What is cleavage?

• Cleavage is a fundamental process in embryology that occurs during the early stages of embryo development following fertilization. It involves the division of cells, leading to an increase in cell number and the formation of a cluster of cells known as blastomeres. These blastomeres come together to form a compact mass called the morula. Cleavage ultimately culminates in the formation of the blastula or blastocyst, depending on the species.
• The process of cleavage is initiated by the sperm during fertilization, which activates the zygote. However, in some cases, such as parthenogenetic eggs, cleavage can begin without fertilization. Cleavage primarily occurs through repeated mitotic divisions, wherein the zygote rapidly undergoes cell division. The result of these divisions is the production of blastomeres.
• It is important to note that during cleavage, there is no significant overall growth in the blastomeres. Instead, as the process progresses, the blastomeres become progressively smaller in size. The embryo as a whole does not increase in size or volume during cleavage.
• The rate of cleavage can vary among different species. For example, in sea urchin eggs, divisions can be observed every 30 minutes. Cleavage continues until the blastomeres form a compact mass called the morula.
• Cleavage is influenced by the concentration of yolk present in the egg. Depending on the amount of yolk, cleavage can be categorized as either holoblastic (total or entire cleavage) or meroblastic (partial cleavage). The vegetal pole of the egg, which contains a higher concentration of yolk, is contrasted with the animal pole, which has a lower concentration.
• One distinctive aspect of cleavage is that it increases the number of cells and nuclear mass without a corresponding increase in cytoplasmic mass. As each subdivision occurs, the cytoplasm is divided among the daughter cells, resulting in approximately half the cytoplasmic content in each subsequent generation of cells. Consequently, the ratio of nuclear material to cytoplasmic material increases with each division.
• Overall, cleavage is a crucial process that transforms a single-celled zygote into a multicellular embryo. It establishes the foundation for further development and differentiation of various cell types within the growing organism.

Characteristics of cleavage

Cleavage exhibits several distinct characteristics during the early stages of embryo development:

1. Cell Division: Cleavage involves the rapid division of the fertilized egg through mitosis. These repeated divisions give rise to numerous cells known as blastomeres.
2. Morula Formation: In certain animals, such as mammals, cleavage leads to the formation of a solid mass of cells called the morula. The morula is a compact cluster of blastomeres.
3. Blastula Formation: In the majority of animals, cleavage results in the formation of a hollow sphere of cells called the blastula. The blastula consists of a single layer of cells surrounding a central cavity called the blastocoel.
4. Cell Proliferation: Cleavage ensures the production of a sufficient number of cells for the subsequent development and construction of tissues and organs within the embryo.
5. Lack of Growth: During cleavage, there is no significant growth in the size or volume of the embryo. The focus is primarily on cell division rather than overall increase in embryo size.
6. Maintained Embryo Shape: The shape of the embryo remains unchanged throughout the process of cleavage. The divisions primarily contribute to the increase in cell number and not alterations in the embryo’s overall form.
7. DNA Increase: Cleavage involves an increase in the amount of DNA as the zygote undergoes successive rounds of cell division. The genetic material is distributed among the blastomeres.
8. Proportional Nuclear-Cytoplasmic Ratio: As cleavage progresses, there is a fixed and balanced ratio between the nuclear material and the cytoplasmic content within each blastomere. This maintains a consistent relationship between these two essential components.

Overall, cleavage is characterized by the rapid division of the fertilized egg, resulting in the generation of numerous blastomeres. These divisions lead to the formation of either a solid mass of cells (morula) or a hollow sphere (blastula), depending on the species. Cleavage provides the necessary cells for subsequent embryonic development, without significant growth or alteration in embryo shape. It also involves an increase in DNA content and establishes a proportional relationship between nuclear and cytoplasmic materials.

Patterns of cleavages

There are mainly two types of cleavage. They are holoblasti cleavage and meroblastic cleavage

A. Holoblastic Cleavage

Holoblastic cleavage, also known as total or complete cleavage, is a type of cleavage where the entire egg undergoes division. This type of cleavage can be characterized by equal-sized or unequal-sized blastomeres.

1. Equal-sized holoblastic cleavage: When the blastomeres resulting from cleavage are of equal size, it is referred to as equal-sized holoblastic cleavage. This means that each division produces blastomeres that are roughly the same in size, resulting in a relatively uniform distribution of cytoplasm among the daughter cells.
2. Unequal-sized holoblastic cleavage: In contrast to equal-sized cleavage, unequal-sized holoblastic cleavage involves the production of blastomeres that differ in size. This can occur due to variations in the distribution of yolk or other factors influencing the distribution of cytoplasm within the egg. The resulting blastomeres may have different amounts of cytoplasm and potential developmental fates.

Holoblastic cleavage can further be classified into four types based on the symmetry of cleavage:

1. Radial cleavage
2. Bilateral cleavage
3. Spiral cleavage
4. Biradial cleavage
5. Rotational cleavage

Holoblastic cleavage plays a crucial role in early embryonic development, dividing the fertilized egg into smaller and more specialized cells. The type of holoblastic cleavage an organism undergoes is influenced by various factors, including the distribution of cytoplasm and yolk within the egg, as well as specific developmental requirements and adaptations of the organism.

1. Radial cleavage

Radial cleavage refers to a specific pattern of cell division in embryonic development, where the cleavage planes create a radial symmetry in the resulting blastomeres. This type of cleavage is observed in organisms such as echinoderms.

In the case of a frog’s zygote, radial cleavage occurs in a distinct manner. The zygote initially divides by a vertical furrow, resulting in two equal-sized blastomeres. The second cleavage furrow is also vertical but appears at a right angle to the first, producing four blastomeres that remain attached to each other. A horizontal cleavage then appears above the equatorial region, dividing the four blastomeres into eight blastomeres. The resulting arrangement forms four radial planes, with each larger blastomere having a smaller blastomere on top of it.

An important characteristic of radial cleavage is that a blastula produced by this type of cleavage can be divided along any meridian to yield two identical halves. This symmetry is seen in echinoderms, a group of organisms that includes sea cucumbers and sea urchins.

During radial cleavage, the blastomeres are arranged in a radial pattern around the central axis of the egg. Any plane passing through the major embryonic axis divides the embryo into symmetrical halves. The spindles during radial cleavage are typically oriented parallel or perpendicular to the polar axis.

While radial cleavage is the typical pattern, there may be slight variations. For instance, in sea urchins, amphioxus, and frogs, the third cleavage may be latitudinal instead of equatorial, resulting in unequal cleavage and the production of two types of blastomeres: micromeres and macromeres. Nevertheless, the overall arrangement still maintains a radial symmetry.

Radial cleavage is significant as it generates indeterminant cells, which have the ability to individually give rise to a complete embryo without predetermined developmental fates. This type of cleavage allows for experimental manipulations, such as isolating a single cell from a developing embryo and observing its potential to develop into a whole embryo, as commonly done in studies involving frog embryos.

2. Biradial cleavage

Biradial cleavage refers to a type of cleavage in embryonic development where the first three division planes are not perpendicular to each other. This pattern of cleavage is observed in organisms such as Polychoerus and Ctenophora.

On the other hand, bilateral cleavage is a type of cleavage that results in bilateral symmetry in the embryo. Blastomeres are arranged bilaterally around the axis of the egg. Bilateral cleavage is found in organisms such as Ascidians (tunicates) and cephalopod mollusks. It is a modified form of radial cleavage where one side of the embryo mirrors the other side in size and arrangement.

In bilateral cleavage, the first cleavage plane passes through the plane of symmetry, which is the median plane of the resulting embryo. The subsequent cleavages are symmetrical to this plane. In Ascidians, for example, the first cleavage plane passes through the animal and vegetal poles, while the second cleavage plane is displaced towards the posterior side of the embryo. This results in four dissimilar blastomeres at the four-cell stage, arranged in a bilateral symmetry. The embryo now exhibits recognizable right and left sides, as well as anterior and posterior ends. Subsequent divisions occur on both the left and right sides, adding to the bilateral symmetry.

In some animals, such as tunicates (Ascidians), cleavage initially shows approximate radial symmetry. However, the second cleavage plane is slightly displaced towards the posterior side of the animal-vegetal axis. This displacement leads to two larger anterior blastomeres and two smaller posterior blastomeres. As a result, subsequent cleavage becomes symmetrical only at the median plane, giving rise to bilateral holoblastic cleavage.

In summary, biradial cleavage occurs when the first three division planes are not at right angles to each other, while bilateral cleavage leads to bilateral symmetry in the embryo. Bilateral cleavage can be a modified form of radial cleavage and is observed in organisms like Ascidians and cephalopod mollusks.

3. Spiral cleavage

Spiral cleavage is a type of cleavage observed in certain organisms where there is a rotational movement of cell parts around the north pole-south pole axis of the egg. This rotational movement leads to a displacement or inclination of the mitotic spindles with respect to the symmetrically disposed radii, resulting in oblique cleavage planes. Each blastomere divides to form one larger cell (macromere) and one smaller cell (micromere). The blastomeres of the upper tier, or micromeres, sit over the junction between each two vegetal blastomeres, due to the oblique position of the mitotic spindles.

The mitotic spindles in spiral cleavage are arranged in a spiral pattern. The direction of the spiral can be clockwise (dextral cleavage, right-handed) or anticlockwise (sinistral cleavage, left-handed). Examples of organisms that exhibit spiral cleavage include Turbellaria, nematodes, rotifers, annelids, and most mollusks except cephalopods.

Here are some key characteristics and features of spiral cleavage:

1. Spiral cleavage is a holoblastic cleavage where blastomeres are slightly displaced, giving a spiral appearance to the embryo.
2. Blastomeres are arranged in a spiral pattern during spiral cleavage.
3. Spiral cleavage occurs in polyclads (flat nemerteans, annelid worms) and most mollusks except cephalopods.
4. The orientation of the spindles prior to cleavage is oblique, leading to the spiral arrangement of blastomeres.
5. Spiral cleavage is a modification of radial cleavage.
6. Unlike in radial cleavage, the spindle in spiral cleavage is slightly inclined to the main axis of the egg.
7. During the second cleavage, the two spindles are inclined to the main axis and in opposite directions to each other.
8. As a result, one daughter blastomere from each cleavage pair lies slightly above the other, while cells opposite to one another are at the same level.
9. The third cleavage is latitudinal, similar to radial cleavage, but the spindles are oblique to the main axis of the egg.
10. In most cases of spiral cleavage, the spindles of the third cleavage are shifted to the right when observed from the animal pole, causing clockwise displacement of the upper tier of cells (dextral spiral cleavage). If the spindles are shifted to the left, the upper tier of cells is displaced in the anticlockwise direction (sinistral spiral cleavage).
11. Spiral cleavage is highly ordered, with each cell occupying a specific position and having a predetermined fate.
12. Spiral cleavage leads to mosaic or determinate development, where each cell is destined to develop into a particular part of the embryo.

In summary, spiral cleavage involves the oblique arrangement of blastomeres in a spiral pattern. It is characterized by rotational movement of cell parts, resulting in oblique cleavage planes and the formation of macromeres and micromeres. Spiral cleavage leads to highly ordered, mosaic development in which each cell has a predetermined fate.

4. Bilateral cleavage

Bilateral cleavage is a type of cleavage observed in certain organisms where the blastula can be cut vertically along one plane to obtain two identical halves, representing the right and left sides. The activity of cleavage on one side is mirrored by activity on the other side, resulting in bilateral symmetry. In most cases, the plane of bilateral symmetry is established by the plane of the first cleavage furrow, which is bilaterally symmetrical. Examples of organisms that exhibit bilateral cleavage include tunicates, Amphioxus (a chordate), amphibians, and higher mammals.

In contrast, biradial cleavage is a specific type of cleavage observed only in ctenophores (comb jellies). During biradial cleavage, the first two cleavage planes are meridional (oriented along the meridional axis). The third cleavage plane is vertical, resulting in the formation of a curved plate of cells arranged in two rows, with each row containing four cells. The central cells are longer than the end cells, giving the curved plate a distinct shape.

In summary, bilateral cleavage is characterized by the ability to divide the blastula into two identical halves along a vertical plane, with mirrored cleavage activity on each side. The plane of bilateral symmetry is established by the first cleavage furrow. Biradial cleavage, on the other hand, is a specific type of cleavage observed in ctenophores, where the cleavage planes result in the formation of a curved plate of cells arranged in two rows.

5. Rotational Cleavage

• Rotational cleavage, as described by Gulyas (1975), is an unusual type of cleavage observed in mammals. It involves a unique arrangement of blastomeres resulting from the second cleavage.
• During the first cleavage in rotational cleavage, two blastomeres are produced through a meridional cleavage, which is similar to other animals. However, the second cleavage is distinctive and leads to an atypical organization of blastomeres.
• In one of the blastomeres, the plane of cleavage passes through the polar axis, maintaining a typical orientation. However, in the other blastomere, the plane of cleavage is perpendicular to the polar axis. This perpendicular orientation means that the cleavage plane in one of the blastomeres is rotated by 90 degrees in relation to the polar axis. This unique pattern of cleavage is why it is called rotational cleavage.
• As cleavage progresses, the blastomeres divide at different rates, leading to an uneven distribution of cells in mammalian embryos at various stages. This unevenness can result in embryos consisting of blastomeres with varying numbers of cells.
• The rotational cleavage observed in mammals is distinct from the more common types of cleavage, such as radial, bilateral, and spiral cleavage. Its specific characteristics contribute to the complexity of early embryonic development in mammals and highlight the diversity of cleavage patterns across different organisms.

B. Meroblastic Cleavage

Meroblastic cleavage is a type of cleavage that occurs in eggs with a large amount of yolk, preventing complete division of the entire egg. Instead, only a portion of the egg undergoes cleavage, while the yolk remains undivided. This type of cleavage is commonly observed in eggs of birds, reptiles, and some fish.

There are two main types of meroblastic cleavage: discoidal cleavage and superficial cleavage.

1. Discoidal Cleavage
2. Superficial Cleavage

Both discoidal cleavage and superficial cleavage are examples of partial or incomplete cleavage due to the presence of a significant amount of yolk. This type of cleavage allows for the formation of a blastoderm or blastodisc, which will give rise to the embryo, while the yolk provides essential nutrients for the developing embryo. The distinct patterns of meroblastic cleavage are adaptations to accommodate the large amount of yolk in the eggs of certain organisms.

1. Discoidal Cleavage

• Discoidal cleavage is a type of meroblastic cleavage that occurs in certain animals such as fish, reptiles, and birds. It is characterized by the presence of a small disc of cytoplasm, known as the blastodisc, located at the animal pole of the egg. Unlike in other types of cleavage, where the entire egg undergoes division, in discoidal cleavage, only the blastodisc undergoes cleavage while the yolk remains undivided.
• The blastodisc is a specialized region of the egg that contains a concentrated amount of cytoplasm and is responsible for forming the embryo. It is positioned at the animal pole, which is the top portion of the egg. During discoidal cleavage, the blastodisc undergoes repeated divisions, resulting in the formation of a layered structure known as the blastoderm.
• The blastoderm consists of multiple layers of cells that are arranged on top of the yolk mass. The cleavage furrows extend only through the blastodisc, with the yolk remaining undivided. This pattern of cleavage creates a disc-shaped structure within the egg, which gives rise to the embryonic tissues. As development progresses, the cells of the blastoderm migrate and differentiate, eventually forming the various organs and tissues of the developing embryo.
• Discoidal cleavage is an adaptation to the presence of a large amount of yolk in the eggs of these animals. The yolk serves as a nutrient-rich reserve that supports the growth and development of the embryo. By confining cleavage to the blastodisc, the embryo can utilize the cytoplasmic resources while leaving the yolk intact for nourishment.
• Overall, discoidal cleavage is a specialized form of meroblastic cleavage seen in fishes, reptiles, and birds. It allows for the partial division of the egg, specifically the blastodisc, while the yolk remains undivided, ensuring proper development and utilization of the yolk reserves during embryogenesis.

2. Superficial Cleavage

Superficial cleavage is a type of meroblastic cleavage that is characteristic of centrolecithal eggs found in insects. In this type of cleavage, the process of segmentation occurs only in the surface layer of the egg, while the central yolk remains undivided. The cytoplasmic region surrounding the yolk is the site of cleavage and gives rise to the embryo.

The term “centrolecithal” refers to the presence of a large central yolk mass within the egg. This yolk serves as a nutrient reserve for the developing embryo. However, due to the concentrated yolk mass, it is not feasible for cleavage to occur throughout the entire egg. Instead, cleavage is limited to the peripheral or superficial region of the egg, where the cytoplasm is located.

Superficial cleavage is further classified into two types based on the potentiality of the egg blastomeres:

1. Determinate cleavage: In some insects, the fate of each blastomere is predetermined early on during cleavage. This means that each blastomere has a specific developmental fate and will give rise to specific structures or tissues in the embryo. The division and differentiation of blastomeres follow a precise pattern, leading to the formation of distinct regions or segments in the developing embryo.
2. Indeterminate cleavage: In other insects, the fate of the blastomeres is not predetermined during cleavage. Instead, the blastomeres have the potential to develop into a wide range of structures or tissues. The division and differentiation of blastomeres are more flexible and can be influenced by various factors, such as signaling molecules and cellular interactions. This type of cleavage allows for greater developmental plasticity and the ability to generate different cell types depending on the conditions and requirements of the developing embryo.

In summary, superficial cleavage is a type of meroblastic cleavage observed in centrolecithal eggs of insects. It involves segmentation only in the superficial layer of the egg, while the central yolk remains undivided. The cleavage can be classified as determinate or indeterminate, depending on whether the fate of the blastomeres is predetermined or flexible, respectively. This unique form of cleavage is adapted to the presence of a concentrated yolk mass and contributes to the development of the embryo within the insect egg.

i. Determinate Cleavage

• Determinate cleavage, also known as mosaic cleavage, refers to a type of embryonic development in which the fate of each blastomere, or cell produced by cleavage, is predetermined. In determinate cleavage, the fertilized egg already contains regions that are earmarked to develop into specific parts of the embryo. These regions are fixed and do not change during subsequent divisions.
• In mosaic eggs, such as those found in ascidians, different regions of the egg are designated to give rise to specific organs or tissues in the embryo. For example, in an ascidian egg, there may be a region that is predetermined to develop into the endoderm, which is the innermost layer of cells in the embryo. If this endoderm region is removed or dissected out from a fertilized ascidian egg, the resulting embryo will lack endoderm formation.
• Cleavages in mosaic or determinate eggs follow a precise and predetermined pattern. Each blastomere has a characteristic position and fate that is unalterable. As the embryo develops, the cleavage divisions serve to separate different regions that will form distinct organs or tissues. The fate of each blastomere is already established, and it contributes to the formation of specific structures in the developing embryo.
• Examples of organisms that exhibit determinate cleavage include nematodes, annelids, mollusks, and ascidians. In these organisms, the cleavage divisions play a crucial role in separating and organizing the predetermined regions, ensuring the proper development of different organs and tissues.
• In summary, determinate cleavage, or mosaic cleavage, is a type of embryonic development where the fate of each blastomere is predetermined. Mosaic eggs have fixed regions that are earmarked to develop into specific parts of the embryo. Cleavage divisions in determinate cleavage follow a precise pattern, separating different regions that will form distinct organs or tissues. Examples of organisms that exhibit determinate cleavage include nematodes, annelids, mollusks, and ascidians.

ii. Indeterminate Cleavage

• Indeterminate cleavage, also known as regulative cleavage, is a type of embryonic development in which the fate of each blastomere is not predetermined. Unlike in determinate cleavage, the fertilized eggs in indeterminate cleavage do not have pre-designated regions for specific organs or tissues. Instead, the developmental potential of each blastomere is flexible and can be influenced by various factors.
• In indeterminate cleavage, if a specific region, such as the endoderm, is removed from a fertilized egg, the resulting embryo can still develop that missing region. For example, if the endoderm region is surgically removed from a sea urchin egg, the subsequent development of the embryo will still include the formation of the endoderm. This demonstrates the plasticity and regulatory nature of indeterminate cleavage.
• In indeterminate cleavage, the cleavage divisions do not separate predetermined regions. Instead, they divide the eggs into segments or smaller cells that have the potential to develop into any organ or tissue. The fate of each blastomere is not fixed and can be influenced by interactions with neighboring cells or signals from the environment. As the embryo continues to develop, cell-to-cell communication and signaling pathways guide the differentiation and specialization of the blastomeres into specific cell types.
• Indeterminate cleavage is observed in various organisms, including certain groups of invertebrates and all vertebrates. In vertebrates, such as mammals, birds, and reptiles, the plan of cleavage is more flexible and adaptable compared to organisms with determinate cleavage. This flexibility allows for greater developmental plasticity and the ability to compensate for changes or perturbations during early embryonic development.
• In summary, indeterminate cleavage, or regulative cleavage, is a type of embryonic development where the fate of each blastomere is not predetermined. Eggs undergoing indeterminate cleavage do not have fixed regions for specific organs or tissues. Instead, the cleavage divisions result in segments or cells that have the potential to develop into any organ. Indeterminate cleavage is flexible and can compensate for changes, and it is observed in certain invertebrates and all vertebrates.

Difference between Holoblastic (complete) cleavage and Meroblastic (incomplete) cleavage

Holoblastic (complete) cleavage and meroblastic (incomplete) cleavage are two different types of cleavage patterns that occur during embryonic development. The main difference between them lies in the distribution of yolk within the egg and the extent of cleavage.

1. Holoblastic (complete) cleavage:
   • Occurs in eggs with sparse, evenly distributed yolk, known as isolecithal eggs.
   • In holoblastic cleavage, the entire egg undergoes cleavage, and the resulting blastomeres are relatively equal in size.
   • Different cleavage patterns can be observed depending on the symmetry: a. Radial cleavage: Found in echinoderms, hemichordates, and amphioxus. The cleavage planes are meridional and perpendicular to the polar axis, resulting in the formation of blastomeres arranged in a radial pattern. b. Spiral cleavage: Seen in annelids, most mollusks, and flatworms. The cleavage planes are oblique to the polar axis, leading to a spiral arrangement of blastomeres. c. Bilateral cleavage: Observed in tunicates. The cleavage planes are vertical, dividing the egg into two identical halves, the right and left. d. Rotational cleavage: Found in placental mammals, nematodes, and marsupials. The cleavage planes in one of the blastomeres are rotated 90 degrees with respect to the polar axis during the second cleavage, resulting in an unusual arrangement of blastomeres.
2. Meroblastic (incomplete) cleavage:
   • Occurs in eggs with a moderate to dense disposition of yolk.
   • The cleavage is limited to the part of the egg that does not contain the yolk, while the yolk-rich region remains uncleaved.
   • Different types of meroblastic cleavage can be distinguished based on the distribution and amount of yolk: a. Telolecithal eggs: Dense yolk throughout most of the cell. Two main cleavage patterns are observed:
     • Bilateral cleavage: Found in cephalopod mollusks. The cleavage planes are vertical, dividing the egg into two equal halves.
     • Discoidal cleavage: Seen in some fish (hagfishes, chondrichthyans, most teleosts), reptiles, birds, and monotremes. The cleavage is limited to a small disc of cytoplasm called the blastodisc, located on top of the yolk. The yolk-rich region remains uncleaved. b. Centrolecithal eggs: Yolk is located in the center of the egg. The cleavage is limited to the peripheral cytoplasm, while the central yolk remains undivided. This type of cleavage is observed in most insects and is referred to as superficial cleavage.

In summary, holoblastic cleavage occurs in eggs with sparse yolk, and the entire egg undergoes cleavage. It can exhibit radial, spiral, bilateral, or rotational patterns. On the other hand, meroblastic cleavage occurs in eggs with moderate to dense yolk, and the cleavage is limited to the non-yolk region. It can be telolecithal (with bilateral or discoidal cleavage) or centrolecithal (with superficial cleavage).

Cleavage Type	Yolk Distribution	Examples
Holoblastic Cleavage	Isolecithal	Radial: Echinoderms, Hemichordates, Amphioxus
		Spiral: Annelids, Most Mollusks, Flatworms
		Bilateral: Tunicates

		Rotational: Placental Mammals, Nematodes, Marsupials
Meroblastic Cleavage	Telolecithal	Bilateral: Cephalopod Mollusks
		Discoidal: Some Fish, Reptiles, Birds, Monotremes
	Centrolecithal	Superficial: Most Insects

Laws of cleavage

The process of cleavage, or cell division, follows certain fundamental principles or laws that govern its patterns and characteristics. These laws help describe and understand the common procedures observed during cleavage. Here are some key laws of cleavage:

1. Sach’s Law: According to Sach’s Law, cells have a tendency to divide into equal daughter cells. Each plane of division also tends to bisect the previous plane at right angles. This law highlights the general principle of equal division during cleavage.
2. Hertwig’s Law: Hertwig’s Law states that the nucleus of a cell always occupies the center of the protoplasm. Additionally, the long axis of the mitotic spindle aligns with the longest axis of the protoplasmic mass. As a result, the division plane tends to cut the protoplasmic mass at right angles to its long axis.
3. Piluger’s Law: Piluger’s Law describes the elongation of the mitotic spindle in the direction of least resistance. This law recognizes that the mitotic spindle tends to align and extend along the path of least resistance within the cell.
4. Balfour’s Law: Balfour’s Law states that the rate of cleavage is influenced by the inverse ratio of the amount of yolk present in the egg. Yolk is a nutrient-rich substance found in certain eggs that can hinder the division of both the nucleus and the cytoplasm. Cleavage in eggs with a higher amount of yolk may be slower or impeded compared to eggs with less yolk.

These laws of cleavage provide insights into the principles that guide the division of cells during embryonic development. They help explain the symmetry, orientation, and rate of cleavage observed in various organisms. By understanding these laws, scientists can gain valuable knowledge about the processes and mechanisms involved in embryogenesis.

The planes of cleavage

During the process of cleavage, the egg undergoes division along different planes, which determine the orientation and arrangement of the resulting cells. Here are the main planes of cleavage:

1. Meridional plane: The meridional plane of cleavage runs along the animal-vegetal axis of the egg, dividing it into two equal halves. This plane bisects both the animal and vegetal poles, resulting in two symmetrical halves.
2. Vertical plane: The vertical plane of cleavage is perpendicular to the meridional plane. It runs from the animal to the vegetal pole, but not through the center of the egg. This plane can result in the formation of cells that are unequal in size.
3. Equatorial plane: The equatorial plane of cleavage divides the egg into two halves at a right angle to the main axis. This plane lies on the equator of the egg, cutting it horizontally into two equal portions.
4. Latitudinal plane: The latitudinal plane is similar to the equatorial plane but lies on either side of the equator. It is also referred to as the transverse or horizontal cleavage plane. This plane divides the egg into two halves along the horizontal axis.

These different planes of cleavage play a crucial role in determining the arrangement and symmetry of cells during the early stages of development. The orientation and position of the cleavage furrows contribute to the formation of specific cell lineages and the overall organization of the developing embryo.

Chemical Changes during Cleavage

During the process of cleavage, significant chemical changes occur in the fertilized egg. Here are some of the key chemical changes that take place:

1. Increase of nuclear material: As cleavage progresses, there is a steady increase in nuclear material, primarily DNA. The cytoplasm of the egg serves as the source of this nuclear material. Both cytoplasmic DNA contained in mitochondria and yolk platelets contribute to this increase in nuclear material.
2. RNA synthesis: Cleavage involves the synthesis of various types of RNA, including messenger RNA (mRNA) and transfer RNA (tRNA). These RNA molecules play essential roles in protein synthesis and gene expression. The synthesis of mRNA and tRNA occurs particularly during the later stages of cleavage.
3. Synthesis of proteins: Cleavage is accompanied by a continuous and remarkable increase in protein synthesis. Proteins are vital molecules that perform a wide range of functions in the developing embryo. As the cleavage progresses, the synthesis of proteins increases, supporting the growth and development of the embryo.

These chemical changes occurring during cleavage are crucial for the proper development and differentiation of cells in the embryo. The increase in nuclear material, synthesis of RNA molecules, and protein synthesis are all fundamental processes that contribute to the complex and intricate changes taking place during cleavage.

Significance of cleavage

Cleavage holds significant importance in the overall process of embryonic development. Its significance can be understood through the following aspects:

1. Subdivision of the Embryonic Substrate: Cleavage divides the initial embryonic substrate, typically a single-celled zygote, into an array of cells called blastomeres. This subdivision is crucial as it increases the number of cells and provides the necessary building blocks for subsequent development.
2. Preparation for Cell Differentiation: Cleavage not only increases the cell count but also sets the stage for the process of cell differentiation. During cleavage, the cells may undergo changes in gene expression and molecular signaling, which lay the foundation for their future specialization into various cell types and tissues. Cleavage thus initiates or prepares the groundwork for the subsequent diversification of cells.
3. Initiation of Cell Differentiation: In some cases, cleavage may even trigger the initiation of cell differentiation. Certain molecular cues or mechanisms activated during cleavage can stimulate specific cells to adopt particular developmental fates and embark on their path of specialization. This early stage of development sets the trajectory for the formation of different tissues and organs.
4. Creation of a Cell Aggregate (Blastula): Cleavage leads to the formation of a cell aggregate known as the blastula. The blastula is typically a hollow sphere of cells surrounding a central cavity called the blastocoel. This structure serves as a crucial developmental stage, as it provides a framework for further morphogenetic processes.
5. Morphological Changes: The cell aggregate formed during cleavage, the blastula, can undergo further morphological changes driven by various morphogenetic processes. These processes, such as gastrulation, invagination, and tissue rearrangement, shape the developing embryo and contribute to the formation of distinct body structures and organs.

In summary, cleavage is of significant importance in embryonic development. It subdivides the embryonic substrate, prepares for cell differentiation, and in some cases, initiates the process of cell diversification. Cleavage also leads to the formation of the blastula, which serves as a foundation for subsequent morphogenetic processes and further morphological changes in the developing embryo.

Cleavage in Different Chordates

1. Cleavage in Amphioxus

Cleavage in Amphioxus follows a holoblastic pattern, meaning the entire zygote undergoes division. The process can be described as follows:

1. First Cleavage: The first cleavage is meridional, dividing the zygote into two equal blastomeres.
2. Second Cleavage: The second cleavage is also meridional but occurs at a right angle to the first cleavage. This results in the formation of four equal blastomeres.
3. Third Cleavage: The third cleavage is latitudinal and takes place slightly above the equatorial plane. It produces eight blastomeres, with four smaller ones known as micromeres located towards the animal pole, and four larger ones called macromeres situated towards the vegetal pole.
4. Fourth Cleavage: The fourth cleavage is meridional and involves all eight cells. It leads to the formation of eight micromeres and eight macromeres.
5. Fifth Cleavage: The fifth cleavage planes are latitudinal. Each micromere is divided into an upper and lower micromere, while each macromere is also divided into an upper and lower macromere. This results in a total of thirty-two blastomeres.
6. Sixth Cleavage: The sixth cleavage planes are nearly meridional and involve all thirty-two cells. This leads to the formation of sixty-four cells.
7. Formation of Blastocoel: At the 64-cell stage, a conspicuous space called the blastocoel is produced at the center of the embryo. This space gradually becomes filled with fluid.
8. Eighth Cleavage: When the eighth cleavage planes occur, the blastula takes on a pear-shaped appearance, and the blastocoel becomes larger.

Overall, the cleavage in Amphioxus involves a series of meridional and latitudinal divisions, resulting in the formation of blastomeres and the development of a blastula with a central fluid-filled cavity (blastocoel).

2. Cleavage in Frog

Cleavage in frogs, specifically the egg of the frog, follows a holoblastic pattern. Here’s a description of the cleavage process in frogs:

1. Egg Characteristics: The frog egg is telolecithal, meaning it contains a significant amount of yolk that is localized towards the vegetal pole. This yolk distribution affects the cleavage process.
2. First Cleavage: The first cleavage plane is meridional and typically occurs around 3 to 3.5 hours after fertilization. It starts at the animal pole and gradually extends towards the vegetal pole, resulting in the egg being bisected along the poles. Two blastomeres of equal size are formed.
3. Second Cleavage: The second cleavage is almost meridional but oriented at right angles to the first cleavage plane. This results in the formation of four blastomeres, but they are not qualitatively identical. Two of the blastomeres contain grey crescent material, while the others do not. Additionally, each blastomere exhibits dark pigment at the animal pole and yellowish yolk towards the vegetal pole.
4. Third Cleavage: The third cleavage is latitudinal and occurs at right angles to the previous cleavage planes, slightly above the equator. It produces eight blastomeres, but they are unequal in size. The animal hemisphere contains four smaller blastomeres called micromeres, while the vegetal part contains four larger blastomeres called macromeres.
5. Fourth Cleavage: The fourth cleavage planes are meridional and involve the micromeres first. The cleavage then progresses slowly towards the yolk-laden macromeres located at the vegetal pole.
6. Synchrony and Asynchrony: In contrast to the synchronous cleavage observed in Amphioxus, frog cleavage exhibits a considerable degree of irregularities and asynchronism in later stages. However, it is consistent that the micromeres continue to divide at a faster rate than the macromeres.
7. Blastocoel Formation: At the eight-celled stage, a small space appears between the four micromeres. This space gradually becomes more prominent and forms the blastocoel, also known as the segmentation cavity. The floor of the blastocoel consists of the macromeres. The blastocoel is eccentrically located and becomes displaced towards the animal pole as development progresses.

In summary, cleavage in frogs is holoblastic, but the presence of a significant amount of yolk affects the distribution and timing of cleavage planes. The resulting blastomeres and the formation of the blastocoel play important roles in subsequent development.

3. Cleavage in Chick

Cleavage in chicks follows a meroblastic pattern, which means that segmentation occurs only in the blastodisc or germinal disc. Here’s a description of the cleavage process in chicks:

1. Meroblastic Cleavage: In chicks, the cleavage is incomplete and limited to the blastodisc. The rest of the egg, including the yolk, does not undergo segmentation.
2. First Cleavage: The first cleavage begins as a meridional furrow near the center of the blastodisc. It starts around 4.5 hours after fertilization when the egg reaches the isthmus of the oviduct. The furrow cuts across the blastodisc and extends towards the vegetal pole, but it does not reach the pole.
3. Second Cleavage: The second cleavage is also meridional but occurs approximately at right angles to the first cleavage plane.
4. Third Cleavage: The third cleavage is vertical, perpendicular to the previous cleavage planes.
5. Fourth Cleavage: The fourth cleavage is also vertical, but the divisions are not synchronous. As a result, eight central cells are surrounded by twelve marginal cells.
6. Irregular Cleavage: From this point onward, the cleavage becomes irregular. A disc composed of smaller cells appears, which remains firmly connected to the underlying yolk. A cleft then appears, separating the disc in the middle from the yolk. The space between the disc and the yolk is called the sub-germinal space.
7. Blastoderm Formation: At the end of segmentation, the disc consists of many-layered small cells that are connected to the yolk only at the periphery. This disc is referred to as the blastoderm, and the cells within it continue to divide.
8. Area Apaca and Area Pellucida: The peripheral part of the blastoderm, in contact with the yolk, contains granular cells known as the area apaca. The inner layer of the blastoderm, with a clear portion, is called the area pellucida.
9. Formation of Future Posterior Side: At one end of the area apaca, cell aggregation occurs, indicating the formation of the future posterior side of the chick.

In summary, cleavage in chicks is meroblastic, occurring only in the blastodisc. The cleavage planes are meridional and vertical, and the resulting blastoderm consists of many-layered small cells connected to the yolk at the periphery. The distinction between the area apaca and area pellucida, as well as the formation of the future posterior side, are notable features of chick cleavage.

4. Cleavage in Rabbit

Cleavage in rabbits is characterized by holoblastic division, meaning the entire egg undergoes cleavage, and it is nearly equal. However, irregularities and asynchrony in cleavage are common, as is the case in other eutherian mammals. Here’s a description of cleavage in rabbits:

1. Alecithal Egg: The rabbit’s egg is small and lacks yolk, belonging to the alecithal type of eggs.
2. First Cleavage: The first cleavage is vertical, resulting in the formation of two blastomeres of unequal size.
3. Second Cleavage: The second cleavage is also vertical but occurs at a right angle to the first cleavage plane.
4. Third Cleavage: The third cleavage is horizontal but slightly above the equator.
5. Rapid and Irregular Divisions: Subsequent divisions occur rapidly and irregularly, leading to the production of numerous blastomeres. These blastomeres cluster together, forming a solid cellular ball known as a morula.
6. Morula Structure: The morula consists of two types of cells: small and large. The large cells are located at the center of the morula.
7. Blastocyst Formation: A cavity appears within the mass of cells on one side, gradually increasing in size and displacing the central cell mass to one side. This stage is known as the blastocyst stage. The inner cell mass remains attached to the outer cell layer, called the trophoblast, of the blastocyst.
8. Blastocoel and Embryonic Knob: The cavity within the blastocyst is called the blastocoel or sub-germinal cavity. It is filled with fluid. The inner cell mass remains attached at the embryonic knob, which is located towards the animal pole. The embryo develops from this embryonic knob.
9. Trophoblast and Placenta Formation: The trophoblast, which surrounds the blastocoel and the embryonic knob, plays a role in the formation of the placenta. The trophoblastic cells that cover the embryonic knob are referred to as the cells of Rauber.

In summary, cleavage in rabbits is holoblastic and nearly equal. The early cleavage divisions are vertical and horizontal, leading to the formation of a morula. The morula then develops into a blastocyst, with an inner cell mass attached to the trophoblast. The blastocyst stage is characterized by the presence of a fluid-filled cavity, the blastocoel, and the embryonic knob, from which the embryo arises. The trophoblast cells, including the cells of Rauber, contribute to the formation of the placenta.

FAQ

References

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