Cleavage and Blastocyst Formation

Cleavage and Blastocyst Formation

The product of fertilization is a one-cell embryo with a diploid complement of chromosomes. Over the next few days, the mammalian embryo undergoes a series of cell divisions, ultimately leading to formation of a hollow sphere of cells known as a blastocyst. At some point between fertilization and blastocyst formation, the embryo moves out of the oviduct, into the lumen of the uterus. The images below demonstrate major transitions in structure during early embryogenesis in cattle. Note that in all of the the early stages, the embryo is encased in its zona pellucida. Embryos from other mammals have a very similar appearance, and the general sequence of stages is seen in all mammals.

Unfertilized oocytes typically fill the entire space inside the zona pellucida, but after fertilization, the one-cell embryo usually is somewhat retracted from the zona pellucida surrounding it. Although not visible in this image, one or two polar bodies are often visible in the perivitelline space, the area between the embryo and the zona pellucida.
The one cell embryo undergoes a series of cleavage divisions, progressing through 2-cell, 4-cell, 8-cell and 16 cell stages. A four cell embryo is shown here. The cells in cleavage stage embryos are known as blastomeres. Note that the blastomeres in this embryo, and the eight-cell embryo below, are distinctly round.
Early on, cleavage divisions occur quite synchronously. In other words, both blastomeres in a two-cell undergo mitosis and cytokinesis almost simultaneously. For this reason, recovered embryos are most commonly observed at the two, four or, and seen here, eight-cell stage. Embryos with an odd number of cells (e.g. 3, 5, 7) are less commonly observed, simply because those states last for a relatively short time.
Soon after development of the 8-cell or 16-cell embryo (depending on the species), the blastomeres begin to form tight junctions with one another, leading to deformation of their round shape and formation of a mulberry-shaped mass of cells called a morula. This change in shape of the embryo is called compaction. It is difficult to count the cells in a morula; the embryo shown here probably has between 20 and 30 cells.
Formation of junctional complexes between blastomeres gives the embryo and outside and an inside. The outer cells of the embryo also begin to express a variety of membrane transport molecules, including sodium pumps. One result of these changes is an accumulation of fluid inside the embryo, which signals formation of the blastocyst. An early blastocyst, containing a small amount of blastocoelic fluid, is shown to the right.
As the blastocyst continues to accumulate blastocoelic fluid, it expands to form - you guessed it - an expanded blastocyst. The blastocyst stage is also a landmark in that this is the first time that two distinctive tissues are present. A blastocyst is composed of a hollow sphere of trophoblast cells, inside of which is a small cluster of cells called the inner cell mass. Trophoblast goes on to contribute to fetal membrane systems, while the inner cell mass is destined largely to become the embryo and fetus. In the expanded blastocyst shown here, the inner cell mass is the dense-looking area at the botton of the embryo.
Eventually, the stretched zona pellucida develops a crack and the blastocyst escapes by a process called hatching. This leaves an empty zona pellucida and a zona-free or hatched blastocyst lying in the lumen of the uterus. Depending on the species, the blastocyst then undergoes implantation or elongates rapidly to fill the uterine lumen.

As mentioned, the developmental progression depicted above for bovine embryos is essentially identical to what all mammalian embryos go through, including humans. For example, the image to the left shows an expanded blastocyst from a dog. This embryo was stained to accentuate the trophoblast and inner cell mass.

The length of time required for preimplantation development varies somewhat, but not drastically, among species. The zona-intact bovine blastocysts shown above were collected 5-6 days after fertilization. The same stages would be seen in mice at about 3.5 days after fertilization.

In addition to the morphological changes in the embryo described here, preimplantation development is associated with that might be called an awakening of the embryonic genome. There is, for instance, little transcription in the embryos of most species prior to the 8 cell stage, but as embryos develops into morulae, then blastocysts, a large number of genes become transcritionally active and the total level of transcription increases dramatically.