New discovery will pave the way for studies of human development and to potentially improve fertility treatment
The early human embryo, or blastocyst, consists of the three cell types necessary for proper implantation and placental development. These cells types are the progenitors of both the embryo, as well as the extra-embryonic lineages required to support the embryo during development.
Immunofluorescence of human nEnd stained for endoderm and basement membrane markers. Left: E-cadherin (green), vimentin (red), GATA6 (white). Middle: AFP (green), collagen IV (red), GATA6 (white). Right: fibronectin (green), vimentin (red), GATA6 (white).
Descendants of the epiblast will give rise to the embryo proper, the trophoblast to the placenta, and the hypoblast (also referred to as the primitive endoderm in other species) to the yolk sac endoderm. While it was previously believed that all embryonic cells came from the epiblast, recent finding suggests that the primitive endoderm also contributes to the future intestinal tract. Naïve human embryonic stem cells (hESCs) resemble the pre-implantation epiblast and a new study from the Brickman group has exploited these cells to develop a stem cell model for human hypoblast.
The key to developing this model was an observation the group made two years ago in the mouse endoderm germ layer (Anderson et al., 2017). The endoderm consists of the progenitor cells that will develop into all the visceral organs, such as lungs, liver, pancreas and some of the intestine. In mammals, endoderm is induced in two waves.
Until recently, the first wave occurring during pre-implantation development was assumed to be cells that would differentiate only to extra-embryonic support structures required for intrauterine development (e.g. the yolk sac). However, as eluded to above, recent data suggests these cells are also the progenitors of the embryonic intestine. The second wave begins later in development as the embryo proper starts form and endoderm induced at this stage are thought to be the founder cells of the embryonic organs. The initial discovery in mouse was based on the observation that the fundamental mechanisms driving the specification of these two lineages are the same and involve similar signalling pathways. The difference was the type of stem cells used to make the endoderm. In other words, whether they started with embryonic stem cells that resemble the blastocyst, or stem cells that resemble the later stages of development that make the embryonic organs.
In this paper, the Brickman group found that the same context dependency applies in human. While other groups have made endoderm from ESCs by using standard hESCs that resemble a later, or more primed stage of development, the scientists leading the study, Madeleine Linneberg-Agerholm and Yan Fung Wong found that by starting with naïve hESCs that resemble the pre-implantation blastocyst they could produce hypoblast. The scientists were then able to use this model to show that aspects of primitive endoderm development deduced in mouse, also apply in human, leading to the subsequent discovery of improved cell culture conditions for naïve hESCs. Most importantly, they were able to take these in vitro hypoblast cells and grow them indefinitely in culture as stem cells that resemble the hypoblast in primate (both cynomolgus monkey and human) pre-implantation embryos. As these cells resemble the earliest stages of endoderm development, and are a blank slate upon which both embryonic (intestine) and extra-embryonic endoderm can be developed, they refer to them as naïve extra-embryonic endoderm (nEnd).
As a stem cell model for human hypoblast, nEnd cells represent the only missing in vitro culture system for the early human embryo. Recent work has led to new models for human trophoblast stem cells, while naïve hESCs closely resemble the epiblast, and now nEnd can be added for the hypoblast. This completes the list of cell types present in the human pre-implantation blastocyst. As a result, this discovery will pave the way for new studies of human development, potentially aiding the future improvements to fertility treatment. Human nEnd could also represent an important tool for generating cell types for regenerative medicine and drug screening.
Linneberg-Agerholm, M., Wong, Y. F., Herrera, J. A. R., Monteiro, R. S., Anderson, K. G. V., and Brickman, J. M. (2019) Naïve human pluripotent stem cells respond to Wnt, Nodal, and LIF signalling to produce expandable naïve extra-embryonic endoderm. Development. doi: 10.1242/dev.180620.