Definition of Cleavage
Cleavage is the elaborated series of rapid and stereotyped patterns of divisions without cell growth and gene expression during early embryogenesis. The daughter cells formed by these divisions are called blastomeres.
Explanation
After fertilization, cleavage is one of the earliest mechanical activities which divides the large volume egg cell into smaller nucleated cells and form a multicellular embryo.
Mid-blastula Transition
At the initial stages, cleavage is rapid, regular, and synchronize. The cell division abruptly halts at the stage where zygotic transcription takes control. This is called mid-blastula transition (MBT).
Characteristics of Cleavage
Cleavage is basically mitotic division and the daughter cells that are formed have genetically similar characters to the parent cell.
In cleavage, the size of the cell becomes smaller and smaller because there is no growth in the interphase.
The somatic cell cycle has four phases i.e., G1, S, G2, and M whereas the cleavage cycle has two phases S and M.
Initially nuclear to cytoplasmic ratio is very small. As cleavage progresses, the nuclear to cytoplasmic ratio increases due to the only synthesis of DNA but no synthesis of cytoplasm.
There is no change of shape because there is no movement in blastomeres.
Mechanism of Cleavage
Cleavage takes place in two parts i.e., karyokinesis and cytokinesis.
Karyokinesis
The division of the nucleus is called karyokinesis. Following events occur during this:
Increase of nuclear material
The nuclear material especially DNA is increased by the synthesis of DNA so each blastomere can have an equal number of it.
Synthesis of RNA
The cleavage can occur without RNA synthesis as it is required for the synthesis of proteins and ribosomes. In the late stages of cleavage, messenger RNA (mRNA) and transfer RNA (tRNA) are synthesized.
Synthesis of proteins
Some proteins are already present but some proteins are synthesized. After three cycles if there is no protein synthesis, cleavage stops completely. So, protein synthesis is essential.
Cytokinesis
Just below the surface of an egg, the ring of microfilaments is formed, composed of actin protein. These contractile rings become shorten to separate the cytoplasm by pulling apart.
The spindles more precisely, the asters form the diffusible factor that acts on the contractile ring and initiates cytoplasmic division. Asters set up conditions necessary for the organization of constriction.
Purpose of cleavage
Cells divide to grow and migrate in specific patterns to make an increasingly more elaborate body.
To work properly, that body needs well-defined axes (such as head and tail).
It also needs a particular collection of many-celled organs and other structures, placed in the best areas along the axes and linked up with one another in the proper ways.
Synthesis of the embryo-derived mRNA.
At first, cells are elastic and can perform any function so by cleavage there is a differential pattern and are assigned for a specific function.
Cleavage enables the regulatory relationship between cytoplasmic volume and the nucleus to be developed.
MCQs
- Which of the following best describes cleavage in embryogenesis?
- A) Growth of cells with gene expression
- B) Series of rapid divisions without cell growth or gene expression
- C) Formation of specialized cells
- D) Synchronized cell enlargement
Answer: B) Series of rapid divisions without cell growth or gene expression
- What are the daughter cells formed during cleavage called?
- A) Zygotes
- B) Embryos
- C) Blastomeres
- D) Gametes
Answer: C) Blastomeres
- What is the term for the stage where zygotic transcription takes control and cell division abruptly halts?
- A) Blastulation
- B) Cleavage arrest
- C) Mid-blastula transition
- D) Differentiation phase
Answer: C) Mid-blastula transition
- During cleavage, what happens to the size of the cell?
- A) It remains constant
- B) It becomes larger due to cell growth
- C) It becomes smaller
- D) It doubles in size
Answer: C) It becomes smaller
- Which phase of the somatic cell cycle is absent in the cleavage cycle?
- A) G1
- B) S
- C) G2
- D) M
Answer: A) G1
- What is the term for the division of the nucleus during cleavage?
- A) Cytokinesis
- B) Karyokinesis
- C) Mitosis
- D) Meiosis
Answer: B) Karyokinesis
- What is the role of RNA synthesis during cleavage?
- A) Synthesis of DNA
- B) Synthesis of proteins
- C) Synthesis of ribosomes
- D) Not required for cleavage
Answer: D) Not required for cleavage
- Which protein is responsible for the formation of contractile rings during cytokinesis?
- A) Actin
- B) Myosin
- C) Tubulin
- D) Collagen
Answer: A) Actin
- What is the main purpose of cleavage during embryonic development?
- A) To induce cell growth
- B) To promote gene expression
- C) To establish body axes and specialized structures
- D) To produce mature cells
Answer: C) To establish body axes and specialized structures
- What happens to cells during cleavage that enables them to perform specific functions later?
- A) They decrease in elasticity
- B) They become undifferentiated
- C) They lose the ability to divide
- D) They undergo differential patterns
Answer: D) They undergo differential patterns
- What is the significance of the mid-blastula transition during cleavage?
- A) It marks the beginning of cell differentiation
- B) It signifies the end of cleavage
- C) It initiates zygotic transcription
- D) It promotes cell growth
Answer: C) It initiates zygotic transcription
- Which part of cleavage involves the synthesis of proteins?
- A) Karyokinesis
- B) Cytokinesis
- C) Both A and B
- D) Neither A nor B
Answer: A) Karyokinesis
- What is the primary factor that determines the division pattern during cleavage?
- A) Cytoplasmic volume
- B) Size of the nucleus
- C) Amount of RNA synthesis
- D) Presence of genetic material
Answer: A) Cytoplasmic volume
- Which phase of the somatic cell cycle corresponds to the cleavage cycle phase S?
- A) DNA synthesis
- B) Growth phase
- C) Preparation for mitosis
- D) Chromosome condensation
Answer: A) DNA synthesis
- What term describes the stage when there is no protein synthesis and cleavage stops completely?
- A) Cleavage arrest
- B) Protein depletion phase
- C) Karyokinetic stasis
- D) Cellular quiescence
Answer: A) Cleavage arrest
Summary:
Cleavage, an essential process in early embryogenesis, involves rapid and stereotyped cell divisions without cell growth or gene expression. The daughter cells formed during cleavage are called blastomeres. Cleavage divides the large volume egg cell into smaller nucleated cells to form a multicellular embryo after fertilization.
The process is characterized by rapid and synchronized division, halting abruptly at the mid-blastula transition when zygotic transcription takes control. During cleavage, the size of the cells decreases due to the absence of growth in the interphase, and the nuclear to cytoplasmic ratio increases as DNA synthesis occurs without cytoplasmic synthesis.
Cleavage occurs in two parts: karyokinesis (nuclear division) and cytokinesis (cytoplasmic division). Karyokinesis involves an increase in nuclear material, RNA synthesis, and protein synthesis. Cytokinesis involves the formation of contractile rings composed of actin protein, which separates the cytoplasm by pulling apart.
The purpose of cleavage is to enable cell growth, migration, and differentiation to form an elaborate body with well-defined axes and specialized structures.
Cleavage also facilitates the synthesis of embryo-derived mRNA and establishes a regulatory relationship between cytoplasmic volume and the nucleus.