyces pombe) total RNA using oligo(dT)-cellulose, and was studied as a model for messenger RNA. The half-life of poly adenylated RNA was measured by two independent methods. (a) The rate of labelling of polyadenylated RNA during incubation of cells with [5-3H]uridine was measured. A half-life of 40-45 min was found by comparing the experimental data with theoretical curves calculated for labelling of RNAs with various half-lives. The influence of precursor-pool specific activity on RNA labelling kinetics is considered. (b) Cells were labelled with [5-3H]uridine then further RNA synthesis was inhibited by addition of 8-hydroxyquinoline. The rate of loos of radioactivity from polyadenylated RNA indicated a half-life of 50 min. The half-life found by these two methods is about one-third of the cell doubling time, and is much longer than previous estimates by indirect methods of yeast messenger RNA half-life. Both experimental methods provided evidence for the existence of tas a half-life of 40-50 min; a much smaller population is probably turning over more rapidly. After inhibition of RNA synthesis by 8-hydroxyquinoline, the rate of total protein synthesis declined much more rapidly than the polyadenylated RNA content of the cells. However, 60 min after inhibition of RNA synthesis there was a small rise in the rate of portein synthesis. These data are interpreted as evidence for mechanisms controlling protein synthesis which operate at the level of messenger RNA translation.     

CELL PROLIFERATION OF CARCINOMA IN BILHARZIAL BLADDER: AN AUTORADIOGRAPHIC STUDY

The rate of cell production in thirty-five cases of carcinoma in Bilharzial bladder was evaluated from the labelling index after in vitro incubation with [³H]TdR. Squamous cell carcinoma was the most frequent histological type in this series and had a median LI of 8.0% which corresponds to a potential doubling time of 5.9 days. In squamous cell tumours the LI increased with the histological grade. Transitional cell tumours had a somewhat greater LI. In all histological types the LI was significantly greater in the deep infiltrating parts of the tumour than in the superficial parts. The discrepancy between the estimated potential doubling time and the growth rate normally attributed to such tumours suggests the existence of an extensive cell loss factor. Areas of focal or diffuse mucosal hyperplasia were associated with increased LI.     

Macromolecules released into the culture medium during the vegetative cell cycle of the unicellular green alga Chlamydomonas reinhardii.

The culture medium of growing Chlamydomonas reinhardii cells contains hydroxyproline-rich glycoproteins, which are mainly liberated during release of the zoospores from the mother-cell wall. Pulse-labelling studies with [3H]proline and [35S]methionine have been performed in order to detect the protein components released by synchronously growing cells at different stages of the cell cycle. When either [3H]proline or [35S]methionine were applied during the phase of cell growth, radioactive label appeared in the released macromolecules after a lag period of 40 min, whereas incorporation into the insoluble part of the cell wall was delayed only by 20 min. When applied at the end of the growth phase, e.g. 13 h after beginning of the illumination period, the radioactive amino acids were incorporated into the cell wall, but radioactive labelling of macromolecules released into the culture medium could not be detected before the zoospores were liberated from the mother-cell wall. Maximal incorporation of [3H]proline and [35S]methionine into the insoluble part of the cell wall was observed during cell division, but essentially no radioactively-labelled macromolecules were released into the culture medium during this time period. Analysis of the macromolecules, which were liberated during cell enlargement, by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed distinct radioactive bands, which were differentially labelled with [3H]proline and [35S]methionine. Among the macromolecules released into the culture medium during cell growth, a component of an apparent Mr 35 000 was preferentially labelled with [3H]proline. This component was also detected after labelling with [35S]methionine, but components of an apparently higher Mr were more prominent after labelling with [35S]methionine. Macromolecules released during the cell-enlargement period of synchronously growing cultures in the presence of [3H]proline contained radioactively-labelled hydroxyproline in addition to proline. These results show that, during cell-wall growth, specific protein components are released into the culture medium and that at least one of these components contains large amounts of proline and hydroxyproline. At least some of these macromolecules seem to be constituents of the cell wall, because during pulse-chase experiments radioactively-labelled macromolecules appeared in the culture medium mainly during the time period when the specific radioactivity of the insoluble inner-cell-wall layer decreased.     

Extracellular matrix alters the relationship between tritiated thymidine incorporation and proliferation of MC3T3-E1 cells during osteogenesis in vitro

Bone cells in vivo exist in direct contact with extracellular matrix, which regulates their basic biological processes including metabolism, development, growth and differentiation. Thus, the in vitro activity of cells cultured on tissue culture treated plastic could be different from the activity of cells cultured on their natural substrate. We selected MC3T3-E1 pre-osteoblastic cells to study the effect of extracellular matrix on cell proliferation because these cells undergo a progressive developmental sequence of proliferation and differentiation. MC3T3-E1 cells were cultured on plastic or plastic coated with ECM, fibronectin, collagen type I, BSA or poly l-lysine and their ability to proliferate was assessed by incorporation of [3H]dT or by enumeration of cells. Our results show that (1) ECM inhibits incorporation of [3H]dT by MC3T3-E1 cells; (2) collagen type I, but not BSA, poly l-lysine or fibronectin also inhibits incorporation of [3H]dT; (3) the level of ECM inhibition of [3H]dT incorporation is directly related to the number of cells cultured, but unrelated to the cell cycle distribution or endogenous thymidine content; (4) the kinetic profile of [3H]dT uptake suggest that ECM inhibits transport of [3H]dT from the extracellular medium, and (5) cell counts are similar in cultures whether cells are grown on plastic or ECM. These results suggest that decreased incorporation of [3H]dT by cells cultured on ECM is not reflective of bone cell proliferation.     

Ultraviolet B irradiation induces epidermal regeneration with rapidly cycling cells

The left flank of hairless mouse skin was irradiated with a minimal erythema dose of ultraviolet B (UVB) light at 297 nm (25 mJcm-2), while the right flank served as untreated control. The alterations in epidermal growth kinetics induced by this UVB dose were studied with the percentage of labelled mitoses (PLM) technique during the period of increased proliferation. Thirty hours after irradiation, when a large cohort of cells appears in S phase, each animal was injected intra-peritoneally with 50 microCi tritiated thymidine [( 3H]-TdR). The number of labelled basal and suprabasal cells, as well as their localization in epidermis were registered in histological sections at short intervals up to 48 h after the [3H]-TdR pulse. Labelled mitoses were also counted in the same specimens. The results showed a four-fold increase of the high initial number of labelled cells in UVB-exposed epidermis within 18 h of the pulse injection, and a six-fold increase after 36 h. In control epidermis, where the starting value of the labelling index was much lower, there was only a three to four-fold increase in the number of labelled cells during the period studied. The PLM and the labelling index data were consistent with an average cell cycle time of approximately 10-12 h for UVB-exposed cells, in contrast to about 30 h for the fastest cycling population in control epidermis. The PLM curve also indicated a prolonged S phase duration in UVB-exposed epidermis compared with controls. In addition, labelled cells were seen in the suprabasal layer as early as 6 h after the [3H]-TdR injection and within 36 h labelled cells had reached the outermost layer of nucleated cells, indicating a reduced transit time through epidermis. The present study shows that a minimal erythema dose of UVB light at 297 nm induced a period of increased transit time through the S phase, combined with rapid cell proliferation, leading to an overall shortening of the epidermal cell cycle time. The cohort of cells labelled with [3H]-TdR 30 h after irradiation seemed to proceed as a wave of partially synchronized cells through the cell cycle for more than two rounds, which is comparable with the cell kinetic perturbations observed in regenerating mouse epidermis.     

Cell kinetics of a rat thyroid transplantable tumour

Changes in morphology and cell kinetics are described in a rat thyroid transplantable tumour (TTT) during the first few transplant generations. The growth of TTT in animals was possible only with an increased circulation level of the thyroid stimulating hormone (TSH). With serial transplantation subcutaneously in isologous animals, the morphology of TTT changed dramatically from that of a follicular tumour in the 3rd passage to become, by the 9th generation, a poorly differentiated tumour with a trabecular arrangement of cells. This change in tumour morphology was accompanied by an increase in the number of proliferating cells--mitotic index (MI), [3H]thymidine labelling index (LI), growth fraction (GF)--and cell loss factor (O) as well as a decrease in the cell cycle time (Tc) and potential population doubling time (TPD). TTT belongs to the class of tumours with a low proliferative activity and might be used in a variety of cell kinetic, radiobiological and chemotherapy studies.     

Cell proliferation kinetics of six xenografted human cervix carcinomas: comparison of autoradiography and bromodeoxyuridine labelling methods

Cell kinetic and histologic parameters of six xenografted tumours with volume doubling times ranging from 6 to 43 d were investigated in order to obtain kinetic information on a panel of tumours to be used in radiobiological studies. The six tumours covered a range of histologies and their DNA indices varied from 2.7 to 1.4. The length of the cell cycle (Tc), potential doubling time (Tpot) and labelling index (LI) were determined by continuous labelling with [3H]TdR and autoradiography in three tumours, Tc varied from 30 to 40 h. Determinations of the length of the S phase (Ts) were found to be less reliable by this method. Data on Ts and LI were also determined in all six tumours using bromodeoxyuridine (Brd) labelling and the single sample method: values of Tpot were slightly longer than those obtained via the autoradiographic method. In addition, multiple samples were taken after BrdU labelling. Tc was determined by fitting the data obtained from mid-S, mid-G2 and mid-G1 windows to curves described by a damped oscillator. Data obtained via the mid-S window were found to be most reliable. Generally, cell cycle times obtained by the BrdU method were longer than those observed with the autoradiographic method. Differences between the two methods could be explained by inaccuracies in the determination of Ts, LI and Tc and differences in the experimental approach. We consider the BrdU labelling method to be a suitable alternative for the time-consuming autoradiography, if data on Ts or Tpot are sufficient. Due to difficulties in the reproducibility of the immunofluorescence staining and asynchronization of cells approximately 10 h after labelling, the method of windows analysis was affected by similar problems to those observed in interpretation of percentage labelled mitosis (PLM) curves. However, the method may serve as an alternative to determine cell cycle times in vitro and, if improved technically, in vivo. Careful comparison of the data obtained from mid-S, mid-G1 and mid-G2 windows may increase the reliability of the determination of cell kinetic parameters.     

Turnover and maturation kinetics in the hairless mouse epidermis. Continuous [3H]TdR labelling and mathematical model analyses

Hairless mice were continuously labelled with 10 microCi of tritiated thymidine ([3H]TdR) every 4 h for 8 d, and the proportions of labelled basal and differentiating cells were recorded separately. The mitotic rate was measured by the stathmokinetic method and the cell cycle distributions were measured by flow cytometry of isolated basal cells at intervals during the labelling period. The mitotic rate of the [3H]TdR-injected animals did not deviate from control values during the first 5 d. Computer simulations of the data based on various mathematical models were made, and three main conclusions were obtained: (1) a large spread in transit times through the G1 phase was found, together with a very narrow distribution in maturation time of differentiating cells; (2) about 20% of the differentiating cells were estimated to leave the basal cell layer directly after mitosis. This is consistent with results obtained from different sets of data; and (3) during continuous labelling more than 90% of the cells are labelled during each passage through the S phase.     

Ultraviolet B irradiation induces epidermal regeneration with rapidly cycling cells

Abstract. The left flank of hairless mouse skin was irradiated with a minimal erythema dose of ultraviolet B (UVB) light at 297 nm (25 mJcm^-2), while the right flank served as untreated control. The alterations in epidermal growth kinetics induced by this UVB dose were studied with the percentage of labelled mitoses (PLM) technique during the period of increased proliferation. Thirty hours after irradiation, when a large cohort of cells appears in S phase, each animal was injected intra-peritoneally with 50 /iCi tritiated thymidine ([³H]-TdR). The number of labelled basal and suprabasal cells, as well as their localization in epidermis were registered in histological sections at short intervals up to 48 h after the [³H]-TdR pulse. Labelled mitoses were also counted in the same specimens. The results showed a four-fold increase of the high initial number of labelled cells in UVB-exposed epidermis within 18 h of the pulse injection, and a sixfold increase after 36 h. In control epidermis, where the starting value of the labelling index was much lower, there was only a three to four-fold increase in the number of labelled cells during the period studied. The PLM and the labelling index data were consistent with an average cell cycle time of approximately 10–12 h for UVB-exposed cells, in contrast to about 30 h for the fastest cycling population in control epidermis. The PLM curve also indicated a prolonged S phase duration in UVB-exposed epidermis compared with controls. In addition, labelled cells were seen in the suprabasal layer as early as 6 h after the [³H]-TdR injection and within 36 h labelled cells had reached the outermost layer of nucleated cells, indicating a reduced transit time through epidermis. The present study shows that a minimal erythema dose of UVB light at 297 nm induced a period of increased transit time through the S phase, combined with rapid cell proliferation, leading to an overall shortening of the epidermal cell cycle time. The cohort of cells labelled with [³H]-TdR 30 h after irradiation seemed to proceed as a wave of partially synchronized cells through the cell cycle for more than two rounds, which is comparable with the cell kinetic perturbations observed in regenerating mouse epidermis.     

Exogenous retinoic acid decreases in vivo and in vitro proliferative activity during the early migratory stage of neural crest cells

We have previously demonstrated that directional migration of neural crest cells (NCC) is associated with a high cell density, resulting from an active cell proliferation. It is also known that treatment with retinoic acid (RA) causes a dose-dependent inhibition of proliferation of some cell types, and that administration of RA during the early stages of embryonic development, induces cranio-facial abnormal patterns corresponding to NCC derivatives. In view of these findings, it was of interest to determine if exogenous RA is a potential modulator of the mitotic rate of NCC, and to explore the hypothesis of an inhibitory effect exerted by RA on the proliferative behaviour of NCC in vivo and in vitro. Homogenates of RA-treated chick embryos showed a low [³H]dT incorporation, indicating a generalized diminution of DNA synthesis. The labelling index (LI=number of labelled cells/total number of cells) revealed that NCC from RA-treated and control embryos had higher values of [³H]dT incorporation than neural tube cells (P < 0.0001). Autoradiographs of RA-treated chick embryos showed a significantly lower [³H]dT incorporation in NCC at the prosencephalic and mesencephalic levels, as well as in the neural tube cells at the prosencephalic, mesencephalic and rhombencephalic levels, than in control chick embryos (P < 0.0001). NCC cultures treated with 1 or 10 μm RA had a significantly lower LI than in cultures treated with 0.1 μm RA or control cultures (P < 0.04). In chick embryos, the mitotic index of NCC was 0.026 for RA-treated and 0.033 for controls, while the duration of the cell cycle was significantly longer in the NCC of RA-treated embryos (～ 40 h) than in controls (～ 25 h). The length of the cell cycle phases of NCC was similar in both experimental conditions, except for G₁ phase, which was significantly longer in the RA-treated group than in controls. These results show that RA blocks DNA synthesis and lengthens the proliferative behaviour of NCC both in early chick embryos and in vitro, effects that could modify the morphogenetic patterns of NCC distribution through a decreased cell population.     

DNA-synthesizing cells in oral epithelium have a range of cell cycle durations: evidence from double-labelling studies using tritiated thymidine and bromodeoxyuridine

Mouse tongue epithelium is characterized by a circadian variation in the number of DNA-synthesizing cells (labelling index, LI). Cells undergoing DNA synthesis were labelled with tritiated thymidine [( 3H]TdR) at 0300 (peak LI) or 1200 h (low LI). The fate of these cells was assessed by injecting animals with bromodeoxyuridine (BrdU) at intervals from 12-48 h after [3H]TdR, to follow them from one cell cycle to the next. Labelling was revealed by combining [3H]TdR autoradiography with immunoperoxidase detection of BrdU in the same sections. A single peak in the appearance of double-labelled cells was seen at 44 h, if [3H]TdR was given at 1200 h; following [3H]TdR at 0300 h, a peak of double labelling was seen at 48 h with the possibility of smaller peaks at 24 h and 36 h. These results show that the 24 h periodicity in LI in this tissue is associated with a predominant cell cycle duration of 44-48 h, but that a few cells cycle more quickly. Double labelling with [3H]TdR and BrdU provides a useful method for establishing cell cycle duration by labelling S-phase cells in successive cell cycles.     

Cell cycle parameters of adult rat hepatocytes in a defined medium. A note on the timing of nucleolar DNA replication

Hepatocytes, isolated from adult (250-350 g) rats, attached and survived well in primary culture on highly diluted (less than 1 microgram/cm2) collagen gel in a synthetic medium without serum or hormones. About 20% of the cells "spontaneously" entered S phase during the first 4 days of culturing, and mitoses were easily demonstrated at the near physiological concentration (1.25 mM) of Ca++ prevailing in the medium. Cultures given 9 nM epidermal growth factor (EGF) and 20 nM insulin 20 h after inoculation showed vigorous DNA synthesis and mitotic activity. Autoradiography of such cells exposed to [3H]thymidine allowed the determination of the following cell cycle parameters: Lag period from EGF/insulin stimulation till onset of increased DNA synthesis, 17 h; rate of entry into S phase (kG1/S), 0.028/h; duration of S phase, 8.4 h; duration of G2 phase, 2.7 h. The peak DNA synthesis (pulse labelling index, 24%) and peak mitotic activity (mitotic index, 1.7%) occurred 35 and 43 h, respectively, after the stimulation with EGF/insulin. These values are comparable to those reported during the in vivo compensatory hyperplasia following partial hepatectomy of adult rats. A marked variation of the intranuclear [3H]thymidine pulse labelling pattern was noted: During the first 1.5 h of the S phase, the labelling was extranucleolar and during the last 1.5 h chiefly nucleolar. The cells survived well in the absence of glucocorticoid, whose effect on cell cycle parameters therefore could be studied. Dexamethasone (25-250 nM) did not appreciably affect the durations of S phase and G2 phase or the pattern of preferential extranucleolar and nucleolar DNA synthesis within the S phase.     

Tracer dose and availability time of thymidine and bromodeoxyuridine: application of bromodeoxyuridine in cell kinetic studies

The present experiments with [14C]-thymidine (TdR) and [3H]-bromodeoxyuridine (BrdU) using mouse jejunal crypt cells show that the upper limit of the tracer dose of TdR is about 0.5 microgram g body weight-1 and that of BrdU is about 5.0 micrograms g body weight-1. Applying these doses, the proportions of the endogenous DNA synthesis attributed to the exogenous DNA precursor are 2% and 9% respectively. For [3H]-TdR doses commonly used in cell kinetic studies this proportion is only 0.1-1.0%, a negligible quantity that does not influence the endogenous DNA synthesis. The maximum availability time of tracer doses of TdR as well as BrdU is 40 to 60 min, the majority of the precursors being incorporated after 20 min. The availability time is the same for TdR doses exceeding the tracer dose by a factor of 80, whereas it is prolonged in the case of BrdU doses exceeding the tracer dose by a factor of 50. BrdU is suitable to replace radioactively labelled TdR in short term cell kinetic studies, i.e. determination of the labelling index or of the S phase duration by double labelling. However, more studies are needed to elucidate how far BrdU can replace TdR in long term studies as shown by differences between the fraction of labelled mitoses (FLM) curves of a human renal cell carcinoma measured with BrdU and [3H]-TdR.