Observations regarding DNA replication sites in human cells in vivo following infusions of iododeoxyuridine and bromodeoxyuridine

In studies using bromodeoxyuridine (BrdUrd) and/or iododeoxyuridine (IdUrd) to label S phase cells in cancer patients, several unique observations were made regarding DNA replication sites and the organization of newly synthesized DNA in post-mitotic cells. While the majority of tumour specimens removed at the end of infusions demonstrated concentration of replication sites around the nuclear membrane, biopsies obtained in leukaemic patients 1 week later demonstrated several distinct patterns of labelling. For example, one, two or all lobes of granulocytes were labelled. Scavenger macrophages bearing labelled leukaemic cells in their cytoplasm were also seen. Sequential IdUrd/BrdUrd labelling of solid tumours showed various patterns of nuclear/nucleolar/membrane labelling, allowing more precise localization of early versus late replication sites.     

Region-specific DNA synthesis in brains of F344 rats following a six-day bromodeoxyuridine infusion

Abstract. Prolonged exposure to certain alkylating chemicals induces glial and meningeal tumours in rats, probably resulting from DNA damage to dividing neural cells. The present work evaluated DNA synthesis in the brains of untreated, young adult male F344 rats in order to define a BrdUrd infusion protocol to more adequately assess proliferation in slowly dividing neural cell populations. BrdUrd (2.5 to 160 mg/ml) was administered for 6 days via subcutaneous osmotic pumps. Clinical toxicity was not observed at any dose. The labelling index (LI; % of cells per brain area that incorporated BrdUrd) and unit length labelling index (ULLI; % of cells per meningeal length that incorporated BrdUrd) were calculated for selected regions by counting labelled neural cells in defined areas of the right hemisphere in coronal brain sections. Intensely stained cells were numerous in the cerebral subependymal layer (LI=35.8%); scattered in cerebral white matter tracts (e.g. corpus callosum and internal capsule; LI=6.2%) as well as cerebral (ULLI=4.2%) and cerebellar (ULLI=3.6%) meninges; and rare in the hippocampus (LI>0.1%). Mildy stained cells were dispersed in the pons (LI=2.1%), deep cerebral (LI=1.8%) and cerebellar (LI=1.0%) grey matter, and thalamus (LI=0.3%). Phenotypically, BrdUrd-positive cells in neuropil were glial cell precursors and their progeny, while those associated with meninges were usually located in the superficial subarachnoid space and appeared to be fibrocytes. Using BrdUrd infusion, LI for glial precursors at these sites ranged from two- to 10-fold higher than those reported previously after a brief parenteral pulse dose. These data indicate that continuous BrdUrd infusion for 6 days by subcutaneous osmotic pump is an efficient means of labelling neural cells throughout the brain.     

Optimal conditions for immunohistochemical determination of the in vitro DNA synthesis labelling index with bromodeoxyuridine in head and neck cancer

The in vitro DNA synthesis labelling index was assessed immunohistochemically in 24 freshly obtained specimens of head and neck cancer using bromodeoxyuridine (BrdUrd) as the DNA precursor to determine the influence of BrdUrd concentration on labelling index (LI). Initially, tumour fragments were incubated in varying concentrations of BrdUrd from 2 to 100 microM for 2 h, and BrdUrd was detected with an anti-BrdUrd monoclonal antibody using immunoperoxidase labelling. There was a dose-response gradient with mean LI varying from 1.6% at 2 microM BrdUrd to 8.8% at 100 microM. The concentration-response gradient best fit a quadratic model when LI was plotted against log BrdUrd concentration (r = 0.65, P less than 0.0001). Eleven additional tumours were then studied to determine whether LI increased for BrdUrd concentrations above 100 microM. The mean LI at 125 microM and at 150 microM in these 11 tumours did not differ from the value at 100 microM, suggesting a plateau at this level. The gradient effect accounted for 17% of the variance in LI, while 60% of the variance was explained by between tumour differences. Within individual tumours, three response patterns were observed: (i) LI rose at a constant rate to the highest concentration tested (n = 8), (ii) the LI plateaued or declined at high BrdUrd concentrations (n = 6); and (iii) there was a biphasic slope slope in which the rate of rise in the LI increased at the higher BrdUrd concentrations (n = 2). The data show that BrdUrd concentration is an important variable in the immunohistochemical assessment of the in vitro LI in head and neck cancer.     

Direct comparison of bromodeoxyuridine and Ki-67 labelling indices in human tumours

Direct comparison of bromodeoxyuridine (BrdUrd) and Ki-67 labelling indices was achieved by selecting similar areas from serial sections of human tumours. Fifteen patients were selected who had been administered BrdUrd in vivo and both proliferation markers were assessed by immunohistochemistry. The data show a good correlation between both BrdUrd LI and MIB-1 LI and T_pot (calculated using the flow cytometry derived duration of S phase) and MIB-1 LI. The contribution of BrdUrd LI to growth fraction varied as a function of proliferation characteristics. In tumours with a high LI, the number of DNA synthesizing cells represented half the growth fraction, whilst in tumours with lower LI's (<10%) the ratio of DNA precursor labelled cells as a function of growth fraction fell to between 10% and 20%. T_pot showed a linear correlation with MIB-1/BrdUrd ratio with a slope approaching unity. It was apparent that both intra- and interpatient variation in proliferation index was greater for BrdUrd labelling than for MIB-1 expression.     

Tissue kinetics in mouse tongue mucosa during daily fractionated radiotherapy

Abstract. The purpose of the present investigation was to quantify cell flux between the distinct layers of the epithelial lining of the ventral surface of mouse tongue during daily fractionated radiotherapy.
A fraction of DNA-synthesizing cells in control epithelium, or at various days through a course of daily fractionated radiotherapy with 3 or 4 Gy per day, was labelled with [³H]dT or BrdUrd, respectively. The labelling indices (LI) in the different epithelial layers were defined histologically after autoradiography, or immunohistochemistry, at intervals between 1 and 10 days after label administration.
In tongue epithelium of untreated mice, the minimum residence time of cells in the germinal layer is 2–3 days. Migration through the functional layers requires an additional 2–3 days before labelled cells are observed in the most superficial layer of nucleated cells. A plateau in LI is observed for several days post-labelling in control epithelium, indicating an equilibrium between loss and proliferation of labelled cells. During fractionated radiotherapy, the minimum time from division to occurrence of labelled cells in the stratum lucidum is less than 2 days, and hence significantly shorter than in control epithelium. In contrast to untreated epithelium, no plateau in the germinal layer LI is seen, indicating that frequently both labelled daughters from dividing labelled cells are being lost from this compartment.
In conclusion, the present data support a recently described model of radiation-induced accelerated repopulation in squamous epithelia, which postulates that the majority of damaged cells undergoes abortive'divisions resulting in two differentiating daughters.     

Ataxia telangiectasia cells exhibit the same radiosensitization response by incorporation of BrdUrd or IdUrd as do normal human cells

Normal and ataxia telangiectasia (AT) human cells were exposed to 10(-5) mole/liter bromodeoxyuridine (BrdUrd) or iododeoxyuridine (IdUrd). High-pressure liquid chromatography (HPLC) measurements showed that up to 26 and 23% of the thymidine in DNA was substituted by BrdUrd in normal and AT cells, respectively. The incorporation of BrdUrd or IdUrd into DNA resulted in radiosensitization in normal and AT cells. When exposed to equal concentrations of BrdUrd and IdUrd, the BrdUrd caused greater radiosensitization than IdUrd in both normal and AT cells.     

Thermal sensitivity and radiosensitization in V79 cells after BrdUrd or IdUrd incorporation

Chinese hamster V79 cells were exposed to 10(-5) moles/liter bromodeoxyuridine (BrdUrd) or iododeoxyuridine (IdUrd) for 16 or 29 hr and then tested for thermal sensitivity, radiosensitivity, and sensitivity to the combined treatments of heat and radiation. BrdUrd and IdUrd treatment of cells resulted in enhanced radiosensitivity which increased with exposure time but had little or no effect on thermal sensitivity. For 42.0 degrees C heating, no effect was observed, while for 45.0 degrees C heating, a small decrease in thermal sensitivity occurred for both 16- and 29-hr exposure times, in the combined treatment of heat and radiation, the presence of BrdUrd or IdUrd resulted in about the same thermal enhancement in radiosensitivity. BrdUrd and IdUrd uptake into cellular DNA were measured using high-pressure liquid chromatography and, after a 29-hr exposure to 10(-5) moles/liter of BrdUrd or IdUrd, approximately 40% of the thymidine was substituted.     

Bromodeoxyuridine and lododeoxyuridine Double Immunostaining for Epoxy Resin Sections

To establish bromodeoxyuridine (BrdUrd)/iododeoxyuridine (IdUrd) double immunostaining for thick sections of epoxy resin-embedded tissues, young hamsters received intra-peritoneal injections of IdUrd and BrdUrd 3 hr and 1 hr before sacrifice, respectively. The intestines were fixed with phosphate-buffered 4% paraformaldehyde and embedded in an Epon-Araldite mixture. The epoxy resin was removed completely by a sodium methoxide/benzene/methanol solution. This epoxy resin removal method was effective for BrdUrd/IdUrd immunostaining using a mono-specific antibody for BrdUrd (Br-3) and a bi-specific antibody for BrdUrd and IdUrd (IU-4), followed by the ABC complex method. Epoxy sections stained with these antibodies showed clear localization of nuclei incorporating the two thymidine analogues with precise morphology of labeled cells. Furthermore, ultrastructural observation of thin sections adjacent to thick sections immunostained for BrdUrd/IdUrd confirmed the cell type and ultrastructural features of cells labeled with these thymidine analogues.     

DNA Ploidy,Bromodeoxyuridine labelling index,S-phase fraction and AgNOR counts in brain tumours

DNA ploidy and the proliferative potential in 88 brain tumours were investigated using the bromodeoxyuridine labelling index (BrdUrd LI), S-phase fraction (SPF) and an argyrophilic nucleolar organizer regions (AgNOR) technique. The study included 65 highly malignant (AIII–AIV), and 23 low-grade (AI–AII) gliomas. One fragment of the tumour was fixed in Carnoy's solution for AgNOR test, while the other fragments were used for flow cytometric determination of the labelling index, SPF and DNA ploidy. For the BrdUrdLl, tumour samples from each patient were incubated in vitro for one hour at 37°C with BrdUrd using a high pressure oxygen method. After fixation and staining, the percentages of BrdUrd-labelied cells (BrdUrdLI) and unlabelled S-phase cells (SPF) were evaluated. The tumours showed variability in the BrdUrdLI values, SPF and AgNOR counts/cell nucleus. However, grade dependent differences in the proliferating rate were only found to exist on the basis of BrdUrdLI and AgNOR counts. The same percentage of DNA aneuploidy (56 %) was found in high-grade as well as in low-grade gliomas. A linear – regression analysis showed a significant correlation between the results of three applied methods: BrdUrdLl, SPF and AgNOR counts.     

A NEW METHOD OF ESTIMATING THE CELL CYCLE TIME IN EPITHELIAL TISSUES OF LONG GENERATION TIME

1. Using standard autoradiographic techniques, the percentage of labelled mitoses and the numbers of labelled cells in the various layers of the epithelium of the hamster cheek pouch have been studied at various times after a single injection of [³H]thymidine. 2. A new method for determining the cell cycle time of the basal layer cells is described and compared with the techniques previously used. This method is based on the rate at which labelled cells leave the basal layer and enter the spinous layer above it. 3. Using this technique, the mean cell cycle time of the basal cells has been estimated as 130 ± 7 hr; a value which compares favourably with the estimate of 135 ± 17 hr determined with similar assumptions from the length of the S period and the labelling index. 4. The technique enables an estimate to be made of the fraction of cells in the basal layer that are proliferating (the growth fraction). This is found to be indistinguishable from 1. 5. The mean time taken by cells in travelling from the basal layer to the keratin, i.e. the transit time through the spinous layer, has been found to be approximately 186 hr.     

Enhancement of chromosome aberrations by the combination of DNA substitution with halogenated deoxyuridine and streptonigrin treatments

We treated CHO cells with streptonigrin (SN) alone, in combination with BrdUrd or IdUrd substitution, and with or without the addition of caffeine. The cells assessed for chromosome damage by SN were in the G₂ period and the magnitude of the damage was expressed as monosubstituted chromatid breaks, bisubstituted chromatid breaks and boundary regions breaks (boundary regions indicate the point of exchange of mono- and bisubstituted chromatids). We found that the combination of BrdUrd or IdUrd substitution with SN treatments produced a remarkable increase in the frequency of breaks over the frequencies observed with the halogenated compound only. The effect was more evident with IdUrd than with BrdUrd, and more dramatic in bisubstituted than in monosubstituted chromatids. The frequency of boundary breaks in cells treated with BrdUrd plus SN was similar to the frequency of breaks in monosubstituted chromatids treated similarly. Conversely, the damage in boundary regions was almost similar to that in bisubstituted chromatids in cells challenged with IdUrd plus SN. The addition of caffeine to BrdUrd-substituted chromosomes gave rise to a marked enhancement of breakages with a gradient of chromatid damage that was: bisubstituted > monosubstituted > boundary regions. A further increase of chromatin breaks maintaining the gradient indicated above was obtained when the cells were treated with BrdUrd plus SN plus caffeine. We propose that BrdUrd and IdUrd substitution alone or in combination with caffeine treatments and with SN in its capacity to bind DNA, give rise to different chromatin structures capable of modulating the DNA damage induced along the chromatin fibril by the active oxygen species liberated by SN-DNA complexes.     

Ki-67 expression and BrdUrd incorporation as markers of proliferative activity in human prostate tumour models

Abstract. The validity of the use of the monoclonal antibodies Ki-67 and anti-BrdUrd to evaluate proliferative activity of human prostate tumour models was studied. Growth of the transplantable PC-82 and PC-EW prostate tumours, as assessed by tumour volume measurements, was significantly correlated with the proliferative activity as reflected by BrdUrd incorporation into DNA ( r = 0.64 and r = 0.78, respectively). The proliferative activity of PC-82 tumours detected by Ki-67 antigen expression paralleled the pattern observed with BrdUrd ( r = 0.51) and a significant correlation ( r = 0.60) between the results obtained with both markers was found. In growing PC-82 and PC-EW tumours only small variations in the Ki-67 and BrdUrd indices were observed. In contrast, Ki-67 expression in regressing PC-82 tumours varied considerably (2.7 ± 2.2%). The BrdUrd index in regressing PC-32 tumours showed less variation (1.3 ± 0.2%), but part of the BrdUrd-positive cells were found in the stromal (murine) part of the regressing tissue. It is concluded that the Ki-67 and BrdUrd proliferation markers are reliable parameters to monitor changes in growth of prostate tumour lines, but that in slow growing or regressing tumours Ki-67 and BrdUrd data should be interpreted with caution.     

Trophic effects of essential fatty acids on pig skin

The daily oral administration of 3 ml of two oils (So-5407 and So-1129) containing essential fatty acids (EFAs) for 16 weeks resulted in a transient increase in cell proliferative activity in the skin of female Large White pigs. The So-5407 oil contained 7% gamma-linolenic acid (GLA) whereas So-1129 was an oil of similar composition, but with no GLA. Hyperplasia of the epidermis was observed after the administration of both oils, and this was characterized by an increase in the size of the rete pegs. The maximum effect occurred at 4 weeks after the start of oil administration, at which time the number of viable cell layers had increased by a factor of approximately 1.5, and mean epidermal thickness (excluding the stratum corneum) was approximately 40% greater than that of the epidermis prior to oil administration. There was a marked increase in the labelling index (LI) of the basal cell layer of the epidermis in pigs receiving So-5407. Maximum LIs were quantified at 4 weeks after the start of administration and were 18.8 ± 1.3% and 13.1 ± 1.7% for pigs receiving So-5407 and So-1129, respectively. After this time the LI declined prgressively and had returned to values within normal limits (P>0.1) by 8 weeks after the start of administration of both oils. A similar pattern of change in the LI was seen in the follicular epithelium, although the peak values at 4 weeks after the start of oil administration of 12.2 ± 1.8% and 10.8 ± 0.9 for the groups receiving So-5407 and So-1129, respectively, were lower than in the epidermis. Labelled cells were also counted in the papillary dermis and maximum values were again seen at 4 weeks after the start of oil administration. Of the two oils, So-1129 had the greatest effect, with the number of labelled cells in the papillary dermis being a factor of three to four-fold higher than in skin prior to oil administration, between 2 and 12 weeks after the start of administration.     

Double labeling with iodo- and bromodeoxyuridine for cell kinetics studies

The rate of progression through the cell cycle was determined in five human glioma cell lines by a new sequential immunohistochemical staining technique. The cells were labeled first with iododeoxyuridine (IdUrd) for 1-3 hr and then with bromodeoxyuridine (BrdUrd) for 30 min. Labeled cells were identified with Br-3, a monoclonal antibody that recognizes only BrdUrd, and with IU-4, an antibody that recognizes both IdUrd and BrdUrd. Each slide was stained sequentially, first with the immunoperoxidase method for Br-3 and then with the alkaline phosphatase-anti-alkaline phosphatase method for IU-4. Cells that were positive only for IU-4 represented the fraction of S-phase cells that passed into the G2 phase during the period of incubation with IdUrd. The rates of progression measured by this method were constant in each cell line and resulted in smaller standard errors than were obtained by measurements from specimens stained singly for IdUrd and BrdUrd in different slides. The duration of the S-phase calculated from this fraction in the five cell lines ranged from 8-13 hr; the estimated potential doubling times were 25-32 hr and were very similar to the actual doubling times.     

Cell proliferation kinetics of the bile duct epithelium of the rat

Abstract. Cell proliferation kinetics of the extrahepatic bile duct were studied by flash and cumulative labelling methods and immunohistochemical techniques. We compared the cell kinetics of the epithelium of the intra- and extra-pancreatic bile ducts and of the bile duct of the ampulla in rats administered intraperitoneally with bromodeoxyuridine (BrdUrd). After a single injection of BrdUrd (flash labelling), labelled cells appeared in the lower portion of the downgrowths of the epithelium in the intra-and extra-pancreatic bile ducts. A gradual accumulation of the labelled cells at the surface epithelium was observed during the cumulative labelling. After cumulative labelling the labelled cells gradually decreased in number and were finally confined to the degenerative cell zone of the surface epithelium 30 days later. Similarly, after a single injection of BrdUrd, the labelled cells in the bile duct of the ampulla appeared at the lower half of the crypt from where they migrated to the upper portion during cumulative labelling. These findings indicate that epithelial cells of the bile duct are renewed at the lower portion of the downgrowths of the epithelium, or crypt, and shed from the surface epithelium or upper portion of the fold. The labelling indices reached 23.83 ± 7.47% in the intra-pancreatic bile duct, 14.74 ± 7.99% in the extra-pancreatic bile duct and 43.42 ± 4.40% in the bile duct of the ampulla at the end of 70 h cumulative labelling. The fluctuating values of the labelling index were higher in the bile duct of the ampulla than in the intra- or extra-pancreatic bile ducts. These results indicate that the bile-duct epithelium undergoes a slower renewal rate than the other parts of the gastrointestinal tract, and that the renewal time of the epithelial cells is shorter at the bile duct of the ampulla than at the intra- or extra-pancreatic bile ducts.     

DNA flow cytometry of human epidermal tumours. Intra- and intertumour variability in ploidy and proliferative characteristics

Tumour ploidy and proliferative characteristics can be estimated by flow cytometric measurements of the nuclear DNA content. This study considers the question whether human epidermal tumours are intrinsically homogeneous with regard to these properties, i.e. whether a single biopsy analysed by flow cytometry is representative of the entire tumour. Analyses of multiple biopsies from ten human epidermal tumours--two kerato-acanthomas (KA), two basal cell carcinomas (BCC), two basosquamous carcinomas (BSC), one Bowen's disease (BO) and three squamous cell carcinomas (SCC)--indicated that both ploidy and proliferation characteristics were reproducible and specific for the peripheries of the different tumours regardless of the histopathologic diagnosis. The tumour centres, however, may deviate considerably from the corresponding periphery. None of the ten tumour peripheries contained more than one cell clone, but six of the ten clones were aneuploid. Both the BO and the KA's were hypodiploid, while one SCC, one BSC and one BCC were hyperdiploid as assessed in their peripheries. The remaining BSC was diploid in its periphery, while both a hypodiploid and a hypotetraploid cell clone were found in the corresponding centre.     

Craniopharyngioma: a case report and comparative galectin histochemical analysis

Craniopharyngioma is a rare benign tumour originating from Rathke's pouch. This paper reports a tumour case studied with a set of markers defining protein–carbohydrate recognition. Expression of endogenous lectins and their reactive glycoligands is under differentiation-dependent control in many cell types. These parameters can be related to the degree of cell differentiation in tumours. Therefore, the expression patterns of endogenous lectins, namely galectins-1, -3, and -7, in the craniopharyngioma case were determined. Galectins-1 and -3 were also used to reveal glycoconjugates in cells and extracellular matrices, an approach that has heretofore relied largely on plant lectins. The staining pattern of craniopharyngioma is compared with that of two other types of ectodermally derived tumours, namely basal and squamous cell carcinomas. Clusters of polygonal and flattened cells with morphological characteristics of differentiated cells in the craniopharyngioma and the majority of poorly differentiated cells in squamous cell carcinomas were reactive with galectin-3. No binding of this probe was observed in cells of basal cell carcinomas and the majority of craniopharyngioma cells. In view of the lack of accessible binding in the basal layer of normal squamous epithelia where proliferative cells (including stem cells) are located, galectin-3 binding could be used to distinguish basal from suprabasal cells of squamous epithelial cells.     

The effect of single versus double-strand substitution on halogenated pyrimidine-induced radiosensitization and DNA strand breakage in human tumor cells

To better understand the mechanism underlying halogenated pyrimidine-mediated cytotoxicity and radiosensitization in human tumor cells, a study was undertaken to determine the influence of unifilar (one DNA strand) versus bifilar (both DNA strands) substitution of thymidine by the halogenated bases 5-iodo-2'-deoxyuridine (IdUrd) and 5-bromo-2'-deoxyuridine (BrdUrd) in HT29 human colon cancer cells. Unifilar labeling was obtained by incubating cells with IdUrd or BrdUrd for one doubling time. Cells were incubated for at least three doublings to approximate bifilar substitution. Only IdUrd caused significant cytotoxicity, which correlated with incorporation into DNA. Both BrdUrd and IdUrd were potent radiosensitizers. Radiosensitization was linearly correlated with incorporation of both bases regardless of the number of strands in which thymidine was substituted. In contrast, the relationship between radiosensitization and DNA double-strand breakage was critically dependent in the case of IdUrd, but not for BrdUrd, on whether substitution was unifilar or bifilar. These findings suggest that incorporation is the best predictor of radiation sensitivity, and that the induction of DNA double-strand breaks alone does not account for radiosensitization mediated by halogenated pyrimidines in these human tumor cells.     

Streaming of labelled cells in the conjunctival epithelium

This study examines epithelial cell streaming and turnover in normal rat bulbar conjunctiva. Twenty seven male adult random-bred Hebrew rats weighing between 250–300 g, were injected i.p. with [³H]-thymidine. Three rats were killed at various times, thereafter from 1 h to 28 days. The enucleated eyes were fixed in formalin, cut into 5 μ thick sections, dipped into liquid emulsion, exposed for three weeks and stained with haematoxylin and eosin. Conjunctival epithelium was scanned from the limbus and outward, using an ocular micrometer grid with 10 x 10 divisions. In each consecutive field the grid was positioned along the basement membrane which was defined as the x-axis. The y-axis extended from the basement membrane outward. The x, y coordinate of each nucleus with three grains or more and its grain content were recorded along the entire epithelium. Conjunctival epithelium is divided into two cell kinetic compartments: a progenitor (P), along the basal and supra basal layer, in which cells proliferate, and a non proliferating Q-compartment, in the layers above. One hour after labelling most of the labelled cells were in the basal and supra basal layers. From then onward labelled cells streamed along both axes. Their x-velocity was 10·5±2·4 μ/day and the y-velocity 9·3 ± 5·4 μ/day. Cells are eliminated at the epithelial surface which is the outer Q-compartment boundary. Basal cell turnover was estimated from grain count dilution curves. The time it takes for the grains in a cell to reach half of their initial value was 8·3 days. It is closely related to the cell's generation time. The present study demonstrates that conjunctival epithelium in the rat streams along two axes, x, and y: 1 The x-axis extends along the basal layer, from the limbus and outward. 2 The y-axis extends from the basal layer into the layers above it. Cells first stream along the x-direction and then turn y-ward. Since cells are ultimately exfoliated from the conjunctival surface, and since the conjunctiva maintains steady state, we propose that stem cells located in the limbus generate transitional cells that stream along the two axes. Macroscopically the limbus is circular, and the stem cells are situated around the cornea. Each stem cell and its streaming progeny can be viewed as a conjunctival epithelial unit. We propose that conjunctival and corneal epithelium, are the descendants of an uncommitted stem cell that generates two differentiation pathways, a corneal and a conjunctival.     

EFFECT OF HAIR PLUCKING ON THE KINETICS OF METHYLCHOLANTHRENE-INDUCED SKIN TUMOURS IN MICE

Both the latent period and the volume doubling time of 20-methylcholanthrene-induced tumours in the skin of SAS/TO mice have been found to be longer for plucked skin than for unplucked skin, even though the cell cycle time of the basal layer cells of the epithelium is much shorter after plucking (50 ± 4 hr) than in unplucked skin (110 ± 10 hr).