Question

Why are the cells of the inner cell mass of the blastocyst called stem cells?

Answer 1

Hint: Stem cells are cells that are totipotent. This means that they are undifferentiated cells which can then undergo differentiation to form the specialised cells of any tissue or organ type of the developing animal.

Complete answer:
The blastocyst is the stage in embryonic development directly following that of the morula. It is an essential part of mammalian development.
The term blastocyst comes from the Greek words blastos – to sprout and cystos – cavity. The blastocyst is a mass of cells consisting of two main layers. The outer layer is the trophectoderm or trophoblast, and the inner layer is also called the embryoblast, and inside of that is the cavity or blastocoel.

The trophoblast does not form cells of the embryo, but instead has precursor cells which form the placenta, providing the connection between the developing embryo/foetus and the mother. The cells of the inner cell mass, on the other hand, undergo several cycles of differentiation producing first the precursor cells, and then finally the end-stage differentiated cells of the body. The first step is the cells of the ICM form two layers, the hypoblast and the epiblast. The cells of the hypoblast then go on to form the different types of endodermal cells, while from the epiblast we get the germ layers which are the ectoderm, endoderm and mesoderm.
The cells of the inner cell mass thus are stem cells in that they can form all the different specialised cell types of the body.

Note: Technically, the ICM cells are pluripotent, not totipotent. They have already undergone some level of differentiation to form the ICM. However, for practical purposes of embryonic they are stem cells.