Morula Stage in Early Embryonic Development

Life has an interesting way of unfolding. There are things that we cannot even imagine and yet they are possible. That itself is the beauty of how things function on this planet. It may be difficult to imagine now, but it is a proven fact that every mammal began its life as a unicellular being. All of us went through the process of life and had to go through certain stages. There are several characteristics to every single stage that we go through before and after our birth. This article will be focused on one such stage that has to take place after the zygote has been formed. Morula is an interesting topic as it takes us into the depth of what happens after the sperm meets the egg and how exactly things that follow take place. 

This concept is crucial in understanding how life functions on the planet. We may not know everything about life on earth but the things that we do know are extremely intriguing. The way we develop into the complex beings that we are from the earlier stages is very interesting to study. This article aims to bring clarity on the topic of Morula and is focused on explaining everything about it. From its composition to a more detailed breakdown of what this state exactly is about. 

Vedantu recommends that students take note of all the key points mentioned in this article making sure that they are not missing out on anything. Use this article as a way to revise this topic even after you have read this once to make yourself more comfortable with it so that you can answer every question based on it with ease. The more you revise and practice, the better you will get at it. 

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 specialized 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 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 the 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 divisions 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 as a mulberry which gives the name Morula. In Latin, Morula means mulberry which characterizes the appearance of this stage. 

Transformation of Morula into Blastocyst

A few days after 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 trophoblasts 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 the 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 from Latin which means mulberry.

We hope that the article provided some really crucial knowledge on the topic of Morula. As mentioned earlier, use this article as a way to revise the topic over and over again so that the concept remains crystal clear. Vedantu offers many more brilliant resources for free that can contribute to your learning, boosting your chance to score more marks. We hope that you take advantage of everything that Vedantu has to offer.