A blastula is a hollow ball of cells that forms early in animal development, just after a fertilized egg undergoes its first rounds of cell division. It represents a key milestone: the moment a single-celled egg has become a multicellular structure with a defined architecture, ready for the dramatic reshaping that builds an actual body. In most animals, the blastula consists of an outer layer of cells surrounding a fluid-filled cavity.
How the Blastula Forms
After a sperm fertilizes an egg, the resulting single cell (the zygote) begins dividing rapidly in a process called cleavage. These divisions don’t involve growth. Instead, the enormous volume of egg cytoplasm gets carved into progressively smaller, nucleated cells called blastomeres. Early on, the dividing embryo looks like a compact cluster of cells, sometimes called a morula (from the Latin word for mulberry).
As cleavage continues, changes kick in. The cells begin pumping fluid inward, opening up a central cavity. Once that cavity forms, the embryo has officially become a blastula. In zebrafish, one of the most-studied lab organisms, the blastula stage spans roughly from the 128-cell stage to the start of the next major phase, encompassing several more rounds of division and potentially reaching over a thousand cells.
Blastula Structure
Every blastula has two main components. The blastoderm is the sheet of cells forming the outer wall. The blastocoel is the fluid-filled cavity inside. This simple arrangement, a hollow sphere of cells enclosing a pocket of fluid, is the basic body plan that sets the stage for everything that follows. The blastocoel isn’t just empty space. It provides a physical environment that allows cells to migrate and rearrange during the next phase of development.
Four Types Across the Animal Kingdom
Not all blastulae look the same. The amount of yolk packed into the egg has a major influence on how cleavage proceeds and what shape the blastula takes. Biologists generally recognize four types.
- Coeloblastula: The classic hollow sphere. Found in frogs, sea urchins, and jellyfish, this is the textbook version with a prominent blastocoel.
- Stereoblastula: A solid ball of cells with little or no internal cavity. Some sponges, roundworms, and mollusks develop this way.
- Discoblastula: A flat cap of cells sitting on top of a large yolk mass. Birds, reptiles, fish, and egg-laying mammals (like the platypus) form this type because their eggs contain so much yolk that the cells can only divide on one side.
- Periblastula: A single outer layer of cells surrounding an inner yolk mass. Found in insects and other organisms whose eggs have yolk concentrated in the center.
Despite these differences in geometry, all four types accomplish the same developmental goal: producing a multicellular embryo poised for the next stage.
The Mammalian Version: The Blastocyst
Mammals put their own twist on the blastula. Instead of forming a simple hollow ball of identical-looking cells, mammalian embryos produce a structure called a blastocyst. The key difference is that the blastocyst contains two distinct cell populations that have already taken on separate roles.
The outer ring of cells, called the trophoblast, will not become any part of the baby’s body. Instead, it forms the embryonic side of the placenta and the surrounding membranes. The inner cell mass, a small cluster tucked to one side of the cavity, gives rise to the actual embryo along with structures like the yolk sac and amnion. This split between trophoblast and inner cell mass is the first true differentiation event in mammalian development. By the 64-cell stage (roughly 13 inner cells and the rest trophoblast), the two groups are fully separate, with neither contributing cells to the other.
In humans, the blastocyst forms around day 5 or 6 after fertilization. It measures about 160 micrometers in diameter, roughly the width of two human hairs side by side. At this point, the embryo is still microscopic and free-floating inside the uterus, preparing to implant.
Why the Blastocyst Matters for Stem Cell Science
Embryonic stem cells are harvested from the inner cell mass of blastocysts. These cells can, under the right laboratory conditions, multiply indefinitely while retaining the ability to develop into virtually any cell type in the body. This property, called pluripotency, is what makes them so valuable for research into tissue repair and disease. Stem cells derived from mouse blastocysts have been studied for decades and provided the foundational knowledge that made human embryonic stem cell research possible.
What Comes Next: Gastrulation
The blastula doesn’t last long. It transitions into the next stage through a process called gastrulation, one of the most dramatic reorganizations in all of biology. During gastrulation, cells on the surface of the blastula begin migrating inward, transforming a single-layered ball into a multi-layered structure called a gastrula.
In humans, gastrulation begins during the third week after fertilization. A groove called the primitive streak appears on the embryo’s surface, establishing for the first time which end will become the head and which will become the tail. Cells detach from the outer layer, change shape, and stream down through this groove. The first wave of migrating cells forms the innermost layer, called endoderm, which eventually builds the lining of the gut, lungs, and liver. The second wave fills in the middle layer, mesoderm, which becomes muscle, bone, blood, and the heart. The cells that remain on the surface become ectoderm, the source of skin, the nervous system, and the brain.
These three layers are the raw material for every organ and tissue in the body. So while the blastula itself is a relatively simple structure, it is the launchpad for all the complexity that follows.