C, Journal of Microscopy, Vol. 106, Pt 2, March 1976, p p . 239-249. Received 15 July 1975; revision received 30 Aunust 1975

Proliferative characteristics of epithelial cells in the pregnant rat uterus

by SANDRAPEEL and B A R B A R A LIDDIARD Human , Morphology, Faculty of Medicine, University of Southampton, Southampton SO9 3 TU

SUMMARY

In the pregnant rat, killed at about mid gestation and 1 h after injection of tritiated thymidine, 40°, of the cells in the epithelium lining the uterine lumen at the implantation site were labelled. Between implantation sites fewer than 200,) of the surface epithelial cells were labelled. A series of rats was given tritiated thymidine on day 12 of pregnancy and killed at intervals in the next 30 h. A percentage labelled mitoses analysis of the epithelium between implantation sites (interconceptual) and within the implantation site (conceptual) showed that cells in either region spent 7 h in DNA synthesis and 1.5 h in the Gz + 4 mitosis phases. The epithelial cells in the conceptual region spent 1.5 h in the GI+ 4 mitosis phases whereas cells in interconceptual regions spent at least 11.5 h in these phases. The average cycle times of cells in conceptual regions was 10 h : in interconceptual regions minimum cycle time was 20 h and the average appeared to be considerably longer. The grain count of the epithelial cells in the conceptual region was rapidly reduced during the 30 h after injection of tritiated thymidine suggesting successive rounds of cell division. In contrast the grain count distribution of cells in interconceptual regions changed only slowly during this time. The percentage of labelled epithelial cells was determined in the animals killed up to 30 h after injection of tritiated thymidine. In both conceptual and interconceptual regions these percentages increased initially as labelled cells produced labelled progeny. In the conceptual region the increase was not maintained after 7 h as cells initially in GI divided to give unlabelled progeny. In the interconceptual region the increase in the percentage of labelled cells continued for 14 h; thereafter the percentage did not significantly alter. The interpretation of these results is discussed in relation to the differences in the kinetic characteristics of the epithelial cells in the two regions and in relation to the morphology of the epithelium lining the uterus during pregnancy. INTRODUCTION

Implantation in the rat occurs about day 6 of pregnancy by attachment of the blastocyst to the antimesometrial aspect of the uterine epithelium. The implantation and its associated decidual response results in occlusion of the lumen at the implantation site, with disappearance of the epithelium. Between implantation sites the lumen remains patent and the epithelium is not lost. The continuity of the lumen is re-established by an epithelial extension antimesometrial to the developing 16

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Sandra Peel and Barbara Liddiard embryo while the chorio-allantoic placenta develops on the mesometrial aspect of the implantation site. The epithelial proliferation involved in the extension of the ‘new’ uterine lumen has been the subject of a previous study (Peel & Bulmer, 1975); one feature of this proliferation is the occurrence, in animals between day 9 and day 13 of pregnancy killed 1 h after injection of tritiated thymidine, of high labelling indices (40-60°,,)in the ingrowing epithelium. At this time only 15-20”,, of the epithelial cells lining the lumen between conceptuses were labelled. Labelling indices for uterine epithelial cells of up to 30°; have been reported by many workers whose findings have been reviewed by Epifanova (1971). More recently, Martin et al., (1973) described high labelling indices (70-80°;) after oestrogen treatment of castrated mice. It was suggested that such high labelling indices reflected a highly synchronized cell proliferation in response to the oestrogen. At the 9-13 day stage of pregnancy the high labelling indices of the uterine epithelial cells within the conceptual region may represent synchronous waves of proliferation although it is highly unlikely that this results from corresponding waves of oestrogenic stimulation (Wotiz et al., 1972). This paper describes further investigations of the surface epithelium lining the uterine lumen, comparing cells in the interconceptual regions with cells in the extension of the new lumen, to examine their proliferative characteristics and determine whether there is evidence of synchrony between days 12 and 13 of pregnancy. MATERIALS AND METHODS

Primagravid Wistar rats of 3-6 months were used with day 0 of pregnancy defined as the day on which spermatozoa were seen in the vaginal smear. The rats were given an intraperitoneal injection of tritiated thymidine (3HTdR, 1 pCi/g body weight sp. act. 5 Ci/mmol: Radiochemical Centre, Amersham, England) on day 12 or day 13 of pregnancy and were killed at various times after the injection, as detailed below. For analysis six specimens of uterus were taken from each animal and fixed in buffered formol saline or Carnoy’s fluid; of these three were of implantation sites (‘conceptual regions’) and three from areas of the uterus between implantation sites (‘interconceptual regions’). Autoradiographs of transverse sections were prepared using Ilford K5 emulsion, with an exposure time of up to 40 days, and Kodak D19 developer. Maximum background grain counts were 4 and mean values were less than 2. Cells with 5 or more grains were assessed as labelled and percentages of labelled epithelial cells were determined from 2000 cells observed on each section. Percentages of labelled mitoses for the conceptual regions were determined from observations of 300 mitoses whereas from interconceptual regions 100 mitoses were analysed. The 50°(;method of Quastler & Sherman (1959) was used to determine the phases of the cell cycle from the percentage labelled mitoses results. Percentages of cells in mitosis (mitotic indices) were determined by counting all the surface epithelial cells on the section. A grain count analysis was performed on at least 100 labelled cells selected at random, but cells with grain counts of more than 30 were not distinguished from each other. RESULTS

Figure 1 shows a low power photograph of a longitudinal section of part of a 12 day pregnant uterus. Lines A and B indicate the regions from which transverse sections were prepared and used in the subsequent comparisons of cells from conceptual (A) and interconceptual (B) regions. The epithelial extension around the antimesometrial aspect of the conceptus is arrowed. 240

Epithelial proliferation in pregnant rat uterus Animals given "TdR at 12, 123 or 13 days of pregnancy and killed 1 h later showed no significant differences in the percentage of labelled epithelial cells within each region (A and B), although there were significantly (P>O.O5) more cells labelled within the conceptual region (Fig. 2). In a series where the animals were killed at frequent successive intervals during this 24 h period (v. infra) there was no significant variation in the mitotic indices of cells in the conceptual region (Fig. 2), although the large standard errors of the means could conceal trends. In the interconceptual region, however, the mitotic indices did show variation throughout the 24 h period. During the first 12 h there was no significant change in mitotic index but in the following 12-h period the values were significantly lower ( P >0.05) than the value observed at 12.5 days. During the first 12 h the differences between the mitotic indices in the two regions, conceptual and interconceptual, were not significant, although the difference between the values was less marked at the end of the initial 12 h period than at the beginning. After 12 h the mitotic indices in the interconceptual regions were significantly lower than those in the conceptual regions. A series of rats were given "TdR on day 12 of pregnancy, between 10 and 12 a.m., and killed 1, 2, 4, 5.5, 7, 10, 12, 14, 17, 20, 23, 26 or 30 h later. Epithelial cells in transverse sections of the two regions were examined to determine the percentage of labelled mitoses at the various times after 3HTdR injection and

A

Fig. 1. This shows a longitudinal section through a uterus from a day 12 pregnant rat ( x 14). The line A represents the area of the uterus from which transverse sections of conceptual regions were taken. The line B

represents the region between the implantation sites from which transverse sections of interconceptual regions were studied. The arrows indicate the epithelium in the conceptual region.

24 1

Sandra Peel and Barbara Liddiard

I

-T

T

f--. +/' OL

T

T

I

i ,I4

,@--/

I

I

I

I2

12.5

13

Doy of preqnoncy

Fig. 2. This shows the percentages of surface uterine epithelial cells labelled with 3HTdR and the percentages of cells in mitosis observed between day 12 and day 13 of pregnancy. Each point is the mean of values from at least three different regions of one animal and standard errors are shown by vertical bars. 0-0, epithelial cells in the conceptual region; ----a, epithelial cells lining the lumen between implantation sites.

the results are shown in Fig. 3. The curves connecting the values were hand drawn. About 20°,, of the mitoses were labelled in the epithelium within the conceptual region 1 h after the injection which indicated that some cells pass through the G2 phase and enter mitosis in less than 1 h. The curve for cells in the conceptual region rose rapidly in the first 2 h suggesting that there is little variation in the G2 phase and that the time spent in mitosis is short. The values for percentage labelled mitoses derived from cells in interconceptual regions 1 and 2 h after 3HTdR injection were significantly lower than those for cells in the conceptual regions (P>0.05). However, when the 50°, method was used to determine the extent of the phases of the cell cycle the average time cells spent in G2 and 4 mitosis (G2+ 4M) in either region was found to be 1.5 h. Using the 50",, value on the descending limbs of the curves the average time spent in DNA synthesis (S) by cells in either region was 7 h. There was a second rapid rise in the percentage of labelled mitoses after 10 h in the epithelium within the conceptual region and values over 60°, were obtained at 12 h. The 500,; method gives reliable estimates of T c from curves which rise substantially above 50°/, and from the 50°, value on the second ascending limb the cycle time of the epithelial cells within the conceptual region appears to be 10 h. The validity of this estimate of T c from the 50°, method is supported by the 10 h period between the start of the experiment and the nadir at 10 h (Fig. 3). By sub-

242

Epithelial proliferation in pregnant rat uterus

*-*-?

(JL,' 1

I

0

8

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I

16 Hours o f f e r 'HTdR

1

I

24

1

1

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Fig. 3. Percentages of mitoses labelled with :3HTdR are shown from observations on rats killed up to 30 h after "TdR injection on day 12 of pregnancy. Each point is the mean of values from at least three different regions from a pregnant rat and standard errors are shown by vertical bars. 0-0, epithelial mitoses in the conceptual region; 0- - - - 0 , mitoses in the epithelium lining the lumen between implantation sites.

tracting the time cells spent in Gz+ iM and S from a cycle time of 10 h the GI 1M phase of the epithelial cells in the conceptual region is 1.5 h. T h e second wave of labelled mitoses did not rise to the same extent as the first, suggesting loss of synchrony of the labelled cohort of cells. It seems likely that further loss of synchrony would not allow any firm conclusions to be drawn about the variation in percentages of labelled mitoses observed in the latter part of the observation period. For cells in the interconceptual region there was no second wave of labelled mitoses before 20 h, an interval which therefore represents the minimum cycle time of the cells in this region. Labelled mitoses occurred with increasing frequency

+

Table 1. Time spent in the various phases of the cell cycle by surface epithelial cells in the conceptual and interconceptual regions. T h e initial labelling index observed 1 h after injection of tritiated thymidine is shown and the fraction of cells in the proliferative cycle (growth fraction) is given. T h e methods used to derive these values are described in the text; the value at * was obtained by using the minimum cycle time Uterine epithelial cells Average Conceptual Interconceptual time (h) spent in region region 1.5 1.5 Gz+ 4M S 7 7 GI :M 1.5 11 5 (minimum) Cell cycle 10 20 (minimum) 0.39 0.58 Mitosis 42", f 5 14.4',, + _ 3 Initial labelling index Growth fraction 0.57 0.45*

+

243

Sandra Peel and Barbara Liddiard after 20 h and the rate of this increase suggests considerable variation in the GI period. The 50°,] method does not give a reliable estimate for cycle time from this curve. The curve shows that the minimum cycle time is 20 h and by 30 h about half of the observed mitoses are labelled. By subtraction, a minimum Gl-tiM phase of 11.5 h can be estimated but the average GI aM phase is probably about 10 h longer. The times cells in each regions spend in the various phases of the cell cycle are summarized in Table 1. Table 1 also compares the percentage of labelled cells observed in the two regions of the epithelium in rats killed 1 h after 3HTdR injection. These values, together with the lengths of the cell cycle time and the DNA synthetic period, were used to calculate the growth fraction (GF) from the formula:

+

GF=

L I obs L I calc'

L I obs; percentage of cells observed labelled 1 h after SHTdR injection.

LI calc;

Th

x 100,

T I 2

where T, = time spent in DNA synthesis ; T, = time spent in cell cycle. It would appear that the growth fraction of the cells in the epithelium within the conceptual region was 0.57. The estimate for cells in the interconceptual region was less accurate because the cell cycle time was less precisely defined, but the growth fraction appeared to be about 0.5. Table 1 also compares the time cells in the two regions spent in mitosis (Tm). The values were calculated from the formula: Tm MI TS

0L

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0

I

I

0

=LI'

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1 16

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24

4 30

Hours after 3HTdR

Fig. 4. Percentages of cells labelled with 3HTdR are shown from observations on rats killed up to 30 h after 3HTdR injection on day 12 of pregnancy. Each point is the mean of values from at least three different regions of one epithelial animal and standard errors are shown by vertical bars. 0-0, cells in the conceptual region; .----a, epithelial cells lining the lumen between implantation sites.

244

Epithelial proliferation in pregnant rat uterus IOOr

Intercmceplus

t lours

Conceptus

I

?

Fig. 5. Histograms showing percentages of cells with grain counts of 5-10, 11-15, 16-20, 21-25, 26-30 and > 30/cell. The values are the means of two

analyses of conceptual regions (histograms on the right) and interconceptual regions (histograms on the left) from rats killed 1,7, 14 and 20 h after 3HTdR. where MI and LI are respectively the mitotic and labelling indices observed at the beginning of day 12. It would appear that cells in the interconceptual region spent about 35 min in mitosis and cells in the conceptual region of the conceptus take about 23 min. In this series of animals killed during the 30 h after injection of "TdR the percentage of labelled cells was also determined (Fig. 4). In the epithelium within the conceptual region there was an increase in the percentage of labelled cells and by 7 h this was significant at the P > 0.05 level. This rate of increase was not then maintained. Towards the end of this observation period there was some variation in the observed percentage of labelled cells. At 20 and 26 h, the values were significantly higher (P>O.O5) than the initial value but at 23 and 30 h the differences were not significant. In the interconceptual region the percentage of labelled cells also increased during the first 7 h after injection but, in contrast to the values for cells within the conceptual region, the increase continued for a further 7 h (Fig. 4). After 14 h the percentage of labelled cells decreased but the values at 20,23,26 and 30 h were significantly higher than the initial labellingindex(P> 0.05). A further comparison of the epithelial cells in the two regions was made by

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Sandra Peel and Barbara Liddiard

Fig. 6. An area of epithelium within the conceptual region, showing labelling of many of the cells in the surface epithelium. The epithelial cells lining the glands (g) are not labelled and glands in this region have not been observed to connect with the surface epithelium. x 360.

counting the silver grains over a sample of cells from rats killed 1, 7, 14 and 20 h after 3HTdR. For ease of representation in Fig. 5 the cells with 5-10, 11-15, 16-20, etc., grains were pooled into groups. One hour after 3HTdR injection over 85", of the cells in either region were heavily labelled (over 30 grains/cell). In the animals killed 7 h after 3HTdR the number of heavily labelled cells in each region was slightly reduced. In the conceptual region this decrease continued rapidly so that by 20 h after 3HTdR there were only about lo0,, of the cells with more than 30 grains. In contrast, in the interconceptual region at 20 h, there were still over 50°,, of the cells with more than 30 grains. DISCUSSION

3HTdR labelling indims Martin et al. (1973), working on ovariectomized mice subjected to oestrogen treatment, related the high percentage of uterine epithelial cells labelled 1 h after 3HTdR injection to synchrony imposed by the oestrogenic stimulation. Zhinkin & Samoshkina (1967) reported high labelling indices for uterine epithelium in the initial stages of pregnancy in the mouse, which decreased during the first week. In the pregnant rat, between day 9 and day 13, labelling indices of 40-60°, have been observed in the epithelium involved in the extension of the 'new' uterine lumen (Peel & Bulmer, 1975). It was thought unlikely that the high values were due to synchronized proliferative activity. This investigation of labelling indices at 12, 12.5 and 13 days of pregnancy did not show evidence of synchrony, i.e. alter-

246

Epithelial prolqeration in pregnant rat uterus nating high and low values. Nor were significant fluctuations in mitotic indices of conceptual regions revealed by examination at nine time intervals between day 12 and day 13. The high labelling indices therefore are not associated with a synchronous population. Phases of the cell cycle Epithelial cells in the conceptual region at the 12-13 day stage have a cycle time of 10 h and spend 7 h in DNA synthesis, as determined by the percentage labelled mitoses method (Fig. 3). The remaining 3 h period is divided equally between the GI and GBphases but about 20min of this 3 h period is spent in mitosis. The analyses suggest that just over half (0.57) of the cells are in the cell cycle. Cell cycle times for uterine epithelial cells have been estimated previously by a variety of methods (for review see Epifanova, 1971). Das (1972) observed the appearance of labelled mitoses after thymidine injection of ovariectomized mice treated 15 h previously with oestrogen. These cells had a cycle time of 11.25 h; the S and GI phases were respectively 6 and 3 h. There is general agreement that oestrogens decrease the cell cycle by decreasing the length of the GI and S phases. The GI period (1.5 h) of epithelial cells within the conceptual region was shorter than that described by Das for oestrogen-stimulated cells. Such a rapid transit of uterine epithelial cells through GI has been described previously by Eide (1975) who studied oestrogen-stimulated uterine epithelium in the neonatal mouse. In animals analysed 12 h after administration of oestradiol the GI period was calculated to be 2.15 h. It was observed that the olive oil used as a vehicle for the oestrodiol had an effect on the cycle times and when an appropriate correction factor was applied the GI period of the uterine epithelial cells was 1.8 h. In the pregnant rat, epithelial cells in both interconceptual and conceptual regions had similar G:! and S phases of 1.5 and 7 h respectively, but the comparatively retarded second wave of labelled mitoses in the interconceptual epithelium suggests that these cells have a longer GI phase than the cells within the conceptual region. The minimum cycle time for cells in the interconceptual region was estimated at 20 h; the minimum time spent in GI was 11.5 h, although the slow rise in the second wave of labelled mitoses suggests there was considerable variation in the GI phase and the majority of the cells spend considerably longer than 11.5 h in this phase. The short G1 phase of epithelial cells within the conceptual region may be associated with oestrogenic stimulation. It is likely, however, that factors other than circulating hormone levels are involved because in the interconceptual epithelium, only a short distance away, the cells spend at least 11.5 h in the GI phase. It is pertinent to note that hormonal environment may have some effect on proliferative activity of the pregnant rat uterine epithelial cell because a drastic reduction in labelling indices occurs after day 14 (Peel & Bulmer, 1975), at a time when progesterone levels are increased (Pepe & Rothchild, 1973). Loss of hormone, as the result of ovariectomy during pregnancy, also affects the proliferative activity of the epithelial cells (Peel & Bulmer, 1975a). A further difference between the cells in the two regions was revealed when the time spent on mitosis (Tm) was calculated for cells at the beginning of day 12. At this time (Fig. 2) the mitotic indices for the two regions were not significantly different but the calculated Tm for cells in the interconceptual region was longer than the T m for cells in the conceptual region. In the latter half of day 12 there were significant differences in the mitotic indices of the two regions. Such a change during day 12 may be related to an alteration in the length of time cells in the interconceptual region spend in mitosis: the value for T m during the latter half of the

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Sandra Peel and Barbara Liddiard day may be reduced. The absence of data on the Ts of cells on day 13 means that the value for T m at this time cannot be determined.

Grain count analysis The grain count analysis showed that 1 h after 3HTdR injection over 85",, of the cells in either region were heavily labelled, with more than 30 grains/nucleus. This means that observations on labelled cells were valid for at least 20 h after the injection, during which time the cells could have divided twice, reducing their grain count by a factor of four. After 20 h, labelling indices observed for the conceptual region became increasingly unreliable due to dilution of the label. The histograms in Fig. 5 show that the distribution of grain counts 1 h after thymidine was similar in each region but as cells with over 30 grains were grouped together it was possible that there were masked differences in labelling intensity. The number of grains over a nucleus depends on many factors. In the two regions studied the cells spent similar lengths of time in DNA synthesis but it is possible that local differences in pool size could give different intensities of labelling. The similarity of the histograms for cells from the two regions 7 h after thymidine injection supports the view that the cells in each region progress through Gz and S at a similar pace. Over the 7-20 h period the cells within the conceptual region rapidly lose their label, suggesting that they undergo rapid repeated cell division, so that at 20 h only about loo, of the cells had 30 or more grains. In contrast there was no such rapid decrease in the number of heavily labelled cells in the interconceptual region. This supports the view that the epithelial cells in the interconceptual region have a longer cycle time than those in the conceptual region. Accumulation of labelled cells The accumulation of labelled cells after 3HTdR injection in the conceptual region (Fig. 4) was in accord with the kinetic data already described. Thus the percentage increased during the first 7 h, as labelled cells produced labelled progeny. This rise did not continue because cells initially in GI divided to give unlabelled daughter cells. That the labelling index at 20 h was significantly higher than the initial level suggests that not all the cells in the epithelium were actively proliferating. The labelling index at 30 h was not significantly different from the initial value, but after 20 h the grain count analysis suggests that these indices become increasingly unreliable. Cells in the interconceptual region also showed an increase in the percentage of labelled cells during the first 7 h after 3HTdR (Fig. 4). However, the values continued to rise until 14 h after 3HTdR. Such an increase could be the result of a long S period (i.e. Gz+ S of 14 h) but the percentage labelled mitoses curve (Fig. 3 ) shows that the Gz + S phases last for 8.5 h. Such an increase would also occur if cells at the end of the GI phase at the time of 3HTdR injection were arrested temporarily and did not progress at the normal rate through S and G2 to mitosis, but only started to produce unlabelled progeny 14 h later. This, however, is unlikely, since no fall in the mitotic index was observed during the initial 12 h (Fig. 2). The increased labelling indices which were observed up to 14 h after 3HTdR could be associated with preferential loss of unlabelled cells by death or migration during this time. Figure 4 also shows that the labelling indices of cells in the interconceptual region did not fall to the initial value, suggesting that not all the cells were in cycle. Cell loss or migration The part played by cell death has not been determined in this investigation. Cell loss was not quantified, but excessive numbers of apparently dying cells were not

248

Epithelial proliferation in pregnant rat uterus observed. There could also be loss of cells by migration from the luminal to the glandular epithelium. Such migration is unlikely in the conceptual region because the glands which were seen in this study did not connect with the surface epithelium end only rarely were cells in them labelled (Fig. 6). In interconceptual regions some glands do connect with the luminal surface but even in this region labelled cells were extremely rare and the glands were not thought to be actively involved in the proliferation occurring during the later stages of pregnancy (Peel & Bulmer, 1975). It has been stressed by other workers that the cells of the glandular and luminal epithelia do not have identical characteristics (Epifanova, 1971; Das, 1972; Martin et al., 1973) and this difference has also been noted during pregnancy (Finn & Martin, 1967; Zhinkin & Samoshkina, 1967; Martin & Finn, 1971; Peel & Bulmer, 1975). A further distinction within the uterine epithelium has now been demonstrated; at day 12 of pregnancy surface epithelial cells in the conceptual regions of the uterus progress from one mitosis to another at least twice as rapidly as do those in the interconceptual regions. The difference in the cycle times of the cells in the two regions is accounted for by differences in the GI phase; cells in either region spend similar times in DNA synthesis and the Gz phase. ACKNOWLEDGMENTS

We are grateful to Professor D. Bulmer for his invaluable advice and criticism and to the colleagues who helped with the illustrations for this paper. References Das, R.M. (1972) T h e effects of estrogen on the cell cycle in epithelial and connective tissues of the mouse uterus. J . Endocr. 55, 21. Eide, A. (1975) The effect of estradiol on the cell kinetics in the uterine and cervical epithelium of neonatal mice. Cell Tissue Kinet. 8, 249. Epifanova, 0.1. (1971) Effects of hormones on the cell cycle. In: The Cell Cycle und Cancer (Ed. by R. Baserga), p. 145. Marcel Dekker, New York. Finn, C.A. & Martin, L. (1967) Patterns of cell division in the mouse uterus during early pregnancy. J . Endocr. 39, 593. Martin, L. & Finn, C.A. (1971) Oestrogen-gestagen interactions on mitosis in targtt tissues. In: Basic Actions of Sex Steroids on Targer Organs (Ed. by P.O. Hubinot, F. Leroy & P. Galand), p. 172. Karger, Basel. Martin, L., Finn, C.A. & Trinder, G. (1973) Hypertrophy and hyperplasia in the mouse uterus after oestrogen treatment: an autoradiographic study. -7. Endocr. 56, 133. Peel, S. & Bulmer, D. (1975) A study of proliferative activity of the uterine epithelium of the pregnant rat in relation to the morphogenesis of the ‘new’ lumen. J . Reprod. Fert. 42 189. Peel, S. & Bulmer, D. (1975a) The effects of late ovariectomy on the proliferation and differentiation of the uterus of the pregnant rat. J . Anat. 119, 569. Pepe, G.J. & Rothchild, I. (1973) Serum progesterone levels in ovariectomizcd rats injected with progesterone and estrone: relation to pregnancy maintenance and growth of decidual tissue. Endocrinology, 93, 1193. Quastler, H. & Sherman, F.G. (1959) Cell population kinetics in the intestinal epithelium of the mouse. Expl. Cell Res. 17, 420. Wotiz, H.H., Scublinsky, A. & Walker, C. (1972) Effect of oestrogen antibodies on pregnancy in the rat. Gynecol. Invest. 3, 124. Zhinkin, L.N. & Samoshkina, N.A. (1967) DNA synthesis and cell proliferation during the formation of deciduomata in mice. 3. Embryol. exp. Morph. 17, 593.

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Proliferative characteristics of epithelial cells in the pregnant rat uterus.

C, Journal of Microscopy, Vol. 106, Pt 2, March 1976, p p . 239-249. Received 15 July 1975; revision received 30 Aunust 1975 Proliferative characteri...
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