Gastrulation morphogenesis in synthetic systems

Abstract

Recent advances in pluripotent stem cell culture allow researchers to generate not only most embryonic cell types, but also morphologies of many embryonic structures, entirely in vitro. This recreation of embryonic form from naïve cells, known as synthetic morphogenesis, has important implications for both developmental biology and regenerative medicine. However, the capacity of stem cell-based models to recapitulate the morphogenetic cell behaviors that shape natural embryos remains unclear. In this review, we explore several examples of synthetic morphogenesis, with a focus on models of gastrulation and surrounding stages. By varying cell types, source species, and culture conditions, researchers have recreated aspects of primitive streak formation, emergence and elongation of the primary embryonic axis, neural tube closure, and more. Here, we describe cell behaviors within in vitro/ex vivo systems that mimic in vivo morphogenesis and highlight opportunities for more complete models of early development.

Keywords: Embryoid; Gastrulation; Gastruloid; Morphogenesis; Synthetic development.

Fig. 1.. Models of embryonic axis formation.

Asymmetry in aggregated pluripotent cells often manifests as localized activation of Wnt/beta-catenin signaling and/or expression of nascent mesoderm marker genes. This site of initial differentiation can be influenced by contact with culture wells (A) or the application of exogenous signaling molecules via combination with source cells (B), microfluidic devices (C), or targeted injections into early blastomeres (D). This axis of asymmetry resembles the embryonic anteroposterior axis, and many 3D cell aggregates subsequently elongate along this axis. Integrated stem cell models in which embryonic and extraembryonic stem cells self-organize into structures resembling post-implantation mouse embryos also recapitulate anteroposterior axis formation through the “anterior” migration of distal/anterior visceral endoderm-like cells (DVE/AVE) and expression of nascent mesoderm markers on the opposite side (E).

Fig. 2.. Models of primitive streak formation.

Mesoderm and endoderm germ layers can arise within pluripotent stem cell aggregates via formation of primitive streak-like structures. Some or all hallmarks of primitive streak formation, including loss of basement membrane, expression of EMT markers, and delamination of motile mesoderm-like cells, can be modeled within 2D micropatterned stem cell cultures (A), 3D stem cell cysts (B), and integrated stem cell models that recapitulate post-implantation mouse development (C).

Fig. 3.. Models of embryonic axis extension.

Naïve embryonic cells explanted from Xenopus (A) and zebrafish (B) embryos can recapitulate both the polarized cell behaviors and the resulting convergence-extension morphogenesis typical of mediolateral cell intercalation (A-B). Depending on culture conditions, elongation of mammalian stem cell aggregates can exhibit hallmarks of convergence-extension and/or accretion, by which new cells are added to a structure’s caudal end (C). In trunk-like structures and other 3D PSC aggregates, these new cells likely arise from neuromesodermal progenitors (NMPs, C), common precursors to both mesodermal and neuroectodermal cell types.