Human embryo replicas have gotten more complex. Here’s what you need to know

The lab-engineered models give scientists a look at human development beyond the first week

A model of a human embryo against a black backdrop. The model has ane exterior ring dotted with bluish light clumps. Wtihin is an oval-shaped purplish blob of cells with orange running through it. Below that are long bluish structures, also with bright orange and white running between cells

A 6-day-old embryo model from Jacob Hanna’s lab in Israel has structures akin to ones found in human embryos 12 days after fertilization. Those structures include a ball of cells (top, purple) that resemble the layer that forms the body, and a hollow circle of cells (bottom, blue) mimicking the yolk sac, all surrounded by a layer of cells similar to a placenta-forming layer in human embryos.