Cell Birth Rate Research Articles

Apoptosis and cancer risk assessment

Apoptosis is one form of physiological or active cell death. The balance between cell proliferation and cell death or apoptosis not only effects organ growth but also has a profound impact on the net increase and growth of initiated cells and preneoplastic and tumor cell populations. With respect to cancer development apoptosis is becoming widely recognized as being an innate tissue defense against carcinogens by inhibiting survival and controlling growth of precancerous cell populations and tumors at different stages of carcinogenesis. Experimental data on cell birth and cell death rates help identify the mode of action of a chemical and can be incorporated into biologically based cancer models. This article describes the quantitation and regulation of apoptosis in rodent liver and how loss of regulation can have a role in hepatocarcinogenesis. A biologically-based mouse liver cancer model is presented and utilized to describe how treatment related growth effects affect the process of carcinogenesis. Advantages and limitations of biologically based cancer models in cancer research and risk assessment are discussed.

Proliferation markers

Types of growth include embryonic, fetal, neonatal, juvenile and mature. Until full differentiation is achieved, cells grow through proliferation from progenitor cells. At maturity, the cellular genome is fixed with committed patterns of cell cycle duration and adaptation, ranging from static to renewing type 3. The static cell type cannot proliferate and adapts through hypertrophy. The renewing type continuously proliferates even without stimulus. In all cell types the processes of differentiation and proliferation are mutually exclusive. Cellular kinetics involve (a) the duration of the cell cycle, (b) the birth rate of cells, and (c) the growth rate fractions. The duration of the cell cycle is 2-4 days. All growth factors (GF) exert their influence during G1 phase. Release a GF by one cell type can influence the proliferation of another (= paracrine stimulation). At the end of G1 is the point of highest sensitivity to toxicity. Tumor suppressor genes act here through tyrosine phosphorylation. During S, the cell replicates its chromosomes. During G2 the immune surveillance and DNA damage repair mechanisms operate. Injured cells stay here longer enabling repair of their damaged DNA. Cell division involves both nuclear (mitosis) and cytoplasmic (cytokinesis) phases giving rise to 2 new cells. The cell cycle has 2 checkpoints. The first involves the G1-S transition and the second the G2-M transition. The types of cell cycle inhibition include (a) cycle- and phase-specific inhibition; (b) cycle-and nonphase-specific inhibition; (c) noncycle-and nonphase-specific inhibition, and finally (d) noncycle, nonphase-, and nonorgan-specific inhibition. Proliferation is a circadian process and it is stimulated by a variety of stimuli which include (1) interference with hormonal feedback pathways; (2) inhibition of the tissue trophic activity; (3) sustained presence of antigenic substances; (4) tissue ischemia; (5) changes of conditions luminally or on surfaces of tissues; (6) sustained cytotoxicity; (7) cell death; and (8) surgical resection. Proliferation can be arrested through senescence, apoptosis, injury or even during the development of immune cells. In the past, tissue/cell kinetics have been studied by tritiated thymidine histoautoradiography. Recently, monoclonal antibodies to proliferation-associated antigens, have been successfully employed. These antigens are cycle-associated proteins and include (1) PCNA; (2) p53; (3) Ki67; (4) AGNOR; (5) Statin; and (6) BrdU. Practical examples are given comparing PCNA and BrdU markers from 3 tissues, i.e. liver, glandular stomach, and uterus, across 2 or 3 strains of rats. Mean values of labeling indices are cited. Within the PCNA marker, 2 different clones are compared from the glandular stomach of SD rats of 2 different ages. Gender and across species comparisons are also made. All these comparisons denote that in every study where markers are used (a) there is a need for a concurrent study control group of the same age; (b) there is a need for in-house control data for this particular organ by species, strain, gender and age; (c) there is ancillary assessment of the trophic status of the target tissue; (d) there is a need for at least 2 different time points during assessment; (e) there is a need for such proliferation data prior to commencing the 2 year rodent bioassay; and (f) that PCNA is the most reliable and versatile of all markers used, capable of rendering good results even from archival specimens.

Germinal‐center cell proliferation in response to T‐independent antigens: A stathmokinetic, morphometric and immunohistochemical study in vivo

Although the nature of the germinal center reaction during responses to T-dependent antigens has been well documented, much less is known regarding the relationship between germinal centers and T-independent antigens. In this study, germinal-center cell proliferation was determined at specific time points in spleens of C3H/HeN mice following immunization with either the type-1, T-independent antigen dinitrophenol-lipopolysaccharide (DNP-LPS), or the type-2, T-independent antigen DNP-Ficoll. A stathmokinetic technique was employed to assess proliferation in terms of germinal center cell birth rate and morphometry was used to measure actual growth and regression of the germinal center cell population. An estimate of the absolute rate of germinal-center (GC) cell proliferation was derived from these two values. In addition, immunohistochemistry was performed to correlate changes in GC cell proliferation with the presence or absence of antigen within GC. Following immunization with both antigens, there was an initial reduction of proliferation within pre-existing germinal centers which manifested as either GC dissociation (DNP-LPS) or a suppression of birth rates (DNP-Ficoll). This was followed by a period of increased GC cell proliferation in animals immunized with DNP-LPS, but not in those exposed to DNP-Ficoll. GC cell proliferation was then measured in mice treated with cyclosporin A from 1 day before to 2 days after immunization with DNP-LPS. In these animals, the expected increase in GC cell birth rates did not take place. Immunohistochemistry showed that DNP-Ficoll and DNP-LPS were present in GC from 1 day after immunization until the end of the experiment on day 7. Treatment with cyclosporin A did not affect the deposition of DNP-LPS in GC. These results show that only some T-independent antigens are able to stimulate GC cell proliferation, and we propose that this is related to their ability to recruit precursors of GC B cells into the GC reaction. In addition, the results indicate that GC proliferation seen in response to a so-called T-independent antigen is at least partly driven by T cell-derived cytokines.

Regulation of murine germinal centre cell proliferation in vivo: a stathmokinetic study examining the effect of differently timed doses of cyclosporin A.

Although studies have identified factors which affect germinal centre cell proliferation in vitro, their relative contributions in vivo remain largely undetermined. In this study, the proliferative rate of germinal centre cells was measured in sheep red blood cell-immunized C3H/HeN mice exposed to variously timed doses of cyclosporin A. Germinal centre (GC) cell proliferation was measured by a stathmokinetic technique to determine GC cell birth rates at specific time points after immunization. Changes in total GC volume were determined by morphometry in order to assess actual growth and regression of the GC cell population. An estimate of the absolute rate of GC cell proliferation was derived from these two values. Following exposure to antigen, there was an initial inhibition of proliferation within pre-existing germinal centres, followed by a rapid rise, then a sustained phase of increased GC cell proliferation. By comparing the effects of the different cyclosporin A treatment regimes, it was possible to deduce that the initial inhibition of proliferation was mediated by a T-cell-derived cytokine, as was the final sustained phase of the proliferative response. The intervening rise in GC cell proliferation, however, was attributable to a contact-dependent signalling mechanism.

Tissue kinetics, ion transport, and recruitment of mitochondria-rich cells in the skin of the toad (Bufo bufo) in response to exposure to distilled water.

Mitochondria-rich cells (MRC) of the amphibian epidermis are responsible for active chloride uptake at low external salinity, and new MRCs are recruited in response to exposure to distilled (deionized) water. The time-course of this recruitment, the tissue kinetics and ion transport have been studied in toads (Bufo bufo) immediately before, and after 2,7, and 14 days exposure to distilled water. General epidermal structure was not affected. However, the numbers of MRCs per mm2 (DMRC) increased throughout the experiment as revealed by staining of epidermal sheets with AgNO3 (Ag) or methylene blue (MB). Part of the increased DMRC was accounted for by an increase in MRC subpopulation(s) that stained neither with Ag nor MB. The cell birth rate (Kb) decreased and cell loss by moulting (Kd) increased without any significant change in epidermal cell pool size, indicating a reduced apoptotic rate. The increase in DMRC was accompanied by a 3-fold increase in Cl- current (ICl). At day-2 there was a transient reduction in the ICl per MRC. H+ secretion was progressively reduced during prolonged exposure to distilled water. Thus, at day-2 MRCs appeared incompletely differentiated as indicated by decreased ICl and H+ flux per MRC, and by the increased proportion of MRCs unstained by Ag or MB. Full Cl- (but not H+) transport capacity, was restored at day-7. We conclude that increased DMRC following exposure to low external Cl-, rather than being due to an increased Kb, is the combined effect of a decreased apoptotic rate and an increased rate of differentiation, where 'morphological differentiation' precedes 'functional differentiation'.

Challenges to Default Assumptions Stimulate Comprehensive Realism as a New Tier in Quantitative Cancer Risk Assessment

The current practice in carcinogen risk assessment of using a linearized multistage model and assuming low-dose linearity is based on several false premises. In many cases linearity at low doses would not be expected based on the interaction between the multiple components in the carcinogenic process. The two-stage growth models, involving multiple mutations and cell birth and death rates, provide one means of exploring these interactions. In addition, if carcinogenesis is considered to be the imbalance between invading substances and defense mechanisms, then the cancer probability depends on how much the substance increases or decreases the number of defenders or their efficiency as well as increasing or decreasing the number of invaders. Challenges to low-dose linearity and other default assumptions have stimulated the development of new risk assessment methodologies as have the need for more realistic estimates of risk, better uncertainty characterization, and greater utilization of cost-benefit analyses, and other tools for risk management decision making. "Comprehensive realism" is an emerging quantitative weight-of-evidence risk. assessment methodology which is designed to reflect all of the relevant and available information and the current state of knowledge about the health risks associated with a substance.

Simultaneous assessment of cell kinetics and programmed cell death in bone marrow biopsies of myelodysplastics reveals extensive apoptosis as the probable basis for ineffective hematopoiesis

Despite hypercellular bone marrows (BM), peripheral cytopenias are the rule in patients with myelodysplastic syndromes (MDS). This study examined the roles played by cell birth and cell death rates in generating this paradox. Cell kinetics from BM biopsies of 35 MDS patients were measured using intravenous infusions of either iododeoxyuridine or bromodeoxyuridine, or both. Degree of apoptosis or programmed cell death (PCD) was estimated using in situ end-labeling of DNA directly from BM biopsies, which were simultaneously double-labeled from proliferation/PCD. MDS were found to be highly proliferative disorders with large numbers of myeloid, erythroid, and megakaryocytic cells synthesizing DNA. Median cycling time (Tc) of myeloblasts was more rapid than that of patients with acute myeloid leukemia (44.1 hr vs. 56.0 hr). Interestingly, most marrow cells of all three lineages in 32 of 34 evaluable cases were undergoing PCD. In 19 of 32 patients, greater than 75% cells were apoptotic. Surprisingly, large numbers of S-phase cells were found to be simultaneously undergoing PCD, as were stromal cells of the BM microenvironment. We conclude that the extensive apoptosis in hematopoietic cells effectively cancels the high birth rate resulting in ineffective hematopoiesis and accounting for deficient bone marrow function.

Growth Kinetics of Enzyme-Altered Liver Foci in Rats Treated with Phenobarbital or α-Hexachlorocyclohexane

A quantitative method based upon a stochastic model for the appearance of initiated cells and their clonal growth was used to estimate cell birth and death rates in enzyme-altered liver foci (EAF). gamma-Glutamyltranspeptidase (gamma-GT)-positive foci were initiated in livers of female SPF Wistar rats by a single application of N-nitrosomorpholine. Serial terminations during and after stop of promoter treatment with either phenobarbital (PB) or alpha-hexachlorocyclohexane (alpha-HCH) provided information on the growth and regression of the EAF. Simultaneous labeling index (LI) measurements were obtained via single injections with [3H]thymidine. No significant increases of the LI were observed with PB or alpha-HCH treatment. Since both agents are strong liver promoters we conclude that the growth of gamma-GT-positive foci is mainly due to a decrease in the rate of apoptosis. Indeed, our analysis supports this conclusion but determines that the abrogation of homeostatic control during promoter treatment is subtle. The ratio of cell death and cell birth rate is found to be decreased only slightly during promoter treatment and slightly increased after stop of promotion. For the mathematical analysis, two distinct focal growth scenarios were employed: (i) volume growth, i.e., all cells within individual foci cycle actively with the same rate, and (ii) surface growth where only cells on the surface of foci cycle actively while interior cells are resting. The model based upon scenario (ii) provides a better fit to the data and is more consistent with the experimental observations indicating heterogeneity of cell division rates within individual foci.

Spatial distribution of mitosis, apoptosis and small blood vessels in malignant diffuse follicular-center-cell lymphomas: a nearest-neighbor analysis.

To better comprehend the relationships between cell birth and cell death in neoplastic disorders, the topography of these events needs to be considered. We applied a computerized nearest-neighbor analysis to malignant, diffuse follicular-center-cell lymphomas in order to examine the spatial distribution of pyknotic (apoptotic) cells/bodies (A) and mitotic figures (M) in relation to capillaries (C) and venules (V). The results revealed a complex dispersion pattern, with significant aggregations of A and M, in addition to an even greater random distribution component. The greatest clustering displayed by A was around capillaries (A-C, 42% reduction of object frequency between the first and second distance class), followed by A-A (33%), A-V (24%) and A-M (12%). Additional values for mitotic figures were: M-M (45%), M-V (33%) and M-C (22%). These findings may reflect the relative importance of inherent properties of the neoplastic and host factors, respectively, in the regulation of cell birth and cell death rates.

Beta-cell proliferation in normal and streptozotocin-treated newborn rats: site, dynamics and capacity.

Regeneration of neonatal beta cells after streptozotocin (STZ)-induced destruction may be due to either replication from pre-existing intra-islet beta cells or extra-islet precursor cells. To further investigate this issue, beta-cell growth was analysed in normal and streptozotocin-treated newborn rats (100 micrograms/g body weight) at several time points during the first 20 days of life. Beta cells were identified by insulin immunostaining, non-isotopic in situ hybridization for rat preproinsulin mRNA, and electron microscopy. Their proliferative activity was recorded by bromodeoxyuridine-pulse labelling. Beta-cell size and total volume were determined by computerized morphometry. In normal rats, there was a threefold increase in total beta-cell volume during the first 5 days of life, with no further expansion till day 20. The bromodeoxyuridine labelling index of the intra-islet beta cells was smaller than that of the extra-islet beta cells (2-3% vs 15-20%). Comparison of the cell birth rate, calculated from the beta-cell labelling index, with the observed increase in beta-cell volume suggested that in normal neonatal rats proliferation of the intra-islet beta-cell population could account for only 10% of the observed expansion. Administration of streptozotocin at birth resulted in more than 90% reduction of the total beta-cell volume at day 2 which then increased to 39% of the normal value by day 20. During this period of partial regeneration, which restored normoglycaemia, the labelling index of intra-islet beta cells was higher than in normal rats (9% vs 2%, p < 0.001), whereas no change was seen in the extra-islet beta-cell labelling index.(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-cell proliferation in normal and streptozotocin-treated newborn rats: site, dynamics and capacity

Regeneration of neonatal beta cells after streptozotocin (STZ)-induced destruction may be due to either replication from pre-existing intra-islet beta cells or extra-islet precursor cells. To further investigate this issue, beta-cell growth was analysed in normal and streptozotocin-treated newborn rats (100 μg/g body weight) at several time points during the first 20 days of life. Beta cells were identified by insulin immunostaining, non-isotopic in situ hybridization for rat preproinsulin mRNA, and electron microscopy. Their proliferative activity was recorded by bromodeoxyuridine-pulse labelling. Beta-cell size and total volume were determined by computerized morphometry. In normal rats, there was a threefold increase in total beta-cell volume during the first 5 days of life, with no further expansion till day 20. The bromodeoxyuridine labelling index of the intraislet beta cells was smaller than that of the extra-islet beta cells (2–3% vs 15–20%). Comparison of the cell birth rate, calculated from the beta-cell labelling index, with the observed increase in beta-cell volume suggested that in normal neonatal rats proliferation of the intra-islet beta-cell population could account for only 10% of the observed expansion. Administration of streptozotocin at birth resulted in more than 90% reduction of the total beta-cell volume at day 2, which then increased to 39% of the normal value by day 20. During this period of partial regeneration, which restored normoglycaemia, the labelling index of intra-islet beta cells was higher than in normal rats (9% vs 2%, p<0.001), whereas no change was seen in the extra-islet beta-cell labelling index. Comparison of cell birth rate with the increase in beta-cell volume indicated that 50–60% of the observed beta-cell growth could result from the intra-islet beta-cell proliferation. These results suggest that replication from pre-existing, surviving beta cells plays an important role in regeneration of neonatal beta cells after destruction by streptozotocin.

Radiation-associated hyperparathyroidism: comparison of adenoma growth rates, inferred from weight and duration of latency, with prevalence of mitosis.

In sporadic parathyroid adenomas, the birth rate of new cells, based on the proportion of S-phase cells at the time of surgical excision, is much too low to account for growth of the tumor from a single cell, as is required by monoclonal origin, even if the mutation occurred in utero, indicating that the rate of cell proliferation has slowed down during the course of the disease. In radiation-associated hyperparathyroidism, the age at irradiation provides a more accurate upper limit to the age of the tumor. The purpose of this study was to relate this age to the prevalence of mitosis as an alternative index of current cell proliferation. In 56 such patients, the geometric mean for the minimum cell birth rate needed for growth from a single cell to the observed size in the time available was 54.4%. In 44 patients, including 31 of the previous 56 and an additional 13, sampling an average of 220,000 cell profiles, 15 mitoses were found, an overall prevalence of 0.15/10(5), which corresponds to a cell birth rate of 2.7%/yr, assuming the duration of mitosis to be 0.5 h. If cases with no mitosis were assigned a value of half the detection limit, the geometric mean mitotic index was 0.360/10(5), and the corresponding cell birth rate was 6.4%/yr. This is more than 8 times smaller than the minimum birth rate required and 20 times smaller than the cell birth rate in meningiomas, suggesting that such extreme reduction of cell birth rate is a unique feature of parathyroid adenomas, rather than a general feature of all benign tumors. The data support the set-point hypothesis, which reconciles the earlier concept of focal hyperplasia with monoclonal origin and provides an alternative nonneoplastic mechanism of etiology for the usual nonprogressive form of the disease.

Cell kinetics of the germinal center reaction ‐ a stathmokinetic study

The changes in splenic germinal center (GC) cell proliferation were measured during primary and secondary responses to a T-dependent antigen in vivo to examine the regulation of the GC reaction. Adult C3H/HeN mice were immunized with sheep red blood cells and boosted 7 or 21 days later. GC cell proliferation was assessed by measurement of GC cell birth rates using a stathmokinetic technique. Actual GC growth and regression were assessed in terms of total splenic volume and number. Pre-existing GC had a mean cell birth rate of 33 cells/1000 cells/h. The GC reactions following each immunization showed a biphasic pattern of changes in cell birth rate, comprising an initial fall immediately succeeded by a transient, but significant, increase. These fluctuations occurred earlier in secondary compared to primary responses. Significant increases in total GC volumes succeeded the peaks of cell birth rate following both primary and early secondary immunization. However, there was a substantially smaller increase following later secondary immunization. We propose that the initial cell birth rate reduction is due to inhibition of pre-existing GC clones and represents one component of the phenomenon of GC dissociation. The succeeding peak birth rate represents early, massive proliferation of newly activated antigen-specific clones. The different patterns of GC expansion, despite similar proliferative responses, may reflect different pathways of differentiation dependent on the timing of antigenic stimulation.

The phantom of tumor treatment - continually rapid proliferation unmasked

Therefore, if the doubling time of clonogens in the tumor towards the end of radiotherapy (T~er) becomes the same as the potential doubling time measured before treatment, this means by definition that the cell loss factor has become zero. The residual tumor is now doubling in only a few days (i.e. similar to Toot) instead of the 50 or 100 days (Tvol) typical of many tumors before treatment. Since the spontaneous cell loss factors in human tumors are usually large, often between 90 and 99~o in the epithelially derived carcinomas, the fall to zero represents a large change. Even if T~er and Tpot are not exactly the same, but are merely similar, both being just a few days, it means that the spontaneous cell loss factor has fallen nearly to zero. How can this be? And more importantly from the practical point of view, when does it occur? More trivially, the rapid proliferation represented by the short values of Tefrderived by many authors from clinical data is not really accelerated proliferation. It is the unmasked birth rate of tumor cells, no longer concealed by a spontaneous cell loss factor. It

Cell kinetics of human gastric mucosa and gastric cancer in organ culture

The cell kinetics of human gastric epithelium in organ culture have been measured using flash labelling with tritiated thymidine and the metaphase arrest technique to estimate cell birth rates. Normal gastric antral and body mucosa have been compared with mucosa showing gastritis and gastric carcinoma. Labelling indices with tritiated thymidine in normal gastric mucosa declined over a 48-h period suggesting that essential growth factors were lacking. Labelling indices and cell birth rates were higher in gastritis than in normal mucosa and highest in gastric carcinoma. Labelling indices were higher in intestinal-type gastric carcinoma than diffuse carcinoma. In metaphase arrest experiments carcinomas showed on average an eightfold increase in resistance to the metaphase-arresting properties of vincristine when compared with normal mucosa. The validity of using the metaphase arrest technique to measure cell birth rate in gastric cancers in view of this vincristine resistance is discussed.

Inhibitory effect of a cholecystokinin antagonist on the proliferative response of the pancreas to pancreatobiliary diversion.

Since pancreatobiliary diversion probably stimulates pancreatic growth by increasing cholecystokinin secretion, the effect of the cholecystokinin antagonist CR-1409 on this adaptive response was tested. Male Wistar rats (n = 108) weighing 220-250g were randomised to receive either pancreatobiliary diversion (n = 60) or sham diversion (n = 48) and thereafter to receive either saline injections or CR-1409 (10 mg/kg/day, subcutaneously). Rats were killed at four, seven, and 14 days postoperatively, when blood was obtained for cholecystokinin assay and the pancreas was assessed for proliferative activity by three techniques: nucleic acid and protein assay, bromodeoxyuridine labelling, and metaphase arrest after vincristine administration (1 mg/kg, intraperitoneally). Pancreatobiliary diversion increased plasma cholecystokinin concentrations by 91% at seven days and 137% at 14 days, irrespective of CR-1409 treatment. Total pancreatic RNA content was doubled by pancreatobiliary diversion at four days (2.15 v 1.07 mg/100 g body weight: p less than 0.001) and at seven days (3.43 v 1.76 mg/100 g: p less than 0.001), and trebled at 14 days (4.27 v 1.32 mg/100 g: p less than 0.001). Pancreatobiliary diversion increased bromodeoxyuridine labelling index from 1.1 to 3.7% at seven days and the cell birth rate from 0.09 to 0.06%. CR-1409 completely abolished this proliferative response and partly prevented the rise in RNA. The results confirm pancreatic hypertrophy and increased acinar cell proliferation after pancreatobiliary diversion. CR-1409 prevents this adaptive growth, probably by blocking cholecystokinin receptors. Bromodeoxyuridine labelling and the metaphase arrest technique may be used to assess pancreatic cell kinetics.

Cell kinetics in parathyroid adenomas: evidence for decline in rates of cell birth and tumour growth, assuming clonal origin

We aimed to provide a cell kinetic explanation for the demonstrated lack of disease progression in most patients with mild, asymptomatic primary hyperparathyroidism. We compared cell birth rates, estimated at the time of adenoma excision, with the lowest birth rates needed to grow tumours of the observed size. Sixty-three patients with primary hyperparathyroidism due to a single chief cell adenoma who had normal renal function were followed up for long enough to demonstrate cure after surgical excision. Fresh adenoma tissue was incubated with tritiated thymidine. The proportion of cells synthesizing DNA was determined directly by radioautography in 18 cases, and indirectly from the regression of label index on rate of DNA synthesis in 45 cases. The birth rate of new cells was calculated assuming the duration of S phase to be 12 hours. The number of cells in each adenoma was estimated both from parenchymal weight and from total DNA content, and the minimum birth rate needed to produce this number of cells from a single cell, beginning in utero, was calculated on an exponential model. The mean observed birth rate of new cells (mean (SD)) was 17.3 (11.1)%/year, and the minimum needed birth rate was 42.8 (21.6)%/year, significantly, (P less than 0.001) higher than the observed birth rate. The rate of mitosis had fallen substantially during the life span of most parathyroid adenomas. To account for this, we propose that the mutation implied by a clonal origin increases the secretory setpoint. Because proliferation, as well as hormone secretion, is influenced by calcium in parathyroid cells, the expected result would be rapid initial growth, slowing down as tumour size reached an asymptotic value corresponding to the total rate of hormone secretion needed to raise the plasma calcium to the new setpoint.

Biologically Based Pharmacodynamic Models: Tools for Toxicological Research and Risk Assessment

The cancer model contains extensive biological detail, but is not fully validated. It is extremely useful for organizing what is known about malignant transformation, for linking measures of tissue dose with biological processes, and for proposing new experiments. It became very clear, as the cancer model was developed, that we lack an adequate understanding of the details of biochemical mechanisms of carcinogen action linking the target-tissue dose of carcinogen with effects on cell birth, death, and mutation rates as specified by the MVK model. This problem extends to noncarcinogenic toxicants such as methotrexate and the organophosphates. In these cases the biologically based descriptions of toxicity stop at the first interaction of the chemical with tissue. The linkage from the primary interaction to ultimate toxic effect is a "black box" that, at present, can only be described empirically rather than in biologically based terms. The models for dioxin and prednisolone do illustrate how biological events several steps removed from the initial tissue interaction of the toxicant can be described in quantitative, biologically based terms. As such they point the way for future development of more realistic descriptions in which the use of purely empirical linkages is reduced. Rigorous organization of current knowledge and specification of a logical sequence of new experiments encompass the major reasons for developing biologically based (structural) descriptions of any process. It bears emphasis that biologically based tissue response models, for cancer, for chloroform cytotoxicity, for functional impairment following organophosphate exposure, etc, are in reality quantitative formulations of an hypothesis or proposed mechanism incorporating the pharmacokinetics of the chemical and the linkage between target-tissue dose and toxic effect(s). The quantitative model allows us to see if the hypothesis is consistent with pertinent data and aids in designing experiments to define, refute, or bolster the general nature of the proposed linkages between tissue dose, tissue response and the whole organism's response to the chemical exposure. In this sense these biologically based models are quantitative descriptions of the current level of understanding of toxic effects in animals. We began this review article by looking at the literature on empirical tissue-response models to educate ourselves on the history of PD modeling. We rapidly found that the clear distinction between PK and PD models described in the literature blurs when one moves to biologically based modeling of tissue disposition and tissue response. Many articles on physiologically based pharmacokinetics turn out on close scrutiny to be as closely related to tissue response as they are to kinetics.(ABSTRACT TRUNCATED AT 400 WORDS)

Respiratory sequelae and lung function after whooping cough in infancy.

The lung function of 31 children, aged 6-13 years, who had whooping cough as infants and 32 control children matched for age, sex, and residence area were compared in a community based cohort study. Family history of obstructive airway disease, smoking habits in the family, atopy, and other background factors examined were similar in the two groups. The ratios of recalled repeated acute respiratory infections did not differ among the groups. Children in the control group were slightly more involved in physical activities. History of obstructive airway disease, findings on chest radiography, and distribution of immunoglobulin concentrations, including IgE, did not differ significantly. Lung function before and after exercise and after inhalation of salbutamol were not different. No impairment of small airways was detected. Our data do not support the hypothesis that whooping cough in itself is a causal factor for later obstructive respiratory disease.

Estimation of Cell Birth Rate in the Wool Follicle Bulb using Colchicine Metaphase Arrest or DNA Labelling with Bromodeoxyuridine

The birth rate of cells in the wool follicle cells (CBR) was measured in two experiments on sheep differing widely in wool growth rate as a consequence of differences in nutritional regime. In Experiment 1, CBR was determined in one sheep on a high and one on a low plane of nutrition by two methods: the colchicine-metaphase arrest method, or from the product of total cell number and cell cycle time (the latter determined by DNA labelling with 5-bromo-2'-deoxyuridine (BrdU) and analysis of the fraction of mitoses labelled curve). CBR by colchicine was 0.73 and 0.56 of that determined using BrdU for the low- and high-plane sheep respectively. Cell cycle times were 26.5 h (low nutrition) to 18.6 h (high nutrition). In Experiment 2, CBR was again measured by the colchicine method, and from the rate of increase in labelled cells following a pulse dose of BrdU, in sheep on two levels of nutrition (n = 4llevel) and with a 3.7-fold difference in wool growth rate. Again, CBR by colchicine was only 0.41 (high nutrition) to 0.74 (low nutrition) of estimates made using the BrdU method, this difference being statistically significant only for sheep on the high plane of nutrition. Nutrition significantly altered CBR determined by colchicine (Pc0.001) and by BrdU (Pc0.034). The rate of fibre growth per follicle measured on two separate occasions was closely related to CBR measured using colchicine (r= 0.83, P&lt; 0.01 and 0.8 1, P c 0.02) or BrdU (r= 0.73, P c 0.04 and 0.64, P&lt; 0.08). It is concluded that CBR is a major determinant of fibre growth, but that colchicine depresses CBR. Previous estimates of mitotic activity in wool follicle bulbs have been made using colchicine and are thus probably underestimated. The technique using BrdU labelling and the subsequent rate of increase in labelled cells appears to be a useful one for estimating follicle cell kinetics.