Cleavage In Biology

The development of a zygote from a single cell to a healthy, multicellular organism is an impressive work of biological processes. Cleavage is a fundamental phenomenon that lies at the core of the growth of living beings. It initiates embryonic development and determines the fate of every living being.

What is Cleavage?
Types of Cleavage
Significance of Cleavage
Practice Problems
Frequently Asked Questions

What is Cleavage?

Cleavage is defined as the cellular divisions observed at the initial stage of zygote formation. The phenomenon is observed after fertilisation, as evidenced by using the term zygote. The process aims to increase cell numbers of the same size rather than growth. These cells skip G1 and G2 for rapid and synchronous cell division. The resultant cells are referred to as blastomere, which further progress to form morula and, subsequently, blastula.

Cleavage serves a function in embryogenesis because it sets the basic skeletal layout and forms the basis for further growth. The structure and layout of cleavage divisions determine the arrangement and fate of cells within a growing embryo.

Alt text: Cleavage

Source: Vedantu

Types of Cleavage

The types of cleavage can be based on the amount of yolk in the egg, degree of reorganisation and cytoplasmic composition. Here are different types of cleavage:

Determinate Cleavage

It is also referred to as mosaic cleavage; the division here is dependent on the potency of blastomere. The fate and determination of the blastomere’s development types are already chosen in this kind of cleavage. This type of cleavage is seen in protostomes.

Indeterminate Cleavage

As compared to the determinate cleavage, the quality of blastomeres is equal in indeterminate cleavage. They can develop into embryos and become complete organisms in isolation. It is seen in deuterostomes.

Holoblastic Cleavage

It involves the complete division of blastomeres, leading to doubling the number of blastomeres. Thus, it is also known as complete cleavage. The holoblastic cleavage is characterised by moderate to low egg yolk quantity. It is observed in animals like amphibians, nematodes, mammals, flatworms, echinoderms and others.

It is of further two types, isolecithal and mesolecithal. The isolecithal is again subdivided into four categories, radial, spiral, bilateral and rotational. The isolecithal cleavage is marked by sparse and even distribution of yolk, while mesolecithal comprises moderate level of vegetal yolk deposition. The radial cleavage is seen in echinoderms and amphioxus, the spiral is seen in annelids, flatworms and molluscs, bilateral is seen in tunicates, and rotational is evident in mammals and nematodes. The mesolecithal cleavage produces only the radial type visible in amphibians.

Meroblastic Cleavage

It is also referred to as partial cleavage, evident in eggs with high yolk concentration. It is of two types, telolecithal and centrolecithal. Telolecithal is observed in eggs with dense yolk throughout the maximum part of the cell, while centrolecithal contains yolk at the centre of the egg. Further, telolecithal is sub-categorized as bilateral and discoidal, while centrolecithal sees superficial cleavage. Bilateral is seen in cephalopods and molluscs with bilateral division.

Discoidal cleavage is observed in reptiles, monotremes, birds and fish. The cleavage furrows remain on top of the yolk without penetrating inside. Here the embryo develops a blastodisc, or a disc-shaped group of cells. Superficial cleavage involves karyokinesis resulting in polynuclear cells. It is observed in arthropods where nuclei move to the outer edge of the egg near the yolk. The plasma membrane develops inward, leading to cell division.

Transitional Cleavage

Many eggs have an uncommon cleavage that is neither normal holoblastic nor normal meroblastic but develops through a stage between the two.

Significance of Cleavage

The cleavage is the first morphological process occurring in the zygote. It holds multiple significance, such as:

Converts single-celled zygote into a multicellular embryo
Preserves quantitative ratio of nucleus and cytoplasm
Preserves cell size
Involves quick division
The products, blastomeres, are necessary for embryo development
Cytoplasmic distribution occurs

Practice Problems

Q1. After how many hours does the first cleavage take place?

A. 12 hours
B. 24 hours
C. 30 hours
D. 72 hours

Ans. C. 30 hours

The cleavage takes place after 30 hours of fertilisation.

Q2. Discoidal and superficial cleavage are which type of cleavage?

A. Equal holoblastic
B. Unequal holoblastic
C. Meroblastic
D. Transitional

Ans. C. Meroblastic

Meroblastic is divided into discoidal and superficial cleavage.

Q3. Which process results in the division of fertilised eggs?

A. Invagination
B. Regeneration
C. Cleavage
D. Oogenesis

Ans. C. Cleavage

Cleavage results in the division of fertilised eggs.

Frequently Asked Questions

Q1. Which type of division is seen in cleavage?
Ans. The cleaving cells undergo mitotic division.

Q2. Why is cleavage called the fractionating process?
Ans. The cleavage lacks the growth phase or interphase, which leads to the shrinking of cells rather than growth. Hence, it is called the fractionating process.

Q3. Which phase of a fertilised egg marks the ending of cleavage?
Ans. The formation of a blastula marks the end of the cleavage process.