Human fetal endometrium--light and electron microscopic study

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.

Regenerative Medicine Approaches in Bioengineering Female Reproductive Tissues

Diseases, disorders, and dysfunctions of the female reproductive tract tissues can result in either infertility and/or hormonal imbalance. Current treatment options are limited and often do not result in tissue function restoration, requiring alternative therapeutic approaches. Regenerative medicine offers potential new therapies through the bioengineering of female reproductive tissues. This review focuses on some of the current technologies that could address the restoration of functional female reproductive tissues, including the use of stem cells, biomaterial scaffolds, bio-printing, and bio-fabrication of tissues or organoids. The use of these approaches could also be used to address issues in infertility. Strategies such as cell-based hormone replacement therapy could provide a more natural means of restoring normal ovarian physiology. Engineering of reproductive tissues and organs could serve as a powerful tool for correcting developmental anomalies. Organ-on-a-chip technologies could be used to perform drug screening for personalized medicine approaches and scientific investigations of the complex physiological interactions between the female reproductive tissues and other organ systems. While some of these technologies have already been developed, others have not been translated for clinical application. The continuous evolution of biomaterials and techniques, advances in bioprinting, along with emerging ideas for new approaches, shows a promising future for treating female reproductive tract-related disorders and dysfunctions.

The development of the human uterus: morphogenesis to menarche

There is emerging evidence that early uterine development in humans is an important determinant of conditions such as ontogenetic progesterone resistance, menstrual preconditioning, defective deep placentation and pre-eclampsia in young adolescents. A key observation is the relative infrequency of neonatal uterine bleeding and hormone withdrawal at birth. The origin of the uterus from the fusion of the two paramesonephric, or Müllerian, ducts was described almost 200 years ago. The uterus forms around the 10th week of foetal life. The uterine corpus and the cervix react differently to the circulating steroid hormones during pregnancy. Adult uterine proportions are not attained until after puberty. It is unclear if the endometrial microbiome and immune response-which are areas of growing interest in the adult-play a role in the early stages of uterine development. The aim is to review the phases of uterine development up until the onset of puberty in order to trace the origin of abnormal development and to assess current knowledge for features that may be linked to conditions encountered later in life. The narrative review incorporates literature searches of Medline, PubMed and Scopus using the broad terms individually and then in combination: uterus, development, anatomy, microscopy, embryology, foetus, (pre)-puberty, menarche, microbiome and immune cells. Identified articles were assessed manually for relevance, any linked articles and historical textbooks. We included some animal studies of molecular mechanisms. There are competing theories about the contributions of the Müllerian and Wolffian ducts to the developing uterus. Endometrium features are suggestive of an oestrogen effect at 16-20 weeks gestation. The discrepancy in the reported expression of oestrogen receptor is likely to be related to the higher sensitivity of more recent techniques. Primitive endometrial glands appear around 20 weeks. Features of progestogen action are expressed late in the third trimester. Interestingly, progesterone receptor expression is higher at mid-gestation than at birth when features of endometrial maturation are rare. Neonatal uterine bleeding occurs in around 5% of neonates. Myometrial differentiation progresses from the mesenchyme surrounding the endometrium at the level of the cervix. During infancy, the uterus and endometrium remain inactive. The beginning of uterine growth precedes the onset of puberty and continues for several years after menarche. Uterine anomalies may result from fusion defects or atresia of one or both Müllerian ducts. Organogenetic differentiation of Müllerian epithelium to form the endometrial and endocervical epithelium may be independent of circulating steroids. A number of genes have been identified that are involved in endometrial and myometrial differentiation although gene mutations have not been demonstrated to be common in cases of uterine malformation. The role, if any, of the microbiome in relation to uterine development remains speculative. Modern molecular techniques applied to rodent models have enhanced our understanding of uterine molecular mechanisms and their interactions. However, little is known about functional correlates or features with relevance to adult onset of uterine disease in humans. Prepubertal growth and development lends itself to non-invasive diagnostics such as ultrasound and MRI. Increased awareness of the occurrence of neonatal uterine bleeding and of the potential impact on adult onset disease may stimulate renewed research in this area.

Endometrial Stem Cell Markers: Current Concepts and Unresolved Questions

The human endometrium is a highly regenerative organ undergoing over 400 cycles of shedding and regeneration over a woman’s lifetime. Menstrual shedding and the subsequent repair of the functional layer of the endometrium is a process unique to humans and higher-order primates. This massive regenerative capacity is thought to have a stem cell basis, with human endometrial stromal stem cells having already been extensively studied. Studies on endometrial epithelial stem cells are sparse, and the current belief is that the endometrial epithelial stem cells reside in the terminal ends of the basalis glands at the endometrial/myometrial interface. Since almost all endometrial pathologies are thought to originate from aberrations in stem cells that regularly regenerate the functionalis layer, expansion of our current understanding of stem cells is necessary in order for curative treatment strategies to be developed. This review critically appraises the postulated markers in order to identify endometrial stem cells. It also examines the current evidence supporting the existence of epithelial stem cells in the human endometrium that are likely to be involved both in glandular regeneration and in the pathogenesis of endometrial proliferative diseases such as endometriosis and endometrial cancer.

Generation of human female reproductive tract epithelium from human embryonic stem cells

Background

Recent studies have identified stem/progenitor cells in human and mouse uterine epithelium, which are postulated to be responsible for tissue regeneration and proliferative disorders of human endometrium. These progenitor cells are thought to be derived from Müllerian duct (MD), the primordial female reproductive tract (FRT).

Methodology/Principal Findings

We have developed a model of human reproductive tract development in which inductive neonatal mouse uterine mesenchyme (nMUM) is recombined with green fluorescent protein (GFP)-tagged human embryonic stem cells (hESCs); GFP-hESC (ENVY). We demonstrate for the first time that hESCs can be differentiated into cells with a human FRT epithelial cell phenotype. hESC derived FRT epithelial cells emerged from cultures containing MIXL1⁺ mesendodermal precursors, paralleling events occurring during normal organogenesis. Following transplantation, nMUM treated embryoid bodies (EBs) generated epithelial structures with a typical MD phenotype that expressed the MD markers PAX2, HOXA10. Functionally, the hESCs derived FRT epithelium responded to exogenous estrogen by proliferating and secreting uterine-specific glycodelin A (GdA).

Conclusions/Significance

These data show nMUM can induce differentiation of hESC to form the FRT epithelium. This may provide a model to study early developmental events of the human FRT.

Ultrastructural dynamics of the human endometrium from 14 to 22 weeks of gestation

In order to elucidate the ultrastructural dynamics of endometrium differentiation, uterine samples of fetuses aged 14 to 22 weeks of gestation (WG) were analyzed. Samples were processed for light (LM), transmission (TEM) and field-emission scanning electron microscopy (FE-SEM). Initial stratification of the uterine wall occurred at 14 WG: endometrial, myometrial, and perimetrial primordia were identified. At this age, the endometrial epithelium was simple columnar to pseudostratified and consisted of microvillous cells. Blood capillaries developed mainly in the stroma and between the myometrium and perimetrium primordia. At 18-20 WG the endometrial epithelium became clearly pseudostratified, with active ciliogenesis and a predominance of microvillous cells. Primordia of tubular glands were present at 20 WG. Microvillous cells still predominated in the endometrial epithelium at 21-22 WG and showed morphological features of apoptosis. The endometrial stroma at this stage was organizing into a thick lamina propria provided with subepithelial capillary plexuses. However, the stroma was formed by still undifferentiated mesenchymal cells during the whole period of study. Our data showed that the epithelial differentiation and distribution in the uterus occur in the human fetus in a similar way as in the adult. The above events are likely the expression of an early developmental patterning and related to future reproductive processes, such as the regulation of gamete passage and blastocyst implantation. Because the structure of the adult uterus is determined by the degree of paramesonephric duct fusion, septum absorption, and differentiation of the uterine primordial layers, our study may contribute toward clarifying to normal urogenital development.

Small cell carcinoma of the uterine corpus: CT appearance

We report a rare case of small-cell carcinoma of the uterine corpus. To our knowledge, this is the first radiological report. Computed tomography (CT) demonstrates a hypodense lesion within the endometrial cavity with nonhomogeneous contrast enhancement.

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.

Carcinoid tumour of the uterine corpus

A carcinoid tumour occurred in the uterine fundus causing an enlargement (12-14 weeks size). The patient did not have the carcinoid syndrome, despite elevated urinary 5-hydroxyindole acetic acid levels. The tumour had a typical organoid pattern with argentaffin-positive cells. Whilst a metastasis from an occult primary lesion cannot be entirely excluded, it is felt that this is a primary tumour most likely arising from resident endocrine cells in the endometrium.