A blastocyst forms when a fertilized egg is in its second phase of growth. This takes place from days five to nine after fertilization. This is a big step for the little organism. Blastocysts follow after the morula phase, which is when the egg becomes a solid ball of cells until day three. At this point, the growing embryo needs space! To compensate for this need, the blastocyst forms a larger, hollow ball of cells with a fluid center to better meet its growing cellular constraints.
The blastocyst thus develops layers. The thin outer layer of cells around it is collectively called the trophoblast. This will eventually become the placenta of the developing infant. The fluid cavity within in named the blastocoel. A blastocyst will span a diameter of about 0.1 to 0.2 mm and undergo a characteristic phase of rapid growth. Whereas the morula comprised of a few dozen cells, the blastocyst will come to encompass hundreds of cells.
Perhaps one of the most important features of blastocyst development is its journey to the uterine wall. The pre-implanted embryo is a sphere made up of the outer trophoblast, the blastocoel cavity, and a cluster of cells in the center called the inner cell mass. But as it grows, the little organism will need more food and a safe space to expand.
Passage to the Uterine Wall
It takes about seven days for the blastocyst to find its way to the mammalian womb. Some drastic steps are taken in making this journey. Once a sperm enters an egg, a tough membrane called the zona pellucida forms around it that is impenetrable to other sperm. This is a protective mechanism against multiple sperms penetrating it. For reference, for any female egg there are between 40 million to 1.2 billion sperm cells competing for it in a single ejaculation. While this shield is helpful, it is not compatible with the growth of the new embryo. Therefore, the egg sheds off the zona pellucida between days five and seven as it begins to differentiate. This starts a rearrangement of cells on the outside and inside of the egg, and the eventual blastocyst forms.
The official name for the process that forms the internal cavity of the blastocyst is cavitation. The fluid is pushed inward by the outer cell layer, and junctions between the cells are formed to pull the cells together to keep the fluid inside the cavity. The trophoblast forms right under the zona pellucida. As its Greek roots imply, the trophoblast layer will “nourish” the growing embryo when it becomes the placenta. Meanwhile, the zona pellucida begins to break down. The growth and division of the blastula stretch and contract the rigid zona pellucida. This is when a sort of “mini birth” occurs. The zona pellucida will split on the side opposite the embryonic pole and “hatch,” or release, the blastocyst. Upon hatching at the end of day five, the blastocyst is ready to find its new place.
This all takes place on the fifth day post-fertilization, after which the blastocyst travels down the fallopian tube. This is made possible with the help of cilia (little “hairs”) that contract, and move the egg into the uterus. Since the egg has to shed off the zona pellucida, it can now implant itself into the inner lining of the uterus called the endometrium. The microvilli on the surface of the trophoblast cells will adhere to the epithelial cells of the uterus through glycoproteins. Once they adhere, the fertilized egg and uterine lining can no longer be flushed out during the menstrual cycle.
5-day blastocyst implantation timeline
Successful implantation of the blastocyst in the uterus is necessary for the fetus to grow. For many women afflicted with infertility, this step is compromised. Implantation rates also decline with female age due to a rising chance of chromosomal abnormality. Successful in vitro fertilization (IVF) transfer rates are about 37.1% for women under the age of 35, and lessen with age. Normally, a human embryo will take four days to travel down the fallopian tube and into the uterine lining. During IVF, implantation will occur between six to ten days after egg retrieval. This is also one to five days after a blastocyst transfer in the recipient.
A summary timeline of normal implantation follows:
- Day 0: the egg is fertilized high up in the fallopian tubes and forms the zona pellucida
- Day 1-3: the egg undergoes division in the solid morula stage
- Day 1-4: the egg continues to move down the fallopian tube and into the uterus
- Day 5: the egg has begun to transform into a hollow blastocyst, and has traveled and implanted itself into the uterine wall near a source of blood (future food!)
- Day 5-9: the blastocyst continues to divide, and still has the outer trophoblast layer, the fluid cavity, and inner cell mass that are the stem cells that will form future fetal tissue
Gastrulation of the blastula
Once the blastocyst stage ends on day nine, the embryo is ready to become a gastrula. Gastrulation is the process of differentiation that cells undergo to form the layers that form its future adult structures. The gastrula is comprised of three germ layers. These germ layers all derive from the inner mass of cells in the blastocyst. The outermost germ layer is called the ectoderm. The ectoderm will give rise to the skin and the nervous system of the growing mammal. The middle layer is called the mesoderm, and gives rise to muscle, bone, connective tissues, and the kidneys. Finally, the endoderm is the inner-most layer that gives rise to the respiratory system, like the lungs, and the gut.
In humans, the embryo forms from the time of fertilization until the end of the eighth week of gestation, when it becomes the fetus.