What is the Difference Between Holoblastic and Meroblastic Cleavage

The main difference between holoblastic and meroblastic cleavage is that the holoblastic cleavage occurs in the absence of a large concentration of yolk in the fertilized egg cell whereas meroblastic cleavage occurs in the presence of a large concentration of yolk. Therefore, in the holoblastic cleavage, the cleavage furrow penetrates the egg completely while, in the meroblastic cleavage, the cleavage furrow does not completely penetrate the egg. Hence, holoblastic cleavage is a type of total or entire cleavage while meroblastic cleavage is a type of partial cleavage. Furthermore, holoblastic cleavage gives rise to different types of special arrangements in animals including bilateral, radial, rotational, and spiral cleavage while meroblastic cleavage gives rise to discoidal and superficial cleavage.

Holoblastic (complete) and meroblastic (incomplete) cleavages are two types of cell divisions or cleavages that occur in the zygote without a significant growth classified based on the amount of yolk present in the fertilized egg.

Key Areas Covered

1. What is Holoblastic Cleavage
     – Definition, Features, Types, Examples
2. What is Meroblastic Cleavage
     – Definition, Features, Types, Examples
3. What are the Similarities Between Holoblastic and Meroblastic Cleavage
     – Outline of Common Features
4. What is the Difference Between Holoblastic and Meroblastic Cleavage
     – Comparison of Key Differences

Key Terms

Cell Division in Zygote, Cleavage Furrow, Holoblastic Cleavage, Meroblastic Cleavage, Yolk

What is Holoblastic Cleavage

Holoblastic cleavage is a type of cleavage that occurs in the fertilized egg in which the cleavage furrow completely penetrates the egg. Here, the cleavage furrow is a furrow that bisects the two poles of the egg by passing through the median axis or center of the egg. Moreover, this complete penetration occurs in the absence of a high amount of yolk in the fertilized egg. Also, the yolk is the nutrient-containing portion of the egg. In addition, the pole of the egg that contains the highest amount of yolk is known as the vegetal pole, and the pole containing the lowest amount of yolk is known as the animal pole.

Furthermore, holoblastic cleavage can be divided into two categories based on the amount of yolk present in the egg. They are the isolecithal and mesolecithal cleavage. The yolk accumulation of the isolecithal eggs is sparse and even. On the other hand, mesolecithal eggs contain a moderate yolk with a vegetal disposition. Moreover, the isolecithal yolk accumulation gives rise to four patterns of cleavage: radial cleavage in echinoderms, hemichordates, and amphioxus; spiral cleavage in annelids, most mollusks, and flatworms; bilateral cleavage in tunicates, and rotational cleavage in placental mammals, nematodes, and marsupials.

Figure 1: Spiral Cleavage

Furthermore, radial cleavage forms spindle axes parallel or at right angles to the polar axis. Spiral cleavage proceeds through the formation of four macromeres or blastomeres within the first two cell divisions, which are not oriented in planes. These macromeres give rise to smaller macromeres with successive cleavage cycles. Also, bilateral cleavage bisects the egg into the left and right halves. The first two cell divisions of the rotational cleavage occur along the meridional axis. In addition, mesolecithal yolk accumulation gives rise to a displaced radial cleavage similarly to those that occur in amphibians and some fish.

What is Meroblastic Cleavage

Meroblastic cleavage is another type of cleavage that occurs in the fertilized egg in which the cleavage furrow incompletely penetrates the egg. The incomplete penetration occurs due to the presence of a high amount of yolk in the egg. Therefore, meroblastic cleavage is a type of unequal cleavage. The two types of meroblastic cleavage are telolecithal and centrolecithal cleavage. Telolecithal eggs contain a dense yolk while centrolecithal eggs contain their yolk in the center of the egg.

Figure 2: Unequal Cleavage

Discoidal and superficial cleavage are the two types of meroblastic cleavage. In the discoidal cleavage, the cell division of the egg gives rise to a disk of cells called the blastodisc on the top of the yolk rather than forming a penetrating cleavage furrow. Telolecithal eggs of monotremes, birds, reptiles, and some fish undergo discoidal cleavage. On the other hand, in the superficial cleavage, the cells only undergo nuclear division, but not cytokinesis. This results in a polynuclear cell. Since the yolk occurs in the center of the egg, these nuclei migrate to the periphery of the egg while the plasma membrane grows inward. The centrolecithal eggs of monotremes undergo superficial cleavage.

Similarities Between Holoblastic and Meroblastic Cleavage

• Holoblastic and meroblastic cleavages are two types of cell divisions or cleavages that occur in the fertilized egg or the zygote.
• They are classified based on the yolk accumulation during the cleavage and they form the blastomeres.
• Also, both give rise to different special arrangements in different phyla of animals.

Difference Between Holoblastic and Meroblastic Cleavage

Definition

Holoblastic cleavage refers to the complete division of an isolecithal or mesolecithal egg into blastomeres while meroblastic cleavage refers to the incomplete cleavage in telolecithal or centrolecithal eggs resulting in unequal blastomeres. Thus, this is the main difference between holoblastic and meroblastic cleavage.

Type of Cleavage

While holoblastic cleavage is a complete cleavage, meroblastic cleavage is an incomplete cleavage.

Cleavage Furrow

Also, another major difference between holoblastic and meroblastic cleavage is that the cleavage furrow goes all the way through the egg in the holoblastic cleavage while the cleavage furrow does not go through the complete egg in the meroblastic cleavage.

Type of Accumulation of the Yolk

Besides, fertilized eggs with a very low amount of yolk undergo holoblastic cleavage while fertilized eggs with a high amount of yolk undergo meroblastic cleavage.

Types Based on the Yolk Distribution

Moreover, holoblastic cleavage occurs in isolecithal and mesolecithal cells while meroblastic cleavage occurs in telolecithal and centrolecithal cells. Hence, this is another difference between holoblastic and meroblastic cleavage.

Patterns

Furthermore, holoblastic cleavage gives rise to radial, bilateral, spiral, and rotational cleavage while meroblastic cleavage gives rise to discoidal and superficial cleavage.

Occurrence

One additional difference between holoblastic and meroblastic cleavage is that the holoblastic cleavage occurs in small eggs containing moderate to sparse yolk while meroblastic cleavage occurs in comparatively large eggs containing a high amount of yolk.

Examples

The cleavage of amphibians, mammals, non-vertebrate chordates, echinoderms, most mollusks, annelids, flatworms, and nematodes are the examples of a holoblastic cleavage while the cleavage of birds and reptiles are examples of meroblastic cleavage.

Conclusion

Holoblastic cleavage is a type of cleavage or cell divisions in the fertilized egg, characterized by the complete penetration of the egg by the cleavage furrow. This occurs due to the presence of a lower amount of yolk in the egg. In comparison, meroblastic cleavage is another type of cleavage characterized by the incomplete penetration of the egg by the cleavage furrow. It occurs due to the presence of a high amount of yolk in the egg. Therefore, the main difference between holoblastic and meroblastic cleavage is the type of penetration of the egg by the cleavage furrow as a result of the amount of yolk present in the fertilized egg.

References:

1. “DEVELOPMENTAL BIOLOGY 3230.” Biology Courses – The University of Utah, Available Here.

Image Courtesy:

1. “Spiral cleavage in Trochus” By Morgan Q. Goulding – Goulding MQ. 2009. (CC BY 2.5) via Commons Wikimedia  
2. “Equal vs unequal cleavage” By C.orosco at en.wikipedia (Public Domain) via Commons Wikimedia