Haemostasis in menstrual endometrium in the presence of an intrauterine device

The histology and ultrastructure of the haemostatic response in the endometrium have been studied in 12 uteri containing an inert or copper wound intrauterine contraceptive device (IUCD). The uteri were removed during the premenstrual phase or during the first 96 hours of menstruation. The results have been compared to earlier observations in non-IUCD influenced endometrium. In all uteri the tissue shedding proceeded more slowly in the presence of an IUCD and considerably fewer haemostatic plugs were found. The platelets in these plugs were more loosely packed and less degranulated, and the plugs contained less fibrin than in the absence of an IUCD. Haemostatic plugs were still observed at times when no such plugs were seen in uteri without an IUCD. Unoccluded vessel lesions were regularly seen and vessels open to the shedding surface were occasionally encountered. These observations suggest that IUCD-induced menorrhagia is caused by the combination of delayed shedding and a decreased haemostatic reaction in the endometrium.

Mechanism of action of IUDs: an ultrastructural view

Endometrial samples from 64 women using different types of IUDs were used to study the ultrastructural changes of the human endometrium as influenced by nonmedicated and copper devices, using SEM and TEM in an attempt to explain the physiological mechanism of their actions. The absence of cytoplasmic macro-apocrine secretory granules with copper devices and their defective separation with nonmedicated IUDs support the theory that the IUD interferes with the carbohydrate metabolism and secretory function of the endometrial cells. It also indicates that IUDs may function through the interference with the blastocyst-endometrial interaction and blastocyst survival. The change in the intrauterine environment due to the absence of normal secretion may also affect sperm capacitation and/or the acrosomal reaction, with subsequent failure of fertilization. The absence of ciliated cells at the site of copper IUD contact zone and their defective ciliation away from it in the presence of normal steroid function may indicate an effect of the copper ions on the estrogen receptors. The asynchronous development of the endometrium and blastocyst mechanism of IUD function was not supported in this study. The occurrence of intermenstrual bleeding with IUDs is governed by 2 factors: 1) the pressure exerted by the IUD causing endothelial vascular injury and release of erythrocytes into endometrial stroma; and 2) the increased stromal hydrostatic pressure which acts as a force against the basal lamina and epithelial integrity. This was proven by the loss of normal microvillous pattern of the endometrial cells even away from the copper IUD, which support the rigidity of the cellular membranes.

[Studies on estradiol occupied and unoccupied receptors in human endometrial cytosol and nucleus: steroid receptors throughout the menstrual cycle and in cases of oral contraceptives administration (author's transl)]

Occupied and unoccupied estradiol (E3) receptors were investigated by exchange assay in cytosol and nuclear extract of human endometrium. Cytosol E2 and progesterone (P) unoccupied receptors and nuclear occupied E2 receptor were measured throughout the menstrual cycle and in the tissues from patients administered oral contraceptives. Sedimentation profiles of these receptors were also studied; and following results were obtained: 1) No occupied E2 receptor was observed in cytosol and almost all of those were of unoccupied type. In nuclear extract, only 0-25% of receptor was of unoccupied type and the other receptor was of occupied type. 2) Highest binding activities of cytosol E2 and P receptors were found in late proliferative phase. Maximum binding sites (Bm) were 630 and 380 f mol/mg protein and dissociation constant (Kd) was 8.9 +/- 0.7 and 4.7 +/- 0.4 x 10(-10) M, respectively. In nuclear extract, binding peaks were observed in late proliferative and late secretory phase. Bm was 1.5 f mole/microgram DNA and Kd was 12.7 +/- 1.0 x 10(-10) M.

Intrauterine release of progesterone: changes in RNA patterns of endometrium

The changes induced in the relative concentrations of the different species of RNA (messenger, ribosomal, transfer and heterogenous) which regulate the metabolic behavior of tissues were studied in the endometrium of normal women and in women wearing a progesterone T IUD for 6-12 months. It was observed that the intrauterine release of progesterone is accompanied by a significant decrease in the content of total RNA (from 78 +or- 8.4 to 64 +or- 6.7 mcg/mg of protein) and in the RNA/DNA ratio (from 0.85 +or- 0.10 to 0.73 +or- 0.08). Although the protein decreased significantly (from 102 +or- 11 to 82 +or- 14 mg/g wet wt.), the DNA content and the Prot/DNA ratio were not altered. Using the affinity chromatography technique in poly(u)Sepharose it was found that the relative concentrations of RNA types were in general drastically modified: mRNA and rRNA decreased (from 2.6 +or- 0.31 to 1.28 +or- 0.15 and from 65.5 +or- 8.0 to 54.8 +or- 6.7 mcg/mg of protein respectively). tRNA content was increased (from 6.2 +or- 0.9 to 11.0 +or- 1.3) while hRNA was unchanged. The probable inhibitory effects of these changes in relation to the normal hormonal stimulation of the human endometrium, as a possible explanation of the mechanism of action of this device, are discussed.

Role of prostaglandins in IUD-associated uterine bleeding--effect of a prostaglandin synthetase inhibitor (ibuprofen)

Prostaglandins have been shown to be increased in the endometrium of women and experimental animals wearing intrauterine devices (IUDs). As prostaglandins may cause increased vascularity and vascular permeability, and as certain prostaglandins (PgI2) inhibit platelet activity, the local generation of prostaglandins may contribute to endometrial bleeding. Thus, the effect of ibuprofen, a prostaglandin synthetase inhibitor, was tested by quantifying menstrual blood loss in 20 women wearing IUDs in a double-blind, 2-period crossover study. Ibuprofen produced a significant reduction in menstrual blood loss; the percentage reduction was greater in women using a Lippes Loop and who had heavier blood loss (39%) than in women using a copper device and who had lighter blood loss (25%). These findings support the contention that prostaglandin synthesis is important in the genesis of IUD-associated menorrhagia and that prostaglandin inhibitors may be useful in the therapy of this condition.

Ultrastructural changes in human endometrium with copper and nonmedicated IUDs in utero

Scanning and transmission electron microscopy were used for a study of the surface and glandular ultrastructure of human endometrium in the presence of different types of IUDs at comparable phases of the menstrual cycle. The aim of the study was to compare the effect of the nonmedicated with the copper and multiload copper devices to further explain the differences in their contraceptive potencies and their mechanism of action. The endometrium was evaluated at and away from the IUD; emphasis was put on the ultrastructure of endometrial gland openings, secretory activity, cellular glycogen content, ciliated cells, microvillous pattern, and kinocilia. The changes of the surface ultrastructure of the endometrium in the presence of copper IUDs were more extensive in this study than those previously reported. There seems to be a direct relationship between the amount of copper incorporated in the device, the degree of ultrastructure changes, and the area of endometrium involved. Copper devices affect the endometrial cells away from the IUD. The altered secretory function with disturbed macroapocrine secretion, the abnormality of ciliated cells, and the defective microvillous growth seem to interfere with the physiologic and functional integrity of the endometrium, reducing the chances of contraception in the presence of copper IUDs.

Estradiol and progesterone receptors in estrogen-primed endometrium

Estradiol and progesterone receptor levels were measured in endometrial samples obtained from patients who were on different dosages of estradiol therapy and from women in the late proliferative phase of a normal menstrual cycle. Samples of blood were collected at the time of biopsy, and the levels of estradiol, estrone, progesterone, follicle stimulating hormone, and luteinizing hormone were measured in the serum. The patients were divided into five groups. The first group (controls) consisted of patients in their late proliferative phase. The patients in groups two, three, and five were receiving estradiol in various doses by pellet therapy, along with a cyclic progestogen each month. The women in the fourth group also had implantation of estradiol pellets but failed to take the progestogen as advised. In our series, the levels of cytoplasmic estradiol and progesterone receptors were markedly elevated in the no progestogen group compared to the controls. There was no significant difference in the levels of the receptors in the groups which took the progestogen as advised.

Ultrastructural alterations in human endometrium caused by intrauterine contraceptive devices

Biopsies of endometrium from women using intrauterine contraceptive devices (IUDs) were examined by electron microscopy to elucidate the possible cause of IUD-associated uterine hemorrhage. The findings reveal the presence of degenerated, shrunken cells in the endometrial glands of IUD users. These cells, termed pyknotic or apoptotic, are more prevalent in tissue from women with complaints of excessive bleeding than in cases without such complaints. The pyknotic cells often show discontinuities of the plasma membrane at their luminal pole, from which cell contents stream into the lumen. Sometimes erythrocytes are also observed in the lumen. A second alteration in the ultrastructure of the endometrium is an abnormal increase in microfilaments in the epithelial cells. These two changes, a large number of pyknotic cells on the one hand and increase in microfilaments on the other, are correlated with excessive bleeding but they do not explain in simple terms of outflow of erythrocytes. Probably some inflammatory mechanism is involved. The increased number of microfilaments might be helping the movement of fluid from healthy to pyknotic cells.

Pseudo-sulfur granules associated with intrauterine contraceptive devices

A structure resembling a sulfur granule found in histologic sections of endometrium from patients with intrauterine contraceptive devices is described. It is distinguished from a true sulfur granule on morphologic grounds and is further characterized as a foreign substance, most probably fragments of the synthetic contraceptive device. It is not associated with clinical disease. Microbiologic cultures of removed intrauterine contraceptive devices show no consistent or predominant growth pattern.

The effect of a synthetic progestogen, ethylnorgestrienone, on hypothalamic-pituitary-ovarian function, cervical mucus, vaginal cytology, and endometrial morphology

The antifertility mechanism of a new progestin preparation, ethylnorgestrienone (13 beta-ethyl-17 alpha-ethynyl-17-hydroxy-gona-4,9,11-triene-3-one) (R2323), was evaluated. The compound was administered orally in doses of 5 mg weekly to seven subjects for a total of nine treatment cycles. Each woman served as her own control and was studied during a normal menstrual cycle followed by a cycle in which she received R2323. Serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol, and progesterone, as well as cervical mucus properties, were studied serially during the control and treatment cycles. Results indicated that all control cycles were ovulatory. Ovulatory gonadotropin patterns were observed in four treated cycles, but preovulatory FSH and LH peaks and progesterone production during the luteal phase were suppressed significantly. Estradiol levels showed an early increase during the follicular phase and a significantly decreased preovulatory peak. Cervical mucus properties were altered and sperm penetration in cervical mucus was inhibited in all treatment cycles. These findings suggest that at least two different factors might be involved in the contraceptive mechanism of R2323: (1) alteration of the ovulatory process and progesterone production, and (2) cervical mucus changes leading to inhibition of sperm migration.

Endometriosis and the development of tuboperitoneal fistulas after tubal ligation

The present study details gross and histologic findings of 79 previously ligated fallopian tubes from 3 groups of patients. Of 20 oviducts removed after documented sterilization failure (group I), 6 revealed a process compatible with endometriosis. Four of nine previously ligated fallopian tubes removed at the Johns Hopkins Hospital (group II) were successfully injected with India ink. In two patients histologic examination demonstrated the India ink in epithelium-lined spaces that lay beyond the muscle of the tubal wall extending from the tubal lumen to the serosal surface. Fifty oviducts were studied in twenty-five patients requesting reversal of their sterilizations (group III). A higher percentage of fistulas was demonstrated in patients with less than 4 cm of remaining proximal tubal segment. Furthermore, most of these fistulas were demonstrated in patients for whom 3 years had elapsed since the original sterilization procedure. Patients sterilzed by laparoscopic cautery methods were observed to have a higher percentage of fistula formation and histologic documentation of endometriosis at the sterilization site as compared with patients sterilized by other methods. Our observations suggest that ligation of the oviduct within 4 cm of the uterine cornu may predispose to the development of endometriosis and subsequent fistula formation in the tip of the ligated oviduct.

[Immunofluorescent analysis of intracellular localization of estrogen and progesterone in normal human endometrial cells (author's transl)]

Intracellular localization and the heat-dependent redistribution of estrogen and progesterone in human endometrial cells have been investigated by a fluorescent steroid-antibody technique. The dispersed endometrial cells were incubated with 5 X 10(-8) M estradiol-17 beta and progesterone in TC medium 199 containing 10% calf serum for 1--3 hr at 4 degrees C or 37 degrees C. An indirect immunofluorescent technique using FITC-labeled anti-rabbit IgG and steroid antibodies raised from rabbits immunizing with estradiol-6-oxime-BSA and progesterone-3-oxime-BSA was applied to the smear specimens. In normal endometrial cells in both proliferative and secretory stages, specific fluorescences to estradiol and progesterone were generally observed in the cytoplasm after incubation with the steroids at 4 degrees C for 1 hr. When these cells were incubated with the steroids at 37 degrees C for 1 hr, cytoplasmic and predominant nuclear fluorescences were detected, whereas the 3 hr-incubation at 37 degrees C resulted in disappearance of cytoplasmic fluorescence, remaining nuclear fluorescence alone. These fluorescences were remarkedly eliminated when endometrial cells were incubated with diethylstilbestrol and R-5020 prior to the incubation with estradiol and progesterone, respectively. These results indicate that the fluorescent steroid-antibody technique used in this study enables us to visualize subcellular localization of estradiol and progesterone possibly bound to receptors in each endometrial cell.

Endometrial biopsy during a cycle of conception

Endometrial biopsies obtained from 18 infertile women during early intrauterine pregnancy were examined histologically. The cycle date on which the biopsies were obtained was determined from the day of ovulation by basal body temperature charts and prior menstrual history. The average cycle dates of the biopsies were similar for the women who delivered at term or who had first-trimester spontaneous abortions. Among the women who delivered at term, the average histologic date assigned to the specimens was significantly earlier than the average cycle date. In contrast, among the women who had early pregnancy loss, the specimens had an average histologic date significantly later than the average cycle date. A histologic date of later than 25 days or later than the cycle date of the biopsy was associated with a poor prognosis for the pregnancy.

Steroid receptors in normal, hyperplastic and malignant human endometria

Oestrogen and progestin receptors are present in the cytosol and nuclear compartments of normal human endometrium, partly associated with their endogenous ligand hormones. Receptor concentrations fluctuate in relation to the menstrual cycle. Hyperplastic endometrium tends to contain high concentrations of cytosol progestin receptor, whereas the levels of cytosol and nuclear progestin receptors in endometrial adenocarcinoma are lower than in non-neoplastic endometrium. The receptor levels seem to decline with decreasing differentiation of the tumour. Progestin treatment extending over several weeks decreases cellular oestrogen and progestin receptor content in both hyperplastic and malignant endometria. Information based on small patient series suggests that patients suffering from advanced or recurrent endometrial carcinoma and having significant concentrations of both receptors in the tumour tend to have a more indolent clinical course than patients with absent or low tumour receptors. Patients whose lesions are progestin receptor-rich more frequently respond to progestin administration than those with receptor-poor tumours. In contrast, patients with advanced or recurrent disease after progestin treatment and with low tumour oestrogen and progestin receptor concentrations respond more often to combination cytotoxic chemotherapy than patients with higher tumour receptor levels. More data are needed about the clinical correlates of receptor determinations in human endometrial carcinoma to confirm these encouraging preliminary results, before the clinical significance of the determinations can be settled. since there are marked differences in the receptor concentrations reported by various investigators, possibly for methodological reasons, comparison of receptor data and treatment results from different groups is sometimes very difficult.

Renin substrate, active and acid-activatable renin concentrations in human plasma and endometrium during menstrual cycles controlled by oral contraceptive preparations

Measurements of active and acid-activatable renin concentrations and renin substrate concentrations in plasma and endometrial samples taken from 31 patients are described. All of these patients were taking oestrogen-containing oral contraceptive preparations. Plasma active renin concentration was lower in these cycles than in normal cycles and the substrate concentrations higher. In the endometrium the concentrations of all of the components of the system were raised by oral contraceptives. As with normal cycles, the maximum potential for production of the angiotensins was in the premenstrual stage of the cycle. It is suggested that this increase in potential is associated with the onset of menstruation.

Effects of contraceptive agents on the biochemical and protein composition of human endometrium

An analysis of the nucleic acid and protein composition of endometrial tissue was undertaken in normal women and in patients using either steroidal oral contraceptives or intrauterine contraceptive devices (IUCD). In the presence of an IUCD, endometrial RNA/DNA and protein/DNA ratios were elevated in the secretory phase of the menstrual cycle compared to values obtained in normal women. In the oral contraceptive group, endometrial RNA/DNA and protein/DNA ratios were below the normal range in the late proliferative phase of the cycle. Typical electrophoretic profiles in the oral contraceptive group were similar to the control group, although a quantitative analysis revealed that the concentrations of certain characteristic uterine proteins were reduced. In the IUCD group, there was a preferential appearance of two proteins with approximate molecular weights of 36,000 and 26,000 daltons. Another protein with a molecular weight of 48,000 daltons, which demonstrated a cyclic change during the normal menstrual cycle, was reduced in both study groups. The results suggest that both agents studied induce changes in the macromolecular composition of the human endometrium which may relate to their contraceptive effect.

An incremental-dose combined oestrogen-progestogen oral contraceptive: cycle control and endometrial changes

A study of bleeding patterns and endometrial histology was made in women using an incremental-dose combined oestrogen-progestogen formulation. The proliferative activity during the first 11 days of treatment was less well-developed, and there was an early appearance of secretory changes (transitional pattern). The increased dose of d-norgestrel resulted in an endometrium showing either early or deficient secretory changes. Menstrual patterns were very satisfactory in terms of cycle length and duration of bleeding.

Endometrial effects of levonorgestrel and estradiol: A Scanning electron microscopic study of the luminal epithelium

Fertile women in the follicular phase possessed an uterine luminal surface with many ciliated cells and with non-ciliated cells, which carried numerous, fairly long microvilli. A moderate number of the non-ciliated cells had an irregular surface with small apical protrusions. Postmenopausal women had an endometrial surface containing rather flat cells. Ciliated cells were seldon encountered, and the non-ciliated cells possessed mostly only few short microvilli. When menopausal women had been wearing estradiol-containing intravaginal rings for three weeks, the uterine surface had developed many ciliated cells, and the non-ciliated cells now possessed many long microvilli. This appearance resembles that appearing during the follicular phase. Fertile women with levonorgestrel-containing subdermal implants or intravaginal rings showed a surface epithelium with few ciliated cells and with non-ciliated cells possessing short and irregular microvilli; that is, an epithelium less developed than that from a cyclic women. Adding estradiol to the levonorgestrel-containing intravaginal rings resulted in an estrogen response with an increase in number and length of the microvilli and an appearance of a few small apical protrusions.

Effects of RU16117, an orally active weak oestrogenic compound, in postmenopausal women

Daily oral administration of 1, 3 or 10 mg of RU16117 (11 alpha-methoxy ethinyl oestradiol) to normal postmenopausal women led to a progressive decrease of basal serum LH levels to 60.4 +/- 17.0, 35.1 +/- 9.1 and 20.1 +/- 2.8% of control (pretreatment values, P less than 0.01), respectively, after 4 wk of drug administration. Although the pattern was similar, the inhibitory effect of RU16117 was even more pronounced on FSH than LH levels: a 50% decrease of basal LH and FSH levels was obtained at the daily 1.8 and 1.2 mg doses of RU16117, respectively. No significant change of basal serum gonadotrophin levels was observed with the daily 0.3 mg dose. Administration of 1 mg of RU16117 every second day or 10 mg once a week led to a relatively small but significant (P less than 0.05) 20--25% decrease of basal serum LH levels after 4 wk of treatment in four out of five women. While daily 0.3 and 1.0 mg doses of RU16117 had no significant effect on the LH response to 100 microgram LHRH, the 3.0 mg dose delayed the response up to 90 min. The 10 mg dose, on the other hand, led to a markedly delayed and reduced response. Treatment for the same period (4 wk) with 1 mg RU16117 every second day or 10 mg once a week led to a small (20--25%, P less than 0.05) inhibition of the LH response to LHRH. At the dose of 10 mg once a week, RU16117 had no or minimal effect on endometrial histology. Since RU16117, an orally active weak oestrogenic compound, has been shown to have anticarcinogenic activity in the rat, the present findings suggest that this new steroid could be useful for the treatment of climacteric symptoms.

[Characteristics of the endometrial estrogen-receptor system in physiological and various pathological states of the body]

The state of endometrial estrogen-receptor system was evaluated under physiological and pathological conditions characterized by changes in estradiol dynamics during the menstrual cycle. It is suggested that external endometriosis is accompanied by changes in the state of the endometrial estrogen-receptor level.

Suppression of concentration of endometrial prostaglandin in early intra-uterine and ectopic pregnancy in women

Concentrations of prostaglandin F2 alpha (PGF2 alpha) and prostaglandin E (PGE) were measured in endometrium from 18 women with ectopic pregnancies. In the nine pregnancies not associated with vaginal bleeding or an intra-uterine contraceptive device (IUCD; intact ectopics), concentrations of PGF2 alpha (12.8 +/- 7.4 (S.E.M.) ng/g) and PGE (4.7 +/- 3.0 ng/g) were similar to those in decidua from nine intra-uterine pregnancies of comparable gestational age (14.4 +/- 4.4 and 8.2 +/- 2.2 ng/g respectively). In both ectopic and intra-uterine pregnancies concentrations of prostaglandins were significantly lower than those found in endometrium throughout the normal menstrual cycle (P < 0.01). In nine ectopic pregnancies with associated vaginal bleeding and/or an IUCD, concentrations of PGF2 alpha and PGE were significantly higher than in the intact group (P < 0.05), although the concentration of PGF2 alpha remained significantly lower than levels in normal secretory endometrium (P < 0.05). These results suggested that suppression of endometrial synsthesis of prostaglandin during early pregnancy may be mediated systemically rather than through a local action of the conceptus.

Actinomyces in the endometrium of IUD users

The colonization of the female genital tract with Actinomyces is closely related to the presence of an IUD in the uterine cavity. This was confirmed in the present study, which shows an Actinomyces frequency of over 3% in users of IUDs, while non-users were free from the opportunistic invader. This frequency was higher with certain types of devices than with others. None of the patients with positive results had pelvic symptoms, but cases of serious infections have been reported in the literature. The length of IUD usage appears to be directly related to the incidence of this colonization. The possible pathogenesis of pelvic actinomycosis is presented; the clinical significance of asymptomatic colonization is discussed.

Studies in menorrhagia: (a) mefenamic acid, (b) endometrial prostaglandin concentrations

Twenty-two women with unexplained heavy menstrual blood loss (average loss for two cycles of >80 ml) were treated with the prostaglandin synthetase inhibitor menfenamic acid during two consecutive menstruations. There was a significant reduction in menstrual blood loss on mefenamic acid therapy, the median loss being 137 ml before treatment and 76 ml while on treatment. Reduction in menstrual loss was achieved in 20 of the 22 patients but varied from a 2% to 78% reduction. The greater the menstrual loss before treatment, the more it was reduced on mefenamic acid therapy. Endometrial concentrations of prostaglandins E2 and F2 alpha in the follicular phase of the cycle were similar whether or not patients had menorrhagia. In the luteal phase, however, 6 of 14 patients with menorrhagia had higher endometrial prostaglandin E2 and F2 alpha concentrations than all 13 controls.

Estradiol and progesterone receptors in human endometrium: normal and abnormal menstrual cycles and early pregnancy

Estradiol and progesterone receptor sites (empty or filled with endogenous hormone)have been measured in the cytoplasm and nuclei of human endometrium. Receptor changes have been observed throughout the normal menstrual cycle. During the preovulatory phase the cytoplasmic estradiol receptor sites do not change while the nuclear receptor sites more than double. Cytoplasmic estradiol receptor sites decrease very early in the secretory phase, whereas the decrease in nuclear sites occurs later. Cytoplasmic progesterone receptor sites more than double during the preovulatory phase and show a large decrease immediately after ovulation, when the concentration of nuclear receptor is at its highest. Thus the total cellular concentrations of both estradiol and progesterone receptors are lowest in the late secretory phase. It was found that they are positively correlated with the concentration of plasma estradiol only during the proliferative phase. The concentration of cytoplasmic progesterone receptor is negatively correlated with 17 beta-hydroxysteroid oxidoreductase activity during the secretory phase. In anovulatory cycles the concentrations of estradiol and progesterone receptors are high, similar to those of the late proliferative phase. "luteal insufficiency" is characterized by a very low concentration of estradiol receptor. Early pregnancy endometrium (8 to 10 weeks' gestation) is characterized by a large concentration of progesterone receptor, exceeding those of any period of the menstrual cycle.

Unoccupied nuclear oestradiol-receptor sites in normal human endometrium

The existence of unoccupied nuclear oestradiol-receptor sites in normal human endometrium was investigated. Nuclei were prepared from endometrial samples obtained by curettage and exposed to [3H]oestradiol, which became maximmaly bound at 0 degrees C within 1 h. This result contrasted with the binding kinetics of oestradiol--receptor complexes, since the exchange of hormone took at least 3 h at 30 degrees C and no displacement occurred at 0 degrees C. Before concluding that the nuclear sites were unoccupied, the presence of endogenous low-affinity ligands was excluded, because the association rate of oestradiol was unchanged after nuclei were stripped from their putative ligands, and the displacement of oestrone bound to nuclear receptor by oestradiol was very slow at 0 degrees C. The available sites had high affinity for oestradiol (KD 1.3 nM) and binding-specificity characteristics of oestradiol receptors. Similar results were observed with crude and purified nuclear preparations. It was concluded that a significant proportion of nuclear oestradiol receptors in normal human endometrium is unoccupied by endogenous hormones.