Comments on 'In vitro culture of cynomolgus monkey embryos beyond early gastrulation'

In vitro investigation of mammalian peri-implantation embryogenesis†

The embryos attach and invade into the uterus and establish the connection with their mother in peri-implantation development. During this period, the pluripotent epiblast cells of embryo undergo symmetry breaking, cell lineage allocation, and morphogenetic remodeling, accompanying with the dramatic changes of transcriptome, epigenome, and signal pathways, to prepare a state for their differentiation and gastrulation. The progresses in mouse genetics and stem cell biology have largely advanced the knowledge of these transformations which are largely hindered by the hard accessibility of natural embryos. To gain insight into mammalian peri-implantation development, great efforts have been made in the field. Recently, the advances in the prolonged in vitro culture of blastocysts, the derivation of multiple pluripotent stem cells, as well as the construction of stem cell-based embryo-like models have opened novel avenues to investigate peri-implantation development in mammals, especially for the humans. Combining with other emerging new technologies, these new models will substantially promote the comprehension of mammalian peri-implantation development, accelerating the progress of reproductive and regenerative medicine.

[The pre-gastrulation embryonic human development: future models and societal concerns]

Infertility, early miscarriages and congenital malformations are major public health issues that are frequent and poorly understood. Until now, what is known about early human development originates from two main sources: studies of human embryos and studies of model animals. Although some molecular mechanisms are conserved, there are specific human features. Thus, it is necessary to study model animals that are close to humans in the phylogenetic classification, which led to the use of pre-established primate cell lineages. Currently, the only human embryos available come from In Vitro Fertilization, which leads to important limitations: these embryos are relatively few and must be destroyed after 14 days. This has led researchers to develop new strategies. Several teams used Embryonic Stem Cells or Induced Pluripotent Stem Cells and their in vitro auto-organization properties to recreate "embryos" and thereby study their development. These new strategies allow a reduced use of human embryos but new questions arise about the legal status of these new research "models". In the future, it would be important to update the different legislations and recommendations of the International Society for Stem Cell Research as science progresses to avoid any failing drift. The respect of recommendations as well as the maintenance of discussions between specialists and the general public will allow a better understanding of early human development and the establishment of innovative strategies to target health challenges.

Monkey Embryos Cultured to 20 Days

Gastrulation is a phase in early mammalian development when the three germ layers are generated and body plan is formed. Although well studied in mice, much less is known about gastrulation in humans. Owing to the lack of access to primary human tissue for study and experimental manipulation, as well as legal and ethical constraints surrounding the use of human embryos, a dissection of the molecular and cellular mechanisms that underlie this process in humans has proven elusive. Nonhuman primates, owing to their relatedness to human species, comprise a tantalizing alternative model system for understanding human biology. Two recent studies have established novel systems to study monkey embryos for 20 days, demonstrating landmark events of early primate embryogenesis with possible relevance to human development. Most strikingly, cells grown in the dish closely resembled cells in in vivo embryos, suggesting that embryo development in a dish might actually be equivalent to that which occurs in vivo. In this piece, the author discusses the tremendous potential of these new methods to unveil insights into mechanisms that mediate primate embryo development. Moreover, repurposing the extended monkey embryo culture methods to create human-monkey embryonic chimeras would aid the development of strategies to create human organs inside livestock species. Finally, the ethical and regulatory issues that emerge from reconsideration of extending time limits for human embryo culture beyond 14 days or primitive streak formation are also briefly considered.