Cleavage

Cleavage is the division of cells that occurs early in embryonic development after fertilisation. A large number of species’ zygotes go through a rapid number of cell cycles with little overall growth, resulting in a cluster of cells that is the same size as the original zygote. 

The blastomeres, which are the individual cells formed from cleavage, group together to form a compact mass known as the morula. 

The cleavage process comes to a close with the creation of the blastula.

Cleavage differs from other forms of cell division in that it increases the number of cells and the amount of nuclear mass-produced without increasing the amount of cytoplasm produced. 

This means that with each subsequent subdivision, the amount of cytoplasm in each daughter cell decreases by approximately half compared to the amount of cytoplasm in the parent cell, and the ratio of nuclear to cytoplasmic material increases.

Characteristics of the cleavage process

(a) Because cleavage entails a sequence of mitotic divisions, offspring cells are genetically identical to their parent cells in all respects.

A mitotic division is repeated until the average cell size characteristic of the parental organism is reached.

(b) The number of divisions is determined by the volume of the egg and the size of a typical somatic cell. Blastomeres are the cells that are produced as a result of this process.

(c) Because there is no growth during the interphase interval during cleavage, the size of blastomeres continues to shrink as the process progresses. As a result, the cleavage is referred to as the fractionating process.

(d) Because the blastomeres do not move during the cleavage, the size, volume, and general form of the blastula stay constant throughout the process.

(e) It is also worth noting that as the cleavage occurs, the nuclear-cytoplasmic ratio continues to rise. Consequently, DNA synthesis happens at a breakneck pace during cleavage, while there is no cytoplasmic synthesis during the breakdown process. The cytoplasmic resources are quickly utilised in the process of DNA replication.

(f)When cleavage occurs, oxygen consumption is likewise quite quick.

(g) Blastomeres exhibit synchronous division in the early stages of development, but this synchrony is lost later on.

(h) Rather than the nucleus, the cytoplasmic yolk (deutoplasm) is responsible for determining the pattern and speed of cleavage. The yolk tends to prevent cleavage from occurring.

Types Of Cleavage

1. Determinate: Determinate cleavage (also known as mosaic cleavage) can be found in the majority of protostomes. As a result, the developmental fate of the cells is determined at an early stage in the embryonic growth process.

2. Indeterminate: Unchanged animal/vegetal cytoarchitectural traits are required for cells to be indeterminate for them to be indeterminate. 

3. Radial: The deuterostomes, which include certain vertebrates and echinoderms, are characterised by radial cleavage in their bodies.

4. Rotational: The process of rotational cleavage begins with a normal initial division along the meridional axis, which results in the formation of two daughter cells. 

5. When this cleavage occurs, one of the daughter cells divides meridionally, whilst the other divides equatorially, and this is the difference between them.

6. Spiral: It is preserved among numerous members of the lophotrochozoan taxonomic group known as Spiralia, which is composed of lophotrochozoans. The majority of spiralians go through equal spiral cleavage, however, some go through unequal spiral cleavage.

Illustrations of cleavage polyembryony

• Gymnosperms are the most common plants with cleavage polyembryony.
• Brassica is a plant that has simple polyembryony.
• The Argemone Mexicana and the Ulmus Americana are examples of mixed polyembryony.
• The adventive polyembryony in citrus, along with Mangifera and Opuntia, is the most common type of polyembryony.
• It has also been observed in several genera of the Pinaceae (e.g., Pinus, Cedrus, and Tsuga), Taxodiaceae (e.g., Sciadopitys and Sequoia), Cupressaceae (e.g., Thuja, Juniperus), and Podocarpaceae (e.g., Thuja, Juniperus) families (e.g. Podocarpus).

What are the lines of cleavage?

• Langer’s lines, also known as Langer lines of skin tension or cleavage lines, are topological lines drawn on a map of the human body to represent skin tension.
• In addition to being parallel to the normal orientation of collagen fibres in the dermis, they are also parallel to the orientation of the underlying muscle fibres.
• It is important to note that Langer’s lines have application in forensic science and the development of surgical methods.

Conclusion

It takes multiple divisions for the group of blastomeres to reach a size that is similar to that of the initial zygote. Between divisions, only new chromatin (nuclear material) is generated, and this occurs at the expense of the cytoplasm (extracellular matrix) (the substance of the cell outside the nucleus). Even though cleavage patterns differ throughout animal groups, they are rather consistent across all individuals of a given species. 

Eggs containing a large amount of yolk, such as those laid by birds, frequently do not divide completely into the yolk-rich region and are referred to as meroblastic. 

The blastomeres in the yolk-free area cleave entirely and form the embryo proper, whereas the blastomeres in the periphery region cleave partially and form the yolk sac. Holoblastic eggs are those that have a small amount of yolk and divide fully.

The cleavage process comes to a close with the creation of the blastula. The cleavage can be either holoblastic (complete or complete cleavage) or meroblastic (particular or partial cleavage) depending on the proportion of yolk in the egg (partial cleavage).