Endothelial mitosis during the initial stages of corpus luteum neovascularization in the cycling adult rat

Nicotine does not affect vascularization but inhibits growth of freely transplanted ovarian follicles by inducing granulosa cell apoptosis

BACKGROUND

There is clear support for an association between smoking and decreased female fecundity and fertility. Cigarette smoke appears to have adverse effects along a continuum of reproductive processes. We therefore studied the effect of nicotine on follicular growth and vascularization of freely transplanted ovarian follicles.

METHODS

We used the skinfold chamber model in Syrian golden hamsters, which allows the in vivo microscopy of follicular grafts. Animals were treated daily with nicotine at doses mimicking low-rate and high-rate smokers (0.2 and 1.0 mg/kg body weight subcutaneously). Saline-treated animals served as controls. To further evaluate the effect of nicotine on angiogenesis, an in vitro aortic ring assay was used.

RESULTS

The re-vascularisation rate of follicles was similar in nicotine-treated animals and controls. During the 7 days after transplantation, nicotine further caused a dose-dependent inhibition of follicular growth. In contrast, the vascularized area and microvessel density were not affected by the nicotine exposure. In vitro aortic ring assays confirmed that nicotine does not influence sprouting and microvessel formation. However, immunohistochemistry for cleaved caspase-3 revealed a large extent of granulosa cell apoptosis within transplanted follicles of high-dose nicotine-treated animals.

CONCLUSIONS

Nicotine as one toxic component of cigarette smoke does not affect vascularization, but adversely influences follicular growth by an increase in apoptotic cell death. As follicular growth is a crucial step in normal ovulation and fertilization, nicotine-induced cell apoptosis may represent one of the mechanisms underlying the well-established link between smoking and fertility disorders.

Changes of messenger RNAs encoding vascular endothelial growth factor and its receptors during the development and maintenance of caprine corpora lutea

The present study was conducted to examine changes of mRNAs encoding vascular endothelial growth factor (VEGF) and its receptors (KDR/Flk-1 and Flt-1), and CD34, which is known to be a specific marker for endothelial cells, during the development and maintenance of the caprine corpora lutea (CL). Effects of a potent GnRH antagonist (GA), which was previously shown to suppress release of luteinizing hormone (LH), on expressions of those mRNAs during the CL development were also investigated. Goats were divided into control (n = 12) and GA-treated groups (n = 6). The goats were treated with saline or GA (50 microg/kg, sc) on days 0 (day of ovulation), 4, and 8 (control only), and CL collected on a subset of goats (n = 3 for each day) on days 0 (no saline), 4, 8, or 14 (control only). Ribonuclease protection assay was performed to quantitate the mRNAs in the CL using specific cRNA probes generated by RT-PCR and in vitro transcription. Level of CD34 mRNA significantly increased from day 0 to 8 (CL development) in the control group (P < 0.05). Long and short forms were detected in the caprine CL by RT-PCR for VEGF mRNA and analyses of their sequences showed that they correspond to mRNAs encoding VEGF(165) and VEGF(121), respectively. Level of VEGF(165) mRNA significantly increased from day 4 to 8 and day 8 to 14 (CL maintenance) in the control group (P < 0.05) while VEGF(121) mRNA did not change during the whole period. Level of KDR/Flk-1 mRNA significantly increased from day 0 to 8 (P < 0.05) while Flt-1 mRNA significantly increased from day 8 to 14 (P < 0.005) in the control group. In the GA-treated group, levels of all of the mRNAs did not alter remarkably as compared with those in the control group. These results suggest that rise of KDR/Flk-1 and VEGF(165) mRNAs during the caprine CL development may be associated with enhanced angiogenesis and that increment of VEGF(165) and Flt-1 mRNAs during the CL maintenance may play nonangiogenic roles. The present study also indicates that the changes of VEGF(165) and KDR/Flk-1 mRNAs during the CL development are probably not regulated by LH.

Cellular proliferation and fibroblast growth factors in the corpus luteum during early pregnancy in ewes

To determine the relationship between cellular proliferation and the presence of FGF-1 and FGF-2 in the ovine corpus luteum (CL) during early pregnancy, ewes received an intravenous injection of bromodeoxyuridine (BrdU) 1 h before slaughter (n = 3/day) on day 12 after estrus (nonpregnant) or on days 12, 18, 24 or 30 after mating (pregnant). The labeling index (LI; number of BrdU-labeled nuclei expressed as a percentage of total nuclei) of each CL was determined by immunohistochemistry and subsequent image analysis. FGF-1 and FGF-2 were immunolocalized by using specific antibodies, and indirect immunoperoxidase detection. Moreover, FGF-2 was immunolocalized by using a primary antibody and fluorescein isothiocyanate (FITC)-labeled secondary antibody, and immunofluorescence was quantified by using an interactive laser cytometer and image analysis. Results demonstrated that the LI was similar for CL of nonpregnant and pregnant ewes on day 12 (4.27 +/- 0.23 vs 5.10 +/- 0.14%) and decreased (P < 0.05) from days 12-30 of pregnancy (2.73 +/- 0.08, 2.02 +/- 0.09 and 1.70 +/- 0.04% on days 18, 24 and 30, respectively). FGF-1 was present in the cytoplasm of large and a few small parenchymal luteal cells, and the distribution and intensity of staining was similar for nonpregnant and pregnant ewes on day 12 as well as across days of pregnancy. In contrast, FGF-2 immunoreactivity was present only in luteal nonparenchymal cells and interstitial areas and was greater (P < 0.05) for pregnant than nonpregnant CL on day 12 (2.34 +/- 0.12 vs 0.14 +/- 0.01%). Although FGF-2 immunoreactivity decreased (P < 0.01) from days 12-30 of pregnancy (0.70 +/- 0.04, 0.22 +/- 0.01 and 0.06 +/- 0.02% on days 18, 24, and 30, respectively), it was highly correlated (r = 0.99, P < 0.01) with luteal LI. We therefore suggest that FGF, and especially FGF-2, play a role in luteal cell proliferation or turnover during early pregnancy, and may thereby contribute to the maintenance of luteal function, which is critical for the successful establishment of pregnancy.

Neovascularization of the corpus luteum of rats during the estrus cycle

In order to elucidate the chronological morphological changes of the corpus luteum (CL) of rats, as a physiological angiogenesis model, the CL of rat ovaries was studied light microscopically using periodic acid methenamine silver staining (PAM) and immunostaining for type IV collagen, laminin, thrombomodulin (TM), factor VIII related antigen (factor VIII) and alpha-smooth muscle actin (alpha-SMA). The CL was also studied electron microscopically. Female Wistar-Imamichi rats were used, which have a regular 4-day estrous cycle. The histological changes of the CL were observed in 6-hour intervals from 4 h before the ovulation to 28 h post-ovulation during the estrous cycle. Once the basement membrane (BM) of the follicle disintegrated following ovulation, developing capillaries entered into the CL and formed a vascular lumen with a surrounding BM, which showed positive for PAM staining, type IV collagen and laminin. The developing capillaries in the CL showed a weakly positive reaction for TM and factor VIII, but were negative for alpha-SMA. However, the appearance of immature pericytes around the well-developed capillary was obvious with electron microscopy. The study reported here provides detailed descriptions of angiogenesis during luteinization. It is concluded that the angiogenesis of the CL begins at the time of destruction of the BM of the ovarian follicle, and that the capillary BM appears when the capillary forms its lumen. Moreover, it was demonstrated that the capillary does not develop into an arteriole during luteinization.

Mitogenic factors of corpora lutea

The mammalian corpus luteum (CL), which plays a central role in the reproductive process because of its production of hormones such as progesterone, appears to be an exceptionally dynamic organ. Its rate of growth and development are extremely rapid and, even when the CL is functionally mature, its rate of cell turnover remains relatively high. Associated with this high rate of cell turnover, the mature CL receives the greatest blood supply per unit tissue of any organ, and also exhibits a relatively high metabolic rate. Although numerous growth factors have been identified in luteal tissue, their role in growth and differentiation of this dynamic organ remains unclear. Recently, while attempting to identify mitogenic factors of ovine and bovine CL, we have found that they produce several mitogens during the estrous cycle as well as pregnancy. The majority of these luteal-derived mitogenic factors are heparin-binding, and although some may represent previously identified factors, several appear to be novel heparin-binding growth factors. Isolation and purification of mitogenic factors produced by the CL will enable us to determine their roles in luteal growth, development and differentiated function, which will contribute to our understanding not only of the regulation of fertility but also of tissue growth and development in general.

Transient and cell-specific expression of tissue-type plasminogen activator and plasminogen-activator-inhibitor type 1 results in controlled and directed proteolysis during gonadotropin-induced ovulation

Proteolytic activity generated by the plasminogen-activator system (PA system) is associated with many biological processes. However, it is not known how the proteolytic activity is regulated in vivo in order to obtain directed proteolysis while, at the same time, protecting unrestrained tissue destruction. Using gonadotropin-induced ovulation as a model, we have studied how two components of the PA system, tissue-type plasminogen activator (tPA) and plasminogen-activator-inhibitor type 1 (PAI-1), are regulated temporally and spatially by gonadotropins, leading to the initiation and termination of a well-directed proteolytic process. In-situ hybridization and in-situ zymography were used to analyze the expression of tPA and PAI-1 mRNA and PA-activity in specific ovarian cell types. Both tPA and PAI-1 were found to be regulated and to have a distinct expression pattern in different ovarian compartments. tPA was expressed in both granulosa and thecal-interstitial cells; the highest levels of tPA mRNA were found in the granulosa cells of preovulatory follicles, just prior to ovulation. Consistent with a role for luteinizing hormone/chorionic gonadotropin (LH/CG) in triggering ovulation, the cells and follicles that actively expressed tPA also contained high levels of LH-receptor mRNA while cumulus cells that contain undetectable amounts of tPA mRNA were devoid of LH-receptor expression. The highest levels of PAI-1 mRNA were found about 6 h before ovulation and mainly in the thecal-interstitial cells and ovarian stroma tissue which encapsulate the follicle. Preovulatory follicles, protruding onto the surface of the ovary with less surrounding stroma tissue, expressed less PAI-1 compared to small non-ovulatory follicles embedded in inner part of the ovary. In-situ zymography also revealed that the PA activity was colocalized to the surface of the ovary just prior to ovulation. Our studies suggest that a proteolytic activity provided by tPA and modulated by PAI-1 is responsible for a controlled and directed proteolysis leading to rupture of selected follicles during ovulation.

Measurement of blood flow in the chick egg yolk sac membrane

Blood vessels of the yolk sac membrane are the transport route by which a chick embryo obtains nutrients from the egg yolk. After 4 days of incubation at 37 degrees C a sparse network of blood vessels are visible within the membrane and they become denser over the following days. The preparation has been used to assess angiogenesis. Using visual inspection to assess the growth of new vessels towards pellets of test substances placed on the membrane surface, responses are judged negative or positive. Laser Doppler flowmetry has been used to investigate the potential of this technique to quantify blood flow through the yolk sac membrane. Measurements have been taken from capillary beds and single larger vessels over a range of temperatures and from avascular yolk and egg white using a Perimed PF3 with PF310 bent-tipped probe. Capillary beds exhibited vasomotion (about one cycle per minute) and oscillations of longer duration (about five cycles per hour). Cardiac pulses, strongly present in larger vessels (mean 90 pu, pulse 75 pu), were not easily identified in the capillary bed. Avascular yolk measurements gave flux values (due to Brownian motion of yolk proteins) which were greater than for capillary beds but without oscillations. Angiogenesis was stimulated by pellets containing 2 micrograms of basic fibroblast growth factor. There were significant increases in single vessel blood flow after 24 h compared with blank control pellets placed on the same egg.

Angiogenesis in the developing corpus luteum of pregnant rats: a stereologic and autoradiographic study

Within the adult mammalian ovary, angiogenesis is associated with development of the corpus luteum (CL). In this study, developing luteal tissue was examined to determine whether its vascularization involves endothelial cell replication and to what extent this proliferation contributes to forming new capillaries. Five rats each at 12, 24, 36, 48, 60, 72, 84, and 96 hr of gestation were given a subcutaneous injection of tritiated thymidine (specific activity 5 Ci/mM; 1 uCi/g body weight). One hour later they were anesthetised with sodium pentobarbitone, and the left ovary was processed for light microscopy. Sections were cut through each ovary until three newly formed CL were recognized; a 1-micron section was taken from the maximum diameter of each CL and processed for autoradiographic demonstration of thymidine labeling in endothelial cell nuclei. The same sections were also examined with stereological techniques to quantitate growth of the vascular compartment. The results show that 36.1 +/- 5.7% of endothelial cells of invading capillary sprouts divide within 12 hr of ovulation; at 24 hr, 29.0 +/- 2.8% are dividing. Within 12 hr after ovulation, blood vessels occupied 5.9 +/- 1.4% of the peripheral space of the ruptured follicle but only 1.6 +/- 0.5% in the center. However, by 36 hr these respective values were 9.3 +/- 1.6% and 8.4 +/- 1.9%. A further peak in endothelial-cell replication (31.2 +/- 5.4%), early on the 3rd day of gestation, corresponded to the very extensive anastomoses within the capillary bed established between this time (13.6% vascularity) and late on the 4th day (about 23%).(ABSTRACT TRUNCATED AT 250 WORDS)