What is the Difference Between Trophoblast and Inner Cell Mass?

The trophoblast and inner cell mass (ICM) are two distinct cell layers that form in the early development of an embryo. They have different functions and cellular compositions:

Trophoblast: This is the outer cell layer of the blastocyst, which is the early stages of embryonic development before implantation into the endometrium of the uterus. The trophoblast does not contribute to the formation of the embryo but develops into a large part of the placenta, playing a crucial role in the correct function of the placenta.
Inner Cell Mass (ICM): This is the cluster of cells inside the blastocyst that will eventually give rise to the definitive embryo, including embryonic stem cells capable of forming all cell types in the body. The ICM forms in the earliest stages of embryonic development and is entirely surrounded by the single layer of trophoblast cells of the trophectoderm. As the embryo develops, the ICM segregates into a bilaminar embryonic disc, which consists of two distinct lineages: the external (dorsal) epiblast and the internal (ventral) hypoblast. The next developmental stage is gastrulation, in which waves of migrating cells convert the ICM into a trilaminar embryonic disc, comprised of three germ layers (ectoderm, mesoderm, and endoderm).

In summary, the key difference between trophoblast and inner cell mass is that trophoblast is the outer cell layer that forms the placenta, while the inner cell mass forms the embryo.