What Are the Types and Characteristics of Embryonic Cleavage?
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.
FAQs on Cleavage in Embryo: Definition, Stages & Importance
1. What exactly is cleavage in the context of embryonic development?
In embryology, cleavage refers to the series of rapid mitotic cell divisions that the zygote undergoes immediately after fertilisation. Unlike normal cell division, these divisions occur without any significant growth in the overall size of the embryo. The primary result is the transformation of a large, single-celled zygote into a multicellular structure called the morula, and subsequently the blastula.
2. What is the difference between holoblastic and meroblastic cleavage?
The main difference lies in how completely the zygote divides, which is determined by the amount of yolk in the egg.
- Holoblastic Cleavage: This is a 'total' or 'complete' cleavage where the division furrow passes through the entire egg. It occurs in eggs with little to no yolk (alecithal or microlecithal eggs), such as in humans and other placental mammals.
- Meroblastic Cleavage: This is an 'incomplete' or 'partial' cleavage where the division is restricted to a small part of the egg, typically the region free of yolk. It occurs in eggs with a large amount of yolk (macrolecithal eggs), such as in birds, reptiles, and fish.
3. What are the key characteristics of the cleavage process?
Cleavage has several unique characteristics:
- It is a series of rapid mitotic divisions.
- There is no growth phase, so the overall size of the embryo does not increase.
- The cells formed during cleavage, called blastomeres, become progressively smaller with each division.
- The ratio of nuclear volume to cytoplasmic volume (nucleo-cytoplasmic ratio) increases.
- Oxygen consumption is very high due to the rapid metabolic rate.
4. How is cleavage different from standard mitotic cell division?
While cleavage involves mitotic divisions, it differs from standard mitosis in a crucial way. In regular mitosis, the daughter cells grow back to the size of the parent cell during the interphase (growth phase) before dividing again. During cleavage, the interphase is very short and lacks the G1 (growth) phase. As a result, the cells (blastomeres) divide without increasing in size, leading to a rapid increase in cell number but no change in the total mass of the embryo.
5. What are blastomeres and what is their significance?
Blastomeres are the individual cells produced by the cleavage of a zygote. The first cleavage divides the zygote into two blastomeres, the second into four, and so on. These cells are significant because they are the fundamental building blocks of the early embryo. Although they start as undifferentiated, they eventually differentiate to form all the tissues and organs of the developing organism.
6. Why doesn't the embryo increase in size during cleavage?
The embryo does not increase in size during cleavage because the cell cycles lack the G1 and G2 growth phases. The zygote is a very large cell, and the purpose of cleavage is to quickly partition its vast cytoplasm into a larger number of smaller, more manageable cells (blastomeres). This rapid division without growth allows for the swift formation of a multicellular blastula, a necessary step before implantation and further development can occur.
7. How does cleavage in a human embryo occur?
Cleavage in a human embryo is holoblastic (complete), radial, and indeterminate. It begins about 30 hours after fertilisation as the zygote travels down the fallopian tube. The divisions continue, forming a solid ball of 16-32 cells called a morula by the end of the fourth day. This morula then enters the uterus and develops into a blastocyst, which is the stage that implants into the uterine wall.
8. What is the main purpose or function of cleavage?
The primary function of cleavage is to convert a single-celled zygote into a multicellular embryo, called a blastula. This process achieves two main goals:
- It produces a large number of cells required for building the future body of the organism.
- It restores the normal nucleo-cytoplasmic ratio that is characteristic of somatic cells, by subdividing the massive cytoplasm of the egg.
9. What developmental stage comes immediately after cleavage?
The stage immediately following cleavage is the formation of the blastula (or blastocyst in mammals). Cleavage ends once the blastula is formed, which is typically a hollow sphere of cells surrounding a fluid-filled cavity called the blastocoel. This marks the transition from simple cell division to the next major phase of embryonic development, which is gastrulation, where cells begin to move and reorganise to form the primary germ layers.
