Blastocoel: Definition & Concept

Instructor: Christine Morgan

Christine has taught college Biology and Anatomy, and has a Master's degree in Anatomy.

Formation of the blastocoel is an important step in development -- it sets the stage for future growth and change in the cells of the early embryo. Learn how the blastocoel helps and protects these cells.

The Blastocoel -- A Great Place to Hang Out

We've all heard the expression 'man cave.' You know, a place to do guy things, get some salty snacks and beverages -- a place to get through the things guys have to get through. You probably didn't know that an embryonic cell cave exists, but it's one of the most important structures in the newly-developing fertilized egg and it's called the blastocoel.

The word part coel (pronounced seel) comes from Greek, meaning cavity, or cave. The process that forms the blastocoel is even called cavitation: the creation of a cave. This special fuid filled space starts forming on about the fifth day after fertilization in the tiny ball of cells that will become a new creature. It's a fully functional cell cave -- full of food and drink and good things to help the future embryo cells grow and get through the the stuff a new life has to get through.

Change is Everywhere

The formation of this blastocoel begins when the early dividing cells of the recently fertilized egg begin to differentiate. This is when they become specialized, as they move around and start to form a new space where there used to be solid cells. These changes, and the presence of the blastocoel, mark the blastula stage of development, which in mammals is called a blastocyst.

Blastocoel Formation
blastocoel formation

Are you seeing a pattern here? Yes, everything to do with this stage is a blast-something - it can be a good way to help you remember the words. This word part always has to do with immature, embryonic cells.

In high-powered microscopic images of the blastula stage, you can clearly see the blastocoel and the cells. Some cells collect in the blastocoel (think: safe in the cave) to become the future embryo -- they are called embryoblasts. Other cells begin to show polarity, with one side that behaves completely different from the other, and move to the outer edge of the blastula.

These polar cells, called trophoblast (tropho means nourish), surround the blastocoel and begin to follow some specific directions. First, they thin out and form tight cell junctions that can seal off the blastula from the environment; this creates a definite inside/outside arrangement for the first time in development.

You may wonder why it's an important step to the blastocoel to have a seal on the outside of the structure, but the blastula will soon break free from the zona pellucida, a remnant of the pre-fertilized egg that has acted as a tough covering. Besides protecting the blastula, the seal also keeps the contents of the newly forming blastocoel from leaking out.

Blastocoel Formation in the Blastocyst
Blastocoel Formation in Blastocyst

Next, trophoblast cells begin showing their nourishing qualities, with the ability to move sodium and chloride ions into the blastocoel cavity that has formed between the outer and inner layers. Remember what sodium and chloride ions combine to form? Yep, they make table salt. It may not seem nourishing to provide salt to the embryoblast cells in the cave; most of us think we should avoid salt even while we're nibbling on chips!

Well, just think about what happens when you eat something salty: you get super thirsty! So, you can probably guess what happens next in the blastula: a super drink. The movement of sodium and chloride from the cells is followed by a rush of water into the cavity through a process called osmosis. This is what provides all the protective fluid that fills the blastocoel and allows for the distribution of nutrients.

Movement of Water by Osmosis

This movement happens because water passes freely through cell membranes, and by osmosis, it'll flow out or in depending on where the least concentration of water is. If there is more salt in the fluid outside a cell compared to the inside, then water will flow out of the cell (because there is less water outside).

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