The morphogenesis and fate of the nucleolar channel system in the human endometrial glandular cell

A Transient Mystery: Nucleolar Channel Systems

The nucleus is a complex organelle with functions beyond being a simple repository for genomic material. For example, its actions in biomechanical sensing, protein synthesis, and epigenomic regulation showcase how the nucleus integrates multiple signaling modalities to intricately regulate gene expression. This innate dynamism is underscored by subnuclear components that facilitate these roles, with elements of the nucleoskeleton, phase-separated nuclear bodies, and chromatin safeguarding by nuclear envelope proteins providing examples of this functional diversity. Among these, one of the lesser characterized nuclear organelles is the nucleolar channel system (NCS), first reported several decades ago in human endometrial biopsies. This tubular structure, believed to be derived from the inner nuclear membrane of the nuclear envelope, was first observed in secretory endometrial cells during a specific phase of the menstrual cycle. Reported as a consistent, yet transient, nuclear organelle, current interpretations of existing data suggest that it serves as a marker of a window for optimal implantation. In spite of this available data, the NCS remains incompletely characterized structurally and functionally, due in part to its transient spatial and temporal expression. As a further complication, evidence exists showing NCS expression in fetal tissue, suggesting that it may not act exclusively as a marker of uterine receptivity, but rather as a hormone sensor sensitive to estrogen and progesterone ratios. To gain a better understanding of the NCS, current technologies can be applied to profile rare cell populations or capture transient structural dynamics, for example, at a level of sensitivity and resolution not previously possible. Moving forward, advanced characterization of the NCS will shed light on an uncharacterized aspect of reproductive physiology, with the potential to refine assisted reproductive techniques.

Transcriptional changes through menstrual cycle reveal a global transcriptional derepression underlying the molecular mechanism involved in the window of implantation

The human endometrium is a dynamic tissue that only is receptive to host the embryo during a brief time in the middle secretory phase, called the window of implantation (WOI). Despite its importance, regulation of the menstrual cycle remains incompletely understood. The aim of this study was to characterize the gene cooperation and regulation of menstrual cycle progression, to dissect the molecular complexity underlying acquisition of endometrial receptivity for a successful pregnancy, and to provide the scientific community with detailed gene co-expression information throughout the menstrual cycle on a user-friendly web-tool database. A retrospective gene co-expression analysis was performed based on the endometrial receptivity array (ERarray) gene signature from 523 human endometrial samples collected across the menstrual cycle, including during the WOI. Gene co-expression analysis revealed the WOI as having the significantly smallest proportion of negative correlations for transcriptional profiles associated with successful pregnancies compared to other cycle stages, pointing to a global transcriptional derepression being involved in acquisition of endometrial receptivity. Regulation was greatest during the transition between proliferative and secretory endometrial phases. Further, we prioritized nuclear hormone receptors as major regulators of this derepression and proved that some genes and transcription factors involved in this process were dysregulated in patients with recurrent implantation failure. We also compiled the wealth of gene co-expression data to stimulate hypothesis-driven single-molecule endometrial studies in a user-friendly database: Menstrual Cycle Gene Co-expression Network (www.menstrualcyclegcn.com). This study revealed a global transcriptional repression across the menstrual cycle, which relaxes when the WOI opens for transcriptional profiles associated with successful pregnancies. These findings suggest that a global transcriptional derepression is needed for embryo implantation and early development.

Modifications of nuclear envelope during differentiation and depolyploidization of rat trophoblast cells

An increased activity of membranes of the nuclear envelope (NE) was observed electron microscopically in the trophoblast cells of the rat placenta. The activity of the membranes was manifested as formation of various NE derivatives, such as the annulate lamellae (AL), the intranuclear tubules, and the concentric membranous structures. At the period of terminal differentiation of the secondary giant trophoblast cells (SGTC) the NE derivatives play active role in subdivision of the initial highly polyploid nuclei into the numerous low-ploidy fragments. (3)H-thymidine labeling showed that attenuation of the DNA replication precedes the nuclear fragmentation. In the course of the nuclear fragmentation the narrow deep NE invaginations subdivide the nucleus into the separate lobes that subsequently are detached from the initial nucleus. By the beginning of the fragmentation, the accumulated membranous structures, i.e. the intranuclear AL, tubules, clusters of pore complexes, etc., seem to be the source of the reserve material that is necessary for formation of the great amount of the NE membranes of the newly formed nuclear fragments. Thus, the intranuclear membranous structures that seem to increase the active surface of the growing endopolyploid nucleus at the earlier stage of differentiation then take part in genome isolation that results in formation of a multinucleate cell with diploid and low-polyploid nuclei. The outer NE membrane of the initial nucleus plays an active role in compartmentalization of cytoplasmic areas around the nuclear fragments within the giant polykaryocyte. Apart from the membranous structures the bundles of intermediate filaments (IF) located in the cytoplasm perinuclear zones seem to participate in the nuclear fragmentation. These processes are likely to provide formation of the giant polykaryocytes incapable for further proliferation.

Morphology of the human endometrium in the peri-implantation period

Although the histology and ultrastructure of the human endometrium are well documented, it is clear that morphometry can reveal subtle changes in cellular biology, especially when combined with suitable sampling and dating of the specimen. These changes reflect functional events, such as protein secretion, which may be studied using the endometrial flushing technique. Although it is likely that further insights into the basic cell physiology of the reproductive tract will result from the use of appropriate in vitro models, the role of endometrial biopsy for clinical monitoring remains essential.

Use of mild periodic acid-thiosemicarbazide-silver proteinate staining for ultrahistochemical demonstration of glycoconjugates in the human uterine epithelium

The human uterine epithelium has been studied by means of mild (0.1, 0.01% respectively) periodic acid (PA) oxidation followed by thiosemicarbazide-silver proteinate staining. Distribution of silver deposits provided evidence for glycoconjugates and was in the case of oxidation with 0.01% PA selective for sialic acid (N-acetylneuraminic acid) complexes. The electron dense silver particles were detectable at the outer side of apical plasma membranes and on the inner surface of lysosomal membranes. Also the marginal zone of lipid droplets reacted regularly. Since the lower concentration of PA proves sialogycoconjugates selectively, the positive staining depicted not only those in the membrane compartments but also as a layer between the hydrophobic content and the hydrophilic cytoplasm. In the granular endoplasmic reticulum of the individual microvillous cells a special glycoprotein of paracrystalline organisation was detected. The functional significance of this substance remains unclear. Nuclear channel systems (NCS) are frequently observed in the epithelial cells in the uterine glands. Similar staining properties of the NCS and nuclear envelope support the hypothesis of its nuclear origin.

An ultrastructural study of the Sertoli cell in the water buffalo (Bubalus bubalis)

The ultrastructure of Sertoli cell in the water buffalo (Bubalus bubalis) was observed in a transmission electron microscope. The nucleus had homogeneous nucleoplasm, scarce heterochromatin and multivesicular nuclear body (MNB). The MNB was composed of numerous vesicles and ribosome-like dense structures. The vesicles varied in size and number and contained a sparse and flocculent substance. In the indentation of the nucleus, aggregates of ribosomes were frequently observed. In the apical and middle region of the cell, long mitochondria and microtubules were distributed parallel to the long axis of the cell. Non-laminated smooth ER and some ribosomes were also recognizable throughout this region. In the basal region, widely-distributed laminated smooth ER was characteristic. Microfilament bundles at ectoplasmic specialization were irregularly arranged. Frequently-emerged nodular processes occasionally separated from basal lamina and formed round structures within Sertoli cytoplasm. Although these characteristics of buffalo Sertoli cell were very similar to those of the bovine studied, the aggregate of ribosomes was more developed in the buffalo.

The effects of steroids on the fine structure of the endometrium

This brief review illustrates the lack of ultrastructural studies on human endometrium, particularly on well-dated material from normal, fertile women. The glandular epithelium, with its triad of unusual organelles in the early luteal phase, poses fascinating problems in cell biology and, probably for this reason, has attracted the most work. Many problems in reproductive biology, in uterine pathology and in the study of unexplained infertility are crying out for detailed study of the luminal epithelium, the stroma and the blood vessels. If this review, by highlighting the gaps in our knowledge, stimulates research into these areas, it will have been successful.

Localization of nucleoside phosphatases (ATPase and 5'-nucleotidase) and nuclear ribonucleoproteins in the human endometric glandular cells during the secretory phase

Endometrial curettings from 3 healthy women between 16th and 19th d of the menstrual cycle were investigated employing ultracytochemical methods. In endometrial glandular cells ATPase and 5'-nucleotidase activities were demonstrated in the nuclei, nucleoli, and nuclear channel system (NCS), i.e., mainly in structures characterized ++as ribonucleoproteins according to the Bernhard method. The functional relationship between the NCS and the molecular biology of the endometrial glandular cell is discussed.

Ultrastructural comparison of endometriotic implants and eutopic endometrium

Ultrastructural features of endometriotic implants suggest an incomplete response to the prevalent hormonal milieu. The cyclic changes, especially in the secretory phase, appear to depend more on the morphologic differentiation of the ectopic implants than on the hormonal stimulus. The endometriotic tissue encompassed a wide range of morphologic development from poorly to highly differentiated glands. Variations occurred from gland to gland and even within the same gland. Complete proliferative development was found only in some of the patients and full secretory transformation was absent in all. The incomplete morphologic response to cyclic hormonal changes may explain the frequent failure of endocrine therapy.

[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.

Action of midcycle contraceptive (R 2323) on the human endometrium

Over 2,148 cycles of midcycle oral administration of R 2323 (50 mg. per day on Days 15,16, and 17), the authors recorded a drug-failure pregnancy rate of 5 per cent and an unusually regular cycle length of 28 +/- 2 days. During this trial, endometrial biopsies obtained in the luteal phase were examined by light and electron microscopy and compared to pretreatment biopsies. Light microscopy indicated a weakly secretory endometrium suggestive of some, albeit low, progesterone impregnation. Ultrastructural examination revealed deleterious changes in the development of the nucleolar channel system and giant mitochondria and a delay in the migration of glycogen granules. This low progesterone impregnation could be explained either by a direct effect of R 2323 on cell ultrastructure or by interference with progesterone availability. It would appear that R 2323 acts as a temporary substitute for progesterone at the receptor level but that it does not induce all the biological manifestations of this hormone, in particular, the endometrial changes required for implantation.