Cleavage in Biology

Cleavage occurs when a series of mitotic divisions occur after egg fertilization. The Zygote undergoes rapid succession to transform the unicellular body into a multicellular organism. The mitotic division leads to numerous smaller nucleated cells called blastomeres, resulting in a hollow spherical body called a blastula.

1.0What is Cleavage?

• All sexually multicellular organism bodies have developed from a single cell formed after the fusion of male (spermatozoa) and female gametes (ovum). The process of further cell division starts called Cleavage just after fertilization. 
• A series of mitotic cell divisions divides the single cell into 16 cells called monomers. Four mitotic divisions follow, converting the single cell into 2, 4, 8, and 16 mesomeres. When the stages of 16 cells are obtained, they transform into a multicellular structure called a morula. 
• This division is further transformed into blastula by successive cell division. This blastula has a single layer of the blastoderm, and there is no ovum growth during blastula formation. The general shape doesn't change for the embryo except for forming a blastocoel cavity. 
• The chemical change of glycogen and Yolk into molecules of nuclear material like DNA, RNA, and nucleoproteins occurs. The sudden cell division increases the number of nuclei, which leads to an unfavorable ratio between the nuclei and the cytoplasm. This is brought to normal by the end of blastulation. 

2.0Patters of cleavage

Characteristics

• Thousands of cells called blastomeres are formed. 
• Cleavage produces a hollow sphere of cells called a blastula. 
• Cleavage forms the cell to create tissue and organs. 
• Embryo shape and size don't change during Cleavage. 
• Cleavage brings a proposition between nuclear and cytoplasmic material. 
• Convert Yolk and glycogen into the cytoplasm, then into a molecule of a nuclear substance.

Principles of cleavage

All cleavages follow common basic principles or laws. They are as follows: 

Sach's law

Sach, in 1877, proposed two rules. 

Rule 1: Cleavage divisions occur uniformly and are subject to the uniform Yolk stored in them. It will only be uniform if the Yolk is evenly stored in them. 

Rule 2: Successive Division is at a right angle to its previous division. 

Hertwig law

O. Hertwig, in 1881, proposed two rules: 

Rule 1: Nucleus and mitotic spindle are found at the centre of blastomeres. 

Rule 2: Spindle fibrosis is the longest axis of the egg. 

Pfluger's Law

Eduard Friedrich Wilhelm Pflüger states that spindle fibres form in the region of less Yolk. 

Balfour's Law

Balfour, in 1885, stated that the Cleavage rate is inversely proportional to the amount of Yolk.

Types of cleavage

Cleavages are of different types due to changes in egg organization. They are of the following types, namely

Radial Cleavage

• Cleavage divides the Zygote in radial symmetry. 
• Their cleavage divisions are at a right angle to their previous division. 
• The successive divisions of the cell are placed just above the previous blastomeres. 
• Thus, the new four blastomeres are arranged just above their previous ones. 
• Thus, partitioning the blastula along any plane produces two identical halves. Examples: Echinodermata, Chordata, Frog. 

Biradial Cleavage

• Cleavage has two different patterns in the mitotic division. 
• The First two mitotic divisions are meridional, and the third division is vertical. 
• Thus, the 8 blastomeres formed don't stand at a right angle to each other. Examples: Polychaeta and Ctenophora.

Bilateral Cleavage

• This Cleavage has two identical halves when the blastula is cut vertically. 
• It can be right or left. This Cleavage occurs due to unequal holoblastic.
• The plane of bilateral symmetry is established by the plane of the first cleavage furrow—for example, in higher mammals, amphibians, and tunicates.

Spiral Cleavage

• This Cleavage has a rotational movement of cell parts around the egg's north or south pole axis. 
• This led to the inclination of the mitotic spindle concerning symmetry radii. 
• Thus, each division produces one bigger cell (macromere) and a smaller cell (micromere). 
• The following Cleavage increases inclination, resulting in an arrangement in a spiral shape. 
• If the rotation is in a clockwise direction, it is called a dextral or right-handed cleavage; else, right-handed Cleavage; otherwise, if it is in an anti-clockwise direction, it is called a sinistral or left-handed Cleavage. Examples: Nematoda, Rotifers, Annelids, Mollusca, and Annelids.

Determinate and Indeterminate Cleavage

Cleavages are also classified according to the potential of the blastomeres to develop for future development. They are classified as determinate and indeterminate Cleavage.

Determinate Cleavage

• In this Cleavage, the area has been marked for the development of a region from different parts of the egg quite early before the onset of Cleavage. In the ascidian egg, a region marked for endoderm development was removed. It resulted in the absence of an endoderm when the embryo was later formed. Examples: Nematodes, Annelids, Mollusks, and Ascidian. 

Indeterminate Cleavage

• In this Cleavage, no area is marked for a region's development. This Cleavage is quite flexible. A region generally used for endoderm development was removed from fertilized eggs of sea urchins, yet the endoderm was developed in the final embryo. This Cleavage cuts the eggs into segments, with each segment potentially developing in any region—for example, All Vertebrates and some species of Echinoderms. 

3.0Planes of Cleavage

Cleavage has also been divided based on Yolk and its distribution in an egg. The following patterns have been observed:

Total or Holoblastic Cleavage

The entire cell is divided by each furrow. This has been further divided into: 

Equal holoblastic

This Cleavage produces blastomeres of equal size in any symmetry. This symmetry can be radial, biradial, spiral, and bilateral. This Cleavage is found in microlecithal and isolecithal eggs. Examples: Amphioxus, Marsupials, and placental mammals.

Unequal holoblastic

This Cleavage produces blastomeres of unequal size in any symmetry. This symmetry can be radial, biradial, spiral, and bilateral. They produce small-size blastomeres called micromeres and large-size blastomeres called macromeres. This Cleavage is found in mesolecithal and moderately telolecithal eggs—for example, Lower Fish and amphibians.

Meroblastic Cleavage

This Cleavage divides the cell partially, resulting in the creation of unequal-size micromeres. This Cleavage is found in eggs with a patch of yolk-free cytoplasm called a blastodisc. The first two or three leaves divide the blastodisc vertically but never reach the bottom of the blastodisc. The yolk part of the ovum is never cut by furrow. 

Discoidal

This type of Cleavage is found in acrolectal and telolecithal eggs. The Cleavage is restricted to the disc-shaped active cytoplasm of the animal pole—for example, bony fish, reptiles, and birds.

Superficial

This type of Cleavage is found in a centrolecithal egg. The Cleavage is restricted to the peripheral cytoplasm of an egg. The zygote nucleus divides without cytoplasm division. So, many nuclei are formed and embedded in the superficial layer of the cytoplasm—for example, Arthropods and Insects.

4.0Metabolic Changes During Cleavage

Chemical change during Cleavage: A significant change occurs during the process. They are namely:

1. Increased Nuclear Material: A steady increase in the genetic material (predominantly DNA) has been seen. The egg's cytoplasm contains mitochondria and yolk platelets that aid in increased nuclear material by acting as a source. A lot of energy is required to facilitate the movement of genetic material towards the pole. These ATP molecules are made in ooplasm and mitochondria through glycolysis and aerobic oxidation of Yolk, glycogen, and other energy-yielding molecules. A continuous supply of deoxyribonucleotides, ribonucleotides, purines, pyrimidines, amino acids, and ribose is needed to synthesize DNA and RNA. 

2. RNA Synthesis: mRNA (messenger RNA) and tRNA (Transport RNA) are largely synthesized during Cleavage.

3. Synthesis of Protein: There has been a steady increase in protein synthesis during the entire process of Cleavage. 

5.0Morulation and Blastulation

• Cleavage splits the fertilized egg into smaller cells called blastomeres. These blastomeres increase in the typical double sequence of 2, 4, 8, 16 and so on. 
• Cleavage forms the layers where a stacking gel loosely joins together each layer. 
• This heap of cohering, sticky blastomeres is known as Morula. It has been named because it resembles mulberry (Morula means mulberry in Latin). 
• The arrangement of blastomeres varies among animals. For example: In a megalecithal egg, a plano convex-like mass of blastomere is formed. 
• The morula stage is followed by the next phase of development, which is called a blastula. Cleavage led to an increase in the number of blastomeres. 
• This blastomere undergoes a rearrangement, arranging itself into a single-cell thick epithelium called blastoderm. A fluid-filled space or cavity called blastocoel appears between the blastomeres. 
• This hollow, spherical and nonepithelial thick embryonic stage is called a blastula. This process of creating a blastula is known as blastulation.

Types of Blastula

There are six types of blastula found in the animal kingdom, with differing factors like the egg size, the amount and distribution pattern of Yolk, and the rate of Cleavage.

1. Coeloblastula: This blastula is hollow, and the blastocoel is surrounded by a single-layer cell. Examples: Echinoderms, Amphioxus, and frogs. 

2. Stereoblastula: This blastula is solid and has no blastocoel. Examples: Annelids, Molluscs, Nemerteans, and some species of planarians. 

3. Discoblastula: This blastula is a multilayered flat disc at the animal pole separated by narrow segmentation from the Yolk. It is found in eggs with a large and developed yolk. Examples are reptiles, Birds, Protosterians, and Fishes. 

4. Blastocyst: This blastula has a regular cleavage and a small cavity inside each cell. Two types of cells are found: the outer layer of epithelial cells containing nutritive cells and the inner mass of the formative cell. Example: Mammals. 

5. Superficial blastula or peri blastula: This blastula has a blastocoel filled with Yeast and surrounded by a peripheral layer of cells. Examples: Insects. 

6. Amphiblastula: This blastula is made up of two different types of structurally different blastomeres. Example: Amphibian

6.0Significance of Blastulation

• Regardless of the shape of the blastula, major presumptive organ forming areas of the future embryonic body are segregated into definite parts of the blastoderm. 
• The exact pattern of arrangement of presumptive organ-forming areas varies from species to species. 
• Blastocoel permits the migration and rearrangement of the major presumptive organ-forming areas during gastrulation.

7.0Cell Cleavage Mechanism

Cleavage occurs through two key events: the division of the nucleus (mitotic karyokinesis) followed by the division of the cytoplasm (cytokinesis). Both events encompass a variety of metabolic processes.

Two coordinated processes occur during cleavage

• Karyokinesis: The mitotic division of the nucleus depends upon the formation of the mitotic spindle. The mitotic spindle is constituted by microtubules of which the tubulin protein is the structural unit. If the egg is treated with the drug colchicine the microtubules are disrupted and karyokinesis is arrested at metaphase. 
• Cytokinesis: Division of the cell depends upon the contractile microfilaments of which protein actin is the structural unit. A ring of microfilament appears in the cortex around the cell where the cleavage furrow is formed. Contraction of the microfilament ring in a purse-string manner deepens the furrow, ultimately cutting the cell into two . Treatment of the egg with cytochalasin B inhibits the organization of the contractile ring of microfilaments so that a cleavage furrow is not formed and cytokinesis does not take place.