Where is a cleavage stage embryo found

Cleavage can take place in two ways: holoblastic total cleavage or meroblastic partial cleavage. The type of cleavage depends on the amount of yolk in the eggs. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage. In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass and an outer layer called the trophoblast.

The inner cell mass is also known as the embryoblast; this mass of cells will go on to form the embryo. At this stage of development, the inner cell mass consists of embryonic stem cells that will differentiate into the different cell types needed by the organism.

The trophoblast will contribute to the placenta and nourish the embryo. The typical blastula is a ball of cells. Visit the Virtual Human Embryo project at the Endowment for Human Development site to step through an interactive that shows the stages of embryo development, including micrographs and rotating 3-D images.

The typical blastula is a ball of cells. The next stage in embryonic development is the formation of the body plan. The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells.

Each of these layers is called a germ layer and each germ layer differentiates into different organ systems. The three germs layers, shown in Figure The ectoderm gives rise to the nervous system and the epidermis.

The mesoderm gives rise to the muscle cells and connective tissue in the body. The endoderm gives rise to columnar cells found in the digestive system and many internal organs. If you could prevent your child from getting a devastating genetic disease, would you do it? Would you select the sex of your child or select for their attractiveness, strength, or intelligence? How far would you go to maximize the possibility of resistance to disease? This is the case no longer: science fiction is now overlapping into science fact.

Many phenotypic choices for offspring are already available, with many more likely to be possible in the not too distant future.

Which traits should be selected and how they should be selected are topics of much debate within the worldwide medical community. The ethical and moral line is not always clear or agreed upon, and some fear that modern reproductive technologies could lead to a new form of eugenics. Eugenics is the use of information and technology from a variety of sources to improve the genetic makeup of the human race.

Ever since, eugenic ideas have not been as publicly expressed, but there are still those who promote them. Efforts have been made in the past to control traits in human children using donated sperm from men with desired traits. In fact, eugenicist Robert Klark Graham established a sperm bank in that included samples exclusively from donors with high IQs. In more recent times, the procedure known as prenatal genetic diagnosis PGD has been developed.

One blastomere is slightly larger and one blastomere is slightly smaller than the others. It was generated by ICSI and transferred but failed to implant. A cell embryo with visible nuclei in some blastomeres. An embryo with more than 10 cells on Day 4. This embryo has not compacted which is unusual at this late stage. Generated by ICSI and cryopreserved.

Correspondingly, several studies have shown that for Day 3 transfers, implantation and live birth rates are positively correlated with an increase in cell number on Day 3, with the 8-cell stage having been a 4-cell embryo on Day 2 having the highest rates van Royen et al. The cleavage stage of the embryo at the time of transfer also seems to have a role in predicting early pregnancy loss. Hourvitz et al. A correlation between cell numbers at distinct observation time points and chromosomal errors has also been reported.

The same pattern was observed by Finn et al. Small portions of cytoplasm enclosed by a cell membrane but usually not containing DNA are often formed during cell division. Fragmentation is therefore defined as the presence of anucleate structures of blastomeric origin Keltz et al. The degree of fragmentation is most often expressed as the percentage of the total cytoplasmic volume.

It was generated by ICSI but not transferred. It was generated by IVF but not transferred. Fragments are concentrated in one area of the perivitelline space PVS. It was generated by ICSI, transferred and implanted. It was generated by IVF and cryopreserved. It was generated by IVF, was transferred but failed to implant. The blastomeres are evenly sized with no visible nuclei. It was generated by IVF but was not transferred. Three views of the same embryo at different focal planes.

Note the importance of assessing the embryo at different focal planes in order to establish the degree and type of fragmentation scattered in this case. The blastomeres are evenly sized and have visible nuclei. The blastomeres are evenly sized but binucleated. It was generated by ICSI but was not transferred. Fragments are scattered in the PVS. Fragments are predominantly concentrated in one area. The blastomeres are unevenly sized. In one focal plane a three to four cells can be seen but in the other two focal planes only one to two cells can be seen b and c.

It is often difficult to make the distinction between a large anucleate fragment and a small nucleated cell. Johansson et al.

It has been shown that a high degree of fragmentation correlates negatively with implantation and pregnancy rates Racowsky et al. Two distinctly different types of fragmentation have been documented by time-lapse analysis in human embryos: definitive fragmentation, characterized as stable persistent fragments clearly detached from blastomeres and pseudo-fragmentation, characterized by a transient appearance during, or shortly after, cell cleavage, but not detected during later development Van Blerkom et al.

Increasing fragmentation also results in reduced blastocyst formation and can influence allocation of cells during differentiation Hardy et al. The spatial distribution of the fragments in the perivitelline space PVS can be differentiated into two patterns, i.

The scattered appearance was found to be correlated with an increased incidence of chromosomal abnormality Magli et al. The higher the degree of fragmentation, the more difficult it is to differentiate between scattered and concentrated fragmentation Figs and Fragmentation is considered to be an essential parameter to include in the evaluation of developing embryos, as embryos with very strong and persistent fragmentation are less likely to be viable.

It has been shown that a high degree of regularity in the blastomere size in embryos on Day 2 is related to increased pregnancy outcome following assisted reproduction treatments Giorgetti et al. Uneven cleavage, i. The relative blastomere size in the embryo is dependent on both the cleavage stage and the regularity of each cleavage division Diagrams 1 and 2. The blastomeres of 2-, 4- and 8-cell embryos should be equal stage-specific embryos, Figs — rather than unequal in size non-stage-specific embryos, Figs — In contrast, blastomeres of embryos with cell numbers other than 2, 4 and 8 should have different sizes as there is an asynchrony in the division of one or more blastomeres Figs — A 3-cell embryo should preferably have one large and two small blastomeres Fig.

These embryos are thereby also considered to be stage specific. However, a 4-cell embryo with one or two blastomers much larger than the others Figs — , a 3-cell embryo with all blastomeres even in size Fig.

A diagram illustrating the expected cell size of a cleavage stage embryo: a human 2-cell embryo should contain two equal blastomeres of the size of the 2-cell stage and are thereby stage specific. The same rule can be applied to 4- and 8-cell embryos. The numbers show proportions of diameter size.

A diagram illustrating the concept of stage-specific versus non-stage-specific cleavage patterns. The dark green color indicates stage-specific cleavage stage embryos, whereas the light green color indicates non-stage-specific cleavage stage embryos. A 2-cell embryo with evenly sized blastomeres and no fragmentation on Day 2.

The blastomeres are stage-specific cell size. The embryo was transferred but did not result in a pregnancy. A 4-cell embryo with evenly sized blastomeres and no fragmentation on Day 2. Notice the clover shape arrangement of the blastomeres. It was transferred and implanted. An 8-cell embryo with evenly sized blastomeres and no fragmentation on Day 3. It was transferred and resulted in a pregnancy. A 2-cell embryo with unevenly sized blastomeres on Day 2. The blastomeres are not stage-specific cell size.

The blastomeres are therefore not stage-specific cell size. A 4-cell embryo with unevenly sized blastomeres on Day 2. One blastomere is indistinct in this view. A 4-cell embryo with unevenly sized and irregular blastomeres with two blastomeres being larger than the other two. Note that the ZP of this embryo is elongated. A 4-cell embryo with unevenly sized blastomeres. An 8-cell embryo with unevenly sized blastomeres.

A 3-cell embryo with one large and two small blastomeres on Day 2. The embryo was transferred but failed to implant. A 5-cell embryo with three large and two small blastomeres. A 6-cell embryo with two large and four small blastomeres. A 7-cell embryo with one large and six small blastomeres. A 3-cell embryo with three blastomeres of the same size at 26 h after insemination. A 5-cell embryo with two large and three small blastomeres instead of three large and two small blastomeres; therefore, not stage-specific cell size.

A 5-cell embryo with two large and three small blastomeres instead of three large and two small blastomeres therefore not stage-specific cell size. A 5-cell embryo with four large and one small blastomeres instead of three large and two small blastomeres; therefore, not stage-specific cell size. A thawed 6-cell embryo with six blastomeres of the same size rather than two large and four smaller blastomeres; therefore, not stage-specific cell size.

A 6-cell embryo with two very large and four very small blastomeres. The extreme size difference between the large and small blastomeres makes this embryo not stage specific.

A 7-cell embryo with three large and four small blastomeres instead of one large and six small blastomeres; therefore, not stage-specific cell size. One blastomere shows multinucleation. The nucleation status is defined as the presence or absence of nuclei in the blastomeres of the cleavage stage embryo.

Ideally, the nucleation status of each blastomere in the embryo should be evaluated as a single nucleus per blastomere Figs — , no nuclei visible or multinucleation Figs — A 4-cell embryo with equally sized, mononucleated blastomeres arranged in a clover shape on Day 2 post-injection.

There is a single nucleus clearly visible in each blastomere. A 4-cell embryo with equally sized, mononucleated blastomeres arranged in a tetrahedron shape on Day 2 post-injection.

A 4-cell embryo with blastomeres of unequal size and with one binucleated blastomere. A 2-cell embryo with blastomeres of unequal size with several four nuclei in one blastomere. The most studied nucleation status is multinucleation, which is defined as the presence of more than one nucleus in at least one blastomere of the embryo Jackson et al. Embryo quality has been shown to correlate with multinucleation, and 4-cell embryos on Day 2 and 8-cell embryos on Day 3 show reduced multinucleation compared with the other cell stages observed on these days Van Royen et al.

Multinucleation is predictive of a decreased implantation potential Jackson et al. Soon after development of the 8-cell or 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.