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Assessment of menstruation in the vervet (Cercopithecus aethiops).

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American Journal of Primatology 69:901–916 (2007)
RESEARCH ARTICLE
Assessment of Menstruation in the Vervet
(Cercopithecus aethiops)
REBECCA L. CARROLL1, KUNIE MAH1, JOHN W. FANTON2,
GWENDALYN N. MAGINNIS2, ROBERT M BRENNER1, AND OV D. SLAYDEN1
1
Division of Reproductive Sciences, Oregon National Primate Research Center,
Oregon Health and Science University, Beaverton, Oregon
2
Division of Animal Resources, Oregon National Primate Research Center,
Oregon Health and Science University, Beaverton, Oregon
Vervet monkeys (Chlorocebus aethiops) are Old World nonhumans that
display attenuated menstruation that requires detection by vaginal swab.
The physiology underlying attenuated menstruation in this species has not
been previously studied. To fill this gap, we evaluated endometrial cell
proliferation, steroid receptor localization and expression of menstruationassociated matrix metalloproteinase (MMP) enzymes in vervets during
natural and artificial menstrual cycles. The artificial cycles were induced
by sequentially treating ovariectomized animals with estradiol (E2) and
progesterone (P). Because menstrual flow is exceptionally light in this
species, menses was detected by vaginal swab. We found that both natural
and artificially cycled animals menstruated 3–5 days after the decline
of P at the end of the cycle. As in other primates, P withdrawal at
the end of artificial cycles triggered endometrial expression of MMPs,
including MMP-1, 2, 3, 7, 10, 11, 13 and 26 transcripts. In both
the natural and artificial menstrual cycle, menstrual sloughing was
restricted to the upper one-fourth of the endometrium, and MMP-1 and
2 were strongly expressed by the stroma of the sloughing zone. MMP-7 was
localized in the endometrial glands during late menses. As in macaques,
epithelial cell proliferation was localized to the functionalis zone during
the estrogen-dominated proliferative phase and to the basalis zone glands
during the P-dominated secretory phase. Regulation of estrogen and
progestin (or estradiol and progesterone) receptors was similar to that
reported for macaques. Because strong similarities exist between the
endometrium of vervets, macaques and women, we conclude that vervets
can provide a useful animal model for studies on hormone regulation of
menstruation. Am. J. Primatol. 69:901–916, 2007. c 2007 Wiley-Liss, Inc.
Key words: menstruation; vervet; hormone action; matrix metalloproteinase enzymes; cell proliferation
Contract grant sponsor: NIH grants; Contract grant numbers: HD07675, HD19182, HD18185 and
RR00163.
Correspondence to: Dr Ov D. Slayden, Division of Reproductive Sciences, Oregon National Primate
Research Center, 505 NW 185th Avenue, Beaverton, OR, 97006, USA. E-mail: slaydeno@ohsu.edu
Received 3 May 2006; revision accepted 28 September 2006
DOI 10.1002/ajp.20396
Published online 9 February 2007 in Wiley InterScience (www.interscience.wiley.com).
r 2007 Wiley-Liss, Inc.
902 / Carroll et al.
INTRODUCTION
Old World nonhuman primates, especially macaques have long been the
experimental animal model of choice for studies of hormone action in the
endometrium. However, macaques are becoming increasingly difficult to obtain
and it is important to assess other primate animal models for studies of the
reproductive tract. The African green monkey or vervet (Chlorocebus aethiops) is
a menstruating nonhuman primate with menstrual cycles of similar duration to
those of women [Hess et al., 1979; Kudolo et al., 1986]. The potential use of vervets
as an alternative to rhesus macaques for studies in reproduction have been
explored previously [Eley et al., 1989; Hess et al., 1979; Kudolo et al., 1986]. Those
studies revealed that menstruation is very light in vervets and that daily testing by
vaginal swab is required to identify menstrual periods. The physiology underlying
the attenuated menstruation in this species has not been previously studied.
In several reviews, we have described morphological and physiological
changes within the endometrium of rhesus macaques during both natural and
artificial menstrual cycles [Brenner & Slayden, 1994; Brenner et al., 2002;
Slayden & Brenner, 2004]. Like women, macaques naturally display 28-day
menstrual cycles, and each cycle is marked by 3–4 days of frank menstrual
bleeding. The macaque uterus is anatomically similar to the human uterus
and the endometrium has been morphologically characterized as having four
well-defined functional zones [Bartelmez, 1951, 1957]. During the follicular or
proliferative phase of the menstrual cycle, ovarian estrogen drives cell proliferation in the upper zones (Zone I–III; functionalis zone). During the luteal or
secretory phase of the cycle, progesterone (P) inhibits mitosis and induces a
secretory state in the functionalis. In women, the basalis zone (Zone IV) adjacent
to the myometrium is relatively unresponsive to cyclic hormonal changes, but
in the macaque the basalis zone’s glandular epithelium proliferates under the
influence of P during the secretory phase of the menstrual cycle [Brenner et al.,
2003; Okulicz et al., 1993; Padykula et al., 1989].
The endometrial functionalis is the zone that bleeds and sloughs during
menses in nonhuman primates and women. In macaques and women, the decline
of P at the end of the cycle is followed by increased expression of matrix
metalloproteinase (MMP) enzymes [Matrisian et al., 1994; Rudolph-Owen et al.,
1998]. The MMPs are capable of degrading the extracellular matrix and play a
role in the dissociation of tissues in the upper endometrial zones (Zones I–III)
during menstruation [Matrisian et al., 1994; Osteen et al., 2003; Zhang &
Salamonsen, 2002].
The cyclic changes in the level of ovarian estradiol (E2) and P also tightly
regulate the expression of steroid receptors in the endometrium [Brenner &
Slayden, 1994]. For instance, in macaques, E2 increases both estrogen receptor
(ER) a and progesterone receptor (PR) in the endometrial glands and stroma
during the proliferative phase of the cycle. During the luteal phase, P greatly
reduces ER in the glandular epithelium and stroma of the functionalis zone.
P also acts to reduce PR in the glands of the functionalis zone, but low levels
of PR protein are retained in the stroma. Interestingly, P does not reduce levels
of ER or PR in the glands of the endometrial basalis zone. A second ER (receptor
beta; ERb) is also expressed in the macaque endometrium. However, levels of ERb
do not appear to change significantly during the menstrual cycle [Brenner &
Slayden, 2004].
Our goal in this study was to assess the cyclic effects of E2 and P on the
regulation of menstruation in the vervet and to compare the menstrual
Am. J. Primatol. DOI 10.1002/ajp
Endometrial Cycles in the Vervet / 903
endometrium in this species with the menstrual endometrium of the well-studied
endometrium of rhesus macaques. For these studies we first evaluated
endometria from a small number of naturally cycling adult vervets. To further
explore the effect of P on endometrial cell proliferation, and P withdrawal
on menstruation, we continued the studies on ovariectomized artificially cycled
vervets, where the timing of P treatment was tightly regulated. Hormone data
from the naturally cycling animals were used to design appropriate Silastic
capsules for delivery of physiologically relevant levels of exogenous E2 and P, and
artificial 28-day cycles were stimulated in the animals by treating them with
implants of E2 and P. We report that the vervet endometrium differentiates into
hormone-responsive functional zones that are similar anatomically to endometrium of macaques and women. Moreover, P withdrawal at the end of the cycle
triggers expression of endometrial MMPs and tissue fragmentation during
menses that is essentially similar to other primates.
MATERIALS AND METHODS
Animal Treatment
Twenty adult female vervet monkeys (mean7SE body weight: 4.670.3 kg)
were purchased from the Caribbean Primate Laboratory (St. Kitts, Eastern
Caribbean). Animal care was provided by the veterinary staff at the Oregon
National Primate Research Center (ONPRC) and all procedures were preapproved by the ONPRC Institutional Animal Care and Use Committee. Except
where indicated all reagents used in this study were purchased from the SigmaAldrich Co (St. Louis, Missouri).
Naturally cycling animals
Blood samples were collected once in every 3 days from six vervets during
natural menstrual cycles. Both blood sampling and assessment of vaginal swabs
(see below) were collected on non-anesthetized animals that were trained to enter
a box with a squeeze mechanism for light restraint. Serum samples from two
natural cycles/animal were assayed for E2 and P to identify the follicular and
luteal phases of the natural menstrual cycle. Because frank (external) menses was
difficult to detect [Hess et al., 1979], the animals were inspected daily by vaginal
swab to detect menstrual bleeding. For this assessment, the animals were held
in light restraint and a cotton swab was inserted fully (2 cm; no speculum was
needed) into the vagina. Presence of blood on the swab was recorded as mense
positive. Animals underwent hysterectomy during the proliferative phase (cycle
day 10; n 5 1), secretory phase (cycle day 21; n 5 1) and the menstrual phase
(cycle days 3–5; n 5 4) of the second natural cycle.
Artificially cycling animals
Fourteen adult animals were ovariectomized during the luteal phase of the
cycle and treated sequentially with Silastic capsules (0.34 cm i.d.; 0.46 cm o.d.;
Dow Corning, Midland, Michigan) filled with crystalline E2 or P to induce
artificial cycles [Slayden et al., 1993]. For these cycles, a 2-cm E2 capsule was
implanted subcutaneously (s.c.) for 14 days to stimulate an artificial proliferative
phase. After 14 days of E2 priming a 3-cm P capsule was implanted for 14 days
(leaving the E2 implant in place) to simulate the secretory phase of the cycle.
Withdrawal of the P implant, leaving the E2 implant in place, completed the cycle.
All the animals were treated with one 28-day cycle and then uteri were collected
Am. J. Primatol. DOI 10.1002/ajp
904 / Carroll et al.
by hysterectomy on days 0 (n 5 2), 1, 2, 3 (n 5 1 each), 4 (n 5 2), 5, 7 (n 5 1 each),
14 (n 5 2) and 21 (n 5 3) of the second cycle (Fig. 1).
Tissue Handling
The uterus from each animal was separated from the oviducts and cervix and
cut into equal quarters along the longitudinal axes. Endometrium from the miduterine corpus from two quarters was further cut into 2 cm thick, full-thickness
cross-sections, lumen to myometrium. These sections were either fixed for
morphological studies and/or frozen for immunocytochemistry (ICC) (details are
given below). Samples for histological analysis were fixed in 2% glutaraldehyde
and 3% paraformaldehyde, embedded in glycol methacrylate sectioned (2 um) and
stained with Gill’s hematoxylin [Bennett et al., 1976]. Endometrium from the
mid-uterine corpus of the other half of each uterus was dissected from the
myometrium under a dissecting scope with fine iris scissors, weighed and frozen
for RNA isolation and analysis with focused cDNA arrays.
Immunocytochemistry
Detailed methods for ICC of nonhuman primate tissues have been previously
published [Brenner et al., 2003; Slayden & Brenner, 2004]. Samples for ICC were
microwave-stabilized, embedded in Tissue Tek OCT (Sakura Finetek USA, Inc.,
Torrance, CA) and frozen in liquid propane as previously described [Slayden
et al., 1995]. Briefly, samples of fresh tissue were microwave irradiated for 7 s
in an Amana Radarrange Touchmatic microwave oven (Amana, Iowa) mounted
in Tissue Tek OCT and dropped into liquid propane, chilled in liquid nitrogen.
Cryosections (5 um) were cut with a Leica CM30505 cryostat (Leica; Wetzlar,
Germany) and thaw-mounted on Superfrost Plus (Fisher Scientific, Pittsburgh,
PA) slides and held at 80 until all sections were cut. The slides were then placed
on wet ice at 51C, and microwave irradiated again for 2 s.
Localization for ER, and PR was accomplished with anti-human ER1D5
(NeoMarkers, Fremont, CA) and anti-human PR JZB 39 (courtesy of Geoffrey
Greene; CA). Proliferating cells were identified with an antibody for Ki 67 (Dako
Corp., Carpinteria, CA). MMPs 1, 2, and 7 were localized using mouse monoclonal
antibodies (MMP1; R&D Systems, Minneapolis, MN; MMP 2 and 7; EMD
Biosciences, San Diego, CA). The working concentration for each antibody (e.g.,
ER1D5, 1/50; JZB 39, 1 mg/ml; Ki 67, 1/200; MMP-1, 4 mg/ml; MMP-2, 4 mg/ml; and
MMP-7, 1 mg/ml) was determined by antibody-specific serial dilution. Positive
staining was detected with an avidin–biotin peroxidase kit (Vector Laboratories,
Fig. 1. Timeline depicting artificial menstrual cycles. During the pretreatment cycle the animals
were treated with E2 alone and then E21P to prime the endometrium for menstruation. Uteri were
collected as shown during the treatment cycle.
Am. J. Primatol. DOI 10.1002/ajp
Endometrial Cycles in the Vervet / 905
Burlingame, CA), after which the slides were treated with 0.05% osmium
tetroxide for 1 min, lightly counterstained with Mayers hematoxylin, and
mounted with Permount (Fisher Scientific).
Validation of ICC involved comparison of staining with each antibody on
endometrium from the macaque and human samples. During the validation of the
ICC for this species, we found that several antibody preparations directed against
human epitopes, and that work well in macaques and human samples, failed in
our trials to produce reliable positive staining in the vervet. Those antibodies
included PR-8 (Neomarkers) and MMP-3 (Neomarkers).
Photomicrography
Low-power micrographs were photographed with an Olympus OM-system
38 mm macro lens (Olympus Optical, Tokyo, Japan) on Ektachrome 64-T film
(Eastman Kodak, Rochester, NY) and digitized with a Polaroid SprintScan 35
scanner. High-power micrographs were captured through Zeiss planapochromatic
lenses with the Optronics DEI-750TD CCD camera (Optronics Engineering,
Goleta, CA). Digital images were adjusted for sharpness and contrast with Adobe
Photoshop (Adobe Systems, Seattle, WA) and photomicrographs were printed
with an Epson Stylus Photo 1200 printer (Epson, Tokyo, Japan).
Total RNA Isolation and Focused Gene Arrays
Samples of endometrium were thawed in ten volumes of TRIzol reagent
(InVitrogen, Carlsbad, CA), immediately homogenized with a Polytron tissue
homogenizer (Brinkman, Instruments, Westbury, NY) and processed following
the standard TRIzol protocol. The TRIzol extracted samples were treated with
RNase-free DNase (Qiagen, Valencia, CA) on RNeasy Mini columns (Qiagen).
Concentration of the eluted RNA was determined by UV absorbance on a 640B
spectrophotometer (Beckman Instruments Inc., Fullerton, CA) and RNA
integrity was checked by fractionation on denaturing agarose gels and visualized
with ethidium bromide.
Total RNA was analyzed on low-density cDNA arrays (GEArray Q Series
Human Extracellular matrix and adhesion molecules kit; SuperArray, Bethesda,
MD) following the manufacturers instructions. The array membranes were prehybridized in 2 ml hybridization buffer for 2 h at 601C. During pre-hybridization,
2 mg total RNA was reverse-transcribed and amplified using the GEArray
AmpoLabeling-LPR kit in the presence of [32P] dCTP (Perkin Elmer, Shelton,
CT). Similar incorporation of [32P]dCTP among the samples was verified by
electrophoresing 1 ml of labeled preparation on a 1% agarose gel, exposing the gel
to X-ray film. The remaining labeled sample was denatured at 941C and
hybridized to the arrays in a roller hybridization incubator (Robbins Scientific,
Sunnyvale, CA) overnight at 601C. After high stringency washing the arrays were
visualized by phosphor imaging with a Bio-Rad Molecular Imager FX (Bio-Rad
Laboratories, Hercules, CA) and quantified with ScanAylze (Stanford University,
Stanford, CA).
For each array the mean intensity for the negative controls (PUC 18 and
no cDNA) was calculated, and this signal level plus two SD of the mean intensity
was considered background level on the blot. The background was subtracted
from signal for each target cDNA and the relative signal for each gene on the
membrane was calculated as the ratio of intensity of target gene to the intensity
of RPL-13a, a ribosomal protein transcript.
Am. J. Primatol. DOI 10.1002/ajp
906 / Carroll et al.
Sample Size Limitations
As indicated above, we were limited in the number of animals available for
these studies, and in some cases only one animal was available at each time point.
Traditional statistical analysis has therefore not been undertaken. On the basis
of our studies in macaques, three clearly defined phases (menstrual, proliferative
and secretory) of the cycle can be identified. We have combined samples when
possible that fit into these phases to provide a database for our descriptive results
below.
RESULTS
Natural Menstrual Cycles
Figure 2 shows the E2 and P concentrations in the naturally cycling vervets.
The mean (7SE) concentration of E2 was during 82736 pg/in the follicular
phase, and 34712 pg/ml in the luteal phase. Serum P levels were o1 ng/ml during
the follicular phase and 5.471.3 during the mid-luteal phase. These values for
E2 and P during these cycles were similar to other menstruating nonhuman
primates including macaques [Brenner & Slayden, 1994]. The average vervet
follicular phase was 16.574.6 days and the average luteal phase was 1573 days.
Frank menstrual bleeding (e.g., menstrual blood on the vulva or cage floor) was
never noted for any of the animals in this study. Bleeding was detectable by
vaginal swab between 2 and 4 days after serum P levels dropped below 1 ng/ml
at the end of the cycle. Menstrual periods averaged 2.470.3 days. On the basis
of these data, the animals displayed a mean (7SE) menstrual cycle length of
28.873.4 days.
Artificial Menstrual Cycles
Because of bleeding variably between days 2 and 4, after serum P levels
dropped below 1 ng/ml at the end of the cycle, we treated ovariectomized animals
with E2 and P implants better control the effects of P in this species. Silastic
capsules were designed to provide serum hormone concentrations within the
normal physiological range for cycling vervets. Treatment with implants of E2
and P resulted in 81722 pg/ml E2 and 570.4 ng P/ml in serum, which were
within the normal range for naturally cycling vervets (see Fig. 2). Similar to the
natural cycles above, no frank menstrual bleeding was noted in the artificially
cycled animals. Mense was detectable by vaginal swab on days 3 through 6 of the
cycle for all the animals. For instance, 0/10 animals were swab-positive on day 2,
6/10 were swab-positive on day 3, 7/10 were swab positive on day 4, 7/10 were
Fig. 2. Hormone profiles from an adult naturally cycling vervet during the menstrual cycle. Values
represent the mean1SE hormone level from six animals. The values have been aligned relative to
the peak E2 value.
Am. J. Primatol. DOI 10.1002/ajp
Endometrial Cycles in the Vervet / 907
swab positive on day 5 and 3/10 were swab positive on day 6. Because all of
the animals began menstruating on either day 2 or early on day 3, the use of
hormone-controlled artificial cycles obviates the need to conduct daily vaginal
swabs to time the onset of menstruation in this species.
Cyclic Changes in Endometrial Histology
The vervet uterus was anatomically similar to the uterus of macaques
[Slayden & Brenner, 2004]. The mean (7SE) uterine weight in the vervet
3.0870.26 g. Figure 3 shows representative photographs of vervet uteri from
animals during the natural menstrual cycle that have been cut along the
longitudinal axis, after the cervix was removed. The cervix (not shown) was
elongate, approximately three times the length of the uterus, and frequently
bisected during hysterectomy. The upper cervical canal collected during
hysterectomy appeared less tortuous than that of macaques. Figure 4 shows
photomicrographs of hematoxylin and eosin-stained sections of endometrium
from vervet monkeys during the natural cycle. Histological analysis of the vervet
endometrium revealed no obvious cycle-specific differences between the naturally
cycling and artificially cycled animals. This observation is similar for macaques,
and women where the only ovarian factors required for menstrual cyclicity are
the ovarian steroids E2 and P [Brenner & Slayden, 1994; Hodgen, 1983].
Therefore the analysis described below applies to both naturally and artificially
cycled vervets.
Secretory phase endometria
During both the luteal phase of the natural cycle and on days 7 and 28 of the
artificial cycle, the glands of the functionalis zone were hypertrophied and deeply
sacculated and the epithelial cells of the secretory phase glands in the functionalis
zone were notably pseudostratified (Fig. 4a and e). Pseudostratification and
glandular sacculation became more notably pronounced during the late secretory
phase (not shown). This is strikingly different from the glands in macaques and
women as they always maintain a simple columnar epithelium [Brenner &
Slayden, 1994]. Mitotic cells were essentially absent from the functionalis zone
during the secretory phase.
To analyze better proliferating cells in the endometrium, we stained sections
from the animals during the artificial cycle for Ki-67 antigen, a marker of
Fig. 3. Macrophotographs of vervet uteri collected at different phases of the natural menstrual
cycle. In each case the cervix and oviducts have been removed and the uterus is cut in half along the
longitudinal axis. In each photo a double-headed arrow shows the endometrial thickness, and a line
has been drawn to indicate the approximate endometrial–myometrial border. The endometrium
was thickest during the secretory phase of the cycle (a). The decline in P levels during the
menstrual phase resulted in menstrual breakdown, which is clearly evident in the inner third of the
menstrual endometrium (b). No bleeding was evident in the proliferative phase endometrium (c).
Am. J. Primatol. DOI 10.1002/ajp
908 / Carroll et al.
Fig. 4. Photomicrographs showing endometrial histology from glycol methacrylate sections of vervet
endometrium from natural menstrual cycles. Similar histology was observed during the artificial
cycle. In the full thickness micrographs (a–d), a black line has been drawn to delineate the
endometrial–myometrial border. (M 5 mitotic cells; Ap 5 apoptotic cells). Secretory phase. The
secretory endometrium was thick and the glands sacculated (a) and there was prominent
peudostratification (PS) of the endometrial glands of the functionalis zone. Peudostratification was
not pronounced in the basalis zone. No mitotic cells were evident in the functionalis zone, but
abundant mitotic cells were observed in the basalis zone during the secretory phase. Menstrual
phase. Menstrual sloughing was evident on days 3–5 of the artificial cycle. During early
menstruation (b, f, j) there was clear evidence of tissue break down in the upper functionalis
zone and apoptotic cells were clearly evident in the basalis zone. During late menstruation,
menstrual breakdown was decreased and the healing endometrial stroma was densely packed,
apoptotic cells were still abundant in the basalis zone. Proliferative phase. During the proliferative
phase the functionalis glands were less sacculated and nonstratified. Abundant mitotic cells and
some apoptotic cells were evident in the functionalis zone. In the basalis zone, there was still
evidence of apoptosis, but little evidence of mitosis. Inset shows an enlargement of a spiral artery.
proliferating cells [Gerdes et al., 1984; Okulicz et al., 1993; Slayden et al., 1993].
Figure 5 shows representative sections stained with Ki-67. During both the mid
(d21) and late (d0) secretory phase, essentially no Ki-67-positive cells were
detected in the glandular epithelium of the functionalis zone, strongly supporting
the lack of mitotic activity in this zone during the secretory phase. In contrast,
3276% of the epithelial cells of the basalis zone were Ki-67 positive during the
secretory phase (days 21 and 28 combined).
The vervet endometrial basalis was not strikingly pseudostratified, and lack
of epithelial stratification provided a marker of lower margin of the functionalis
zone for counting Ki-67-positive cells. Unlike the functionalis zone, the basalis
zone also contained abundant mitotic cells. Secretory phase cell proliferation was
confirmed on the animal from the natural luteal phase (above), which had 31% of
the cells in the basalis zone epithelial cells positive for Ki-67 (not shown). In this
Am. J. Primatol. DOI 10.1002/ajp
Endometrial Cycles in the Vervet / 909
Fig. 5. Photomicrographs showing Ki-67 imunostaining, during the artificial menstrual cycle in the
vervet. Functionalis zone Ki-67 positive cells were minimal at the end of the secretory phase (a) and
early menstrual phase (b, c) of the cycle. Functionalis zone cell proliferation increased during late
mense (d) and was maintained at a high level throughout the proliferative phase on days 5, 7 and 14
(e, l and m). P suppressed functionalis zone Ki-67 by mid-secretory phase (n, d 21). In contrast, in
the basalis zone, Ki-67 positive cells were abundant during both the mid- (r) and late secretory
phase (a).
respect, the vervet is very similar to macaques in which P stimulates basalis zone
cell proliferation. Also like macaques, the vervets had well-developed endometrial
spiral arteries. In macaques, these arteries grow during the secretory phase.
In the vervet, Ki-67-positive cells were observed in the endometrial stroma
surrounding these arteries (not shown) indicating that these vessels grow during
the secretory phase as in macaques and other primates.
Menstrual phase endometria
During the natural menstrual cycle, menstrual breakdown was clearly
evident during fresh tissue dissection of the uterine cavity during the menstrual
phase of the natural (Fig. 3b). In general, menstruation was preceded by
endometrial shrinkage and then stromal tissue fragmentation in the upper
functionalis zones of the natural cycle (Fig. 3a–c). This was also strikingly evident
by day 3 of the artificial cycle (Fig. 5c). In the Vervet menstrual flow was very
light, detectable only by swab, and menstrual breakdown was more limited to the
very uppermost endometrial zones than observed in macaques [Rudolph-Owen
et al., 1998]. Menstruation was slightly more prolonged in vervets during the
natural cycle and some breakdown was still evident during late menses (Fig. 4c
and g) 5 days after P levels declined below 1 ng/ml.
Am. J. Primatol. DOI 10.1002/ajp
910 / Carroll et al.
Premenstruation and menstruation was associated with a cessation of cell
proliferation in both the functionalis and basalis zones as evident by the absence
of mitotic and Ki-67 positive cells on day 3 after P withdrawal (Fig. 5). By
late menses on day 4 after P withdrawal, Ki-67-positive cells were evident in the
healing glands in the functionalis zone. At this time there was a striking increase
in the number of apoptotic cells noted in the basalis zone (Fig. 4c).
Proliferative phase
By day 8 of the natural follicular phase, the vervet endometrium was fully
healed (Fig. 3c) and the glands displayed abundant mitotic cells (Fig. 4d). This
morphological state was also evident after 7 and 14 days of E2 in the proliferative
phase of the artificial cycle. Like other nonhuman primates and women, the
proliferative-phase endometrium exhibited tubular non-sacculated glands with
abundant mitotic cells in the functionalis zone. Analysis by Ki-67 ICC revealed
that on days 7 and 14 of the proliferative phase the abundance of Ki-67-positive
cells in both the glands and stroma was greatly increased compared with the
secretory and menstrual phases of the cycle (Fig. 5). Counts of proliferative
phase cells (days 7 and 14 combined) revealed that 4878% of the epithelial cells
were Ki-67 positive. In contrast, E2 did not stimulate cell proliferation (assessed
by mitotic cells or Ki-67 staining) in the basalis zone. This is similar to the
regulation observed in macaques, where E2 fails to stimulate basalis zone growth
[Padykula et al., 1989].
Regulation of ER and PR
Figure 6 shows ICC staining for ER and PR in the vervet endometrium. In
general, this pattern of ER and PR staining was very similar to that seen in other
nonhuman primates and women. Photomicrographs of ER and PR staining
during the artificial cycle are shown in Figure 6. Staining from vervets in the
natural cycle was identical to staining in the artificial cycle.
ER
ER was most strikingly regulated in the functionalis zone during the cycle.
For instance, ER staining was very strong in functionalis zone during the
proliferative phase of the cycle (Fig. 6a and d). By day 21 of the cycle (day 7 of the
secretory phase), P suppressed ER in the stroma of the functionalis zone (Fig. 6b).
Progesterone-induced downregulation of ER in the functionalis stroma and
glands was more pronounced at the end of the artificial cycle (Fig. 6c). Withdrawal
of P during the menstrual period resulted in a gradual increase in ER staining in
functionalis zone stroma and ER expression was clearly upregulated in the glands
of the functionalis zone by late mense (not shown). In contrast, in the basalis
zone, the glands maintained ER expression throughout the cycle.
PR staining
Like ER, PR staining was strongly regulated in the functionalis zone during
the cycle (Fig. 6g–i), the abundance of strong PR-positive cells was greatest in both
the stroma and glands of the upper zones (Fig. 6g and j) during the late
proliferative phase. By day 21 of the cycle, P treatment greatly suppressed PR in
the stroma and glandular epithelium (Fig. 6h). This downregulation of PR was
even more evident at the end of the secretory phase (Fig. 6i) After P withdrawal at
the end of the cycle, there was a striking increase in stromal PR in the functionalis
zone stroma (not shown) that was essentially the same as observed at mid-cycle.
Am. J. Primatol. DOI 10.1002/ajp
Endometrial Cycles in the Vervet / 911
Fig. 6. Photomicrographs showing estrogen receptor (ER), progesterone receptor (PR) immunostaining in the vervet. In the functionalis zone both ER (a) and PR (g) were strong in the glandular
epithelium and stroma. During the early secretory phase (d 21) stromal ER (b) and PR (h) were
strikingly suppressed. P-induced suppression of ER and PR in both the glands and stroma was
evident by the end of the cycle (c and i). In the basalis zone, ER and PR staining were maintained
in the glandular epithelium. Stromal ER was suppressed by P in the basalis zone. PR staining
in the basalis zone was variable. h—glandular looks very similar to i, more so than to g.
As seen with ER, PR was maintained in the glands of the basalis zone.
Stromal PR staining in the basalis zone was more variable and appeared to be
suppressed by P during the secretory phase (Fig. 6j–l).
MMPs During Menstruation
In rhesus macaques and women, menstruation is associated with an abrupt
increase in the endometrial levels of several MMP enzymes after P withdrawal
during the artificial cycle. To demonstrate that MMPs are involved in menses in
the vervet, we analyzed samples of RNA for MMP transcripts by focused gene
array. Results of this analysis are shown in Figure 7. Compared to the secretory
phase of the cycle, withdrawal of P during the artificial menstrual phase resulted
Am. J. Primatol. DOI 10.1002/ajp
912 / Carroll et al.
Fig. 7. Endometrial matrix metalloproteinase (MMP) transcript expression in artificially cycled
vervets detected by focused gene array. Compared with the end of the secretory phase, expression of
MMP-1, -2, -3, -7, -10, -11, -13, was significantly increased during menses. Tissue inhibitor of
metalloproteinase-1 (Timp-1) but not Timp-2 was also elevated during menses. The levels of these
enzymes declined to baseline after menses during the proliferative phase of the cycle.
in a 30–100-fold increase in expression of MMP-1, 2, 3, 7, 10 and 11. Several other
MMP transcripts including MMP-12, 13 and 26 were also elevated, but to a lesser
extent. Expression of these enzymes then declined back to baseline during the
proliferative phase of the artificial cycle. This pattern of MMP expression was
very similar to that previously published for macaques [Rudolph-Owen et al.,
1998].
Immunostaining for MMP-1, -2 and -7 during the artificial menstrual cycle
is shown in Figure 8. MMP-1 and -2 were localized in the stroma and the
fragmenting zone during menstruation on day 3 after P withdrawal. MMP-1 and
-2 were greatly decreased by day 5. Similar strong staining was observed during
early menstruation in the natural cycle (not shown). No specific MMP staining
was observed in the non-menstruating animals in either the natural or artificial
cycle.
MMP-7 showed a strong signal in the glandular epithelium and not the
stroma during late menses. Glandular expression of MMP-7 is unique compared
with other MMPs, which are primarily stromal. Epithelial MMP-7 staining was
also noted in macaques [Rudolph-Owen et al., 1998] and may correlate with the
beginning of glandular remodeling at the end of the menstrual phase.
DISCUSSION
This study is the first assessment of experimentally induced, hormonal
control of menstruation in the vervet. Compared to other primates including
women [Jabbour et al., 2006], macaques [Slayden et al., 2001] and baboons,
[D’Hooghe et al., 1994], vervets display exceptionally light menstrual bleeding
that requires daily testing by vaginal swab to identify menstrual periods [Hess
et al., 1979]. However, in most other respects, the vervet is very similar to other
nonhuman primates.
Am. J. Primatol. DOI 10.1002/ajp
Endometrial Cycles in the Vervet / 913
Fig. 8. Photomicrographs showing immunostaining for matrix metalloproteinase (MMP)-1, -2 and
-7 in the vervet endometrium. Stromal staining was clearly evident for MMP-1 and 2 during menses.
Staining for MMP-7 increased during late menses but was localized the glandular epithelium.
For instance, as in other nonhuman primates, E2 increased staining for ER
and PR, both well-recognized endpoints of estrogen action. P during the luteal
phase inhibited this action of E2 in the functionalis zone and suppressed ER and
PR. This basic regulation by E2 in the vervet endometrium is identical to the wellrecognized pattern in other nonhuman primates and women.
One limitation was the availability of animals for evaluation in this study.
For instance, minimal sample size prevented a quantitative analysis of ER and PR
staining. To reduce the variability among animals in this study, we deliberately
treated animals with artificial cycles to control progestin levels as well as the
timing of P withdrawal. This resulted in a robust effect on cell proliferation
and MMP expression. For instance, E2 during the proliferative phase of the cycle
stimulated epithelial cell proliferation only in the functionalis zone of
the endometrium. During the secretory (luteal) phase of the cycle P stimulated
cell proliferation that was restricted to the basalis zone. During the luteal phase
P-suppresses proliferation in the functionalis zone. This cyclic pattern of
functionalis–basalis cell proliferation was identical to that seen in macaques
[Brenner et al., 2003] but quite different from the pattern seen in women, where
P is not reported to stimulate basalis zone proliferation.
Another striking change is the expression of MMPs, which increased as much
as 200-fold in some cases during menses. Like other menstruating primates,
menstrual bleeding and sloughing in the vervets was associated with the
expression of MMP enzymes (e.g., MMP-1, 2, 3, 7 10, 11) during the menstrual
phase of the cycle. Immunostaining revealed that MMP-1 and -2 proteins were
strictly localized to the stromal cells in the sloughing zone of the endometrial
functionalis. This is consistent with reports on other primates, including women
and macaques, that the decline of P at the end of the cycle triggers the expression
of MMP enzymes [Matrisian et al., 1994; Rudolph-Owen et al., 1998].
Am. J. Primatol. DOI 10.1002/ajp
914 / Carroll et al.
The MMPs are capable of degrading the extracellular matrix and play a role
in the menstrual dissociation of endometrial tissue in women [Matrisian et al.,
1994; Osteen et al., 2003; Zhang & Salamonsen, 2002] and macaques. Expression
of MMP-1, -2, -3, -10, -11, and -14 in the rhesus macaque endometrium peaks
on days 2–4 of the luteal–follicular transition (LFT), then declines spontaneously
after menstruation during the proliferative phase of the cycle even though P is
absent [Rudolph-Owen et al., 1998]. Similar MMP enzymes are upregulated in
the vervet. Paradoxically, PR consensus sequences have not been identified on the
genes for these menstruation-associated MMPs. Therefore, P mediated regulation
of endometrial MMPs appears to be indirect.
Our studies show that similar MMP enzymes are expressed in both macaques
[Rudolph-Owen et al., 1998] and vervets, therefore, differences in MMP
expression do not explain light menstruation in the vervet. Our studies further
indicate that ovarian function is not responsible for attenuated menstruation
in this species. Similar light menses was observed during artificial cycles where
ovarian steroid hormone levels were tightly controlled. Why, therefore, do vervets
display exceptionally light menstrual bleeding?
It is possible that vervet-specific anatomy may play a role in light menses
observed in this species compared with the macaque. Our observation that the
upper cervix was frequently cut in half during hysterectomy suggests that it
is longer relative to the macaque. Some of the menstrual material may seep into
the vaults of the cervical mucosa and never reach the vulva. Although difficult to
quantify, the level of fragmentation in the vervet endometrium appeared to be
restricted to the upper fourth or less of the functionalis zone. Moreover, at each
time point in the artificial cycle there was less tissue sloughing than seen
in other macaques or menstruating species. It is currently not known what
physiological factors limit the expression of MMPs and menstruation to the upper
functionalis zone. Like other menstruating nonhuman primates, vervets possess
endometrial spiral arteries that vascularize the upper endometrial zones. These
arteries appear to be critical to normal menstruation in primates [Markee, 1950].
Additional studies are needed to extensively analyze arterial patterns in vervets
and to determine if differences in arterial function are responsible for reduced
tissue sloughing in this species. However, reduced tissue volume and an elongated
cervix may combine to be the reason that the external bleeding associated with
menstruation is attenuated in this species. Despite minimal menstrual discharge
in this species, strong similarities between the vervet and macaque endometrium
indicate that vervets can be a useful animal model for studies of menstruation,
including analysis of the role of MMPs in endometrial remodeling, and
identification of zone-specific factors that regulate proliferation in the nonhuman
primate endometrium.
CONCLUSIONS
Cyclic regulation of the vervet endometrium is very similar to that seen in
macaques and women. P withdrawal at the end of the cycle induces menstrual
breakdown marked by upregulation of the same MMP family typically observed
in other nonhuman primates. The lack of external bleeding appears primarily
because of the reduced amount of tissue that is sloughed off during the menstrual
process. Therefore, this species could provide a viable animal model in which to
study mechanisms that underlie the zonal differences in MMP expression, steroid
receptor regulation and control of proliferation.
Am. J. Primatol. DOI 10.1002/ajp
Endometrial Cycles in the Vervet / 915
ACKNOWLEDGMENTS
We gratefully acknowledge the Animal Care Technicians in the Division of
Laboratory Animal Medicine for care of the animals during this study.
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