Whole-mount sections displaying microvascular and glandular structures in human uterus using multiphoton excitation microscopy

Novel microarchitecture of human endometrial glands: implications in endometrial regeneration and pathologies

BACKGROUND

Human endometrium remains a poorly understood tissue of the female reproductive tract. The superficial endometrial functionalis, the site of embryo implantation, is repeatedly shed with menstruation, and the stem cell-rich deeper basalis is postulated to be responsible for the regeneration of the functionalis. Two recent manuscripts have demonstrated the 3D architecture of endometrial glands. These manuscripts have challenged and replaced the prevailing concept that these glands end in blind pouches in the basalis layer that contain stem cells in crypts, as in the intestinal mucosa, providing a new paradigm for endometrial glandular anatomy. This necessitates re-evaluation of the available evidence on human endometrial regeneration in both health and disease in the context of this previously unknown endometrial glandular arrangement.

OBJECTIVE AND RATIONALE

The aim of this review is to determine if the recently discovered glandular arrangement provides plausible explanations for previously unanswered questions related to human endometrial biology. Specifically, it will focus on re-appraising the theories related to endometrial regeneration, location of stem/progenitor cells and endometrial pathologies in the context of this recently unravelled endometrial glandular organization.

SEARCH METHODS

An extensive literature search was conducted from inception to April 2021 using multiple databases, including PubMed/Web of Science/EMBASE/Scopus, to select studies using keywords applied to endometrial glandular anatomy and regeneration, and the references included in selected publications were also screened. All relevant publications were included.

OUTCOMES

The human endometrial glands have a unique and complex architecture; branched basalis glands proceed in a horizontal course adjacent to the myometrium, as opposed to the non-branching, vertically coiled functionalis glands, which run parallel to each other as is observed in intestinal crypts. This complex network of mycelium-like, interconnected basalis glands is demonstrated to contain endometrial epithelial stem cells giving rise to single, non-branching functionalis glands. Several previous studies that have tried to confirm the existence of epithelial stem cells have used methodologies that prevent sampling of the stem cell-rich basalis. More recent findings have provided insight into the efficient regeneration of the human endometrium, which is preferentially evolved in humans and menstruating upper-order primates.

WIDER IMPLICATIONS

The unique physiological organization of the human endometrial glandular element, its relevance to stem cell activity and scarless endometrial regeneration will inform reproductive biologists and clinicians to direct their future research to determine disease-specific alterations in glandular anatomy in a variety of endometrial pathological conditions.

The New Era of Three-Dimensional Histoarchitecture of the Human Endometrium

The histology of the endometrium has traditionally been established by observation of two-dimensional (2D) pathological sections. However, because human endometrial glands exhibit coiling and branching morphology, it is extremely difficult to obtain an entire image of the glands by 2D observation. In recent years, the development of three-dimensional (3D) reconstruction of serial pathological sections by computer and whole-mount imaging technology using tissue clearing methods with high-resolution fluorescence microscopy has enabled us to observe the 3D histoarchitecture of tissues. As a result, 3D imaging has revealed that human endometrial glands form a plexus network in the basalis, similar to the rhizome of grass, whereas mouse uterine glands are single branched tubular glands. This review summarizes the relevant literature on the 3D structure of mouse and human endometrium and discusses the significance of the rhizome structure in the human endometrium and the expected role of understanding the 3D tissue structure in future applications to systems biology.

Three-dimensional understanding of the morphological complexity of the human uterine endometrium

The fundamental morphology of the endometrial glands is not sufficiently understood by 2D observation because these glands have complicated winding and branching patterns. To construct a large picture of the endometrial gland structure, we performed tissue-clearing-based 3D imaging of human uterine endometrial tissue. Our 3D immunohistochemistry and layer analyses revealed that the endometrial glands form a plexus network in the stratum basalis and expand horizontally along the muscular layer, similar to the rhizome of grass. We then extended our method to assess the 3D morphology of tissue affected by adenomyosis, a representative "endometrium-related disease," and observed its 3D morphological features, including the direct invasion of endometrial glands into the myometrium and an ant colony-like network of ectopic endometrial glands within the myometrium. Thus, further understanding of the morphology of the human endometrium based on 3D analysis will lead to the identification of the pathogenesis of endometrium-related diseases.

Pilot Study: Quantitative Photoacoustic Evaluation of Peripheral Vascular Dynamics Induced by Carfilzomib In Vivo

Carfilzomib is mainly used to treat multiple myeloma. Several side effects have been reported in patients treated with carfilzomib, especially those associated with cardiovascular events, such as hypertension, congestive heart failure, and coronary artery disease. However, the side effects, especially the manifestation of cardiovascular events through capillaries, have not been fully investigated. Here, we performed a pilot experiment to monitor peripheral vascular dynamics in a mouse ear under the effects of carfilzomib using a quantitative photoacoustic vascular evaluation method. Before and after injecting the carfilzomib, bortezomib, and PBS solutions, we acquired high-resolution three-dimensional PAM data of the peripheral vasculature of the mouse ear during each experiment for 10 h. Then, the PAM maximum amplitude projection (MAP) images and five quantitative vascular parameters, i.e., photoacoustic (PA) signal, diameter, density, length fraction, and fractal dimension, were estimated. Quantitative results showed that carfilzomib induces a strong effect on the peripheral vascular system through a significant increase in all vascular parameters up to 50%, especially during the first 30 min after injection. Meanwhile, bortezomib and PBS do not have much impact on the peripheral vascular system. This pilot study verified PAM as a comprehensive method to investigate peripheral vasculature, along with the effects of carfilzomib. Therefore, we expect that PAM may be useful to predict cardiovascular events caused by carfilzomib.

Biological applications of second harmonic imaging

Second Harmonic Generation (SHG) microscopy dates back to 1974, but effective biological use of the technique has a history of barely 10 years. It is now widely used to image collagen in many different applications, and is becoming useful for imaging myosin and some polysaccharides. A separate line on research has focussed on SHG dyes, which can provide high-speed indication of membrane potential and are now in use in neurobiology. This review looks at the progress to date in these different fields.

Two-photon microscopy of vital murine elastic and muscular arteries. Combined structural and functional imaging with subcellular resolution

Understanding vascular pathologies requires insight in the structure and function, and, hence, an imaging technique combining subcellular resolution, large penetration depth, and optical sectioning. We evaluated the applicability of two-photon laser-scanning microscopy (TPLSM) in large elastic and small muscular arteries under physiological conditions. Elastic (carotid) and muscular (uterine, mesenteric) arteries of C57BL/6 mice were mounted in a perfusion chamber. TPLSM was used to assess the viability of arteries and to visualize the structural components elastin, collagen, nuclei, and endothelial glycocalyx (EG). Functionality was determined using diameter changes in response to noradrenaline and acetylcholine. Viability and functionality were maintained up to 4 h, enabling the assessment of structure-function relationships. Structural vessel wall components differed between elastic and muscular arteries: size (1.3 vs. 2.1 microm) and density (0.045 vs. 0.57 microm(-2)) of internal elastic lamina fenestrae, smooth muscle cell density (3.50 vs. 1.53 microm(-3)), number of elastic laminae (3 vs. 2), and adventitial collagen structure (tortuous vs. straight). EG in elastic arteries was 4.5 microm thick, covering 66% of the endothelial surface. TPLSM enables visualization and quantification of subcellular structures in vital and functional elastic and muscular murine arteries, allowing unraveling of structure-function relationships in healthy and diseased arteries.

Systems biology and clinical cytomics: The 10th Leipziger Workshop and the 3rd International Workshop on Slide-Based Cytometry, Leipzig, Germany, April 2005

Despite very significant technical and software improvements in flow cytometry (FCM) since the 1980's, the demand for a cytometric technology combining both quantitative cell analysis and morphological documentation in Cytomics became evident. Improvements in microtechnology and computing permit nowadays similar quantitative and stoichiometric single cell-based high-throughput analyses by microscopic instruments, like Slide-Based Cytometry (SBC). SBC and related techniques offer unique tools to perform complex immunophenotyping, thereby enabling diagnostic procedures during early disease stages. Multicolor or polychromatic analysis of cells by SBC is of special importance not only as a cytomics technology platform but also because of low quantities of required reagents and biological material. The exact knowledge of the location of each cell on the slide permits repetitive restaining and reanalysis of specimens. Various separate measurements of the same specimen can be ultimately fused to one database increasing the information obtained per cell. Relocation and optical evaluation of cells as typical SBC feature, can be of integral importance for cytometric analysis, since artifacts can be excluded and morphology of measured cells can be documented. Progress in cell analytic: In the SBC, new horizons can be opened by the new techniques of structural and functional analysis with the high resolution from intracellular and membrane (confocal microscopy, nanoscopy, total internal fluorescence microscopy (TIRFM), and tissue level (tissomics), to organ and organism level (in vivo cytometry, optical whole body imaging). Predictive medicine aims at the detection of changes in patient's state prior to the manifestation of the disease or the complication. Such instances concern immune consequences of surgeries or noninfectious posttraumatic shock in intensive care patients or the pretherapeutic identification of high risk patients in cancer cytostatic therapy. Preventive anti-infectious or anti-shock therapy as well as curative chemotherapy in combination with stem cell transplantation may provide better survival chances for patient at concomitant cost containment. Predictive medicine-guided optimization of therapy could lead to individualized medicine that gives significant therapeutic effect and may lower or abrogate potential therapeutic side effects. The 10th Leipziger Workshop combined with the 3rd International Workshop on SBC aimed to offer new methods in Image- and Slide-Based Cytometry for solutions in clinical research. It moved towards practical applications in the clinics and the clinical laboratory. This development will be continued in 2006 at the upcoming Leipziger Workshop and the International Workshop on Slide-Based Cytometry.

Recent advances in endometrial angiogenesis research

This review summarises recent research into the mechanisms and regulation of endometrial angiogenesis. Understanding of when and by what mechanisms angiogenesis occurs during the menstrual cycle is limited, as is knowledge of how it is regulated. Significant endometrial endothelial cell proliferation occurs at all stages of the menstrual cycle in humans, unlike most animal models where a more precise spatial relationship exists between endothelial cell proliferation and circulating levels of oestrogen and progesterone. Recent stereological data has identified vessel elongation as a major endometrial angiogenic mechanism in the mid-late proliferative phase of the cycle. In contrast, the mechanisms that contribute to post-menstrual repair and secretory phase remodelling have not yet been determined. Both oestrogen and progesterone/progestins appear to have paradoxical actions, with recent studies showing that under different circumstances both can promote as well as inhibit endometrial angiogenesis. The relative contribution of direct versus indirect effects of these hormones on the vasculature may help to explain their pro- or anti-angiogenic activities. Recent work has also identified the hormone relaxin as a player in the regulation of endometrial angiogenesis. While vascular endothelial growth factor (VEGF) is fundamental to endometrial angiogenesis, details of how and when different endometrial cell types produce VEGF, and how production and activity is controlled by oestrogen and progesterone, remains to be elucidated. Evidence is emerging that the different splice variants of VEGF play a major role in regulating endometrial angiogenesis at a local level. Intravascular neutrophils containing VEGF have been identified as having a role in stimulating endometrial angiogenesis, although other currently unidentified mechanisms must also exist. Future studies to clarify how endometrial angiogenesis is regulated in the human, as well as in relevant animal models, will be important for a better understanding of diseases such as breakthrough bleeding, menorrhagia, endometriosis and endometrial cancer.

Morphological events in the primate endometrium in the presence of a preimplantation embryo, detected by the serum preimplantation factor bioassay

BACKGROUND

Hormonal modulation of the endometrium towards receptivity is well established; however, the role of embryonic stimuli in modulation of the endometrium prior to implantation, especially in primates, is unknown. The aim of the present study was to evaluate the endometrial histology when the embryo was present in its vicinity prior to implantation.

METHODS

Preimplantation factor (PIF) bioassay was used as a tool to detect the presence of an embryo in the uterine lumen of mated bonnet monkeys (Macaca radiata) (n=9). The control group comprised seven non-mated animals. The specificity of the PIF bioassay for the presence of an embryo was tested by studies in pregnant humans and monkeys. The effects of embryonic stimuli on the endometrial morphology were analysed by routine haematoxylin-eosin staining. The expressions of CD34, an endothelial cell marker, alpha-smooth muscle actin (alpha-SMA), a marker for blood vessel maturation, and prolactin, a marker of endometrial decidualization, were studied by immunohistochemistry.

RESULTS

That PIF is embryo specific was established by its presence in sera of pregnant humans, monkeys and also in embryo culture media. Six mated bonnet monkeys were found to be PIF positive. Morphologically, the endometria from these PIF-positive animals showed the presence of the pre-epithelial plaque reaction, increased angiogenesis and stromal compaction. The significantly increased number of CD34- and alpha-SMA-positive blood vessels (P<0.05) in the endometria of PIF-positive animals indicated increased angiogenesis in response to embryonic stimuli. The endometrial expression of immunoreactive prolactin was also significantly increased (P<0.05) in the PIF-positive animals, indicating decidualization.

CONCLUSIONS

Using PIF as a marker to detect early pregnancy in bonnet monkeys, we have shown that the embryo induces a pre-epithelial plaque type of reaction, increased angiogenesis and decidual reaction in the endometrium prior to implantation.

Cytomics--new technologies: towards a human cytome project

BACKGROUND

Molecular cell systems research (cytomics) aims at the understanding of the molecular architecture and functionality of cell systems (cytomes) by single-cell analysis in combination with exhaustive bioinformatic knowledge extraction. In this way, loss of information as a consequence of molecular averaging by cell or tissue homogenisation is avoided.

PROGRESS

The cytomics concept has been significantly advanced by a multitude of current developments. Amongst them are confocal and laser scanning microscopy, multiphoton fluorescence excitation, spectral imaging, fluorescence resonance energy transfer (FRET), fast imaging in flow, optical stretching in flow, and miniaturised flow and image cytometry within laboratories on a chip or laser microdissection, as well as the use of bead arrays. In addition, biomolecular analysis techniques like tyramide signal amplification, single-cell polymerase chain reaction (PCR), and the labelling of biomolecules by quantum dots, magnetic nanobeads, or aptamers open new horizons of sensitivity and molecular specificity at the single-cell level. Data sieving or data mining of the vast amounts of collected multiparameter data for exhaustive multilevel bioinformatic knowledge extraction avoids the inadvertent loss of information from unknown molecular relations being inaccessible to an a priori hypothesis.

CHALLENGE

It seems important to address the challenge of a human cytome project using hypothesis-driven molecular information collection from disease associated cell systems, supplemented by systematic and exhaustive knowledge extraction. This will allow the description of the molecular setup of normal and abnormal cell systems within a relational knowledge system, permitting the standardised discrimination of abnormal cell states in disease. As one of the consequences, individualised predictions of further disease course in patients (predictive medicine by cytomics) by characteristic discriminatory data patterns will permit individualised therapies, identification of new pharmaceutical targets, and establishment of a standardised framework of relevant molecular alterations in disease. This special issue of Cytometry, on new technologies in cytomics, focuses on prominent examples of this presently fast-moving scientific field, and represents one of the preconditions for the formulation of a human cytome project.

A three-dimensional study of endometrial microvessels in women using the contraceptive subdermal levonorgestrel implant system, norplant®

Recent evidence points towards a substantial disturbance of the process of angiogenesis within the endometrium in women who are exposed continuously to low dose contraceptive progestogens. This results in the patchy appearance of abnormally small and abnormally large, thin-walled vessels in the superficial regions of exposed endometrium. Three-dimensional pictures were developed from digitised images of serial tissue sections of endometrium in which microvascular endothelial cells were labelled with antibodies to the endothelial cell surface antigen, CD34 and their basement membranes labelled with anti-Collagen IV antibodies. Microvessels from endometrium exposed to continuous low-dose levonorgestrel from a subdermal implant system (Norplant) displayed considerable variations in size and shape. No spiral arterioles were identified. Some microvessels showed considerable dilatation, distortion and variability in the presence of surrounding basement membrane components. Other endothelial structures included narrow, solid cords of endothelial cells, without basement membranes, which often connected with normal or abnormal vessels containing lumens. Some areas, especially deeper in the tissue, contained microvessels of normal size and shape surrounded by basement membrane. These images have revealed an overall picture of great variability in superficial endometrial vascular structures in some women using a low-dose levonorgestrel implant system which appears substantially different from that seen in normal endometrium.