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Postpartum regeneration of the guinea pig endometriumRelationship to serum estradiol and progesterone concentrations.

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THE ANATOMICAL RECORD 210:41-44 (1984)
Postpartum Regeneration of the Guinea Pig Endometrium:
Relationship to Serum Estradiol and Progesterone
Department of Anatomy, East Carolina University School of Medicine,
Grienuille, NC 27834
The pattern of cell mitotic activity in the uterus of postpartum, pregnant guinea pigs was correlated with the associated changes in
serum progesterone and estradiol levels between days 1-8 of pregnancy.
Stromal and glandular epithelial mitotic patterns mimicked the associated
fluctuations in serum progesterone and estradiol levels, respectively. No
changes in myometrial mitotic activity were observed. The luminal epithelial
mitotic index was apparently sensitive to the relative change in the ratio of
progesterone-to-estradiol in circulation. These studies indicate that postpartum, endometrial preparation for blastocyst implantation depends on the proper
priming of the uterus by both progesterone and estradiol in the guinea pig.
In order for blastocyst implantation to SUCceed, the uterine endometrium must be properly prepared and sensitized to support
subsequent differentiation and growth of the
conceptus (Psychoyos, 1973; Finn, 1977) by
ovarian steroid hormones. In virgin or nongravid females, estrogen serves to stimulate
uterine epithelial and glandular growth,
whereas progesterone apparently subserves
these functions relative to stromal preparation for implantation (Finn, 1977). However,
in the postpartum pregnant female, the endometrial trauma induced by parturition necessitates that both epithelial and stromal
growth be reestablished, in additon to proper
hormonal conditioning of the endometrium,
in order for a subsequent implantation reaction to occur. Considering that in the rat and
guinea pig blastocyst implantation occurs at
approximately 6 days postovulation, the endometrial changes that occur in postpartum
pregnant animals must be rapid and dramatic with respect to cellular reorganization.
Since previous studies with the guinea pig
have indicated that the estrogen-induced,
uterine hyperemia associated with the nidatory process in this species is present in the
postpartum pregnant female (Garris and
Whitehead, 1981), it was of interest to determine the pattern of stromal, glandular, and
epithelial cell growth relative to the cyclic
changes in circulating progesterone and estradiol levels in this model system. The fol-
0 1984 ALAN R. LISS, INC.
lowing study was undertaken to determine
the pattern of cell mitotic activity in the postpartum, pregnant guinea pig uterus relative
to the timing of blastocyst implantation and
circulating ovarian steroid hormone levels.
Adult female guinea pigs (Dunkin-Hartley)
weighing between 600 and 1,000 g were used
in this study. All animals were maintained
under a controlled photoperiod of 14 hr light/
day (lights on 0600 hr) with food and water
available ad libitum. Breeding was accomplished by placing a postpartum female with
a breeder male overnight. The use of this
model in our colony increases the rate of
successful breedings to nearly loo%, with day
1 of pregnancy denoted by either the presence of sperm in the vaginal lavage or a
sperm plug. Females were then removed to a
separate cage until used.
Between 1000 and 1200 h r on days 1-3 or
5-8 postpartum, each animal received a n IP
injection of colchicine (4mgkg) at 2 hr prior
to sacrifice. Blood samples were collected by
either direct cardiac puncture or by decapitation a t the end of each experiment. Blood
was allowed to clot and stored at 5°C overnight, and the serum was collected following
centrifugation and stored a t -20°C until asReceived November 15, 1983;accepted March 26, 1984
sayed. Serum 170-estradiol (E) and progesterone (P) levels were estimated by radioimmunoassay as previously described
(Garris and Whitehead, 1981) using E antibody GDN244 and P antibody GDN337. All
samples were run in duplicate and are expressed uncorrected for procedural loss ( <
5%). Intraassay and interassay variability
was less than 5 and 8%, respectively. The
specificity of the P antibody has been previously described (Gibori et al., 1977), with assay sensitivity ranging between 0 and 30 ngl
ml. The specificity of the E antiserum has
been reported (Korenman et al., 1974).
Uterine horns were removed from each female guinea pig and prepared for histological
analysis using conventional techniques for
paraffin-embedded tissue samples. Mid-uterine segments that did not possess placental
scars were serially sectioned (8 pm) and
stained with hematoxylin and eosin. The
number of epithelial, glandular, stromal, and
myometrial cells that were observed to be
arrested in the metaphase stage of mitosis
were counted from 3-5 random sections of
each uterus. All data were analyzed as group
means (k SEMI. Intergroup differences were
analyzed using the Student’s t-test with a
significance level set a t P < 0.05.
The changes in the mitotic index of epithelial, glandular, stromal, and myometrial cells
in the postpartum, pregnant guinea pig
uterus are depicted in Figure 1. The epithelial cell mitotic activity was elevated between days 1-5 postpartum and subsequently
declined between days 6-8. The mitotic pattern of glandular epithelial cells exhibited a
decline in activity between days 1-3 postpartum followed by a peak in cellular division
rate on day 5 postpartum. Subsequently, the
mitotic index of the glandular cells declined
through day 8 postpartum to basal rates.
Stromal cells exhibited minimal mitotic activity between days 1-3 postpartum, after
which time a rapid rise to peak division rates
was observed between days 5-7. This rise in
stromal mitotic activity was temporally associated with the timing of blastocyst implantation in this species (Garris, 1984a), and
rapidly declined to near basal rates by day 8
postpartum. The myometrial mitotic index
remained relatively constant between days
1-8 postpartum with no significant changes
observed relative to the timing of blastocyst
implantation in these animals.
l a
D a y Post-Partum
Fig. 1. The mean ( * SEM) number of mitotic cells in
the luminal epithelium, glandular epithelium, stroma,
and rnyometrium of the postpartum, pregnant guinea
pig uterus is represented. The number of animals per
group is indicated in parentheses. Intergroup differences
are denoted as a vs. b, b vs. c, or c vs. d: P 4 0.05; a vs.
c: P < 0.01; a va. d: P < 0.001.
The changes in serum progesterone and
estradiol between days 1-8 postpartum are
represented in Table 1. Serum progesterone
rose steadily between days 1 and 5 postpartum before reaching constant, elevated levels
between days 5 and 7 before subsequently
declining to intermediate levels on day 8. In
contrast, serum estradiol levels decreased between days 1-3 postpartum before demonstrating a midluteal elevation between days
5 and 6 in association with the expected time
of blastocyst implantation. Serum estradiol
subsequently fell to basal levels between days
7 and 8.
The results of the present studies indicate
that the endometrial changes associated with
cellular growth and division in the postpartum, pregnant guinea pig occw in close as-
TABLE 1. Changes in serum progesterone and estradiol levels between days 1 and 8 in the postpartum,
pregnant guinea pig
Progesterone (ng/ml)
Estradiol (pg/ml)
6.8 k1 16.9 kz
13.1 k3 12.5 k3
All values are represented as group means
3: P 4 0.05; 1 vs 3: P < 0.01.
Dav Dostuartum
15.6 kz
2.2 f 1
25.6 k3
18.6 k 3
22.4 k3
15.8 k3
22.8 k 3
5.2 +'
15.7 k2
6.7 +'
+ SEM for (N) animals. Intergroup differences are denoted by 1 vs 2 or 2 YS
sociation with the concomitant changes in
circulating progesterone and estradiol levels.
The pattern of mitotic activity exhibited by
the glandular epithelium paralleled the temporal changes in serum estradiol levels. In a
similar manner, stromal cell multiplication
followed (approximately 24-48 hr) the temporal elevations observed in serum progesterone levels, especially between days 3 and
7 postpartum. Myometrial cell mitosis did
not demonstrate any rate changes representative of the fluctuations in serum ovarian
steroid hormone levels. The pattern of mitotic activity in the epithelial cells appeared
to vary with the changes in both estradiol
and progesterone levels. The day 1-2 decline
in the epithelial cell mitotic index followed
the mild decline in serum estradiol levels,
during which time serum progesterone concentrations increased. The elevated epithelial mitotic activity between days 3-5
postpartum preceeded the subsequent elevation in serum estradiol levels between days
5 and 6, and declined to basal rates between
days 6-8 as serum estradiol levels declined
and progesterone concentrations rose. Thus,
the temporal correlations observed in the cellular mitotic activity and serum steroid levels suggest that while the glandular
epithelial and stromal changes are apparently modulated by estradiol and progesterone, respectively, the luminal epithelium
changes occur relative to the fluctuating ratio of estradiol-to-progesterone in circulation.
The results of the present study are in close
agreement with respect to the site of action
of estradiol and progesterone in the guinea
pig uterus (Everett, 1962) and the effects of
each relative to the cellular mitotic activity
(Marcus, 1974a; Mehrotra and Finn, 1974).
Of interest is the observation that in the
postpartum guinea pig uterus, the uterine
hyperemia which occurs in association with
blastocyst implantation between days 5 and
7 (Garris and Whitehead, 1981; Garris,
1984a) is accompanied by a rapid rise in
stromal cell mitotic activity. The concomitant elevation in both serum progesterone
and estradiol levels during this time suggest
that the hormonal modulation of the endometrial vascular response (Garris and Whitehead, 1981)and mitotic index are both causal
and interdependent events. Similar correlations have been reported for the mouse (Finn,
1977) and rat (Marcus, 1974b; Krueger et al.,
1975; Kang et al., 1975).
These studies suggest that the pattern of
endometrial cell mitosis in the postpartum
guinea pig is very similar to that observed in
the virgin, nongravid condition (Marcus,
1974a; Mehrotra and Finn, 1974).The ability
to mimic the cyclic pattern of mitotic activity
in the endometrial layers of ovariectomized
guinea pigs with ovarian steroids (Marcus,
1974a) indicates that the temporal aspects of
uterine mitotic activity observed in the present study were a result of the relative fluctuations in serum progesterone and estradiol
levels. Considering both the rapidity of the
endometrial changes that occur postpartum
and the associated response elicited from the
endometrial vascular bed by ovarian steroids
(Garris and Whitehead, 1981), it is concluded
that the guinea pig uterus is capable of returning to a near normal endometrial condition that is acceptable for successful nidation
within 5 to 6 days postpartum. This is supported by the previous observations that
blastocyst implantation is successful following postpartum breeding in the guinea pig
(Garris and Whitehead, 1981) and that various other uterine parameters which are recognized to participate in the nidatory process
also occur in association with endometrial
reorganization in this species (Garris, 198413).
These changes are apparently under the di-
rect influence of the combined actions of progesterone and estradiol.
Everett, J. (1962) The influence of oestriol and progesterone on the endometrium of the guinea-pig in vitro. J.
Endocrinol., 24:491-496.
Finn, C.A. (1977)The implantation reaction. In: Biology
of the Uterus. R.M. Wynn, ed. Plenum Press, New
York, p. 245.
Garris, D.R. (1984a) Ultrastructural aspects of the appositional stage of blastocyst implantation. Gynecol. Obstet. Invest., 17:lO-17.
Garris, D.R. (1984b) Uterine blood flow, pH and pCOz
during nidation in the guinea pig: Ovarian regulation.
Endocrinology, 124:1219-1224.
Garris, D.R., and D.S. Whitehead (1981) Uterine blood
flow and timing of blastocyst implantation in the
guinea pig. Am. J. Physiol., 241:E142-E145.
Gibori, G., E. Antczak, and I. Rothchild (1977) The role
of estrogen in the regulation of luteal progesterone
secretion in the rat after day 12 of pregnancy. Endocrinology, 100:1483-1489.
Kang, Y.-H., W.A. Anderson, and E.R. DeSombre (1975)
Modulation of uterine morphology and growth by estradiol-170 and an estrogen antagonist. J. Cell Biol.,
Korenman, S.G., R.H. Stevens, L.A. Carpenter, M. Robb,
G.D. Niswender, and B.M. Sherman (1974) Estradiol
radioirnmunoassay without chromatography: Procedure, validation and normal values. J. Clin. Endocrinol. Metab., 38t718-720.
Krueger, W.A., W.J. Bo, and P.C. Hoopes (1975) A circadian rhythm of mitotic activity in the uterine luminal
epithelium of the rat: Effect of estrogen. Anat. Rec.,
Marcus, G.J. (1974a) Hormonal control of proliferation
in the guinea-pig uterus. J. Endocrinol., 63:89-97.
Marcus, G.J. (1974b)Mitosis in the rat uterus during the
estrous cycle, early pregnancy and early pseudopregnancy. Biol. Reprod., 10:447-452.
Mehrotra, S.N., and C.A. Finn (1974) Cell proliferation
in the uterus of the guinea-pig. J. Reprod. Fertil.,
Psychoyos, A. (1973) Hormonal control of ovo-implantation. Vitam. Horrn.. 31 :201-256.
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progesterone, guinea, estradiol, postpartum, endometriumrelationship, concentrations, pig, serum, regenerative
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