What is Morula?
Morula is a solid mass of cells called blastomeres. The morula is an early stage of embryonic development. A Morula is formed by a series of cleavage or cell divisions of the zygote . A sperm fertilizes an egg and forms a zygote. This process is called fertilization. This zygote undergoes cell divisions and forms four-cell stage, eight-cell stage and sixteen-cell stage. At this stage, the sixteen cells rearrange themselves to form a mulberry like structure. This is called morula. The word morula comes from the Latin word “morus” meaning mulberry. Morula consists of cells called blastomeres that are present in form of a solid ball inside zona pellucida. The precursor of morula is a zygote and morula further develops and gives rise to blastula .
What is Blastula?
Blastula is a hollow sphere of cells formed after the morula stage. Like morula, it is also a part of the early stage of embryonic development. It consists of cells or blastomeres that form the outer layer of a fluid filled cavity . These outer layer cells are called blastoderm and the fluid filled cavity is called blastocoel. Blastula comes from a Greek word “blastos” meaning sprout. The morula stage develops into blastula by a process called blastulation. The blastula in mammals is called blastocyst. The blastocyst consists of embryoblast and trophoblast. The embryoblast gives rise to different structures of the fetus whereas the trophoblast gives rise to the extra embryonic tissues. Blastula is an important stage in embryogenesis that gives rise to gastrula by process called gastrulation .
What are the major differences between Morula and Blastula?
There are several differences between the two early stages of embryonic development, Morula and Blastula. These differences are described below:
- The formation from precursors: The zygote develops into morula and the morula further develops into blastula.
- Structure: Morula is solid in structure whereas Blastula is hollow. Morula is completely packed with cells whereas blastula is not.
- Composition: Morula is formed when zygote is cleaved rapidly by cell division whereas blastula is composed of a layer of spherical cells called blastomeres surrounding a fluid filled cavity.
- Gives rise to: Morula develops in blastula by the process of blastulation whereas blastula further develops into grastula by a process called gastrulation. In grastula, the germ layers of embryo are formed .
- Presence of cavity: Morula has no cavity present. On the other hand, blastula is filled with a cavity called blastocoel.
- Presence of blastoderm: Morula has no blastoderm in it whereas blastula has a well-defined blastoderm.
- Presence of trophoblast: Morula does not have trophoblast whereas Blastula has trophoblast cells.
- The time of occurrence after fertilization: Morula is formed approximately three to four days after fertilization whereas blastula is formed after morula (five or more days) after fertilization.
- Role in in-vitro fertilization (IVF): When the morula (day four) are transferred for in-vitro fertilization, the success rate of in vitro fertilization is low. When the blastulas (blastocysts, day five) are transferred for in vitro fertilization, the success rate in comparatively much higher than that of morula . Although there are several other factors that also affect the rate of success in in-vitro fertilization.
The differences between morula and blastula described above are listed in a tabular form below:
Although both morula and blastula are the stages of early development of the embryos, there are several major differences. The most important difference between morula and blastula is that structurally morula is solid and completely filled with cells whereas blastula has a layer of cells that surround a fluid filled cavity called blastocoel.
The morula develops from the cell division of zygote and blastula is formed later from morula by the process of blastulation. The blastula further develops into gastrula by a process called gastrulation and forms germ layers. Morula occurs three to four days after fertilization of sperm and eggs whereas blastula occurs after five days of fertilization.
This difference in time from fertilization is used in in-vitro fertilizations where the blastulas have a more success rate of pregnancy than the morula stage.
Author: Debashree Basu
Debashree received her PhD in Biochemistry and specialized in infectious diseases. She is a Research Scientist whose main focus is bacteria, yeast and mammalian cells. She published articles in peer reviewed journals.