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Summary. Samples from corpus haemorrhagicum, mid-cycle corpus luteum (CL) and late-cycle CL were tested for their abilities to stimulate neovascularization of chorioallantoic membranes (CAM) of developing chicks. Responses were graded from 0 to 4 (4 being the greatest response). Luteal tissue implants from each stage of the oestrous cycle stimulated growth of CAM blood vessels, and vascular responses increased with age of CL. Implants from late-cycle CL were typically graded 3 or 4. Luteal tissues from several stages of development were also incubated for 6 h in serum-free medium containing no hormone, LH, PGF-2α or both hormones. Media conditioned by luteal tissues were assayed for progesterone and tested for their ability to stimulate mitogenesis and migration of bovine aortic endothelial cells in vitro. All media conditioned by luteal tissues stimulated mitogenesis and migration of endothelial cells, but media from late-cycle CL exhibited the greatest activity. Luteinizing hormone significantly increased in-vitro secretion of a factor(s) that stimulated migration of endothelial cells. PGF-2α alone had no effect on production of endothelial cell mitogen or migrationstimulating factor(s) from luteal incubations; however, the ability of LH to enhance secretion of the migration-stimulating factor(s) was blocked by PGF-2α. This study demonstrates that angiogenic activity of bovine luteal tissues increases with age of the CL and in-vitro secretion of angiogenic factor is responsive to hormones known to regulate luteal function.

Keywords: angiogenesis; corpus luteum; cow; oestrous cycle

Functions of corpus luteum (CL) are influenced by numerous factors including hormones, growth and angiogenic factors, nutritional plane and dietary supplements such as arginine (Arg), a semi-essential amino acid and precursor for proteins, polyamines and nitric oxide (NO). The aim of this study was to determine if Arg supplementation to ewes fed different planes of nutrition influences: (1) progesterone (P4) concentrations in serum and luteal tissue, (2) luteal vascularity, cell proliferation, endothelial NO synthase (eNOS) and receptor (R) soluble guanylate cyclase β protein and mRNA expression and (3) luteal mRNA expression for selected angiogenic factors during the estrous cycle. Ewes (n = 111) were categorized by weight and randomly assigned to one of three nutritional planes: maintenance control (C), overfed (2× C) and underfed (0.6× C) beginning 60 days prior to onset of estrus. After estrus synchronization, ewes from each nutritional plane were assigned randomly to one of two treatments: Arg or saline. Serum and CL were collected at the early, mid and late luteal phases. The results demonstrated that: (1) nutritional plane affected ovulation rates, luteal vascularity, cell proliferation and NOS3, GUCY1B3, vascular endothelial growth factor (VEGF) and VEGFR2 mRNA expression, (2) Arg affected luteal vascularity, cell proliferation and NOS3, GUCY1B3, VEGF and VEGFR2 mRNA expression and (3) luteal vascularity, cell proliferation and the VEGF and NO systems depend on the stage of the estrous cycle. These data indicate that plane of nutrition and/or Arg supplementation can alter vascularization and expression of selected angiogenic factors in luteal tissue during the estrous cycle in sheep.

Scrapie in sheep is spread laterally by placental transmission of an infectious misfolded form (PrPSc) of a normal prion protein (PrPC) used as a template in PrPSc formation. We hypothesized that PrPC would be expressed in uterine and placental tissues and estradiol-17β (E₂) would affect uterine PrPC expression. PrPC expression was evaluated in the uterus of long-term ovariectomized (OVX) ewes treated with an E₂ implant for 2–24 h and in uteroplacental tissues from day 20 to day 30 of pregnancy. Expression of PrPC mRNA and PrPC protein increased in the uterus after E₂ treatment of OVX ewes. In the maternal placenta, expression of PrPC mRNA and PrPC protein were unchanged, but in the fetal membranes (FM) PrPC mRNA and PrPC protein expression increased from day 20 to day 28. In the nonpregnant uterus, PrPC protein was immunolocalized at apical borders of the surface epithelium, in outer smooth muscle layers of large blood vessels, and in scattered stromal cells of the deep intercaruncular areas of the uterus. In the maternal placenta, PrPC protein was immunolocalized in the cytoplasm of flattened luminal epithelial cells apposed to the FM, whereas in the FM PrPC protein was in trophoblast cells and was also in several tissues of the developing embryo during early pregnancy. These data linking estrogen stimulation to increases in PrPC expression in uteroplacental tissues suggest that PrPC has a specific function during the estrous cycle and early pregnancy. Future studies should determine whether or not estrogen influences PrPC expression in other tissues, such as the nervous system and brain.

Placental vascular development (angiogenesis) is critical for placental function and thus for normal embryonic/fetal growth and development. Specific environmental factors or use of assisted reproductive techniques may result in poor placental angiogenesis, which may contribute to embryonic losses and/or fetal growth retardation. Uterine tissues were collected on days 14, 16, 18, 20, 22, 24, 26, 28, and 30 after mating and on day 10 after estrus (nonpregnant controls) to determine vascular development and expression of several factors involved in the regulation of angiogenesis in the endometrium. Compared with controls, several measurements of endometrial vascularity increased (P<0.001) including vascular labeling index (LI; proportion of proliferating cells), the tissue area occupied by capillaries, area per capillary (capillary size), total capillary circumference per unit of tissue area, and expression of factor VIII (marker of endothelial cells), but capillary number decreased (P<0.001). Compared with controls, mRNA for placental growth factor, vascular endothelial growth factor receptors, angiopoietins (ANGPT) 1 and 2, ANGPT receptor TEK, endothelial nitric oxide synthase, and hypoxia-inducible factor 1α increased (P<0.05) during early pregnancy. Vascular LI was positively correlated (P<0.05) with several measurements of vascularity and with mRNA expression of angiogenic factors. These data indicate that endometrial angiogenesis, manifested by increased vascularity and increased expression of several factors involved in the regulation of angiogenesis, is initiated very early in pregnancy. This more complete description of early placental angiogenesis may provide the foundation for determining whether placental vascular development is altered in compromised pregnancies.

This study was conducted to evaluate the expression of endothelial nitric oxide synthase (eNOS) in ovarian follicles and corpora lutea (CL) throughout the estrous cycle in sheep. Three experiments were conducted to (1) immunolocalize eNOS protein, (2) determine expression of mRNA for eNOS and its receptor guanylate cyclase 1 soluble β3 (GUCY1B3), and (3) co-localize eNOS and vascular endothelial growth factor (VEGF) proteins in the follicles and/or CL throughout the estrous cycle. In experiment 1, ovaries were collected from ewes treated with FSH, to induce follicular growth or atresia. In experiment 2, ovaries were collected from ewes treated with FSH and hCG to induce follicular growth and ovulation. In experiment 3, ovaries were collected from superovulated ewes to generate multiple CL on days 2, 4, 10, and 15 of the estrous cycle. In experiments 1 and 2, the expression of eNOS protein was detected in the blood vessels of the theca externa and interna of healthy ovarian follicles. However, in early and advanced atretic follicles, eNOS protein expression was absent or reduced. During the immediate postovulatory period, eNOS protein expression was detected in thecal-derived cells that appeared to be invading the granulosa layer. Expression of eNOS mRNA tended to increase in granulosa cells at 12 and 24 h, and in theca cells 48 h after hCG injection. In experiment 3, eNOS protein was located in the blood vessels of the CL during the estrous cycle. Dual localization of eNOS and VEGF proteins in the CL demonstrated that both were found in the blood vessels.

Utero-placental growth and vascular development are critical for pregnancy establishment that may be altered by various factors including assisted reproductive technologies (ART), nutrition, or others, leading to compromised pregnancy. We hypothesized that placental vascularization and expression of angiogenic factors are altered early in pregnancies after transfer of embryos created using selected ART methods. Pregnancies were achieved through natural mating (NAT), or transfer of embryos from NAT (NAT-ET), or IVF or in vitro activation (IVA). Placental tissues were collected on day 22 of pregnancy. In maternal caruncles (CAR), vascular cell proliferation was less (P<0.05) for IVA than other groups. Compared with NAT, density of blood vessels was less (P<0.05) for IVF and IVA in fetal membranes (FM) and for NAT-ET, IVF, and IVA in CAR. In FM, mRNA expression was decreased (P<0.01–0.08) in NAT-ET, IVF, and IVA compared with NAT for vascular endothelial growth factor (VEGF) and its receptor FLT1, placental growth factor (PGF), neuropilin 1 (NP1) and NP2, angiopoietin 1 (ANGPT1) and ANGPT2, endothelial nitric oxide synthase 3 (NOS3), hypoxia-inducible factor 1A (HIF1A), fibroblast growth factor 2 (FGF2), and its receptor FGFR2. In CAR, mRNA expression was decreased (P<0.01–0.05) in NAT-ET, IVF, and IVA compared with NAT for VEGF, FLT1, PGF, ANGPT1, and TEK. Decreased mRNA expression for 12 of 14 angiogenic factors across FM and CAR in NAT-ET, IVF, and IVA pregnancies was associated with reduced placental vascular development, which would lead to poor placental function and compromised fetal and placental growth and development.

Sheep were fed a maintenance (M) diet with adequate (A) Se or high (H) Se concentration from 21 days before breeding to day 135 of pregnancy. From day 50 to day 135 of pregnancy (tissue collection day), a portion of the ewes from ASe and HSe groups were fed restricted (R; 60% of M) diet. Fetal ovarian sections were stained for: 1) the presence of proliferating cell nuclear antigen (a marker of proliferating cells) to determine the proportion of proliferating primordial follicles, or the labeling index (LI; percentage of proliferating cells) for primordial, primary, secondary and antral follicles, stromal tissues, and blood vessels; 2) factor VIII (a marker of endothelial cells) or 3) a presence of apoptotic cells/bodies. The number of proliferating primordial follicles and the LI of primordial follicles was decreased by R and/or HSe diets. The LI was similar for theca and granulosa cells, and for secondary or antral follicles, but was greater in secondary and antral than in primordial and primary follicles. R diet and/or Se affected the LI in all follicle types, in stromal tissues and blood vessels. A dense network of blood vessels was detected in the areas containing secondary to antral follicles, medulla, and hilus, but areas containing primordial follicles were poorly vascularized. The number of apoptotic cells was minimal. These results demonstrate that nutrient restriction and/or Se level in the maternal diet affected cellular proliferation in follicles, blood vessels, and stromal tissues in fetal ovaries. Thus, plane of nutrition and Se in the maternal diet may impact fetal ovarian development and function.

Corpora lutea and blood samples were collected from superovulated ewes 0, 4, 8, 12 and 24 h after prostaglandin F_2α (PGF) analog injection on day 10 of the estrous cycle. Changes in vascular cell and fibroblast composition, apoptosis and mRNA expression for several angiogenic factors in the corpus luteum (CL) were determined. While peripheral progesterone concentration decreased at 24 h after PGF injection, CL weight did not change. The area of positive BS-1 lectin staining (endothelial cell marker), smooth muscle cell actin (SMCA; pericyte and SMC marker), collagen type 1 (fibroblast marker), and the rate of cell death changed in luteal tissues after PGF treatment. In association with these cellular changes, mRNA for several angiogenic factors including vascular endothelial growth factor (VEGF) and receptors (Flt and KDR), basic fibroblast growth factor (FGF2) and receptor, angiopoietin (ANGPT) 1 and receptor Tie-2, endothelial nitric oxide synthase (NOS3), and angiotensin II receptor 1 (AT1) were altered. Changes in endothelial cell marker expression were positively correlated with changes in VEGF and NO systems. In addition, changes in mRNA expression for VEGF, Flt and KDR were positively correlated with changes in ANGPT2, Tie-2, and NOS3, indicating a functional relationship. This data demonstrates that after an initial increase, the endothelial component of the vascular bed decreases during PGF-induced luteal regression. However, SMCA expression remained high during luteal regression, potentially indicating a role of pericytes and vascular SMC in luteolysis, likely to regulate tissue remodeling and to maintain the integrity of larger blood vessels. Further, it appears that early regression may increase collagen type 1 production and/or expression by fibroblasts. Expression of angiogenic factors is influenced by PGF-induced luteolysis and may serve to maintain vascular structure in order to aid luteal regression.

To evaluate the role of gap junctions in the regulation of progesterone secretion, two experiments were conducted. In Experiment 1, luteal cells obtained on days 5, 10, and 15 were cultured overnight at densities of 50×10³, 100×10³, 300×10³, and 600×10³ cells/dish in medium containing: (1) no treatment (control), (2) LH, or (3) dbcAMP. In Experiment 2, luteal cells from days 5 and 10 of the estrous cycle were transfected with siRNA, which targeted the connexin (Cx) 43 gene. In Experiment 1, progesterone secretion, Cx43 mRNA expression, and the rates of gap junctional intercellular communication (GJIC), were affected by the day of the estrous cycle, cell density, and treatments (LH or dbcAMP). The changes in progesterone secretion were positively correlated with the changes in Cx43 mRNA expression and the rates of GJIC. Cx43 was detected on the luteal cell borders in every culture, and luteal cells expressed 3β-hydroxysteroid dehydrogenase. In Experiment 2, two Cx43 gene-targeted sequences decreased Cx43 mRNA expression and progesterone production by luteal cells. The changes in Cx43 mRNA expression were positively correlated with changes in progesterone concentration in media. Thus, our data demonstrate a relationship between gap junctions and progesterone secretion that was supported by (1) the positive correlations between progesterone secretion and Cx43 mRNA expression and GJIC of luteal cells and (2) the inhibition of Cx43 mRNA expression by siRNA that resulted in decreased production of progesterone by luteal cells. This suggests that gap junctions may be involved in the regulation of steroidogenesis in the ovine corpus luteum.

To characterize early fetal placental development, gravid uterine tissues were collected from pregnant ewes every other day from day 16 to 30 after mating. Determination of 1) cell proliferation was based on Ki67 protein immunodetection; 2) global methylation was based on 5-methyl-cytosine (5mC) expression and mRNA expression for DNA methyltransferases (DNMTs) 1, 3a, and 3b; and 3) vascular development was based on smooth muscle cell actin immunolocalization and on mRNA expression of several factors involved in the regulation of angiogenesis in fetal membranes (FMs). Throughout early pregnancy, the labeling index (proportion of proliferating cells) was very high (21%) and did not change. Expression of 5mC and mRNA for DNMT3b decreased, but mRNA for DNMT1 and 3a increased. Blood vessels were detected in FM on days 18–30 of pregnancy, and their number per tissue area did not change. The patterns of mRNA expression for placental growth factor, vascular endothelial growth factor, and their receptors FLT1 and KDR; angiopoietins 1 and 2 and their receptor TEK; endothelial nitric oxide synthase and the NO receptor GUCY13B; and hypoxia inducing factor 1 α changed in FM during early pregnancy. These data demonstrate high cellular proliferation rates, and changes in global methylation and mRNA expression of factors involved in the regulation of DNA methylation and angiogenesis in FM during early pregnancy. This description of cellular and molecular changes in FM during early pregnancy will provide the foundation for determining the basis of altered placental development in pregnancies compromised by environmental, genetic, or other factors.