The frequency of early reproductive losses, according to leading world experts, reaches 31%, but taking into account only clinically confirmed pregnancies, it is lower, about 18–20%.

The pathogenesis of habitual miscarriage is associated with a violation of endometrial receptivity. It is regulated by complex intermolecular and intercellular interactions and depends on the synchronicity of embryo and endometrial development.

Due to the expression of a large number of signaling molecules that carry out paracrine, autocrine, intracrine, and juxtacrine regulation of intra- and intercellular interactions, the development and “behavior” of the blastocyst occurs, as well as pregnancy recognition and subsequent adaptation of the mother's body to it.

More than 70 years have passed since the appearance of morphological endometrial research, but it is not enough to assess endometrial receptivity in women with reproductive losses, therefore a search for new molecular markers is required.

Violation of endometrial receptivity occupies an essential place in the genesis of reproductive failures [1]. Successful endometrial implantation requires interaction at all three levels of regulation (genetic, proteomic, and histological) [2][3]. In order to fully evaluate the function of the endometrium, it is necessary to evaluate various proteins simultaneously with the receptors for sex steroids [1].

Chronic endometritis is accompanied by a disorder of endometrial receptivity and includes damage to the surface epithelium, a violation of the maturation of pinopodia by the time of the “implantation window”, followed by the Eрα overexpression [4]. Stroma fibrosis can lead to asynchrony of the receptor apparatus and disruption of secretory endometrial transformation. All this disrupts the pre-pregnancy restructuring of the endometrium and leads to the formation of habitual miscarriage or infertility [5].

Under the influence of steroid hormones, the transcriptional activity of genes changes, which leads to the formation of pinopodia, changes in their morphological structure, the appearance of epithelial cells, increased secretion of growth factors, and expression of other molecular markers of endometrial receptivity.

In order to assess the transcriptional activity of genes, Paramonova and Kogan use quantitative determination of the corresponding mRNAs. Northern blot analysis makes it possible to identify and study single mRNA molecules [6][7].

MicroRNAs and their target genes also play an important role in embryo implantation and growth. MicroRNA-145 is an irregular microRNA with repeated implantation failures, which performs such physiological functions as differentiation, cell proliferation, and migration/invasion into the cell [8].

A number of authors suggest that the receptivity of the endometrium depends only on the level of progesterone and various growth factors (leukemia inhibitor factor (LIF), leukemia inhibiting factor receptor (LIFR), and avß3-integrin).

The suppression of the NOX 10 genes leads to a decrease in the number of pinopodia. They regulate the proliferation of endometrial stromal cells and the morphogenesis of epithelial cells. The embryo and the endometrium carry out signaling interaction at the site of the formation of pinopodia. In addition, growth factors are expressed on pinopods, which play a key role in the development of blastocysts and transplantation [9].

However, Li et al. (2022) in their studies showed that pinopodia were not accurate markers of the implantation window [10].

According to a number of authors, the formation of new tissues is often accompanied by the formation of neovascularization and immune regulation. Toll-like receptor 4 (TLR4) is widely expressed in the maternal-fetal interface and is both a membrane receptor and an activator of the nuclear factor Kappa-B (NF-kB). TLR4 regulates, through Nf-kB, the production of inflammatory factors such as tumor necrosis factor α (TNFα), interleukin-6 (IL-6), and interleukin-1β (IL-1β), which affects embryo implantation and the physiological environment of the uterus [11].

Vascular endothelial growth factor (VEGF) is a cytokine that promotes angiogenesis, which regulates vascular permeability and promotes the proliferation of endothelial cells.

LIF is a widely active pleiotropic cytokine that has a regulatory effect on the endocellular mass, trophoblast cells, and endometrial epithelial cells of embryos during embryo implantation [12][13].

Immunohistochemical staining of the endometrium revealed that, normally, the LIF content in the glandular and superficial epithelium of the endometrium was higher than in the stroma. The content of LIF in epithelial cells increases in the middle and late secretory phase, when the number of pinopodia is maximum.

It was found that in chronic endometritis, LIF expression prevailed in the endometrial stroma [14].

Vascular endothelial growth factor receptor-1 (VEGFR-1) is involved in macrophage recruitment and angiogenesis at the implantation site. Macrophages have a high plasticity potential and can change their functions depending on changes in the microenvironment in tissues. Taking into account the function and phenotype, macrophages can be classified into activated type M1 and alternatively activated type M2. The balance between macrophages M1 and M2 in the middle of the secretory phase is crucial for the process of embryonic implantation.

The study demonstrated that the ratio of M1/M2 was reduced in patients in the group with defective endometrial susceptibility compared with the control group.

Some studies have also shown that during implantation, activated M1 macrophages produce inflammatory cytokines and mediators, such as IL-6, IL-1β, TNF-α and nitric oxide, causing pro-inflammatory reactions and contributing to the attachment of the embryo to the decidua. These results confirm the importance of macrophage polarization balance for embryonic implantation [15][16].

“Implantation markers (LIF, avß3-integrin, E-catherins, HOX genes, pinopodia) are regulated by the direct or indirect influence of sex hormones through their receptors, therefore, in order to determine the ‘implantation window’ and predict the onset of pregnancy, an immunohistochemical study of the expression of estrogen and progesterone receptors in the endometrium is used”.

Mclean and Hayashi showed that progesterone promoted the production of mediators necessary for pregnancy by binding to receptors and thereby stimulating gene expression. By the 20th–22nd day of the menstrual cycle, sex hormone receptors in the epithelial cells of the endometrial stroma disappear, and the content of progesterone and estradiol in the blood increases.

However, not only the number of estrogen and progesterone receptors is important but also their functional usefulness and ratio. In the secretory phase, the expression of estrogen receptors α decreases, and that of estrogen receptors β increases [17][18][19][20].

During the implantation window, the main topographical criteria of the receptive endometrium are a uniform three-layer structure, the presence of correct peristaltic waves from the inner pharynx to the mouths of the fallopian tubes, and the presence of subendometrial blood flow according to color flow mapping [21].

All the data presented in the literature review are important for understanding the complex and multi-stage molecular biological processes of conception, the action of which is realized in the endometrium and mediates its receptivity. Despite a large number of studies, to date, there is no unique marker for predicting the assessment of endometrial receptivity.

Authors’ contribution:

I.M. Ordiyants — development of research design;

T.A. Bazieva — data acquisition and analysis, text writing;

B.A. Dzhabrailova — review of publications on the topic of the article;

R.U. Tunguzbieva — review of publications on the topic of the article.

Conflict of interest. Authors declares no conflict of interest.