Embryo implantation in the laboratory: an update on current techniques

Abstract

BACKGROUND The embryo implantation process is crucial for the correct establishment and progress of pregnancy. During implantation, the blastocyst trophectoderm cells attach to the epithelium of the endometrium, triggering intense cell-to-cell crosstalk that leads to trophoblast outgrowth, invasion of the endometrial tissue, and formation of the placenta. However, this process, which is vital for embryo and foetal development in utero, is still elusive to experimentation because of its inaccessibility. Experimental implantation is cumbersome and impractical in adult animal models and is inconceivable in humans. OBJECTIVE AND RATIONALE A number of custom experimental solutions have been proposed to recreate different stages of the implantation process in vitro, by combining a human embryo (or a human embryo surrogate) and endometrial cells (or a surrogate for the endometrial tissue). In vitro models allow rapid high-throughput interrogation of embryos and cells, and efficient screening of molecules, such as cytokines, drugs, or transcription factors, that control embryo implantation and the receptivity of the endometrium. However, the broad selection of available in vitro systems makes it complicated to decide which system best fits the needs of a specific experiment or scientific question. To orient the reader, this review will explore the experimental options proposed in the literature, and classify them into amenable categories based on the embryo/cell pairs employed. The goal is to give an overview of the tools available to study the complex process of human embryo implantation, and explain the differences between them, including the advantages and disadvantages of each system. SEARCH METHODS We performed a comprehensive review of the literature to come up with different categories that mimic the different stages of embryo implantation in vitro, ranging from initial blastocyst apposition to later stages of trophoblast invasion or gastrulation. We will also review recent breakthrough advances on stem cells and organoids, assembling embryo-like structures and endometrial tissues. OUTCOMES We highlight the most relevant systems and describe the most significant experiments. We focus on in vitro systems that have contributed to the study of human reproduction by discovering molecules that control implantation, including hormones, signalling molecules, transcription factors and cytokines. WIDER IMPLICATIONS The momentum of this field is growing thanks to the use of stem cells to build embryo-like structures and endometrial tissues, and the use of bioengineering to extend the life of embryos in culture. We propose to merge bioengineering methods derived from the fields of stem cells and reproduction to develop new systems covering a wider window of the implantation process.

Building a stem cell-based primate uterus

“…The co-culture of embryos alongside endometrial epithelial monolayers has long been suggested as a model for implantation and to gauge mechanisms of apposition and competence of blastocysts for maternal–embryo interactions 76 – 84 . Similarly, the culture of embryos with endometrial stromal cells provides opportunities for investigating aspects of trophoblast invasion at the earliest stages 82 , 85 – 89 .…”

Expression and significance of miR-30d-5p and SOCS1 in patients with recurrent implantation failure during implantation window

“…Author details 1 Reproductive Medical Center, Department of Obstetrics and Gynecology, Xiangya Hospital, Central South University, Changsha 410008, Hunan, China. 2 Department of Obstetrics, Xiangya Hospital, Central South University, Changsha 410008, Hunan, China.…”

“…Infertility is a global reproductive health problem that affects 10-15% of the reproductive-age couples [1]. After 40 years of development, in vitro fertilization (IVF) and embryo transfer (ET) have become an effective and common therapeutic approaches for couples with infertility [2].…”

The flip side of the coin: is the endometrium ready for IVM-derived embryo implantation?

“…The window of implantation (WOI) is a short specific time interval, corresponding to the midsecretory phase in the endometrial cycle when the endometrium becomes receptive to the blastocyst. Within this timeframe, an emerging concept is that the proper molecular crosstalk between the endometrium and the blastocyst is of utmost relevance to ensure proper implantation [11]. WOI development is regulated by gland development and function, decidualization, immune cells, a wide variety of cytokines, growth factors, prostaglandins, enzymes, cell adhesion molecules, and ECM proteins and pinopodes (also known as uterodomes) [11,12].…”

“…Within this timeframe, an emerging concept is that the proper molecular crosstalk between the endometrium and the blastocyst is of utmost relevance to ensure proper implantation [11]. WOI development is regulated by gland development and function, decidualization, immune cells, a wide variety of cytokines, growth factors, prostaglandins, enzymes, cell adhesion molecules, and ECM proteins and pinopodes (also known as uterodomes) [11,12]. A series of genes have been linked to endometrial receptivity.…”