Cyclin/PCNA immunostaining as an alternative to tritiated thymidine pulse labelling for marking S phase cells in paraffin sections from animal and human tissues

Paraffin sections from animal or human tissues fixed in different fixatives were submitted to immunostaining with the mouse monoclonal antibody 19A2, developed by Ogata et al. (1987a) against cyclin/PCNA. Detection of the bound antibody was performed by the indirect method with biotinylated sheep antibody and streptavidin-biotin-peroxidase complexes. No, or faint, nuclear staining was seen in material fixed in ethanol, Bouin, Bouin-Hollande, Carnoy or formaldehyde, whereas readily detectable immunocytochemical reaction was constantly observed over nuclei of methanol-fixed tissues. Hydrolysis with 2 N HCl prior to immunocytochemistry (as currently performed to render incorporated BrdU accessible to antibodies) somewhat improved the results with Bouin or Carnoy and markedly augmented the intensity of the peroxidase reactions in formaldehyde and in methanol-fixed tissues. The distribution of the positive nuclei in the two latter cases coincided with the proliferative compartment. On the other hand, double labelling with [3H]-thymidine and with the cyclin/PCNA antibody revealed that in methanol-fixed tissues the cyclin/PCNA labelling index did not differ by more than 6% from the [3H]-thymidine index. Besides the two labels overlapped in a proportion of labelled cells that was in reasonable agreement with expectation considering cells flow in and out of S phase since the time of [3H]-thymidine injection. This indicates that both labels recognize the same cells in this material. In contrast, in formaldehyde-fixed tissues, the cyclin/PCNA labelling index markedly exceeded the [3H]-thymidine labelling index. From this it is concluded that cyclin/PCNA immunostaining can be used: (1) In formaldehyde-fixed tissues (including existing material stored as paraffin blocks): for defining and mapping the proliferative (or germinative) compartment. (2) In methanol-fixed tissues as a substitute to the [3H]-thymidine autoradiographic labelling index. From this, a method is proposed (derived from classical 'double-labelling' technique) for measuring S phase duration in tissues fixed at a known interval time after a single labelling with [3H]-thymidine (or BrdU) and submitted to cyclin/PCNA immunocytochemical detection and to autoradiography (or to BrdU immunostaining).     

Changes of colon epithelium proliferation due to individual aging with cyclin proliferating cell nuclear antigen (PCNA/cyclin) immunostaining compared to [3H]-thymidine radioautography

We report a change in the proliferative activity of mouse colonic epithelium due to development and aging. In order to measure the proliferative activity, colonic epithelium was immunostained for cyclin proliferating cell nuclear antigen (PCNA/cyclin), which appears from the Gl to the S phase of the cell cycle, and compared with labeling obtained by [³H]-thymidine radioautography. Litter mice of six age groups from the fetal period (embryonic day 19), newborn period (postnatal day 1), suckling period (postnatal day 5), weaning period (postnatal dy 21), adult period (2 month old) to the senescent period (11 month old) were examined by immunohistochemistry. The descending colons were fixed in methacarn (method-Carnoy) and embedded in paraffin. Sections were stained for PCNA/cyclin activity using 19A2 monoclonal antibody and the avidin-biotin peroxidase complex (ABC) technique. For radioautography, litter mice of nine age groups using in vivo intraperitoneal administration of [³H]-thymidine. The labeling indices of colonic epithelial cells in the proliferative zone were then analyzed and compared between the two investigative methods. Our results show that the prliferative activity of mice colon was high in the fetal and newborn periods and almost constant from the suckling period to senescence, as demonstrated by both PCNA/cyclin immunohistochemistry and [³H]-thymidine radioautography. The labeling index seen by PCNA/cyclin immunohistochemistry was, however, higher than that seen by [³H]-thymidine radioautography.     

S phase duration measurement by combined PCNA/cyclin immunostaining and radioautography after a single pulse-labelling with 3H-thymidine

A method for measuring S phase duration is described and evaluated that combines single pulse labelling with 3H-thymidine (TdR), detected by radioautography, and proliferating cell nuclear antigen (PCNA)/cyclin immunostaining to replace the second pulse labelling of the classical double-labelling method. Conditions were set up in which nuclei showing one or both types of label were readily distinguished, hence allowing to verify that cell fluxes in and out of S phase were equal. S phase durations thus measured in different tissues of the mouse were concordant with those obtained by the double 3H-TdR labelling or from labelled mitoses curves. Our method might be used with archived samples of methanol-fixed cells or tissues, singly labelled with 3H-TdR or with bromodeoxyuridine.     

Cell Kinetic Studies of Chick Brain Cells In Culture: an Autoradiographic Study With [3H]- and [14C]-Thymidine

Labelling index, S-phase duration and cell-cycle time of proliferating brain cells from 6-day-old chick embryos in culture were investigated autoradiographically after labelling with [³H]- and/or [¹⁴C]-thymidine. the dissociated cells were cultured in the absence or in the presence of brain extract from 8-day-old chick embryos. Cultures contained essentially two cell types, which could be easily distinguished by the size of their nuclei: small nuclei identified as belonging to precursor cells of neurons and large nuclei corresponding to astroglial cells. the labelling index of astroglial cells (16.4%) was about 2 times higher than that of the neuronal cells (9.9%). Under the influence of brain extract the labelling index of neuroblasts was nearly doubled while that of the astroglial cells remained nearly unchanged. From double-labelling experiments with [³H]- and [¹⁴C]-thymidine, the same S-phase duration of about 7 hr was found for both cell types cultured with or without brain extract. A cell-cycle duration of 39 hr for neuronal and of 29 hr for astroglial cells was found. the cycle times remained constant under the influence of brain extract. From the measured data mentioned above, a growth fraction of 50% (neuroblasts) and 68% (astroglial cells) was calculated in control cultures without brain extract. After addition of brain extract, the growth fraction increased for both cell types (neuroblasts: 92%; astroglial cells: 80%). the results demonstrate that more cells proliferate in the presence of brain extract, but the durations of the S-phase and the cell cycle remain unchanged.     

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.     

Genesis and evolution of high-ploidy tumour cells evaluated by means of the proliferation markers p34cdc2, cyclin B1, PCNA and 3[H]-thymidine

Although cell polyploidization is not an infrequent event in mammalian cells and is common in tumours, the mechanisms involved are not well understood. Using the murine B16 cell line as a model, we evaluated the role of some key proteins involved in cell cycle progression: p34^cdc2, cyclin B1 and PCNA. By means of flow cytometry, we showed that both in modal- and in high-ploidy subpopulations, almost all cells were p34^cdc2-positive. In the modal-ploidy subpopulation only 17.1% cells were cyclin B1-positive and 85.6% PCNA-positive; in contrast, in the high-ploidy subpopulation up to 91.8% cells were cyclin B1-positive and 97.3% cells were PCNA-positive (P < 0.001). Immunofluorescence microscopy showed that PCNA was located in the nucleus; p34^cdc2, both in the nucleus and cytoplasm; and cyclin B1 yielded a cytoplasmic spotted pattern with a perinuclear reinforcement. After a 24-h incubation with ³[H]-thymidine followed by withdrawal of the isotope, high-ploidy cells remained labelled 8 days after thymidine withdrawal, in contrast to modalploidy cells. Taken together, our results suggest that polyploid cells are not quiescent, their cell cycle is longer than that of the modal-ploidy population, and they maintain cyclin B1 throughout the cycle, which may contribute to their genesis by impeding the exit from mitosis.     

Does ex vivo labelling of proliferating cells in colonic and vaginal mucosa reflect the S-phase fraction in vivo?

Summary The ex vivo labelling of DNA-synthesizing epithelial cells in colonic and vaginal mucosa was compared with in vivo labelling. For this purpose, in vivo S-phase cells were labelled with [³H]thymidine (Tdr) and ex vivo labelling was continued by culturing tissue specimens in bromodeoxyuridine (BrdU). Various methods of tissue culture were employed in order to improve diffusion of medium (and BrdU) in the tissue. BrdU and ³H-TdR labelling were evaluated by immunohistochemistry and autoradiography respectively. Ex vivo labelling resulted in a patchy distribution of labelled cells, which did not correspond with the ³H-TdR labelling pattern obtained in vivo. Under the described conditions ex vivo labelling does not appear to be a reliable for estimation of the proliferative activities in vivo.     

Genistein abolishes nucleoside uptake by cardiac fibroblasts

Genistein, a soy isoflavone, is reported to exert significant beneficial action in several human disorders, which has generated immense interest in the mechanisms underlying its effects on diverse cellular processes. It has anti-proliferative action on many cell types, an effect generally attributed to tyrosine kinase inhibition. In this study, genistein was found to cause total inhibition of [³H]-thymidine incorporation into DNA and a modest reduction in [³H]-proline incorporation into protein in primary cultures of cardiac fibroblasts. The decrease in [³H]-thymidine incorporation was not associated with a decrease in cell proliferation but correlated exactly with low intracellular levels of [³H]-thymidine. Genistein dramatically reduced [³H]-thymidine but not [³H]-proline uptake by these cells in which the equilibrative nucleoside transporter may be the major route of nucleoside uptake. The effect was irreversible and was demonstrable in pulmonary fibroblasts as well. The findings suggest that nucleoside uptake mechanisms may be a novel target of genistein action in cardiac fibroblasts and point to serious limitations in using genistein to assess the role of tyrosine kinase in cell proliferation by the standard technique of [³H]-thymidine incorporation.     

Comparative analysis of different approaches to investigate cell kinetics

The potential of different methods to investigate proliferative activity of cell populations was analysed for non-Hodgkin's lymphomas. Cells in S phase and all cycling cells were determined on cell suspensions obtained from fresh lymph node material by [3H]-thymidine autoradiography [( 3H]TdR LI), a monoclonal antibody to bromodeoxyuridine (BrdU LI), and the monoclonal antibody Ki67. A good correlation was observed between the values of [3H]TdR LI and BrdU LI (rs = 0.90; P less than 0.01), [3H]TdR LI and S phase (rs = 0.62; P less than 0.01) and [3H]TdR LI and Ki67 (rs = 0.64; P less than 0.01) in individual lymphomas. Using the median values obtained from the different approaches as cut-off points to define slowly and rapidly proliferating tumours, the best agreement was observed between [3H]TdR LI and BrdU LI (91%) and poorer agreements, even though statistically significant, were observed between [3H]TdR LI and S phase (73%) or Ki67 (76%). In conclusion, the kinetic information derived from different approaches was more or less concordant and newly proposed approaches should be directly and carefully verified for their prognostic relevance before using them as alternatives to conventional methods.     

Interferon inhibition of thymidine incorporation into DNA through effects on thymidine transport and uptake

Replenishment of medium after 72 hr of growth of HeLa-S3 cells in dense suspension cultures increased [³H]-thymidine uptake into cells and incorporation into DNA, with the levels reaching a peak ～ 12 hr following medium change; β interferon inhibits the enhanced uptake of [³H]-thymidine and labeling of DNA in a dose-dependent manner. Some reduction in these processes is observed at a concentration as low as 1 u/ml, and ～ 75% inhibition at 640 u/ml. Kinetic analysis has revealed that the rate of labeling of the acid-soluble pool with [³H]-thymidine, measured either at 22°C, or 37°C, is reduced in interferon-treated (640 u/ml, 24 hr) HeLa-S3 cells. At 22°C, the initial rate of thymidine transport at a high (500 μM) thymidine concentration, determined within the first 30 sec of [³H]-thymidine addition was depressed by 44% in interferon-treated HeLa cells. At 37°C, labeled precursors accumulate in acid-soluble material for ～ 8 min after the addition of [³H]-thymidine, after which an apparent equilibrium level is attained. At this temperature, the rate of thymidine uptake and the apparent equilibrium level attained were depressed by 70% in interferon-treated HeLa cells. The reduced incorporation of [³H]-thymidine into DNA in interferon-treated HeLa-S3 cells can be largely explained by interferon inhibition of thymidine transport and phosphorylation.     

Effects of soluble factors and extracellular matrix on DNA synthesis and surfactant gene expression in primary cultures of rat alveolar type II cells

Proliferation and differentiation of epithelial cells are thought to be regulated by soluble factors in extracellular fluid and insoluble components of the extracellular matrix. We have examined the combined effects of soluble factors and an extracellular matrix (EHS matrix) on DNA synthesis, cell proliferation, and surfactant protein gene expression in primary cultures of alveolar type II epithelial cells. Cells on EHS matrix cultured in DMEM containing insulin, cholera toxin, EGF, aFGF, 5% rat serum, and 15-fold concentrated bronchoalveolar lavage fluid (D-GM) formed larger aggregates than cells cultured on the same substratum in DMEM containing 5% rat serum (D-5). Cells cultured in D-GM on EHS matrix incorporated more [³H]-thymidine than cells on the same substratum in D-5, with an eight-fold increase seen on day 4 of culture. This increase in [³H]-thymidine incorporation was accompanied by a labeling index of greater than 65% of the cells. Cell counts showed that exposure of type II cells on EHS matrix to D-GM resulted in increased cell number on day 4 of culture. [³H]-thymidine autoradiography combined with immunostaining with anti-cytokeratin, anti-SP-A, and anti-vimentin antibodies demonstrated that the proliferating cells were epithelial cells that contained SP-A. Type II cells cultured on plastic in D-GM also showed increased [³H]-thymidine incorporation compared to cells cultured in D-5. The level of [³H]-thymidine incorporation by cells on plastic, however, was significantly less than that seen in cells cultured in the same medium on EHS matrix. Type II cells cultured on EHS matrix in D-GM had a decreased abundance of mRNAs for SP-A and SP-C than cells cultured on EHS matrix in D-5 as determined by Northern analysis. This inhibition was reversed by switching from D-GM to D-5 on day 4 and culturing the cells for an additional 4 days. In contrast, SP-B mRNA was increased in response to D-GM. This increase was not reversed by switching from D-GM to D-5 on day 4. These results suggest that the interaction of soluble factors and extracellular matrix components has a strong influence on type II cell proliferation, which were partially associated with the reversible inhibition of lung tissue-specific protein mRNAs. Their dynamic interplay among the type II cell, the extracellular matrix, and growth factors may determine multicellular functions and play an important role in normal lung development and in the repair of the lung epithelium following injury.     

Discrimination of two fibroblast progenitor populations in early explant cultures of hamster gingiva

Summary Numerous metabolic studies have demonstrated heterogeneity of fibroblast populations in culture, yet little is known about the structure of fibroblast populations in adult tissues in vivo. To determine if populations of both cycling and non-cycling cells are present in gingiva, hamsters were labelled with [³H]-thymidine to label cycling cells in vivo, and explanted biopsies were subsequently incubated with bromodeoxyuridine to label cycling cells in vitro. Cycling cells were identified by combined immunohistochemistry and radioautography. Fibroblasts were recognized by the presence of vimentin and the absence of keratin as determined by immuno-fluorescence. The largest proportion of cells were double-labelled with [³H]-thymidine and bromodeoxyuridine (43.8%) indicating the presence of actively cycling populations that maintained their proliferative status upon explantation. Cultures also exhibited a second population of cells labelled only with bromodeoxyuridine (38.7%) that did not cycle in vivo, but retained the capacity for proliferation in vitro. However, limiting dilution analysis of single-cell suspensions revealed only a single class of progenitors capable of forming large colonies in vitro. Approximately 1 in 190 plated cells was capable of colony-formation, indicating that, upon explantation, a subset of the cycling cells in vitro exhibits extensive proliferative capacity. There was also a small population of cells unlabeled with either [³H]-thymidine or bromodeoxyuridine (9.4%) that appeared to be terminally differentiated. Different substrates, including glass and thin films of gelatin and collagen, did not significantly alter the fraction of cells labelled with [³H]-thymidine. These data demonstrate the existence of 2 separate progenitor-cell populations with different capacities for proliferation in vivo and in vitro.     

cAMP and PMA enhance the effects of IGF-I in the proliferation of endometrial adenocarcinoma cell line HEC-1-A by acting at the G1 phase of the cell cycle

The present study was undertaken to determine whether endometrial cancer cell line HEC-1-A differ from nontransformed cells, in that the cAMP and protein kinase C pathways may enhance IGF-I effects in mitogenesis by acting at the G₁ phase of the cell cycle instead of G₀. Immunofluorescence staining of HEC-1-A cells using the proliferating cell nuclear antigen (PCNA) monoclonal antibody and flow cytometric analysis determined that HEC-1-A cells do not enter the G₀ phase of the cell cycle when incubated in a serum-free medium. Approximately 51% of the cells were in G₁, 12% were in S and 37% in G₂ phase of the cell cycle prior to treatment. Forskolin and phorbol-12-myristate 13-acetate (PMA) were used to stimulate cAMP production and protein kinase C activity, respectively. IGF-I, forskolin and PMA each increased (P <0.01) [³H]-thymidine incorporation in a dose and time dependent manner. The interaction of forskolin and PMA with IGF-I was then determined. Cells preincubated with forskolin or PMA followed by incubation with IFG-I incorporated significantly more (P <0.01) [³H]-thymidine into DNA than controls or any treatment alone. It is concluded that forskolin and, to a lesser extent, PMA exert their effect at the G₁ phase of the cycle to enhance IGF-I effects in cell proliferation.     

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.     

Enhancement of phospholipid hydrolysis in vasopressin-stimulated BHK-21 and H9c2 cells

The hydrolysis of phospholipids in vasopressin-stimulated baby hamster kidney (BHK)-21 and H9c2 myoblastic cells was investigated. Phosphatidylcholine and phosphatidylethanolamine in these cells were pulse labelled with [³H]glycerol, [³H]myristate, [³H]choline or [³H]ethanolamine, and chased with the non-labelled precursor until linear turnover rates were obtained. When cells labelled with [³H]glycerol or [³H]myristate were stimulated by vasopressin, no significant decrease in the labelling of phosphatidylcholine was detected, but the labelling of phosphatidic acid was elevated. However, the labellings of phosphatidylethanolamine and its hydrolytic product were not affected by vasopressin stimulation. When the cells were pulse labelled with [³H]-choline, vasopressin stimulation caused a decrease in the labelled phosphatidylcholine with a corresponding increase in the labelled choline. The apparent discrepancy between the two types of labelling might be explained by the recycling of labelled phosphatidic acid back into phosphatidylcholine, thus masking the reduction in the labelled phospholipid during vasopressin stimulation. Alternatively, the labelled choline produced by vasopressin stimulation was released into the medium, thus reducing the recycling of label precursor back into the phospholipid and making the decrease in the labelling of phosphatidylcholine readily detectable. Further studies revealed that vasopressin treatment caused an enhancement of phospholipase D activity in these cells. The presence of substrate-specific phospholipase D isoforms in mammalian tissues led us to postulate that the differential stimulation of phospholipid hydrolysis by vasopressin was caused by the enhancement of a phosphatidylcholine-specific phospholipase D in both BHK-21 and the H9c2 cells.Abbreviations BHK-21 cells baby hamster kidney-21 cells     

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

Abstract. The present experiments with [¹⁴C]-thymidine (TdR) and [³H]-bromo-deoxyuridine (BrdU) using mouse jejunal crypt cells show that the upper limit of the tracer dose of TdR is about 0.5 µg g body weight^-1 and that of BrdU is about 5·0 µg 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.     

Periodic acid incubation can replace hydrochloric acid hydrolysis and trypsin digestion in immunogold-silver staining of bromodeoxyuridine incorporation in plastic sections and allows the PAS reaction

Summary We have examined the possibility of improving the present methods of detecting bromodeoxyuridine (BrdU) and for combining the PAS reaction with the BrdU detection by means of immunogold-silver staining (IGSS). This was done in testes fixed in Carnoy or Bouin, and in parts of the small intestine which were fixed in Carnoy or periodate-lysine-paraformaldehyde (PLP). All tissues were embedded in a mixture of glycol methacrylate and butanediol-monocrylate. It was found to be impossible to carry out BrdU detection using HCl hydrolysis and trypsin digestion in combination with a PAS reaction. However, incubation of the plastic sections in periodic acid for a period of 30 minutes appeared to make it possible to eliminate the HCl denaturation step and to carry out a specific PAS reaction. Moreover, after incubation in periodic acid, trypsin digestion was no longer required to make the BrdU label accessible in GMA-embedded sections, nor to re-expose the antigenic sites in plastic sections of tissues fixed with cross-linking fixatives. In this way the loss of cell structures, which is inevitable when trypsin is used, can be avoided. Now a BrdU detection with improved morphology can be combined with the PAS reaction in the same plastic section in order to stain tissue carbohydrates. This is important for tumour diagnosis, where the PAS reaction can be very useful.     

Effect of CO2 on labelling of nerve and liver cells by nucleic acid precursors

In vivo experiments in mice demonstrated that 5% CO₂ content in the air inhaled did not change the labelling in autoradiograms from animals injected with [³H]uridine, [³H]orotic acid, [³H]hypoxanthine, [³H]lysine or [³H]cytidine. At 20% CO₂ content there was a significant decrease in labelling of brain cells with [³H]uridine and [³H]cytidine, but not following [³H]lysine; there was no labelling of nerve cells with [³H]orotic acid or [³H]hypoxanthine, but a control group was not included. The labelling of choroid plexus and hepatocytes was independent of the CO₂ concentration. A comparison of in vivo and in vitro experiments at 20% CO₂ content showed a similar significant decrease in labelling of brain cells with [³H]uridine and [³H]cytidine. It is concluded that a metabolic change is the most appropriate explanation of the CO₂ effect.     

Mathematical model analysis of mouse epidermal cell kinetics measured by bivariate DNA/anti-bromodeoxyuridine flow cytometry and continuous [3H]-thymidine labelling

Abstract. In a previous study the epidermal cell kinetics of hairless mice were investigated with bivariate DNA/anti-bromodeoxyuridine (BrdU) flow cytometry of isolated basal cells after BrdU pulse labelling. The results confirmed our previous observations of two kinetically distinct sub-populations in the G₂ phase. However, the results also showed that almost all BrdU-positive cells had left S phase 6–12 h after pulse labelling, contradicting our previous assumption of a distinct, slowly cycling, major sub-population in S phase. The latter study was based on an experiment combining continuous tritiated thymidine ([³H]TdR) labelling and cell sorting. The purpose of the present study was to use a mathematical model to analyse epidermal cell kinetics by simulating bivariate DNA/BrdU data in order to get more details about the kinetic organization and cell cycle parameter values. We also wanted to re-evaluate our assumption of slowly cycling cells in S phase. The mathematical model shows a good fit to the experimental BrdU data initiated either at 08.00 hours or 20.00 hours. Simultaneously, it was also possible to obtain a good fit to our previous continuous labelling data without including a sub-population of slowly cycling cells in S phase. This was achieved by improving the way in which the continuous [³H]TdR labelling was simulated. The presence of two distinct sub-populations in G₂ phase was confirmed and a similar kinetic organization with rapidly and slowly cycling cells in G₁ phase is suggested. The sizes of the slowly cycling fractions in G₁ and G₂ showed the same distinct circadian dependency. The model analysis indicates that a small fraction of BrdU labelled cells (3–5%) was arrested in G₂ phase due to BrdU toxicity. This is insignificant compared with the total number of labelled cells and has a negligible effect on the average cell cycle data. However, it comprises 1/3 to 1/2 of the BrdU positive G₂ cells after the pulse labelled cells have been distributed among the cell cycle compartments.     

Utilization of the label from bacterial [3H] DNA by isolated barley roots and embryos under different conditions

[³H] DNA fromEscherichia coli and [³H] thymidine were applied, in sterile conditions, on isolated barley embryos and on roots excised from these embryos, both cultivated in the liquid medium and on halves of barley seeds, through the endosperm bridge. In embryos and roots, the labelled compounds were applied in 1.5% sucrose + 0.2 SSC alone, or together with either unlabelled thymidine or DEAE-dextran. Similar labelling indices were found after [³H] thymidine and [³H] DNA treatment which shows that the activity of [³H] DNA is utilized during the S phase. After application of [³H] thymidine, only cell nuclei in S phase were labelled. After the application of [³H] DNA an extranuclear label, in addition to the labelling of nuclei in the S phase, was observed in some experimental variants. The density of label above labelled nuclei after [³H] DNA treatment sharply decreased when unlabelled thymidine or DEAE-dextran was added, while the density of label above nuclei labelled by [³H] thymidine decreased when unlabelled thymidine but not DEAE-dextran was added. The labelling of nuclei with the label from [³H] DNA is the result of degradation of exogenous DNA reutilization of low molecular weight products. Extranuclear labelling is most probably due to the polymerous or partly degraded DNA.