The ultrastructural response of human endometrium to medroxyprogesterone acetate

Progesterone Threshold Determines Nucleolar Channel System Formation in Human Endometrium

Nucleolar channel systems (NCSs), micron-sized organelles specific to nuclei of human endometrial epithelial cells (EECs), are robust markers of the midluteal phase under the apparent control of progesterone. To gain further insight into the role of progesterone in NCS formation, we quantitatively assessed their sensitivity to oral contraceptive pills (OCPs) using immunofluorescence-based detection of NCSs. Comparison of endometrial biopsies and serum progesterone levels on cycle day (CD) 10 and 20 (LH +6/7) of 6 naturally cycling women and 6 OCP users demonstrated that OCPs interfered with NCS formation on CD20, their natural peak presence. Although this confirmed prior observation based on electron microscopic sampling, OCPs unexpectedly induced limited but distinct amounts of NCSs already on CD10, when they are never present in natural cycles. Thus, OCPs can cause secretory changes in the endometrium during the proliferative phase. In a novel finding, robust NCS formation on CD20 was dependent on a 4 ng/mL progesterone threshold but did not correlate linearly with serum progesterone levels. Given the threshold being close to that serving as evidence for ovulation, NCSs can serve as ovulation markers.

Premature formation of nucleolar channel systems indicates advanced endometrial maturation following controlled ovarian hyperstimulation

STUDY QUESTION

Is there a shift in the timing of nucleolar channel system (NCS) formation following controlled ovarian hyperstimulation (COH)?

SUMMARY ANSWER

NCSs appear prematurely following COH compared with natural cycles.

WHAT IS KNOWN ALREADY

During natural cycles, NCSs of endometrial epithelial cell (EEC) nuclei are exclusively present during the window of implantation and are uniformly distributed throughout the upper endometrial cavity.

STUDY DESIGN, SIZE, DURATION

Prospective two-cohort study. Cohorts I and II each consisted of seven volunteers for the duration of three menstrual study cycles that were separated by at least one wash-out or rest cycle, between December 2008 and May 2012.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Participants were recruited from a pool of healthy oocyte donors. Consecutive endometrial biopsies were obtained during the same luteal phase on cycle days (CD) 16, 20 and 26 for Cohort I, and on CD14, 22 and 24 for Cohort II, following random assignment to a natural cycle group, a COH cycle group (using a GnRH antagonist), or a COH cycle group receiving luteal phase hormonal supplementation (COH + S). The day of oocyte retrieval was designated CD14 in COH cycles and the day of the LH surge was designated CD13 in natural cycles. Prevalence of NCSs in the nuclei of EECs was quantified using indirect immunofluorescence with an antibody directed against a subset of related nuclear pore complex proteins that are major constituents of NCSs. Progesterone and estradiol levels were measured on the day of each endometrial biopsy.

MAIN RESULTS AND THE ROLE OF CHANCE

The natural cycle group exhibited peak NCS prevalence on CD20 [53.3%; interquartile range (IQR) 28.5-55.8], which rapidly declined on CD22 (11.8%; IQR 6.3-17.6), CD24 (2.5%; IQR 0.0-9.2) and CD26 (0.3%; IQR 0.0-3.5), and no NCSs on CD14 and 16 defining a short NCS window around CD20. In contrast, in COH and COH + S cycles, NCS prevalence was high already on CD16 (40.4%; IQR 22.6-53.4 and 35.6%; IQR 26.4-44.5, respectively; P = 0.001 compared with CD16 of the natural cycle group, Mann-Whitney), whereas no significant difference in NCS prevalence was detected on any of the other five CDs between the three groups (P > 0.05).

LIMITATIONS, REASONS FOR CAUTION

The cohort size was small (n = 7) but was offset by the all-or-none presence of NCSs on CD16 in natural versus COH and COH + S cycles and the fact that each subject served as her own control.

WIDER IMPLICATIONS OF THE FINDINGS

Premature appearance of NCSs and hence maturation of the endometrium following COH is consistent with previous studies based on histological dating but contradicts studies based on mRNA expression profiling, which reported a lag in endometrial maturation. However, this is the first study of this kind that is based on consecutive endometrial biopsies within the same cycle and that reports such clear-cut differences: no versus robust NCS presence on CD16. Our observation of advanced endometrial maturation following COH may contribute to the reduced implantation rates seen in fresh compared with frozen and donor IVF-embryo transfer cycles. Therefore, the NCS window could serve as a sensitive guide for timing of embryo transfer in frozen and donor cycles.

STUDY FUNDING/COMPETING INTEREST(S)

The study was supported by the March of Dimes Birth Defects foundation (1-FY09-363 to U.T.M.); Ferring Pharmaceuticals, Parsippany, NJ; East Coast Fertility, Plainview, NY and the CMBG Training Program (T32 GM007491 to M.J.S.). We report no competing interests.

Quantification of nucleolar channel systems: uniform presence throughout the upper endometrial cavity

OBJECTIVE

To determine the prevalence of nucleolar channel systems (NCSs) by uterine region, applying continuous quantification.

DESIGN

Prospective clinical study.

SETTING

Tertiary care academic medical center.

PATIENT(S)

Forty-two naturally cycling women who underwent hysterectomy for benign indications.

INTERVENTION(S)

NCS presence was quantified by a novel method in six uterine regions-fundus, left cornu, right cornu, anterior body, posterior body, and lower uterine segment (LUS)-with the use of indirect immunofluorescence.

MAIN OUTCOME MEASURE(S)

Percentage of endometrial epithelial cells (EECs) with NCSs per uterine region.

RESULT(S)

NCS quantification was observer independent (intraclass correlation coefficient 0.96) and its intrasample variability low (coefficient of variation 0.06). Eleven of 42 hysterectomy specimens were midluteal, ten of which were analyzable with nine containing >5% EECs with NCSs in at least one region. The percentage of EECs with NCSs varied significantly between the LUS (6.1%; interquartile range [IQR] 3.0-9.9) and the upper five regions (16.9%; IQR 12.7-23.4), with fewer NCSs in the basal layer of the endometrium (17 ± 6%) versus the middle (46 ± 9%) and luminal layers (38 ± 9%) of all six regions.

CONCLUSION(S)

NCS quantification during the midluteal phase demonstrates uniform presence throughout the endometrial cavity, excluding the LUS, with a preference for the functional luminal layers. Our quantitative NCS evaluation provides a benchmark for future studies and further supports NCS presence as a potential marker for the window of implantation.

The nucleolar channel system reliably marks the midluteal endometrium regardless of fertility status: a fresh look at an old organelle

OBJECTIVE

To determine whether nucleolar channel systems (NCSs) in the midluteal endometrium are associated with overall fertility status and/or with unexplained infertility.

DESIGN

Retrospective and prospective clinical studies.

SETTING

Repository of stored specimens from prior multicenter study and private infertility center.

PATIENT(S)

Retrospective study that included 97 women (49 fertile couples, 48 infertile couples) who had been randomized for endometrial biopsy during the midluteal or late luteal phase. The prospective study included 78 women with a variety of infertility diagnoses.

INTERVENTION(S)

Endometrial biopsies were obtained and assessed for the presence of NCSs by indirect immunofluorescence.

MAIN OUTCOME MEASURE(S)

The presence of NCS was graded semiquantitatively and dichotomized as normal versus low or absent.

RESULT(S)

Normal presence of NCS was significantly associated with the midluteal phase compared with the late luteal phase (80% vs. 29%). However, there was no association between presence of NCS and fertility status or between presence of NCS and unexplained infertility.

CONCLUSION(S)

Midluteal phase endometrium consistently forms NCSs regardless of fertility status, including unexplained infertility. This indicates a possible role for the NCS in initiating the window of endometrial receptivity. However, the consistent presence of NCSs across several different types of infertility challenges the likelihood that inadequate secretory transformation is a cause of infertility.

Optically Clear Endometrial Nuclei

The ultrastructure of optically clear endometrial nuclei is presented. These nuclear alterations have been found in spontaneous abortion, term pregnancy, endometriosis, and uterine choriocarcinoma.

Nuclear pore complex proteins mark the implantation window in human endometrium

Nucleolar channel systems (NCSs) are membranous organelles appearing transiently in the epithelial cell nuclei of postovulatory human endometrium. Their characterization and use as markers for a healthy receptive endometrium have been limited because they are only identifiable by electron microscopy. Here we describe the light microscopic detection of NCSs using immunofluorescence. Specifically, the monoclonal nuclear pore complex antibody 414 shows that NCSs are present in about half of all human endometrial epithelial cells but not in any other cell type, tissue or species. Most nuclei contain only a single NCS of uniform 1 microm diameter indicating a tightly controlled organelle. The composition of NCSs is as unique as their structure; they contain only a subset each of the proteins of nuclear pore complexes, inner nuclear membrane, nuclear lamina and endoplasmic reticulum. Validation of our robust NCS detection method on 95 endometrial biopsies defines a 6-day window, days 19-24 (+/-1) of an idealized 28 day cycle, wherein NCSs occur. Therefore, NCSs precede and overlap with the implantation window and serve as potential markers of uterine receptivity. The immunodetection assay, combined with the hitherto underappreciated prevalence of NCSs, now enables simple screening and further molecular and functional dissection.

Morules with optically clear nuclei in ovarian borderline endometrioid tumor

Optically clear nuclei (OCN) have been observed in morules of some neoplasms and in some conditions unrelated to the development of the morules. We first report a case of ovarian borderline endometrioid tumor (BET) showing the morules associated with OCN. The patient was a 47-year-old premenopausal woman with a left ovarian cystic tumor, atypical endometrial hyperplasia, and elevated serum levels of FSH, LH, estradiol, and CA 125. The resected ovarian tumor measured 6 cm in diameter, and showed a papillary growth. Histologically, the ovarian tumor was consistent with BET, and the morules with OCN were scattered. Immunohistochemically, OCN were proven to be rich in biotin. An aberrant nuclear expression of beta-catenin was observed in both the tumor cells and the morular cells. Our case may suggest the possibility that the appearance of OCN with or without morules in ovarian tumors is related to endometrioid differentiation of the tumor cells, and should be recognized as a diagnostic clue of ovarian endometrioid tumors. Although female sex hormones have been reported to play a role in the occurrence of OCN, the participation of beta-catenin mutation has also been suggested.

Intranuclear endoplasmic reticulum induced by Nopp140 mimics the nucleolar channel system of human endometrium

Exogenous expression of the characteristic repeat domain of the nucleolar chaperone Nopp140 induces the formation of intranuclear structures, termed R-rings. Here, the R-rings are identified as extensive stacks of membrane cisternae in the otherwise membrane-free nucleus. They consist of bona fide endoplasmic reticulum (ER) containing integral membrane proteins of the smooth and rough ER. Although lacking nuclear pore complexes and lamina, the R-rings derive specifically from the inner nuclear membrane. These findings are consistent with the idea that all transmembrane proteins synthesized in the ER and the outer nuclear membrane can freely diffuse through the pore membrane domain into the inner membrane. Uniquely, the soluble transfected Nopp140 is directly involved in the generation of these membrane stacks as it localizes to the electron dense matrix in which they are embedded. The only well-documented example of intranuclear membrane proliferation is the nucleolar channel system of the postovulation human endometrium. The transient emergence of the nucleolar channel system correlates precisely with the readiness of the endometrium for the implantation of the fertilized egg. The nucleolar channel system exhibits an ultrastructure that is indistinguishable from R-rings, and nuclei of human endometrium harbor Nopp140 and ER marker containing structures. Therefore, the nucleolar channel system appears to be identical to the R-rings, suggesting a role for Nopp140 in human reproduction.

Somatic but not germline mutation of the APC gene in a case of cribriform-morular variant of papillary thyroid carcinoma

We report a case of cribriform-morular variant (C-MV) of papillary thyroid carcinoma (PTC) in a 27-year-old woman. In addition to conventional cytologic features of typical PTC, the fine-needle aspirate showed numerous epithelial cells with abundant, eosinophilic, very elongated cytoplasm. Microscopically, the tumor was encapsulated and highly cellular and exhibited a mixture of cribriform, follicular, papillary, trabecular, solid, and spindle cell patterns of growth, with morular foci showing peculiar nuclear clearing (biotin-rich nuclei). The cells were cuboidal or tall, with frequent nuclear pseudostratification and abundant eosinophilic cytoplasm. The nuclei were usually hyperchromatic, with grooving, pallor, and pseudoinclusions. Angioinvasion and foci of capsular invasion were observed. Immunohistochemically, the neoplastic cells showed reactivity for thyroglobulin, epithelial membrane antigen, low- and high-molecular-weight cytokeratins, vimentin, neuron-specific enolase, CD15, estrogen and progesterone receptors, and bcl-2 protein. Molecular genetic analysis of the APC gene revealed a mutation in exon 15 at codon 1309 in tumoral tissue but not in peripheral lymphocytes. These findings support a relationship between the morphologic pattern of the C-MV of PTC and the APC gene and the existence of this variant as a sporadic counterpart of familial adenomatous polyposis-associated thyroid carcinoma.

Formation of intranuclear crystalloids and proliferation of the smooth endoplasmic reticulum in schwann cells induced by tellurium treatment: association with overexpression of HMG CoA reductase and HMG CoA synthase mRNA

Administration of tellurium (Te) in weaning rats causes a well-established demyelinating neuropathy induced by the inhibition in myelinating Schwann cells (SC) of the synthesis of cholesterol, a major component of the myelin sheath, at the level of squalene epoxidase. We have used this experimental model of Te neuropathy to study the biogenesis and reorganization of the endomembranes of the nuclear envelope and endoplasmic reticulum (ER) in response to Te treatment by ultrastructural analysis and in situ hybridization for the detection of HMG CoA reductase and synthase mRNA, which encode key enzymes in cholesterol synthesis. The adaptive response of myelinating SC to cholesterol depletion includes cell hypertrophy, the formation of tubular invaginations of proliferating nuclear membranes giving rise to peculiar nuclear inclusions termed crystalloids, and, at the cytoplasmic level, the formation of lamellar bodies of rough ER, proliferation of the smooth ER, and overexpression of HMG CoA reductase and synthase mRNAs. The changes revert after withdrawal of Te treatment. Our results show that the biogenesis and structural organization of both endomembrane systems change dynamically upon Te-induced cholesterol depletion, indicating that this constituent plays a critical role in the organization of nuclear envelope and ER compartments in SC. The results also suggest that the HMG CoA reductase, an integral membrane protein of ER, provides the signal for the extensive membrane assembly. While the physiological meaning of crystalloid remains to be clarified, the hypertrophy of the smooth ER may represent a cytoprotective mechanism involved in detoxification of the neurotoxic agent or its metabolic derivates.

Morules with biotin-containing optically clear nuclei in colonic tubular adenoma

Morules have been reported in pulmonary endodermal tumors (PET) resembling fetal lung, in thyroid carcinoma, and in endometrial and colonic neoplasms. A morule has biotin-containing optically clear nuclei (OCN) in PET and thyroid carcinoma. Biotin-containing OCN have been also reported in endometrial tissue during pregnancy and in endometrioid carcinoma of the ovary, and it has been postulated that morules or OCN develop under the influence of female sex hormones. The authors report here the first case, to their knowledge, of morules with OCN in a colonic adenoma from a 68-year-old man. The colonic polyp consisted of ordinary tubular adenomatous tissue and morules. Many cells in the morules contained OCN. The OCN were immunopositive for biotin and reacted with streptavidin. The neoplastic cells in the morules were immunopositive for oncofetal antigens. Serum levels of female sex hormones were within the normal range, and no cells in the adenoma were immunopositive for receptors for progesterone and estrogen. The results indicate that OCN are rich in biotin and that morules may be embryologically immature elements that develop independently of influence by female sex hormones.

The nuclear channel system in endometrial adenocarcinoma exposed to medroxyprogesterone acetate

The nuclear channel system (NCS), giant mitochondria and subnuclear glycogen form a triad of ultrastructural features observed in normal human endometrial epithelium in response to progestational steroids. Both the giant mitochondria and subnuclear glycogen have been described in endometrial adenocarcinoma, but the NCS has not. This article reports the development of the NCS in adenocarcinoma treated with medroxyprogesterone acetate. Previous studies suggest that the NCS in normal tissue is a response to the acyl group in the 17-beta position of the D-ring of some progestational steroids, such as medroxyprogesterone acetate. Medroxyprogesterone acetate was administered to 12 postmenopausal women with endometrial adenocarcinoma. Hysterectomies were performed 8 to 20 days after treatment. Pretreatment specimens were also obtained on 8 of the 12 patients. Using standard electron microscopy procedures, light microscopy on plastic semithin sections was first used to confirm the presence of tumor. Thin sections of malignant endometrium were prepared and evaluated ultrastructurally for progestational alterations. Abnormal giant mitochondria and subnuclear glycogen were found both before and after treatment. The third element of the triad, the NCS, was not observed in any of the available pretreatment biopsies, but was seen in three of the treated specimens. Thus it appears that the NCS is a response to the given progesterone therapy.

Pulmonary blastoma: an ultrastructural and immunohistochemical study with special reference to nuclear filament aggregation

A case is presented of pulmonary blastoma occurring in the right upper lobe of a 25-year-old man without distinct clinical features and laboratory abnormality. Light microscopic analysis revealed that the tumor was composed of branching glands and morulae embedded in a primitive but bland mesenchyme. Immunohistochemically the epithelial cells were immunoreactive for cytokeratins, S-100 protein, protein gene product 9.5, chromogranin A, calcitonin, and Ki-67 (MIB-1); the mesenchymal cells were immunoreactive for vimentin, actin, cytokeratins, and Ki-67; and all the tumor cells were negative for p53, estrogen receptor protein, and human chorionic gonadotropin beta. Characteristically, many epithelial cells contained optically clear nuclei which were immunoreactive for biotin (M743). Electron microscopic analysis revealed that the optically clearing change was due to replacement of the central area of the nuclei by a mass of parallel-arranged 7- to 10-nm filaments, and biotin-immunoreactive products were mainly localized in the nuclear matrix. Additionally, spherical bodies were identified in the cytoplasm of the nuclear filament-aggregated cells, suggestive of an intimate pathogenetic association of the two morphological abnormalities. The similarity of the aggregated nuclear filaments to those observed in gestational endometrium and ovarian endometrioid carcinoma implies that a similar mechanism plays a role in the pathogenesis of these abnormalities.

Morule with biotin-containing intranuclear inclusions in thyroid carcinoma

One thousand and sixty cases of thyroid carcinoma were reviewed to compare morules with squamous metaplasia clinicopathologically and immunohistochemically. Morules and squamous metaplasia were found in five (0.47%) and 32 cases (3.0%) respectively. The five patients with morules were all female (age 20-36 years) including four with papillary carcinoma and one with follicular carcinoma. The 32 patients with squamous metaplasia consisted of 30 females and 2 males (age 14-78 years), all of whom had papillary carcinoma except for one follicular carcinoma. The morules demonstrated characteristic 'optically clear nuclei' (OCN), which ultrastructurally showed filamentous structures in the nuclei. The OCN were immunohistochemically demonstrated to contain intranuclear biotin. Furthermore, the morule often accompanied with the OCN was positive for Ulex Europaeus agglutinin l (UEA-l) but negative for bovine muzzle epidermal keratin (EK). On the contrary, squamous metaplasia unaccompanied with the OCN was negative for UEA-l, but positive for EK. Follow-up information revealed that one of the five patients with morules had died of the disease, one was alive with pulmonary metastasis, and three were disease-free. Eight of 32 patients with squamous metaplasia had died of the disease; of the others who were alive, four patients have had recurrence.

Peculiar nuclear clearing composed of microfilaments in papillary carcinoma of the thyroid

BACKGROUND

Papillary carcinoma of the thyroid frequently occurs in young women. Four cases of papillary carcinoma were found in women in their 20s, and the disease was characterized by unusual morphologic and clinical features.

METHODS

Four cases were analyzed clinically, grossly, microscopically, ultrastructurally, and immunohistochemically and compared with control cases of papillary carcinoma.

RESULTS

The tumors histologically showed various appearances with trabecular, solid cribriform, follicular, and papillary patterns. A striking finding was that stratified cancer cell nests with prominent nuclear clearings were scattered in the tumors. Ultrastructurally, the nuclear clearings were composed of fine thread-like fibrils, and they were bound unselectively to various antibodies against hormones and intermediate filaments. The four patients also had common clinicopathologic features as follows: they were all women in their 20s and their preoperative serum thyroglobulin levels were all normal despite the large size of their thyroid tumors. The tumors were grossly well encapsulated, and there was no lymph node metastasis, except in one patient who had only one positive node.

CONCLUSIONS

The peculiar nuclear features were distinct from intranuclear cytoplasmic inclusions or ground-glass nuclei, and these have not been reported previously to the authors' knowledge in conventional thyroid neoplasms. These tumors might represent another subtype of papillary thyroid carcinoma.

Pulmonary endodermal tumor resembling fetal lung: a clinicopathologic study of five cases with immunohistochemical and ultrastructural characterization

Pulmonary endodermal tumor resembling fetal lung describes an uncommon neoplasm of the lung which also has been referred to as pulmonary blastoma and adenocarcinoma of fetal lung type. We describe five cases which fall within a narrow band on the spectrum of pulmonary neoplasms with both epithelial and mesenchymal features. These five cases all occurred as well-defined masses visible on chest radiograph in middle-aged females, and were treated by surgical excision. Histopathologically, low and high grades of malignancy are found. Well-formed racimose glands with cytoplasmic vacuolization resemble endometrioid carcinoma. Neoplastic columnar cells have abundant glycogen in the cytoplasm. Morules of cells within the glands have optically clear nuclei. Ultrastructurally, the optically clear nuclei are occupied by a filamentous substructure of chromatin. Multiple neuroendocrine hormones are present in low-grade malignancy. Nuclear pleomorphism, lymphatic invasion, multifocal necrosis, lack of mesenchyme at the pulmonary interface, and restricted neuroendocrine expression suggest high-grade malignancy. A mesenchymal stroma surrounding the glands is an intrinsic part of the neoplasm, but the stroma does not appear malignant, and did not form part of the metastasis in the single case where a metastasis occurred. Stromal cells show fibroblastic and myofibroblastic differentiation. Pulmonary endodermal tumor resembling fetal lung typically is a low-grade malignancy, with a better prognosis than the majority of lesions sometimes described as pulmonary blastoma or adenocarcinoma of fetal lung type.

Mechanism of action of gestagens

In the mid-1950s it was shown that ovulation can be inhibited by oral administration of progesterone or of synthetic progestogens such as norethynodrel. The idea that progestogens may interfere with fertility even when administered in doses so small, that they do not invariably inhibit ovulation was launched in the mid-1960s by a group of Mexican investigators. Since then a large number of studies have been conducted on the mechanism of action of gestagens when used as contraceptives. The objective of this communication is to review the information collected so far on this topic.

Nucleolar canalicular structure in extrauterine endometriosis

Under the influence of progesterone the unique intranuclear structure known as the nucleolar canalicular structure (NCS) develops in the human endometrium. This organelle was not described previously outside the uterus. In this report the cases of two patients in whom the NCS appeared in extrauterine endometriosis are presented.

The Depo-Provera debate. Commentary on the article "Depo-Provera, a critical analysis"

A widely publicized article has in recent months caused a great deal of concern among individuals interested in responsible promotion of family planning. The article contains a long series of factual errors, distortions and biased quotations. This commentary presents evidence, based on current knowledge, that Depo-Provera is a satisfactory contraceptive with several advantages and some disadvantages, and poses no more unresolved problems than oral contraceptives. There is no evidence that, at contraceptive doses, it increases the risk of cancer, impairs bone mineralization, "shocks" the hypothalamus, damages the liver or the immune system, or causes premature aging. Studies to date have not shown damaging effects on infants exposed to the drug in utero or via breast milk. To most women, disruption of the menstrual cycle, the major side effect, is not a health hazard. Finally, women in various parts of the world have shown to be quite capable of choosing for themselves whether or not the advantages of the drug can overcome the disadvantage of almost certain menstrual disturbance.

Ultrastructural changes in endometriotic tissue during danazol treatment

Electron microscopic examinations were carried out on 29 representative endometriotic glands obtained from 16 biopsies. Patients with endometriosis diagnosed by laparoscopy and confirmed by laparoscopic biopsy prior to hormonal treatment received danazol, 800 mg daily. The tissue samples were obtained after 2, 4, and 6 months of usage of the drug to study the ultrastructural effect of this treatment. The glandular epithelium appeared to be arrested in what looked like the late proliferative stage of the normal menstrual cycle. The ciliated cells remained unchanged for the most part. The findings were correlated with possible effects of danazol on the target tissue, but caution is required in the interpretation of ultrastructural effects of steroids until detailed knowledge of the metabolic pathways is available.

[Ultrastructural investigations on the nuclear channel system in the human endometrium (author's transl)]

The Nuclear Channel System (NCS) is a distinct endometrial pattern as observed by transmission electron microscopy. The NCS was identified in the present study in cell nuclei of endometrial glands during mid and late proliferation. These observations do not support the concept that the NCS develops at the ovulation, and is therefore not a typical post ovulatory structure. It is suggested that there are relations between hormonal factors and the development of the NCS which are as yet not clarified.