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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:


  • The first happens in 30 hours post fertilization. The first cleavage divides the zygote longitudinally into two blastomeres, with one being a bit larger than the other. 

  • The second cleavage occurs within forty hours after fertilization and is once again longitudinal. It is at 90 degrees to the plane of the first cleavage, creating four blastomeres. 

  • The third cleavage happens about 72 hours after fertilization. This time again the division is longitudinal, but at 90 degrees to both first and the second divisions.  

  • During these stages, the young embryo moves slowly down the fallopian tube towards the uterus. At the end of the fourth day when the embryo reaches the uterus, the morula has thirty-two cells. 

  • Also, cleavage is radial in the human zygotes. This means that the blastomeres are arranged in a radial plane around the polar axis.


Significance of the Cleavage

  • The cleavage converts a unicellular zygote into a multicellular embryo.

  • It produces a large number of cells or blastomeres that are needed for the building of the baby’s body.

  • The  size of the cell and the ratio of nucleo-cytoplasma is cleavage. 

  • During cleavage, the blastomeres go through quick mitotic division, after which there is no growth of blastomeres.

  • Cleavage facilitates the distribution of cytoplasm among the blastomeres.


Factors That Cleavage Of An Embryo Depend On:

  • Cleavage can form in two ways - holoblastic (total) cleavage or meroblastic (partial) cleavage. 

  • The type depends on how much yolk the egg contains. In placental mammals like humans, where nourishment to the egg is provided by the mother’s body, the eggs have a very small amount of yolk and undergo holoblastic cleavage. Other species (such as birds) have a lot of yolk in the egg to nourish the embryo during development. These species undergo meroblastic cleavage.

  • The pattern of embryonic cleavage is determined by two major parameters: the amount of yolk protein, its distribution within the cytoplasm, and factors inside the egg cytoplasm and the timing of its formation.


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.

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FAQs on Cleavage - Embryo

1.What happens to cells when embryos undergo cleavage?

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?

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. 

3.What stage follows the cleavage stage in embryo development?

After the cleavage has produced over 100 cells, the embryo is called a blastula. The blastula refers to a spherical layer of cells (the blastoderm) that surround a fluid-filled or yolk-filled cavity. Mammals form a structure called the blastocyst at this stage. The blastocyst is characterised by an inner cell mass that is distinctly placed from the surrounding blastula. At the cleavage stage, the cells divide without any increase in mass. A large single-celled zygote divides itself into multiple smaller cells. An individual cell inside the blastula is called a blastomere.

4.What are the characteristics of cleavage?

Some of the characteristics of the cleavage are:


  • The daughter cells are genetically similar to the parental cell as the cleavage involves a series of mitotic divisions.

  • A number of divisions depending upon the volume of the egg and typical somatic cell. 

  • Oxygen consumption is very rapid during cleavage.

  • Cleavage is called the fractionating process as there is no growth during this interphase period. In this phase, the size of blastomeres gets smaller and smaller. 

  • The size, volume and general shape of the blastula remain the same throughout the cleavage.

  • The pattern and speed of cleavage are determined by the cytoplasmic yolk and not the nucleus. 

5.What is an embryo?

Embryo refers to the early developmental stage of an animal when it is in the egg or in the uterus of the mother. In humans, the term is used until the end of the seventh week following conception after it is called a fetus.


When the ovum unites with the sperm a zygote or fertilized egg is formed. The zygote undergoes a series of cell divisions called cleavages during its journey down the fallopian tube. After several cleavages, the cells form a blastula. The blastula then gets attached to the uterine lining, leading to the formation of a placenta. The nutrient from the mother is transferred to the growing embryo by the placenta. In some animals, the embryo gets its nourishment from the yolk.


Through gastrulation, the embryo differentiates into three types of tissue: 


  • The ectoderm: Producing the skin and nervous system.

  • The mesoderm: Development of connective tissues, muscles, bones and the circulatory system.  

  • The endoderm: Formation of the digestive system, lungs, and urinary system. 


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