Key Difference – Holoblastic vs Meroblastic Cleavage
Holoblastic cleavage is referred to entire embryonic cell cleavage while meroblastic cleavage is referred to partial embryonic cell cleavage. This is the key difference between Holoblastic and Meroblastic Cleavage.
Cleavage is defined as the division of cells during the early embryonic stage. This occurs once the fertilization step is completed and the zygote is formed. Cleavage is initialized with the activation of cyclin defendant kinase. Two types of cleavages are found depending on the amount of yolk present in the egg. They are holoblastic cleavage or meroblastic cleavage.
CONTENTS
1. Overview and Key Difference
2. What is Holoblastic Cleavage
3. What is Meroblastic Cleavage
4. Similarities Between Holoblastic and Meroblastic Cleavage
5. Side by Side Comparison – Holoblastic vs Meroblastic Cleavage in Tabular Form
6. Summary
What is Holoblastic Cleavage?
Holoblastic cleavage is defined as a type of cleavage that takes place in the embryonic cells that do not contain a large amount of yolk (moderate to sparse amount of yolk). This type of cleavage takes place in isolecithal cells. Isolecithal refers to even distribution of yolk in the cytoplasm of the mammalian ovum.
Holoblastic cleavage could be of four major cleavage types; bilateral holoblastic, radial holoblastic, rotational holoblastic and spiral holoblastic. Bilateral holoblastic cleavage is said to be the first type of cleavage that bisects the zygote into two halves; left and right. Radial cleavage is characterized by the arrangement of the blastomeres of each upper tier directly over those of the next lower tier resulting in radial symmetry around the pole to pole axis of the embryo.
Figure 01: Holoblastic Cleavage
During rotational holoblastic cleavage, the normal first division that takes place along the meridional axis and then rotated in 90 degrees and gives the other cells. Spiral holoblastic cleavage occurs in a spiral manner around the pole to pole axis of the embryo.
What is Meroblastic Cleavage?
Meroblastic cleavage is defined as a type of cleavage that takes place in a fertilized egg cell with a large amount of yolk and undergoes a partial cleavage. Meroblastic cleavage could be categorized into two sections; discoidal cleavage and superficial cleavage.
During discoidal cleavage, the cleavage furrow that is developed does not penetrate the yolk. This type of cleavage could be commonly found in species such as monotremes, avians, reptiles and fish that contains telolecithal eggs.
Figure 02: Meroblastic Cleavage
During superficial cleavage, the process of mitosis takes place without cytokinesis. Superficial cleavage results in a polynuclear cell. Here, the yolk is positioned at the center of the egg cell where the nuclei get migrated to the periphery of the egg.
What are the Similarities Between Holoblastic and Meroblastic Cleavage?
- Both Holoblastic and Meroblastic Cleavage are two types of cleavages.
- Both take place during the embryonic stage.
- Both are triggered by cyclin-dependent kinase complex.
- Both cleavages end with the formation of the blastula.
What is the Difference Between Holoblastic and Meroblastic Cleavage?
Holoblastic vs Meroblastic Cleavage |
|
| Holoblastic cleavage is defined as a type of cleavage that takes place in the embryonic cells that contain a moderate or sparse of yolk in the ovum. | Meroblastic cleavage is defined as a type of cleavage that takes place in a fertilized egg cell with a large amount of yolk and undergoes partial cleavage. |
| Mitosis | |
| No mitosis occurs in holoblastic cleavage. | Mitosis takes place in meroblastic cleavage. |
| Amount of Yolk in the Egg | |
| Smaller amount of yolk is present in eggs that show holoblastic cleavage. | Larger amount of yolk is present in eggs that show meroblastic cleavage. |
| Type of Cleavage | |
| Holoblastic cleavage results in a complete cleavage. | Meroblastic cleavage results in a partial cleavage. |
| Subtypes | |
| Bilateral holoblastic, radial holoblastic, rotational holoblastic and spiral holoblastic are types of holoblastic cleavage. | Discoidal cleavage and superficial cleavage are types of meroblastic cleavage. |
| Synonyms | |
| Holoblastic cleavage – Total cleavage and complete cleavage are synonyms of holoblastic cleavage. | Incomplete cleavage or partial cleavages are synonyms for meroblastic cleavage. |
| Examples | |
| Most of the deuterostomes and protostomes such as amphibians, mammals, echinoderms, annelids, flatworms, nematodes, etc show holoblastic cleavage. | Monotremes, avians, reptiles show meroblastic cleavage. |
Summary – Holoblastic vs Meroblastic Cleavage
Cleavage is defined as the division of cells that takes place during early embryology. It is of two types; holoblastic cleavage and meroblastic cleavage. This depends on the amount of yolk present in the egg. Holoblastic cleavage results in complete cleavage while meroblastic cleavage results in partial cleavage. The final result of both cleavages is a blastula. This is the difference between holoblastic cleavage and meroblastic cleavage.
Reference:
1.“Holoblastic Cleavage.” Study.com, Study.com. Available here
2.“Cleavage.” Cleavage. Available here
3.“Meroblastic Cleavage.” Study.com, Study.com. Available here
Image Courtesy:
1.’Spiral cleavage in Trochus’By Morgan Q. Goulding (CC BY 2.5) via Commons Wikimedia
2.’Equal vs unequal cleavage’By C.orosco – Own work (Public Domain) via Commons Wikimedia
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