A blastocyst looks like a tiny hollow ball of cells, roughly the size of the period at the end of this sentence. It measures between 150 and 250 micrometers in diameter, which is just barely visible to the naked eye. Despite its small size, a blastocyst has a surprisingly organized structure with distinct layers and a fluid-filled center that give it a recognizable appearance under a microscope.
The Basic Shape and Structure
Under a microscope, a blastocyst resembles a hollow sphere. Two layers of cells form its walls: an outer layer that is only one cell thick, and an inner layer that is three to four cells thick. The center of the sphere is filled with fluid, forming a cavity called the blastocoel. This fluid-filled space is one of the most recognizable features when viewing a blastocyst through a microscope, giving it the look of a tiny, slightly lopsided bubble.
Clustered along one side of this hollow ball is a dense clump of cells called the inner cell mass. If you imagine a snow globe, the inner cell mass would be like a small mound stuck to the inside wall. This clump eventually develops into the embryo. The thin outer ring of cells surrounding the entire sphere is the trophoblast, which later forms the placenta. Even at this early stage, the two cell types look noticeably different: the inner cell mass appears as a compact, darker cluster, while the outer layer looks like a smooth, thin shell encircling everything.
The Protective Shell
In its early stages, a blastocyst is still enclosed within the zona pellucida, a transparent, gel-like shell that has surrounded the egg since before fertilization. Under a microscope, this shell looks like a clear, slightly thick ring around the outside of the cell ball. Think of it as a protective casing, similar in appearance to the membrane around a soap bubble but more rigid. At this point, the blastocyst looks like a sphere within a sphere, with a visible gap between the outer cells and the shell as the blastocyst grows and the shell begins to thin.
How It Changes as It Expands
A blastocyst doesn’t stay the same size. Over the course of roughly two days (days 5 through 7 after fertilization), it grows from about 65 micrometers to 200 micrometers as fluid fills the central cavity and the cell count climbs from around 47 cells to 129 or more. This expansion is visible and dramatic under magnification. The blastocyst progresses through recognizable stages:
- Early blastocyst: The fluid cavity has just started forming and takes up less than half the volume. The structure still looks relatively compact.
- Full blastocyst: The cavity now fills the entire interior, and the sphere looks plump and round.
- Expanded blastocyst: The cavity has grown so large that the blastocyst stretches beyond its original size, and the zona pellucida (the outer shell) visibly thins out, like a balloon being inflated.
What Hatching Looks Like
One of the most visually striking moments in blastocyst development is hatching. As the blastocyst continues to expand, it eventually ruptures the thinned zona pellucida and squeezes through the opening. Under a microscope, this looks like a ball of cells pushing through a crack in a shell. The outer trophoblast cells send projections into the shell first, creating a small breach, and then the rest of the blastocyst herniates outward.
During hatching, the blastocyst often takes on a figure-eight or hourglass shape, with part of the cell mass bulging outside the shell while the rest remains inside. The inner cell mass can be seen stretching across the opening, with cells passing through the gap in what looks like a slow-motion escape. Once fully hatched, the blastocyst sheds the shell entirely and appears as a bare, free-floating cluster of cells ready to attach to the uterine lining.
How IVF Clinics Grade Appearance
If you’re going through IVF, you’ll likely hear your blastocyst described with a code like “4AA” or “3AB.” This grading system, developed by embryologist David Gardner, scores three things you can see under a microscope: how expanded the blastocyst is, what the inner cell mass looks like, and what the outer cell layer looks like.
The first number (1 through 6) describes expansion. A 1 means the cavity is just beginning to form. A 3 is a full blastocyst with the cavity completely filling the interior. A 4 means it has expanded beyond its original size with a thinning shell. A 5 is actively hatching, and a 6 has fully hatched out of the shell.
The first letter grades the inner cell mass. An “A” means many cells tightly packed together, appearing as a dense, well-defined clump. A “B” means several cells that are more loosely grouped, looking a bit scattered. A “C” means very few cells, which appears as a sparse, barely visible cluster.
The second letter grades the outer cell layer. An “A” here means many cells forming a smooth, cohesive ring around the blastocyst. A “B” means fewer cells forming a looser, less uniform layer. A “C” means very few large cells with visible gaps between them.
So a blastocyst graded “4AA” is an expanded blastocyst with a tightly packed inner cell mass and a smooth, well-formed outer layer. It looks round, plump, and organized. A “3BC,” by contrast, would be a full but not yet expanded blastocyst with a loosely grouped inner cluster and a patchy outer ring. The visual difference between high-grade and low-grade blastocysts is genuinely noticeable, even to an untrained eye looking at microscope images.
How It Compares to Earlier Embryo Stages
Before the blastocyst stage, a developing embryo looks like a solid clump of identical-looking cells, somewhat like a tiny raspberry. There’s no visible cavity and no way to distinguish one group of cells from another. The transition to the blastocyst stage is a clear visual shift: the interior opens up as fluid accumulates, the cells sort themselves into two distinct populations, and the structure suddenly has obvious architecture. It goes from looking like a cluster of marbles to looking like a hollow ball with a dense knot of cells on one side. This is why the blastocyst stage is considered the first point at which the embryo shows real structural organization.