# Human Blastoid: A Next-Generation Model for Reproductive Medicine?

## TL;DR

This paper explores blastoids, synthetic embryo-like structures from stem cells, as a potential model for studying early human development and improving reproductive technologies.

## Contribution

The paper reviews blastoids as a novel model for human embryogenesis and highlights their potential for advancing reproductive medicine.

## Key findings

- Blastoids may help study early human development without ethical issues of using embryos.
- SES could offer objective methods for embryo evaluation in assisted reproductive technologies.
- Current models still need refinement to accurately mimic human blastocysts.

## Abstract

Early human embryogenesis remains poorly investigated. At the same time, the problems of studying early human development are relevant due to the large number of unsuccessful pregnancies and early embryonic mortality. However, the use of human embryos in studies has ethical and legal restrictions, whereas animal models have differences in the mechanisms of early development, which reduces their suitability. As an alternative, models of embryos from pluripotent stem cells, which are useful for imitating the early stages of human development, have been proposed. Such models were called blastoids. Using them may become beneficial not only for studying early human development but also for developing new approaches in the field of assisted reproductive technologies, in particular, for identifying pathologies associated with conception and the first weeks of pregnancy.

Human early embryogenesis remains unexplored due to limited access to human embryos for research purposes. Meanwhile, the number of natural early pregnancy terminations remains significant, and solving the problem requires a deep understanding of the developmental mechanisms of this period. Although assisted reproductive technologies (ART) utilize up-to-date approaches in culturing human embryos in vitro, characterization of the embryos is still based on visual evaluation and subjective assessment. In addition, embryonic development in animal models, such as rodents and cattle, correlates poorly with human embryonic development. Synthetic embryology presents a promising new approach for studying human embryos involving the creation of embryos without the use of haploid germ cells. Instead, diploid pluripotent stem cells (PSCs) in a given state of pluripotency, which is maintained under conditions of induction and/or inhibition of certain signaling pathways, are used. Synthetic embryo systems (SES) may become a successful alternative model for studying fundamental processes of human early preimplantation embryogenesis, exploring new methods of objective embryo qualification, and personalized approaches in ART. However, the question of whether SES models can be considered as full-fledged mimics of the embryo remains open. This review examines human blastocyst-like structures known as blastoids. It discusses their use as models, as well as the parameters that need to be modified to more accurately simulate the human blastocyst.