Introduction to Morula

Morula is a very early embryo stage and it consists of 16 cells. These 16 cells which are known as blastomeres define the morula stage. It is a solid ball that consists of the zona pellucida. The morula stage is a stage in the process of blastulation and occurs before the formation of the blastula. The morula occurs after 3 - 4 days of fertilization of the egg by the sperm. Hence, it is one of the important steps in the process of embryo development and the later birth of a child. During this stage, most of the cells in the morula are totipotent and have the capability to produce an entire living organism as it can differentiate into different types of cells with specialised functions and thus is one of the primary steps in embryo development.


What is Morula?

For the development of multicellular organisms, there are various methods of reproduction. Sexual reproduction is one of the types of reproduction and usually occurs via fertilisation. In this process, an egg cell is fertilized by a sperm cell and which after undergoing different processes results in the development of the embryo. An embryo is the early stage of the development of a multicellular organism, especially mammals such as human beings. Embryonic development is the part of the life cycle of the multicellular organism and begins right after fertilization further leading to the formation of the different structures of the body such as tissues and later the complex organ development. 

Each of the embryos initially starts as a zygote which is a single cell that resulted from the fusion of the gametes. In the early stages of embryonic development, the single cell resulting from the fertilization, the zygote, undergoes many rapid cell divisions, a process known as cleavage, that forms a blastula, which is similar to a ball of cells. After this, the cells in the blastula stage of the embryo development start their rearrangement into layers in a process known as gastrulation. Successively these layers will further develop into the different parts of the developing multicellular organism like the nervous system, connective tissues and the organs. 

It is clear that the process of cleavage follows fertilization. For many species, the zygotes undergo a rapid cell division without increasing in size and mass. They are actually cleaved in such a way that all the cells that are cleaved are of the same size and the cluster of cells that is present is in total the same size as the zygote. The different cells that are derived from the cleavage are known as blastomeres and the cleavage process continues until the formation of the blastula. Before the formation of the blastula, the morula stage is reached when there are in total 16 cells formed after the cleavage. Hence, after fertilization as the zygote starts to cleave over time it goes to the stage of morula which as per the introduction and morula definition is identified by the cells formed before transforming into the blastula. 


Morula Stage

From the explanation and morula meaning it is easy to consider the morula and blastocyst/blastula as the same. Morula is different from a blastocyst. The first difference between morula and blastula is that morula is a stage that occurs before blastocyst formation. From the explanation of what is morula, morula is a stage of 16 cells that occurs 3 - 4 days after fertilization whereas blastocyst is the stage that occurs after 4 - 5 days of fertilization. Also, morula cells are totipotent and the stage is an arrangement of cells in a spherical shape as compared to blastocyst which has a cavity present inside the zona pellucida along with the inner cell mass. As mentioned, the primary difference between morula and blastula is that blastocyst (in human beings) or blastula (in other animals) is a stage following morula, hence when and only when the morula is left untouched and allowed to progress while remaining implanted, it eventually develops into a blastocyst. The image given below shows the difference and the transformation:


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The stage of morula definition is achieved by the series of division because of the cleavage of the early embryo, beginning from the single-cell stage of a zygote. From the given image it appears that as the zygote gets divided into 16 cells it appears a mulberry which gives the name morula. In Latin, morula meaning mulberry which characterizes the appearance of this stage. 


Transformation of Morula Into Blastocyst

A few days after the fertilization the cells on the outer part of the morula become bound together in a tight formation, a formation of desmosomes and gap junctions. As they get into the tight formation of desmosomes and gap junctions they become nearly indistinguishable in a process known as compaction. The cells that are present on the outside and the inside undergo differentiation and the outer cells go to become trophoblast and the inner cells lead and become inner cell mass progenitors. There is the formation of a cavity inside the morula through the active transport of the sodium ions from trophoblast cells and the osmosis of water. This leads to the formation of a hollow ball of cells that are known as blastocysts.

The outer cells of the blastocyst become the first embryonic epithelium also known as the trophectoderm. Some of the cells, however, will always be there in the interior and will lead to becoming the inner cell mass (ICM) and are pluripotent i.e. that can differentiate into different types of cells. In mammals, the inner cell mass will finally form the “embryo proper” during which time the trophectoderm forms the placenta and the extraembryonic tissues. But this is different for different living organisms. For example, for reptiles, the inner cell mass is different. Also, the embryonic stages are prolonged and are divided into four parts.


Conclusion

It is clear from the given information in the article that morula is a stage in the early stages of embryonic development. It occurs after the zygote formation which is formed after fertilization. When the single-celled zygote undergoes cleavage and divides itself into a number of cells to become the blastula, it passes through the morula stage. This stage is important for the transition of the zygote to the blastula and can lead to the halting of the development of the embryo. Thus, it is a very important stage in childbirth and usually occurs after 3 - 4 days of fertilization. The morula cells look like mulberry and hence, the stage derives its name morula from Latin which means mulberry.

FAQs (Frequently Asked Questions)

1. What is the Difference Between Blastula and Morula?

Ans: Morula is an intermediate stage in between the single-cell zygote stage and the cleaved cell stage of the blastula. Morula is the combined solid mass of cells, which are known as blastomeres, formed by the cleavage of the single-cell zygote which in turn is formed after fertilization of the ovum by the sperm. On the other hand, a blastula is a ball of cells consisting of two layers that are formed by the dynamic arrangement of the blastomeres. Also, the blastula has a cavity around which the cells are arranged in a formation that is absent in the case of the morula.

2. Is Morula an 8-Cell Stage?

Ans: After the formation of the zygote via fertilisation, the zygote undergoes a process known as cleavage. In this, the single-celled zygote gets divided into a cluster of cells after a rapid cell cycle event. This does not result in the overall growth of the cell mass, but only the number of cells increases resulting in the cluster of cells of the same size as the original zygote. After undergoing cleavage, the zygote before reaching the blastula stage passes through a 16 cell stage. This 16 cell stage at first is known as morula without any significant cavity. As the morula further develops, a cavity is formed arranging the cells around it, which is known as the blastocyst or the blastula stage. Hence, the morula is a 16 cell stage, not an 8 cell stage.