Empty Horn Research Articles

P-glycoprotein expression and localization in the rat uterus throughout gestation and labor.

Uterine tissues contain the efflux transporter P-glycoprotein (P-gp, encoded by Abcb1a/1b gene), but little is known about how it changes through gestation. Our aim was to investigate the expression profile and cellular localization of P-gp in the pregnant, laboring and post-partum (PP) rat uterus. We propose that during pregnancy the mechanical and hormonal stimuli play a role in regulating myometrial Abcb1a/1b/P-gp. Samples from bilaterally and unilaterally pregnant rats were collected throughout gestation, during labor, and PP (n=4-6/gestational day). RNA and protein were isolated and subjected to quantitative PCR and immunoblotting; P-gp transcript and protein were localized by in situ hybridization and immunohistochemistry. Expression of Abcb1a/1b gene and membrane P-gp protein in uterine tissue (1) increased throughout gestation, peaked at term (GD19-21) and dropped during labor (GD23L); and (2) was upregulated only in gravid but not in empty horn of unilaterally pregnant rats. (3) The drop of Abcb1a/1b mRNA on GD23 was prevented by artificial maintenance of elevated progesterone (P4) levels in late gestation; (4) injection of the P4 receptor antagonist RU486 on GD19 caused a significant decrease in Abcb1 mRNA levels. (5) In situ hybridization and immunohistochemistry indicated that Abcb1/P-gp is absent from myometrium throughout gestation; (6) was expressed exclusively by uterine microvascular endothelium (at early gestation) and luminal epithelium (at mid and late gestation), but was undetectable during labor. In conclusion, ABC transporter protein P-gp in pregnant uterus is hormonally and mechanically regulated. However, its substrate(s) and precise function in these tissues during pregnancy remains to be determined.

Short-Length and High-Aperture-Efficiency Horn Antenna Using Low-Loss Bulk Anisotropic Metamaterial

An empty horn antenna is filled with a bulk anisotropic metamaterial to enhance its gain and aperture efficiency. Radiation patterns in both the E-plane and the H-plane are improved owing to the anisotropic permittivity and the anisotropic permeability. Aperture efficiency over 70% is achieved despite of its short longitudinal length $\sim {\lambda _0}$ . Insertion loss below 0.18 dB and energy reflection below $ - 20~\hbox{dB}$ are obtained, which are quite small compared to most metamaterial-based antennas. The metamaterial-filled horn is fabricated and tested, and the measured results agree very well with the simulated results.

Distension of the uterus induces HspB1 expression in rat uterine smooth muscle

The uterine musculature, or myometrium, demonstrates tremendous plasticity during pregnancy under the influences of the endocrine environment and mechanical stresses. Expression of the small stress protein heat shock protein B1 (HspB1) has been reported to increase dramatically during late pregnancy, a period marked by myometrial hypertrophy caused by fetal growth-induced uterine distension. Thus, using unilaterally pregnant rat models and ovariectomized nonpregnant rats with uteri containing laminaria tents to induce uterine distension, we examined the effect of uterine distension on myometrial HspB1 expression. In unilaterally pregnant rats, HspB1 mRNA and Ser(15)-phosphorylated HspB1 (pSer(15) HspB1) protein expression were significantly elevated in distended gravid uterine horns at days 19 and 23 (labor) of gestation compared with nongravid horns. Similarly, pSer(15) HspB1 protein in situ was only readily detectable in the distended horns compared with the nongravid horns at days 19 and 23; however, pSer(15) HspB1 was primarily detectable in situ at day 19 in membrane-associated regions, while it had primarily a cytoplasmic localization in myometrial cells at day 23. HspB1 mRNA and pSer(15) HspB1 protein expression were also markedly increased in ovariectomized nonpregnant rat myometrium distended for 24 h with laminaria tents compared with empty horns. Therefore, uterine distension plays a major role in the stimulation of myometrial HspB1 expression, and increased expression of this small stress protein could be a mechanoadaptive response to the increasing uterine distension that occurs during pregnancy.

Stretch-Induced Uterine Myocyte Differentiation During Rat Pregnancy: Involvement of Caspase Activation1

Proliferation, differentiation, and apoptosis are three major processes by which the pregnant uterus maintains homeostasis to accommodate the growing fetus. We demonstrated previously that caspase activation in the pregnant rat myometrium at midgestation coincides with the transition from uterine hyperplasia to hypertrophy. We hypothesized that this transition was induced by stasis of myometrial blood flow (and subsequent hypoxia/ischaemia insult) resulting from acute myometrial stretch induced by a growing embryo. Therefore, we measured the expression of active caspase 3 and two hypoxia markers (transcription factor HIF1A and pimonidazole hydrochloride) in pregnant rat myometrium. To investigate the effect of gravidity we used unilaterally pregnant rats. Caspase 3 was activated only in the gravid horn of the unilaterally pregnant animals on Gestational Days 12-15. This activation was associated with high levels of HIF1A and pimonidazole immunostaining, which were limited to the circular myometrial layer of the gravid horn, indicative of hypoxia within this tissue. To isolate the effect of myometrial stretch applied by the growing fetus, we inserted an expandable polymer tube (intra-uterine expandable tube [IUET]) into the empty horn of Day 13 and Day 20 unilaterally pregnant rats. Tissue was collected 2, 14, and 24 h later. In the IUET-stretched empty horn, cleaved caspase 3 was activated at midgestation (Day 14), but not at late gestation (Day 21). We speculate that hypoxia resulting from mechanical stretch may activate caspase 3 within the pregnant myometrium only in the context of a specific endocrine environment.

Mechanical stretch regulates hypertrophic phenotype of the myometrium during pregnancy

The adaptive growth of the uterus is a critical event that involves changes in cellular phenotypes throughout pregnancy. In early pregnancy, uterine growth is due to hyperplasia of uterine smooth muscle cells (SMCs) within the myometrium; however, the major component of myometrial growth occurs after mid-gestation. This study sought to test the hypothesis that increase in myometrial growth seen during late pregnancy is due to SMC hypertrophy caused by mechanical stretch of uterine tissue by a growing fetus(es) by providing direct measurements of individual SMC size. We employed a stereological approach to calculate the average cell volumes of uterine myocytes through diameter measurements using the Stereoinvestigator statistical software. Uterine tissues were collected from nonpregnant Wistar rats, as well as from gravid and nongravid horns of unilaterally pregnant animals on gestational days (d) 8 (early gestation), 14 (mid-gestation), 19 (late gestation), 22 (term), and 4 days post partum. Anti-caveolin-1 immunostaining was used to clearly delineate SMC boundaries. The stereological analysis revealed that the dramatic increase in myometrial growth seen during late gestation (d19-22) is due to a threefold increase in the size of uterine myocytes. A significant increase in SMC volumes was detected in the gravid uterine horn as compared with the corresponding empty horn of unilateral term pregnant animals (day 22, mean cell volume 1114 vs 361 microm(3), P<0.05), indicating the effect of uterine occupancy. The restriction of the hypertrophy to cells within the gravid horn suggests that it may be a response to the biological mechanical stretch of uterine walls by the growing fetus(es) and placenta(s).

Uterine Stretch Regulates Temporal and Spatial Expression of Fibronectin Protein and Its Alpha 5 Integrin Receptor in Myometrium of Unilaterally Pregnant Rats1

The adaptive growth of the uterus during pregnancy is a critical event that involves increased synthesis of extracellular matrix (ECM) proteins and dynamic remodeling of smooth muscle cell (SMC)-ECM interactions. We have previously found a dramatic increase in the expression of the mRNAs that encode fibronectin (FN) and its alpha5-integrin receptor (ITGA5) in pregnant rat myometrium near to term. Since the myometrium at term is exposed to considerable mechanical stretching of the uterine wall by the growing fetus(es), the objective of the present study was to examine its role in the regulation of FN and ITGA5 expression at late gestation and during labor. Using myometrial tissues from unilaterally pregnant rats, we investigated the temporal changes in Itga5 gene expression in gravid and empty uterine horns by Northern blotting and real-time PCR, in combination with immunoblotting and immunofluorescence analyses of the temporal/spatial distributions of the FN and ITGA5 proteins. In addition, we studied the effects of early progesterone (P4) withdrawal on Itga5 mRNA levels and ITGA5 protein detection. At all time-points examined, the Itga5 mRNA levels were increased in the gravid uterine horn, compared to the empty horn (P < 0.05). Immunoblot analysis confirmed higher ITGA5 and FN protein levels in the myometrium, associated with gravidity (P < 0.05). Immunodetection of ITGA5 was consistently high in the longitudinal muscle layer, increased with gestational age in the circular muscle layer of the gravid horn, and remained low in the empty horn. ITGA5 and FN immunostaining in the gravid horn exhibited a continuous layer of variable thickness associated directly with the surfaces of individual SMCs. In contrast to the effects of stretch, P4 does not appear to regulate ITGA5 expression. We speculate that the reinforcement of the FN-ITGA5 interaction: 1) contributes to myometrial hypertrophy and remodeling during late pregnancy; and 2) facilitates force transduction during the contractions of labor by anchoring hypertrophied SMCs to the uterine ECM.

The expression of transforming growth factor β in pregnant rat myometrium is hormone and stretch dependent

From a quiescent state in early pregnancy to a highly contractile state in labor, the myometrium displays tremendous growth and remodeling. We hypothesize that the transforming growth factor beta (TGFbeta) system is involved in the differentiation of pregnant myometrium throughout gestation and labor. Furthermore, we propose that during pregnancy the mechanical and hormonal stimuli play a role in regulating myometrial TGFbetas. The expression of TGFbeta1-3 mRNAs and proteins was examined by real-time PCR, Western immunoblot, and localized with immunohistochemistry in the rat uterus throughout pregnancy and labor. Tgfbeta1-3 genes were expressed differentially in pregnant myometrium. Tgfbeta2 gene was not affected by pregnancy, whereas the Tgfbeta1 gene showed a threefold increase during the second half of gestation. In contrast, we observed a dramatic bimodal change in Tgfbeta3 gene expression throughout pregnancy. Tgfbeta3 mRNA levels first transiently increased at mid-gestation (11-fold on day 14) and later at term (45-fold at labor, day 23). Protein expression levels paralleled the changes in mRNA. Treatment of pregnant rats with the progesterone (P4) receptor antagonist RU486 induced premature labor on day 19 and increased Tgfbeta3 mRNA, whereas artificial maintenance of elevated P4 levels at late gestation (days 20-23) caused a significant decrease in the expression of Tgfbeta3 gene. In addition, Tgfbeta3 was up-regulated specifically in the gravid horn of unilaterally pregnant rats subjected to a passive biological stretch imposed by the growing fetuses, but not in the empty horn. Collectively, these data indicate that the TGFbeta family contributes in the regulation of myometrial activation at term integrating mechanical and endocrine signals for successful labor contraction.

A quantitative study of rat uterine sympathetic innervation during pregnancy and post partum

In mammals, pregnancy induces a transient and extensive degeneration of uterine sympathetic innervation. We used the models of unilateral oviduct ligation and in oculo myometrium transplant in pregnant rats to address the role of stretching forces and/or hormone milieu in the loss of sympathetic innervation. The sympathetic fibres of the uterine horn and in oculo myometrial transplants were quantified on tissue sections processed by the glyoxylic acid technique. In normal pregnant rats, the density of uterine horn innervation was significantly reduced at late pregnancy and recovery took place during post partum. The empty horn of pregnant rats showed no significant changes in density of myometrial innervation during pregnancy or post partum. In oculo myometrial transplants were organotypically reinnervated in virgin animals. When the transplants were exposed to gestational hormonal milieu, few or no fibres were observed to the end of pregnancy; however, a significant increase at post partum was observed. Results showed that both the effects of stretching and the hormone milieu derived from the fetus-placenta complex play a role as inductors of changes on sympathetic myometrial innervation during pregnancy and support the idea that immature muscular uterine fibres are more susceptible to the effects of pregnancy than those originating from adult animals.

The effect of divergent selection for uterine capacity on fetal and placental development at term in rabbits: maternal and embryonic genetic effects.

The aim of this work was to study the effects of the genotype of the dam, the embryo, or their interactions on prenatal growth by performing double-reciprocal embryo transfers between two lines of rabbits divergently selected for uterine capacity. Females from high (n = 53) and low (n = 48) lines were slaughtered at 72 h of gestation, and recovered embryos were transferred to the oviducts of recipient does from the high (n = 23) and low (n = 19) lines. Each recipient doe received eight embryos from the high line into one oviduct and eight embryos from the low line into the other. Recipient does were slaughtered on d 28 of gestation. The percentages of live fetuses at 28 d of gestation were 89.2 and 74% for high and low recipient lines, respectively. Length and weight of the empty uterine horn and weight of the full uterine horn were not affected by either the recipient or by donor line. Fetal weight was affected by the recipient line but not by the donor line. Fetuses gestated in high recipient does were 7% heavier (P < 0.10) than those in the low recipient does. There was a donor and a donor x recipient interaction effect on fetal placental weight. Fetal placental weight was heavier (7%, P < 0.01) for embryos from the low line. Embryos from the high line gestated in low-line uteri showed a lower fetal placenta weight than did low-line embryos gestated in high-line uteri and low-line uteri (P < 0.05). Linear regression coefficients of fetal weight at term on fetal placental weights differed (P < 0.05) for the high and low donors (4.33 +/- 0.28 and 3.41 +/- 0.29 respectively). A significant effect of the donor genotype on individual placental length was observed (P < 0.05), which might have resulted from a smaller individual placental length of low-line embryos gestated high-line uteri (P < 0.10). Neither donor nor recipient lines affected maternal placental weight or available space for fetuses. Fetuses and their fetal placentae were heavier when receiving more than four blood vessels than when receiving less than three blood vessels (13 and 17% respectively, P < 0.05). Neither recipient nor donor genotype affected the number of blood vessels arriving at each live fetus. Thus, fetal weight depends on the maternal genotype, whereas fetal placental weight depends on the embryo genotype in these two lines of rabbits divergently selected for uterine capacity.

The role of NGF in pregnancy-induced degeneration and regeneration of sympathetic nerves in the guinea pig uterus

In the guinea pig, pregnancy is associated with a generalised depletion of noradrenaline in uterine sympathetic nerves and, in the areas of the uterus surrounding the foetus, by a complete degeneration of sympathetic nerve fibres. These pregnancy-induced changes have been interpreted as a selective effect of placental hormones on the system of short sympathetic fibres arising from the paracervical ganglia. An alternative explanation is that pregnancy affects the neurotrophic capacity of the uterus. We measured NGF-protein levels in the guinea pig uterine horn, tubal end and cervix at early pregnancy, late pregnancy and early postpartum, using a two-site enzyme-linked immunosorbent assay. For comparative purposes the distribution and relative density of noradrenaline-containing sympathetic nerve fibres were assessed histochemically, and tissue levels of noradrenaline were measured biochemically, using high-performance liquid chromatography with electrochemical detection. In all the uterine regions analysed, NGF-protein levels showed a decline at term pregnancy, but in no case was this change statistically significant. After delivery, NGF-protein levels showed a marked increase in the cervix as well as in both the fertile and empty horns. These results suggest that alterations in NGF-protein do not account for the impairment of uterine sympathetic innervation during pregnancy, but may contribute to their recovery after delivery.

Histochemical demonstration of a concomitant reduction in neural vasoactive intestinal polypeptide, acetylcholinesterase, and noradrenaline of cat uterus during pregnancy

Noradrenaline, acetylcholinesterase, and vasoactive intestinal polypeptide (VIP) were visualized in uterine nerves of cats by histochemical techniques. Alterations were followed in different regions of the organs at various stages of pregnancy and compared with the situation in non-pregnant controls. Positively stained nerve fibres, the adrenergic type being particularly well developed, were found along the muscle bundles and around blood vessels in the smooth muscle layers, as well as in the mucosa, of both uterine horns and cervix. The nerve supply was especially prominent in the upper part of the cervix. The distribution of VIP-immunoreactive and acetylcholinesterase-positive nerve fibres resembled each other, but they were less numerous than the adrenergic fibres. In the course of pregnancy there was a marked reduction in the number of all positively reacting nerves, so that almost no fibres were visible in the uterine horns near term. A small number of positive nerve fibres was found to remain, however, in the wall of the sterile (empty) horn during unilateral pregnancy. The reduction was less prominent in the cervix, particularly its lower part. Distinct changes were encountered already during early and mid pregnancy in those parts of the uterine wall distended by the growing conceptus, where almost no fibres were seen. The nerve supply was more intact in the non-distended portions located between the fetuses, and especially in the empty horn of unilateral pregnancy. No overt reduction in the number of positively stained nerve fibres was found in the cervix at these pregnancy stages. The results show that marked alterations take place in the uterine autonomie innervation during such an entirely physiological event as pregnancy. There is reason to assume that the histochemical observations reflect both structural and functional alterations in the innervation related both to the type of nerves involved and to the localization of the conceptus.

Uterine fibrinolysis and unilateral insertion of devices in rats.

The validity of a frequently used experimental model for evaluating the effect of different intrauterine devices (IUDs) on uterine fibrinolysis in rodents was investigated in 42 rats. In this model, an IUD is inserted into one horn by a surgical procedure including laparotomy and uterotomy, while the other empty horn is used as a control. In the present investigation, the fibrinolytic activity in different parts of the IUD-containing and control horn was related to the localization of the IUD. The fibrinolytic activity as determined according to Todd's histochemical fibrin slide method was significantly increased in the endometrium in direct contact with the IUD as well as in the uterine segment opposite to the IUD. The enhancement of fibrinolysis in the opposite segment of the control horn was independent of the localization of the IUD, suggesting that neurogenic mechanisms may underlie the stimulation of uterine fibrinolysis in the empty horn. It was concluded that this experimental model is less suitable for evaluating the effect of IUDs on uterine fibrinolysis, since the presence of an IUD in one uterine horn also influences the fibrinolytic activity of the other horn.

Effect of uterine distension and oestrogen treatment on gap junction formation in the myometrium of the rat.

Gap junctions were present in the myometrium of 1 out of 5 rats on Day 21 of pregnancy, 4 out of 5 on the morning of Day 22, 7 out of 7 during parturition and 1 out of 3 on Day 1 post partum. Their frequency (number per mm membrane) at these time periods increased from 0.30 to 2.20 and 6.48 and then decreased to 0.31. Ten ovariectomized post-partum rats were fitted with an intrauterine balloon in one horn for recording pressure changes. Control rats maintained continuous pressure cycles. In rats given 7.5 micrograms oestradiol-17 beta the frequency of intrauterine pressure cycles decreased from about 50 per hour to 4.3 per hour 15 h later, but the maximum rate of rise of uterine pressure cycles increased significantly from 7.2 +/- 0.95 to 11.3 +/- 1.85 mmHg sec-1 (n = 5, P less than 0.05). The overall number of gap junctions per mm membrane in the oestrogen-treated rats was 7.37 in the horns equipped with balloons and 1.03 in the empty horns, compared with 0.97 and none respectively in the control rats. The increased rate of rise of pressure in the oestrogen-treated myometrium indicated improved coupling between cells and this was associated with a high frequency of gap junctions. Both oestrogen treatment and distension (from the balloon) appeared to cause some gap junctions to form, but numbers equivalent to those at parturition were only obtained in animals in which these two treatments were combined.

Pregnancy induces degenerative and regenerative changes in the autonomic innervation of the female reproductive tract.

The uterus is supplied with an extensive system of adrenergic nerves. The neurobiological properties of this innervation have been investigated in a series of studies primarily using the guinea-pig as model. The guinea-pig uterus is supplied from three different sources: the paracervical plexus, containing short adrenergic neurons; the inferior mesenteric ganglion; and a cranial source, probably the aorticorenal plexus, via nerves in the uterine suspensory ligaments. The nerve density is higher in the tubal end of the uterine horn and in the cervix than in the main part of the uterine horn. The turnover rate of transmitter is lower in the uterus than in a control organ, such as the heart. Noradrenaline levels in the uterus, but not the heart, are influenced by alterations in the endocrine milieu, e.g. during the oestrous cycle and after treatment with sex steroids. In the uterine tissue surrounding the conceptus during pregnancy, there is an early and drastic decay in various functional parameters related to the adrenergic nerve plexus and primarily reflecting a local, pregnancy-induced axonal degeneration. In the main part of the empty horn, in unilateral pregnancy, there is an extensive decay in various adrenergic functional parameters; these, however, reflect changes in a nerve plexus that has an essentially intact structure. No sign of functional impairment is seen in the adrenergic nerves of the uterine cervix. The increased turnover rate and reduced transmitter content in this region during late pregnancy may reflect increased frequency of firing of the adrenergic nerves. The tubal end of the uterine horn shows no signs of altered sympathetic function. This is the only part of the uterine horn that appears unaffected by pregnancy. The deficient recovery post partum of the changes in the uterine horn that previously contained the fetuses suggests permanent damage to the adrenergic nerve plexus after a pregnancy. The post partum recovery of changes seen in the previously empty horn, however, is more pronounced but still incomplete by comparison with the innervation before pregnancy. Studies on the adult human uterus indicate that similar events to those described for the guinea-pig model occur in human pregnancy.

Changes in ornithine decarboxylase during early implantation in the rat.

Ornithine decarboxylase activity was compared in the right and left horns of unilaterally ovariectomized rats on Days 4 and 5 of pregnancy. It was found that enzyme activity in the pregnant horn was significantly increased over that of the contralateral empty horn by 0400 h on Day 5 before implantation of the blastocyst occurred and reached maximum levels by 0900 h. Similar changes were not observed in the intact horns of uteri from unilaterally ovariectomized rats mated with vasectomized males even though eggs were present. However, intraluminal injection of arachis oil at 1000 h on Day 5 into those horns, a treatment producing massive deciduomas, led to large increases in ODC activity in the treated horn within 5 h. Similar increases were not induced by laparotomy. The data indicates that the decidual response induced by the blastocyst has already commenced by 0400 h on Day 5, in agreement with previous conclusions based on studies of RNA synthesis.

Uptake and metabolism of [3H]norepinephrine in uterine nerves of pregnant guinea pig

Myometrial tissue slices from virgin, pregnant (uni- or bilateral pregnancy), and puerperal animals were incubated in media containing [3H]norepinephrine ([3H]NE), and the neuronal and extraneuronal uptake was estimated. The metabolic fate of [3H]NE was elucidated by the chromatographic separation of [3H]NE, [3H]normetanephrine and 3H-labeled acid metabolites. An early and extensive reduction in both total and neuronal uptakes occurred in the myometrial regions surrounding the fetuses, and at term pregnancy no neuronal [3H]NE uptake at all was found in tissue slices from the fetus-containing horns. Concomitantly, the fraction of unchanged [3H]NE decreased with a corresponding increment in its metabolites. Similar changes, though less extensive, also occurred in the empty horn in unilateral pregnancy. The total amount of neuronal uptake in the cervix seemed unchanged compared to virgin animals. Earlier and present data support the following suggestion that in uterine horns harboring fetuses, an extensive axonal degeneration occurs; in those devoid of fetuses (unilateral pregnancy), the nerve terminals are to a great extent structurally intact, but functionally damaged. Recovery of the pregnancy-induced impairment of the adrenergic nerve plexus is slow and incomplete.

Variations in the level of uterine norepinephrine during pregnancy in the guinea pig

The uterine smooth musculature is innervated by short adrenergic neurons which are unique in that their transmitter content varies during pregnancy and under the influence of exogenous sex hormones. Uterine norepinephrine was measured fluorometrically at 25 stages throughout pregnancy in the guinea pig, which was chosen because one of the pregnant uterine horns often is devoid of fetuses and hence not subject to mechanical tension. During the initial 10 days of pregnancy, the total organ content of norepinephrine was almost doubled whether the horn contained conceptuses or not. In the horn with conceptuses, norepinephrine was subsequently reduced in the course of the remaining gestation time to reach near-zero levels at term (about 65 days post coitus). In the empty horn, the amine level was constantly elevated until the last two weeks, during which it fell to the same low values as in the contralateral horn. Cervical norepinephrine exhibited the same variations during pregnancy as the empty horn. It is suggested that the characteristic variations in uterine norepinephrine reflect a hormonal influence on the system of short adrenergic neurons, with the addition of mechanical factors in that horn containing fetuses.

Fetal and placental weights in experimentally delayed implantation pregnancy.

Fetal and placental weights from rats in which implantation was delayed by ovariectomy and progesterone-estrone treatment were heavier than those from untreated rats at comparable days of gestation. When these hormones were given to ovariectomized pregnant rats with a normal preimplantation interval, the fetuses weighed the same as those from 2- or 10-day delayed rats. When blastocysts from either normal or delayed rats were transplanted to the empty horn of a unilaterally pregnant recipient, the body weights of the fetuses were not significantly different from those of normal rats. Only rats which were spayed and given exogenous hormones had fetuses and placentae heavier than normal controls. It is concluded that the heavier fetal and placental weights of delayed rats are not due to prolongation of the preimplantation period but are associated with ovariectomy and exogenous hormone treatment. (Endocrinology 86: 131, 1970)