From which cellular structure do mouse embryonic stem (ES) cells originate?

Mouse embryonic stem (ES) cells originate from the inner cell mass of early embryos, specifically from a blastocyst stage. The blastocyst is a hollow ball of cells that forms early in embryogenesis and contains two distinct groups of cells: the outer trophoblast, which contributes to the placenta, and the inner cell mass, which is destined to develop into the embryo itself.

The inner cell mass has the unique capability to give rise to various cell types of the body because these cells are pluripotent. This means they can differentiate into nearly all cell types, making them essential for developmental biology studies as well as regenerative medicine. In contrast, other options like the outer cell mass and placenta do not give rise to ES cells, as they are primarily involved in supporting the embryo rather than being the source of pluripotent stem cells. Adult tissues, while they contain stem cells, are not the source of embryonic stem cells. Therefore, the correct origin of mouse embryonic stem cells is indeed the inner cell mass of early embryos.