Table of Contents
- What Is Cleavage in Biology?
- Cell Cleavage Mechanism
- Types of Cleavage
- Significance
- Frequently Asked Questions (FAQs)
What Is Cleavage in Biology?
Cell division during the early stages of the embryo’s development after fertilisation is referred to as cleavage in embryology. Zygotes of several species possess rapid cell cycle progression without considerable overall growth, resulting in a group of cells of identical size as the initial zygote. The diverse cells produced by cleavage are known as blastomeres, and they group together to form a solid mass known as the morula. The development of the blastula, or the blastocyst in animals, indicates the termination of cleavage.
The mitotic division begins as the zygote travels through the isthmus of the oviduct, termed cleavage, towards the uterus and produces 2, 4, 8, and 16 daughter cells (blastomeres). A morula is an embryo that has 8 to 16 blastomeres. As it progresses into the uterus, the morula continues dividing and develops into a blastocyst.
The transformation from fertilisation to cleavage results from the activation of a mitosis-promoting factor (MPF).
Cleavage of Zygote
- Human zygote cleavage begins inside the fallopian tube. It is holoblastic, dividing the zygote fully into blastomeres or daughter cells.
- After fertilisation, the first cleavage occurs about 24 to 30 hours later. It creates two blastomeres by longitudinally dividing the zygote (one mildly larger than the other).
- The second cleavage takes place forty hours later.
- After fertilisation, there is a third cleavage approximately 72 hours later. During these early cleavages, the young embryo progresses down the fallopian tube towards the uterus.
- The embryo enters the uterus at the end of the fourth day. It is referred to as morula and resembles a mulberry. There are 32 cells in this solid morula. The cleavage is radial and of an indeterminate kind in human zygotes.
Cell Cleavage Mechanism
The zygote begins cleaving once fertilisation occurs, and a new organism starts to develop. Cleavage furrow refers to the area where cleavage begins.
Two coordinated mechanisms combine to produce cleavage.
Karyokinesis, or the division of the nucleus during mitosis, is the first of these cyclic mechanisms. The mechanical force behind this division is the mitotic spindle, which has microtubules made of tubulin (a protein that comprises the sperm flagellum).
Cytokinesis, or cell division, is the second phase. An actin-based contractile ring of microfilaments serves as the mechanical force behind cytokinesis.
The initiation of zygotic transcription and the termination of cleavage coincides. This transitional stage in non-mammals is known as the mid-blastula transition and is regulated by the nuclear-to-cytoplasmic ratio.
Types of Cleavage
During the cleavage period, there is a significant degree of reorganisation, and the cytoplasmic contents primarily determine the types of cleavage.
Determinate Cleavage
Determinate cleavage, also known as mosaic cleavage, is a type of cleavage based on the potency of blastomeres where each blastomere has a predetermined developmental fate and is not qualitatively equal. Many protostomes exhibit this type of cleavage.
Each blastomere, formed during early embryonic cleavage, has an early developmental fate that prevents it from developing into a complete organism.
Indeterminate Cleavage
The term “indeterminate cleavage” refers to a type of cleavage based on the potency of blastomeres where the blastomeres are qualitatively equal, and each can develop into an entire embryo when isolated. Deuterostomes usually possess this type of cleavage.
It is considered indeterminate when an embryonic cleavage yields cells that can individually develop into an entire organism and possess the whole complement of intact animal features.
Holoblastic Cleavage
With each cleavage in holoblastic cleavage, the number of blastomeres doubles since the zygote and blastomeres completely divide during the cleavage.
The cleavage can be either holoblastic or meroblastic, primarily depending on the yolk amount in the egg. When there is a moderate to low amount of yolk in an egg, the holoblastic form of cleavage is usually observed. Animals, including mammals, amphibians, echinoderms, flatworms, annelids, nematodes, and others, produce eggs that divide holoblastically.
Four main types of cleavage can be seen in isolecithal cells, microlecithal cells or mesolecithal cells when there is not enough yolk present: rotational holoblastic, radial holoblastic, bilateral holoblastic, and spiral holoblastic cleavage.
Meroblastic Cleavage
The fertilised egg cell may experience partial or meroblastic cleavage when there is a high concentration of yolk present. The discoidal and superficial cleavages are the two main types of meroblastic cleavage.
Discoidal Cleavage
Monotremes, reptiles, birds, and fish with telolecithal egg cells (yolk located at one end) often exhibit discoidal cleavage. The cleavage furrows do not penetrate the yolk in discoidal cleavage. On top of the yolk, the embryo develops a disc-shaped group of cells known as a blastodisc.
Superficial Cleavage
Karyokinesis, but not cytokinesis, occurs during superficial cleavage, producing a polynuclear cell. Arthropods with centrolecithal egg cells (yolk concentrated at the centre of the cell) exhibit superficial cleavage. The nuclei move to the outer edge of the egg, where the yolk is located, and the plasma membrane develops inward, dividing the nuclei into separate cells.
Transitional Cleavage
Many eggs have an unusual cleavage that is neither usual holoblastic nor meroblastic but instead takes a stage between the two.
Significance
- It preserves the nucleo-cytoplasmic proportion and cell size of the species.
- It transforms a single-celled zygote into a multicellular embryo.
- The blastomeres divide quickly during cleavage, after which they stop growing.
- Large cells, or blastomeres, produced by cleavage are necessary for the development of the progeny.
- The cytoplasm is distributed among the blastomeres as a result of cleavage.
Related Links:
- Cleavage Biology MCQs for NEET 2021
- Difference between Radial and Spiral Cleavage
- Difference between Cleavage and Mitosis
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