Cleavage - Embryo

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Cleavage in Human Zygote

Human Biology deals with the major section that is reproduction among human beings. There are several functions carried inside a mother before she gives birth to a baby. The first stage is fertilisation, which further leads to the zygote division that forms a single cell ball. This phenomenon is called cleavage in the human zygote. This solid ball of cells is known as the morula, which further converts into hollow balls of cells known as a blastula. The article below will understand the cleavage Embryo, its development, and its characteristics in detail. Also, it will clear you with the process after fertilisation. 

Cleavage of Fertilized Egg

The fertilised egg is known as a zygote that undergoes various stages to form a baby’s body. According to the study of embryology, the first few divisions in the zygote is called cleavage. The early cleavage is longitudinal but at different angles. Below points describe the cleavage in the human zygote:

  • For the first time, the zygote undergoes the longitudinal division, which is along the plane. 

  • The division is again longitudinal for the second time but at 90 degrees from the first division plane.

  • Again the third-time division is longitudinal but at 90 degrees to both first and second divisions. However, it is equatorial in position to the first two. 

Blastomeres

With the early cleavage process and division of cells, the separate cells formed are called blastomeres. The first divisions of the zygote are very frequent and simultaneous that occur within 30 hours after fertilisation. However, with time, the cells start developing independently, and simultaneity is also lost. There is little growth among different parts of the cells. 

Characteristics of Blastomeres

  • These are formed due to frequent cleavage in embryonic development. 

  • The size of the blastomeres remains the same after cleavage. It is the chromatin material that is introduced among cells for their development.

Where Does Early Cleavage Take Place?

When an egg is fertilised, it undergoes various divisions. However, this cleavage of the fertilized egg first occurs when it travels down from the fallopian tube. When an egg is about to enter the mother womb’s uterus, it goes under cleavage to develop into a cell group. This blastulation takes place as the embryo is growing and making its way to the uterine cavity. 

The process of cleavage occurs before implantation when a unicellular zygote turns into a two-celled embryo. Thus the further division takes place to develop various body organs. It is how cleavage in embryos takes place. 

Characteristics of Cleavage

  • Egg cleavage undergoes rapid cell division, which is a mitotic division. Hence the daughter cells formed have similar characteristics to the parent cell. 

  • The early division is a rapid process that occurs within 30 hours after an egg is fertilised. 

  • Cleavage forms a spherical and multicellular development stage which is known as a blastula. The process of formation of multiple cells is known as blastulation. 

  • Cleavage in embryos continues until an average cell size as that of the parent cell is achieved. However, the volume of the egg will decide the number of divisions a cell undergoes. 

Types of Cleavage

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The above diagram shows two different and most common types of cleavage observed among organisms, i.e. holoblastic and meroblastic cleavage. 

  • Determinate

It is a mosaic division of the zygote that occurs in protostomes. It is the first development of the fate of cells to develop further into embryos. 

  • Indeterminate

A cell is said to be regulative when it has cytoarchitectural features. When an embryo divides, it can further form individual organs, further developing to form an organism.

  • Holoblastic

Under this cleavage, a zygote undergoes complete cleavage and hence the number of cells doubles. It is the case of longitudinal cleavage and embryo development. Holoblastic cleavage takes place in human beings. 

  • Meroblastic

When a large amount of yolk is present, it undergoes partial cleavage. Its two major types are discoidal and superficial.  

Cleavage in Chick Embryo

Cleavage in a chick embryo is a complete ten stage cycle development process to form an egg. It is a type of meroblastic cleavage which has a huge amount of yolk inside. It is partial cleavage when a mother hen forms blastula in the early stage. The same process of cleavage is observed among reptiles. 

Fun Facts

  • Consumption of oxygen is very rapid when an egg undergoes cleavage after fertilization.

  • The separate cells formed after cleavage are of the same size, volume, and shape because they do not move. 

  • Human reproductive cleavage is the holoblastic type with no yolk. 

  • Cleavage is a fractionating process.

FAQ (Frequently Asked Questions)

1. What happens to Cells when Embryos undergo Cleavage?

Ans: The cells undergo division to form multicellular organisms, with each part developing individually. However, there is no increase in mass, but the zygote undergoes division to multiple cells of a similar kind. It is the general case of cleavage in the human zygote. However, other organisms lay eggs under meroblastic cleavage, which is a partial embryo development case. Humans undergo Holoblastic division as they do not have egg yolk. 


As a result, different cells combine to form a compact mass which we call a morula. Once the blastula is formed, the stage of cleavage stops as a result. However, it is different from the basic cell division as there is no increase in cytoplasmic mass. 

2. Why does Cleavage take Place?

Ans: Cleavage is the basic division of cells to form a multicellular organism. It is to ensure that no more sperms will enter to fertilise the same egg. As the cleavage takes place, acrosomal reactions take place at one location of the membrane. On the other location of the membrane, eggs release proteins. Thus it avoids other sperms to fertilise the egg. However, if cleavage does not take place, multiple sperms will fertilise the same egg and results to form polyspermy.


Moreover, cleavage organises embryos into developing regions. Thus, it helps the egg reach the next stage of implantation to develop these cells into individual organs further.