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Morphometric analysis of the uterine endometrium of swine on days 12 and 16 postestrus.

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THE ANATOMICAL RECORD PART A 270A:59 – 66 (2003)
Morphometric Analysis of the Uterine
Endometrium of Swine on
Days 12 and 16 Postestrus
DIANNA M. BLACKWELL,1,3 ROBERT C. SPETH,2,3 AND
MARK A. MIRANDO1,3*
1
Department of Animal Sciences, Washington State University,
Pullman, Washington
2
Department of Veterinary Comparative Anatomy, Pharmacology and Physiology,
Washington State University, Pullman, Washington
3
Center for Reproductive Biology, Washington State University,
Pullman, Washington
ABSTRACT
The uterine endometrium of swine is comprised of luminal epithelial, glandular epithelial, and stromal cells that secrete the luteolysin, prostaglandin F2␣ (PGF2␣), during late
diestrus. However, which of these cells contribute the most to luteolytic PGF2␣ secretion is
unknown because the cellular composition of the endometrium has not been quantified.
Therefore, this study quantified the cellular composition of the endometrium on days 12 and
16 postestrus by histologic and morphometric analyses. On day 12, the endometrium consisted predominantly of stromal cells (47% of total cell quantity) and glandular epithelial cells
(37%), whereas luminal epithelial cells represented only 16% of the total of the three cell
types. The number of glandular epithelial cells tended to increase (P ⬍ 0.10) between days 12
and 16, such that they comprised 45% of the endometrium by day 16, while the number of
stromal and luminal cells did not change and accounted for 45% and 10% of the cells,
respectively. Luminal epithelial cells had a 58% greater cross-sectional area (P ⬍ 0.001) than
glandular epithelial cells, whereas glandular epithelial cells had a 22% greater area (P ⬍
0.001) than stromal cells. Glandular epithelial cells decreased (P ⬍ 0.001) in cross-sectional
area between days 12 and 16, whereas the area of luminal epithelial and stromal cells
remained unchanged. These results indicate that the porcine endometrium is comprised
predominantly of stromal and glandular epithelial cells that are likely to contribute substantially to endometrial PGF2␣ secretion during luteolysis. The contribution of glandular epithelium to luteolytic PGF2␣ secretion probably increases during diestrus as the number of
these cells increases. Anat Rec Part A 270A:59 – 66, 2003. © 2003 Wiley-Liss, Inc.
Key words: endometrium; uterus; epithelial cell; stromal cell; morphometry;
pig
Prostaglandin F2␣ (PGF2␣) is released from the uterine
endometrium in a highly pulsatile manner during the
luteolytic period of late diestrus in domestic ungulates to
terminate corpus luteum (CL) function, and oxytocin is
the major known stimulus for pulsatile secretion of PGF2␣
(Silvia et al., 1991; Mirando et al., 1996; McCracken et al.,
1999). In ruminants, epithelial cells are entirely responsible for the oxytocin-stimulated PGF2␣ secretion, and
stromal cells are completely unresponsive to oxytocin (Asselin et al., 1996). In contrast, stromal cells are the porcine
endometrial cells that are the most responsive to oxytocin,
whereas luminal epithelial cells are completely unresponsive and glandular epithelial cells display an intermediate
response (Uzumcu et al., 1998; Braileanu et al., 2001; Hu
©
2003 WILEY-LISS, INC.
et al., 2001a, b). This pattern of response apparently is
opposite to that for expression of oxytocin and oxytocin
Grant sponsor: National Institutes of Health; Grant number:
HD30268.
*Correspondence to: Mark A. Mirando, National Research Initiative Competitive Grants Program, 1400 Independence Ave.
SW, Stop 2241, U.S. Department of Agriculture, Washington, DC
20250-2241. Fax: (202) 205-3641. E-mail: mmirando@reeusda.gov
Received 9 April 2002; Accepted 15 August 2002
DOI 10.1002/ar.a.10182
60
BLACKWELL ET AL.
receptors in these cells (Boulton et al., 1995, 1996). Although stromal cells showed a greater PGF2␣ secretory
response to oxytocin than did luminal or glandular epithelial cells (Uzumcu et al., 1998; Braileanu et al., 2001; Hu
et al., 2001a, b), stromal cells apparently secrete less total
PGF2␣ per cell than do epithelial cells, especially compared to those of the luminal surface (Zhang et al., 1991;
Uzumcu et al., 1998; Hu et al., 2001a, b), because of the
greater basal secretion of PGF2␣ from epithelial cells in
the absence of oxytocin stimulation. This would seem to
indicate that stromal cells make a relatively minor contribution to luteolytic PGF2␣ secretion in swine despite being
more responsive to oxytocin than the two types of epithelial cells. However, this suggestion completely ignores the
relative contribution of each cell type to the total morphological composition of the endometrium. If stromal cells
comprise a substantial portion of the endometrium in pigs,
then they likely also would contribute significantly to luteolytic secretion of PGF2␣ during late diestrus. However,
to our knowledge morphometric analysis of the cell types
in the endometrium has not been reported for pigs or any
other species.
It was hypothesized that stromal cells are a major constituent of the endometrium in cyclic gilts. Therefore, the
primary objective of this study was to determine quantitatively the tissue and cellular composition of the endometrium during the luteolytic period of late diestrus in
pigs (i.e., days 12–16) using standard accepted stereological morphometric procedures (Chalkley, 1943; Abercrombie, 1946; Weibel, 1979; Mirando et al., 1989). A secondary
objective was to determine if the tissue and cellular composition of the endometrium changed as the luteolytic
period progressed from just before the onset of endometrial responsiveness to oxytocin and pulsatile PGF2␣ secretion (i.e., on day 12) to the middle of luteolysis on day
16 (Carnahan et al., 1996; Edgerton et al., 1996).
MATERIALS AND METHODS
Animals
Fourteen crossbred peripubertal gilts (Landrace, Yorkshire, Large White, Hampshire, and Duroc) were observed
daily in the presence of an intact boar for standing estrous
behavior. Gilts that showed at least one normal interestrous interval of 18 –24 days were allotted, in a completely randomized design, to undergo hysterectomy 12
(n ⫽ 8) or 16 (n ⫽ 6) days after the second or third estrus.
At hysterectomy, the uterus was excised, immediately and
carefully trimmed of the mesometrium, and weighed.
Three samples of tissue, approximately 4 – 6 cm in length,
were obtained from one randomly selected uterine horn
approximately 5 cm from the uterine body, 5 cm from the
utero-tubal junction, and from the center of the uterine
horn. Each sample was placed immediately in 100 ml of
phosphate-buffered (pH 7.4) 4% paraformaldehyde fixative. Additional tissue from each of the three regions of the
uterus was collected, placed in plastic bags, and kept on
ice. The tissue samples on ice were transferred to the
laboratory where measurements of specific gravity were
obtained by weighing 4- to 6-g portions and then submerging them in a known volume of deionized water to determine volume displacement. Duplicate determinations
were made on each region for a total of six measurements
per uterus.
Histological Preparation of Tissue
Paraformaldehyde fixative was replaced with fresh fixative 1 and 3 hr after initial tissue immersion, at which
times approximately 0.5 cm was trimmed from each end of
the tissue to increase the ratio of fixative volume to tissue
mass and to permit increased access of fixative to the
center of the tissue while maintaining integrity of the
gross tubular anatomy of the tissue. After 48 hr, both ends
of the tissue were trimmed to obtain a cross section of
tissue approximately 0.5–1 cm in length from near the
center of the tissue sample. Tissues were stored in 70%
ethanol until cleared, infiltrated, and embedded in paraffin. Tissues were sectioned at 3 ␮m, and two complete
cross sections, 30 ␮m apart, were obtained. After being
mounted on glass slides, tissue sections were deparaffinized, cleared, stained with Mayer’s hematoxylin, counterstained with eosin/phloxine, and then dehydrated (Humason, 1979), after which coverslips were applied using a
drop of Richard Allen mounting medium (Richard Allen
Scientific, Kalamazoo, MI).
Histological and Morphometrical Analyses
The cross-sectional areas of stromal, glandular epithelial, and luminal epithelial cells (Fig. 1) were measured
using a Nikon Labophot microscope (Nikon USA, Melville,
NY) attached to an MCID image analysis system (Imaging
Research, Inc., St. Catherine, Ontario, Canada). After
magnification at 80⫻, images were displayed on a 33-cm
video monitor. A minimum of 35 cells (range ⫽ 35–105) of
each type was quantified on each tissue section obtained
from the three regions of the uterus. The cellular crosssectional area and total scanned area were measured in
pixels, and data were then converted to area by determining the number of pixels per unit area using a micrometer
slide for calibration.
The proportion (percentage area) of uterus consisting of
endometrium, myometrium, and perimetrium (Fig. 1) was
determined by light microscopy at 100⫻ magnification
using a 100-point ocular graticule by the method of Chalkley (1943), as described previously for testicular tissue
(Mirando et al., 1989). Four determinations, spanning the
cross section from perimetrium to uterine lumen, were
made on each section. The proportions (percentage area)
of luminal epithelial, glandular epithelial, and stromal
tissue within the endometrium were determined similarly, with four determinations made on each section.
Calculations
All calculations were made such that measurements of
variables for each of the three regions of the uterus contributed equally to the mean values calculated. Uterine
volumes (cm3) were obtained by dividing the weight of
each uterus by its mean specific gravity measurement.
Volumes of endometrium, myometrium, and perimetrium
were then obtained by multiplying the mean proportion of
each tissue within each uterus by the total volume of each
respective uterus (Mirando et al., 1989). Total volumes of
luminal epithelium, glandular epithelium, and stroma
within the endometrium of each uterus were determined
similarly by multiplying the mean proportion of each tissue component within each uterus by the total volume of
endometrium within each uterus. The true cell count
within a given scanned volume of endometrial tissue was
determined from the crude cell count, section thickness,
MORPHOMETRY OF PORCINE UTERINE ENDOMETRIUM
61
Fig. 1. Representative hematoxylin and eosin/phloxine-stained photomicrographs showing uterine cross sections from day 12 (left column)
and day 16 (right column) pigs at 40⫻ (A) and 400⫻ (B) magnification.
The uterine lumen, endometrium (ENDO), myometrium (MYO), and pe-
rimetrium (PERI) are indicated in the top panel. Luminal epithelial cells
(LEC), glandular epithelial cells (GEC), stromal cells (SC), and blood
vessels (BV) of the endometrium are indicated by arrows in the bottom
panel.
and cellular diameters (Abercrombie, 1946; Weibel, 1979;
Mirando et al., 1989), whereby cellular diameters were
calculated from the cellular cross-sectional area of individual cells using the formula for a circle, as described
previously (Mirando et al., 1989). The numbers of luminal
epithelial, glandular epithelial, and stromal cells per
uterus were then obtained from the product of the true cell
count per scanned volume of endometrial tissue and volume of endometrial tissue component per uterus (i.e.,
number of stromal cells per uterus ⫽ stromal cells per unit
volume stroma ⫻ stromal volume per uterus, etc.). Although the stromal, glandular epithelial, and luminal epithelial cells examined in the present study were not
spherical, a previous study demonstrated that morphometric analyses derived from diameters calculated from
cellular components that were highly irregular in shape
yielded similar results to those derived from diameter
measurements made on spherical cellular components
(Mirando et al., 1989).
Statistical Analyses
Data were subjected to least-squares analysis of variance (ANOVA) for a completely randomized design using
the general linear models (GLM) procedure of the Statistical Analysis System (SAS, 1996). Data for uterine
weight, specific gravity, uterine volume, volumes of uterine tissue components (i.e., endometrium, myometrium,
and perimetrium), volumes of endometrial tissue components (i.e., stroma, glandular epithelium, and luminal epithelium), and numbers of cells were analyzed for a oneway treatment structure with day as the main effect. Data
for cell size (i.e., cross-sectional area of stromal, glandular
epithelial, and luminal epithelial cells) were analyzed for
a split-plot ANOVA with a three-way treatment structure.
Day was the whole-plot effect, gilt was nested within day,
and the subplot effects were region of the uterus (i.e.,
anterior, middle, and posterior) and cell type. Data for
proportion (percentage area) of uterine tissue components
62
BLACKWELL ET AL.
Fig. 3. Mean (⫹ SEM) volumes of endometrium, myometrium, and
perimetrium on days 12 and 16 postestrus in cyclic gilts. The volumes of
endometrium, myometrium, and perimetrium did not differ between days
12 and 16.
Fig. 2. Mean (⫹ standard error of mean (SEM)) proportion of uterine
endometrium (top) and myometrium (bottom) in the anterior, middle, and
posterior regions of the uterus on days 12 and 16 postestrus in cyclic
gilts. There was a lower (P ⫽ 0.06) proportion of endometrium and more
(P ⬍ 0.07) myometrium on day 16 than on day 12. The anterior region of
the uterus also contained a smaller (P ⬍ 0.01) proportion of myometrium
and a greater (P ⬍ 0.01) proportion of endometrium than did the middle
or posterior regions.
and endometrial tissue components were analyzed for a
split-plot ANOVA with a two-way treatment structure.
Day was the whole-plot effect, gilt was nested within day,
and the subplot effect was region of the uterus. Because
values for proportion of perimetrium and luminal epithelium approached zero, angular (i.e., arc sin) transformations were initially performed for these two characteristics. However, these transformations had only a miniscule
effect on the outcome of the ANOVAs, and therefore, only
results for the untransformed data were utilized and reported. All tests of hypotheses were performed using the
appropriate error terms according to the expectation of the
mean squares (Snedecor and Cochran, 1980).
RESULTS
Mean weight and volume of the uterus did not differ
significantly between days 12 and 16 (610 ⫾ 61 g vs. 695 ⫾
70 g and 577 ⫾ 59 cm3 vs. 662 ⫾ 68 cm3, respectively).
Mean specific gravity of the uterus also did not vary significantly between day 12 (1.06 ⫾ 0.01 g/cm3) and day 16
(1.05 ⫾ 0.01 g/cm3). The proportions of endometrium and
myometrium (Fig. 2) varied by day (P ⬍ 0.07) and region
(P ⬍ 0.01). There was a lower (P ⫽ 0.06) proportion of
endometrium and more (P ⬍ 0.07) myometrium on day 16
than on day 12. The anterior region of the uterus also
contained a greater (P ⬍ 0.01) proportion of endometrium
and a lower (P ⬍ 0.01) proportion of myometrium than did
the middle or posterior regions. The volumes of endometrium, myometrium, and perimetrium did not differ between days 12 and 16 (Fig. 3). The proportion of luminal
epithelium did not vary by day or region (Fig. 4). The
proportion of glandular epithelium varied by region (P ⬍
0.05), with the posterior region possessing a smaller proportion of glandular epithelium than the middle or anterior regions of the uterus (Fig. 4); however, the interaction
of region by day (P ⫽ 0.05) indicated that variation among
regions of the uterus was primarily due to a decrease in
glandular epithelium in the posterior region on day 16.
The proportion of endometrial stroma also varied (P ⬍
0.01) by region of the uterus, with the posterior region
possessing the greatest proportion of stroma, the middle of
the uterus containing the least, and the anterior region
having an intermediate amount (Fig. 4). Within the endometrium, the volumes of stroma, glandular epithelium,
and luminal epithelium did not vary significantly by day
(Fig. 5).
The size of individual luminal epithelial, glandular epithelial, and stromal cells varied by cell type (P ⬍ 0.001)
with an interaction of day by cell type (P ⬍ 0.01). Across
both days, luminal epithelial cells were 58% larger (P ⬍
0.001) than glandular epithelial cells, and glandular epithelial cells were 22% larger (P ⬍ 0.001) than stromal cells
(Fig. 6). Glandular epithelial cells also decreased (P ⬍
0.01) in size between days 12 and 16, whereas the size of
luminal epithelial and stromal cells remained unchanged
(Fig. 6). Cell size also varied significantly by region of the
uterus (cell ⫻ region interaction, P ⬍ 0.001); glandular
epithelial cells were of similar size throughout the uterus,
whereas luminal epithelial cells were largest near the
uterine body, smallest near the anterior region of the
uterus, and intermediate in size near the middle of the
uterine horn (Fig. 6). In contrast, stromal cells were larger
in the posterior and anterior regions of the uterine horn
than toward the middle region. Of the three endometrial
cell types examined, the endometrium was comprised predominantly of stromal cells (47%) on day 12 and, to a
lesser degree, of glandular epithelial cells (37%), whereas
luminal epithelial cells represented only 16% of the three
endometrial cell types (Fig. 7). The number of glandular
MORPHOMETRY OF PORCINE UTERINE ENDOMETRIUM
63
Fig. 5. Mean (⫹ SEM) volumes of luminal epithelium, glandular epithelium, and stroma within the endometrium on days 12 and 16 postestrus in cyclic gilts. Volumes of endometrial tissue components did not
vary significantly by day.
Fig. 4. Mean (⫹ SEM) proportions of luminal epithelium (top), glandular epithelium (middle), and stroma (bottom) within the endometrium in
the anterior, middle, and posterior regions of the uterus on days 12 and
16 postestrus in cyclic gilts. Toward the posterior region of the uterus,
the endometrium contained a greater (P ⬍ 0.01) proportion of stroma
than did the middle or anterior regions of the uterine horns. In contrast,
the proportion of glandular epithelium did not vary by region on day 12,
but on day 16, the endometrium nearest the uterine body possessed the
smallest (P ⬍ 0.05) proportion of glandular epithelium. The interaction of
region by day (P ⫽ 0.05) indicated that variation among regions of the
uterus was primarily due to a decrease in glandular epithelium in the
anterior region on day 16.
epithelial cells tended to increase (P ⬍ 0.10) 54% between
days 12 and 16. As a result, they comprised 45% of the
three endometrial cell types, while stromal cells also accounted for 45% and luminal cells accounted for the remaining 10% (Fig. 7).
DISCUSSION
This study demonstrated that the uterine endometrium
of cyclic pigs is comprised predominantly of stromal and
glandular epithelial cells, whereas luminal surface epithelial cells constituted only 10%–16% of the three endometrial cell types during the luteolytic period of late diestrus.
Previous studies with pigs demonstrated that oxytocin
induced maximal response from endometrial stromal
cells, did not stimulate PGF2␣ secretion from luminal epithelial cells, and promoted an intermediate response from
glandular epithelial cells (Uzumcu et al., 1998; Braileanu
et al., 2001; Hu et al., 2001a, b). However, the contribution
of oxytocin-induced PGF2␣ release from stromal and glandular cells was questioned because these cells secrete less
PGF2␣ than do luminal epithelial cells (Uzumcu et al.,
1998; Braileanu et al., 2001). Based on the prevalence of
Fig. 6. Mean (⫹ SEM) cross-sectional area of luminal epithelial,
glandular epithelial, and stromal cells on days 12 and 16 postestrus (top)
within the endometrium in the anterior, middle, and posterior regions
(bottom) in cyclic gilts. Luminal epithelial cells were larger (P ⬍ 0.001)
than glandular epithelial cells, and glandular epithelial cells were larger
(P ⬍ 0.001) than stromal cells. Glandular epithelial cells also decreased
(P ⬍ 0.001) in size between days 12 and 16, whereas the size of luminal
epithelial and stromal cells remained unchanged. Glandular epithelial
cells were of similar size throughout the uterus, whereas the size of
luminal epithelial and stromal cells varied (P ⬍ 0.001) by region of the
uterus.
the stromal and glandular cells determined in the present
study and their closer proximity to the uterine vasculature, it is suggested now that these two cell types are
likely to contribute the most to endometrial secretion of
PGF2␣ during luteolysis in pigs.
The contribution of glandular epithelium to luteolytic
secretion of PGF2␣ probably increases during diestrus as
64
BLACKWELL ET AL.
Fig. 7. Mean (⫹ SEM) number of luminal epithelial, glandular epithelial, and stromal cells per uterus on days 12 and 16 postestrus in cyclic
gilts. The number of glandular epithelial cells tended to increase (P ⬍
0.10) between days 12 and 16, but the numbers of luminal epithelial and
stromal cells did not change significantly.
the glands proliferate and the number of these cells increases, as indicated by the results of the current study.
The uterine endometrium of the pig responds to cues of
ovarian steroids with orderly cycles of tissue proliferation,
glandular secretion, and prostaglandin production (Corner, 1921; Geisert et al., 1990a, b, 1994; Gray et al.,
2001a). The results of the present study indicated that the
number of glandular cells increased 54% between days 12
and 16 after estrus, results that generally are consistent
with glandular proliferation from early to mid-diestrus
reported previously (Corner, 1921; Erices and Schnurrbusch, 1979). The ability of glandular epithelial cells to
secrete PGF2␣ also increases sharply between days 12 and
16 (Uzumcu et al., 2000). Taken together, these results
indicate that the total amount of PGF2␣ released by the
glandular epithelium increases substantially as diestrus
progresses. Moreover, the increase in PGF2␣ release from
glandular epithelial cells between days 12 and 16
(Uzumcu et al., 2000) occurred in spite of a decrease in size
of these cells.
In the endometrium of several species, both epithelial
and stromal cells are capable of producing prostaglandins
(Schatz et al., 1987; Smith and Kelly, 1987, 1988; Fortier
et al., 1988; Zhang et al., 1991; Uzumcu et al., 1998, 2000;
Braileanu et al., 2001; Hu et al., 2001a, b), although there
is discrepancy as to which cell type is the major source of
prostaglandin synthesis. Some reports indicate that stromal cells are almost completely responsible for the PGF2␣
release from the human endometrium (Gal et al., 1982) or
that glandular epithelial and stromal cells from the human endometrium secrete similar amounts of PGF2␣
(Schatz et al., 1987), whereas others reported that glandular epithelial cells secrete more PGF2␣ than do stromal
cells (Lumsden et al., 1984; Smith and Kelly, 1987, 1988).
Similarly, glandular epithelial cells of sheep and cattle
endometrium secrete substantially more PGF2␣ than do
stromal cells, whereas stromal cells are the major source
of PGE2 (Fortier et al., 1988; Cherny and Findlay, 1990).
Working with porcine endometrium, Zhang et al. (1991)
reported that glandular cells secrete more PGF2␣ than
PGE, whereas stromal cells secrete more PGE than
PGF2␣; however, the two cell types secrete similar
amounts of PGF2␣ when obtained from the endometrium
of cyclic pigs. Some studies from our laboratory (Uzumcu
et al., 1998) have obtained results similar to those of
Zhang et al. (1991), although the results of most experiments indicate that luminal epithelial cells secrete the
greatest quantity of PGF2␣, stromal cells secrete the least,
and glandular epithelial cells release an intermediate
amount (Uzumcu et al., 1998; Braileanu et al., 2001).
Collectively, those results suggest that stromal cells make
a relatively minor contribution to luteolytic PGF2␣ secretion in swine despite being more responsive to oxytocin
than the two types of epithelial cells. However, results of
the present study indicate that stromal cells probably are
a major source of luteolytic PGF2␣ pulses during late
diestrus in pigs because they comprise such a substantial
portion of the endometrium. Moreover, stromal cells are in
closest proximity to the uterine vasculature, and the
PGF2␣ released from them may enter the circulation more
readily than that secreted from the luminal surface epithelium or the superficial glands.
Although stromal cells are most responsive to oxytocin
in vitro and epithelial cells of the luminal surface are least
responsive (Uzumcu et al., 1998; Braileanu et al., 2001;
Hu et al., 2001a, b), those results seemingly are in contrast to results of studies that localized oxytocin receptor
mRNA primarily within the luminal epithelium of the
porcine endometrium and minimally within stromal cells
(Boulton et al., 1995). However, luminal epithelial cells
also synthesize substantially more oxytocin than do stromal cells (Boulton et al., 1996). Thus, oxytocin released by
luminal epithelial cells in vitro may act in an autocrine
manner to either desensitize cultured luminal epithelial
cells to exogenous oxytocin or elevate basal PGF2␣ secretion. Either of these effects would reduce the apparent
responsiveness of cultured luminal epithelial cells to oxytocin, and both effects were recently reported to occur in
luminal epithelial cells from the pig endometrium (Hu et
al., 2001b). In contrast, stromal cells did not exhibit noticeable desensitization of the response to oxytocin (Hu et
al., 2001b), which also may contribute to these cells displaying a greater response to oxytocin in vitro than do
luminal epithelial cells.
In addition to an increase in the number of glandular
epithelial cells between days 12 and 16, the proportion of
endometrium occupied by glandular epithelium varied
among regions of the uterus in the present study. Keys
and King (1989) reported that the height of the luminal
epithelial cell layer was low on days 10 and 13 of the
estrous cycle but increased by days 16 –19, observations
that agreed with those of Crombie (1972) and King et al.
(1982). Taken together, these results indicate that substantial histological remodeling of the uterine endometrium occurred in the absence of changes in uterine weight
or volume. A similar conclusion was also reached by Harney and Bazer (1990). Although endometrial glands are
known to proliferate primarily during the follicular phase
(Corner, 1921; Gray et al., 2001a), they also undergo hyperplasia and hypertrophy during pregnancy in sheep,
cattle, and swine (Gray et al., 2001a), which suggests
induction of glandular proliferation by progesterone. Consistent with this suggestion, the results of the present
study also indicate that extended exposure to progesterone, as diestrus progresses, may promote a substantial
increase in number of glandular cells between days 12 and
16 during the diestrous period. How this could be achieved
is not clear because progesterone receptors were not de-
MORPHOMETRY OF PORCINE UTERINE ENDOMETRIUM
tectable in the superficial glandular epithelium of the
porcine endometrium after day 12 postestrus (Geisert et
al., 1994) or in the ovine endometrium on days 6 –13
postestrus (Spencer and Bazer, 1995), although it could be
mediated by underlying stromal cells, as occurs for the
response of epithelium to estrogen in mice (Cooke et al.,
1997; Kurita et al., 2000). Alternatively, proliferation of
glandular epithelium during diestrus may occur through
the concerted actions of prolactin and estradiol, as proposed to occur during pregnancy in pigs (Gray et al.,
2001a).
Several studies have shown that the growth of porcine
conceptuses depends on their position within the uterus.
For example, Hagen et al. (1980) reported that runts of the
litter occupied the middle portion of the uterine horns
during fetal development. Uterine effects on fetal size did
not result from fetal crowding because Dziuk (1968, 1985)
reported that the capacity of the domestic pig averages
about 14 live fetuses, a level of productivity infrequently
attained by the pig. Perry and Rowell (1969) found that as
the number of fetuses increased, fetuses at the anterior
end of the uterus tended to have an advantage in growth
and were 5%–10% heavier than those in the middle of the
uterus. Fetuses toward the anterior end also tended to
have an increased advantage over those at the cervical
end (Perry and Rowell, 1969). Similarly, placental weights
were greater at day 30 of gestation for fetuses toward the
oviductal end of the uterus and were less toward the
cervical end of the uterus (Wise et al., 1997). The distribution of epithelial and stromal cells within the endometrium and their corresponding secretions may be critical
to conceptus growth. Because uterine glands recently
were shown to be essential for embryonic growth and
survival in sheep (Gray et al., 2001b, c), growth of pig
conceptuses could be directly related to the proportion of
glandular epithelium within various regions of the uterus.
In the present study, the proportion of glandular epithelium was lowest and the proportion of endometrial stroma
was greatest near the uterine body. However, the suggestion that conceptus growth could be related to distribution
of glandular epithelium and stroma within the endometrium must be tempered by the fact that results of the
present study were obtained from diestrous phase endometrium of cyclic pigs. Results for endometrium of pregnancy could differ as the endometrium undergoes remodeling (Harney and Bazer, 1990).
In summary, these results indicate that the porcine
endometrium is comprised predominantly of glandular
epithelial and stromal cells. Therefore, these two cell
types are likely to contribute substantially to endometrial
secretion of prostaglandin F2␣ during luteolysis in pigs.
The contribution of glandular epithelium to luteolytic release of prostaglandin F2␣ probably increases during
diestrus as the glands proliferate and the number of these
cells increases between days 12 and 16 postestrus. Alterations in uterine morphology occurred during late diestrus
that were entirely attributable to remodeling of the uterus
because neither weight nor volume of the uterus changed
significantly during the luteolytic period. The latter results have potential implications in embryonic and fetal
growth if changes in uterine morphology occur similarly
during pregnancy.
65
ACKNOWLEDGMENTS
The authors are grateful to Drs. Ann A. Wiley and
Frank F. Bartol, Auburn University, Auburn, Alabama,
for advice on histological preparation of the uterine tissues, and to Dr. Troy L. Ott, University of Idaho, Moscow,
Idaho, for photomicrography. The authors also thank Dr.
Kwan-Hee Kim and the Histology Core Laboratory at
Washington State University for assistance with histological preparation of the tissue. The authors are indebted to
the members of M.A. Mirando’s laboratory for assistance
with surgery and to the staffs of the Washington State
University Swine Center and the Experimental Animal
Laboratory Building for assistance in care and handling of
animals.
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