Translating Embryogenesis to Generate Organoids: Novel Approaches to Personalized Medicine

Abstract

The astounding capacity of pluripotent stem cells (PSCs) to differentiate and self-organize has revolutionized the development of 3D cell culture models. The major advantage is its ability to mimic in vivo microenvironments and cellular interactions when compared with the classical 2D cell culture models. Recent innovations in generating embryo-like structures (including blastoids and gastruloids) from PSCs have advanced the experimental accessibility to understand embryogenesis with immense potential to model human development. Taking cues on how embryonic development leads to organogenesis, PSCs can also be directly differentiated to form mini-organs or organoids of a particular lineage. Organoids have opened new avenues to augment our understanding of stem cell and regenerative biology, tissue homeostasis, and disease mechanisms. In this review, we provide insights from developmental biology with a comprehensive resource of signaling pathways that in a coordinated manner form embryo-like structures and organoids. Moreover, the advent of assembloids and multilineage organoids from PSCs opens a new dimension to study paracrine function and multi-tissue interactions in vitro. Although this led to an avalanche of enthusiasm to utilize organoids for organ transplantation studies, we examine the current limitations and provide perspectives to improve reproducibility, scalability, functional complexity, and cell-type characterization. Taken together, these 3D in vitro organ-specific and patient-specific models hold great promise for drug discovery, clinical management, and personalized medicine.

Keywords: Bioengineering; Embryology; Tissue Engineering.

Graphical abstract

Figure 1

Stem Cell-Derived Embryo Models Recapitulate Mammalian Embryonic Development (A) Illustration of the key stages of mammalian embryogenesis, showing the progression of embryonic development of the conceptus inside a reproductive tract in vivo. (B) Schematic of embryo-like structures generated from pluripotent stem cells in vitro, which can recapitulate the key events of the post-implantation process. These embryo-like structures include the blastoids (modeling the blastocyst stage), Post-implantation Amniotic Sac Embryoid (PASE) and ESC- and TSC-derived (ETS) embryo-like structures (modeling amniotic cavity formation and specification of mesoderm and endodermal layer), and gastruloids (modeling gastrulation and lineage specification with the formation of structures resembling somites and neural tube and early organogenesis). The bottom arrows represent a simplified version of the starting material that is mixed with PSCs to generate the embryo-like structures in a dish (see also Table 1).

Figure 2

Directed Differentiation of PSCs to Generate Organoids of Three Different Germ Layers with the Strategies for Functional Validation and Its Biomedical Applications A simplified roadmap for directed differentiation of PSC using growth factors from different signaling pathways (Wnt, BMP, TGF-β, Notch) to generate 3D cell culture models. (A) Signaling factors can also promote PSCs to a defined lineage bifurcating from ectoderm and PS. Complex organoid structures (assembloids) can be generated with vascularization or nervous system or coupling multi-endodermal structures. Faded color in the arrow denotes the ability to generate organoids from tissue-specific adult stem cells but not yet from PSCs (see also Table 2 for features of organoids). (B) An overview of different available tools for functional characterization of 3D culture models. (C) The potential applications of organoids in biomedical research.

Figure 3

Applications of PSC-Derived Organoids in Personalized Medicine and Clinical Management (A) The disease-causing variants are cataloged in several public clinical databases and are used for functional studies. (B) Different variants can be generated in PSCs using the CRISPR-based genome editing toolbox, which can be further differentiated to the organoid of the desired lineage. The organoids represent each patient/variant, and high-throughput drug screening and cytotoxicity studies can be performed to develop a personalized drug. (C) Schematic showing the use of organoids in high-throughput genetic screens using CRISPR to identify novel gene targets that get frequently mutated to cause cancer. Furthermore, these organoids can be xenotransplanted to develop an in vivo model for mouse tumorigenesis studies. The resulting tumors can be further cryopreserved as organoids for drug screening and mutagenesis studies.