The inner cell mass (ICM) is small, but these few cells are destined for greatness. Learn about the formation of this developmental milestone that comes after fertilization.
Great Things Come in Small Packages
So no one has ever answered the question, ‘Which came first, the chicken or the egg?’ One thing’s for sure, though, when you look at the eggs we eat there is nothing in there that would become a chicken: yellow goo, clearish goo, but no chick parts. That’s because, if you eat commercial eggs, it’s never been fertilized and no chick has started to grow in there.
It may surprise you to know that, after fertilization does happen, scientists can tell on about the fourth day (for most mammals) which cells are going to become the future fetus and which cells are destined for support roles.
The inner cell mass (ICM) is the name for the cells destined to become the new organism (the parts), and is one of the first group of cells that becomes specialized, or differentiated from a little ball of cells called the morula: a Latin word for a clump of cells that look like a mulberry.
Changes on the Way
The morula is the first stage of cell division you can see after the initial split of the fertilized egg, and divides to about 16 cells. These cells are all rounded and look pretty much the same, and still contained within the zona pellucida, a tough covering that is a remnant of the pre-fertilized egg and keeps them all safe from the outside world. But soon these cells begin to change, and two to four cells at the center of the morula become noticeably different from those on the outer portion of the group.
On high-powered microscope images, you can see the cells to the outside begin to compact and thin out, forming cell junctions to hold them close, and moving out against the zona.
They are soon called the trophoblast because of these changes, and their actions allow a fluid-filled space called the blastocoel to form where the few center cells will divide and grow to become the ICM (think: parts in the goo).
You can think of the word tropho to mean nourish: the trophoblast will produce sugars, proteins and other helpful things for the care of the ICM as it grows and changes in the goo. The cells of the inner cell mass are also known as embryoblast cells, because these are the ones that develop into the tissues of the embryo and some of its membranes. The appearance of the ICM and blastocoel are visual signs that the end of the morula stage has arrived, and the structure is now called a blastula.
No Time to Slow Down
The blastula stage, which in you and me and other mammals is known as the blastocyst, ends up with around 150 cells, and the embryoblast is always outnumbered by trophoblast in about a 1:10 ratio. The blastula is still surrounded by the zona pellucida, but will soon break free so it can move to the uterine lining for implantation.