Cleavage

Introduction

• After fertilization is complete, the development of multicellular organisms proceeds through a process called cleavage.
• Cleavage is the rapid mitotic division of the zygote.
• This process transforms the zygote into a morula (16 to 32 cells) and then into a blastula.
• There is no growth phase during cleavage due to the absence of interphase between successive divisions, causing the size of the blastomeres to decrease progressively.
• The morula stage is characterized by a large number of cells, while the blastula stage features a hollow mass of cells. Each individual cell in this mass is called a blastomere.

Factors Influencing Cleavage

The plane and pattern of cleavage are influenced by:

1. The position of the zygote nucleus and mitotic spindle at the point of sperm entry.
2. The amount and distribution of yolk.
3. Cytoplasmic rotation.

The first cleavage in a frog’s egg was observed by Swammerdam in 1738.

Planes of Cleavage

There are different planes of cleavage:

Meridional Plane

The cleavage furrow passes through the center of the egg, from the animal pole to the vegetal pole, dividing the egg into two equal halves.

Example: Seen in the first and second cleavage of a frog’s egg and the first cleavage of a chick egg.

Vertical Plane

The cleavage furrow runs parallel to the meridional plane but slightly away from the egg’s center.

Example: Observed in the third cleavage in chick eggs.

Equatorial Plane

The cleavage furrow forms at a right angle to the meridional plane along the equator of the egg.

Example: Seen during the third cleavage of a sea urchin egg.

Latitudinal Plane

Similar to the equatorial plane, but the cleavage furrow passes through the egg on either side of the equator along its latitude.

Example: Observed in the eighth cleavage of a frog egg.

Types of Cleavage

Cleavage can be broadly classified into two main types:

Determinate Cleavage

Also known as mosaic cleavage, this type is based on the predetermined developmental fate of blastomeres.

• Each blastomere has a specific and fixed developmental outcome and is not quantitatively equal.
• Many protostomes, such as annelids, mollusks, and nematodes, exhibit this type of cleavage.
• The blastomeres formed during early embryonic cleavage cannot develop into a complete organism if separated.  

Indeterminate Cleavage

Indeterminate cleavage is a type of cleavage where blastomeres are qualitatively equal and capable of developing into a complete embryo if isolated.

• Deuterostomes typically exhibit this type of cleavage.
• Such embryos’ blastomeres retain the potential to develop into an independent organism, with all animal features intact.
• Found in the eggs of echinoderms and vertebrates, such eggs are termed regulative eggs.

Cleavage Patterns Based on Amount and Distribution of Yolk

Holoblastic

Greek Meaning: Holos means complete.

In holoblastic cleavage, each cleavage furrow completely divides the entire egg due to less or moderate amounts of yolk.

Equal Holoblastic

Occurs in alecithal (minimal or no yolk) and microlecithal (small amount of yolk) eggs.

Each cleavage furrow divides the egg so that blastomeres of equal size are produced.

Unequal Holoblastic

Unequal-sized blastomeres are produced in mesolecithal and moderately telolecithal eggs (e.g., some bony fishes and amphibians).

Due to the increased concentration of yolk in the vegetal hemisphere:

• The smaller blastomeres are called micromeres.
• The largest, yolk-laden blastomeres are called macromeres.  

Meroblastic

Greek Meaning: Meros means part.

Meroblastic cleavage occurs where the furrow doesn’t divide the egg completely, being restricted to a small cytoplasmic part of the egg. This happens in eggs with large amounts of yolk (e.g., telolecithal and centrolecithal eggs).

Discoidal Meroblastic

Occurs in macrolecithal and highly telolecithal eggs (e.g., fish, reptiles, birds, and monotremes).

Cleavage is restricted to a small, disc-shaped active cytoplasmic area while the mass of yolk remains undivided.

Superficial Meroblastic

Occurs in centrolecithal eggs of insects.

Cleavage occurs only in the peripheral cytoplasm, with the yolk remaining undivided and centrally located.

Yolk and Cleavage

Yolk generally inhibits cleavage.

Blastomeres formed in the relatively yolk-free animal pole are smaller than those in the yolk-rich vegetal pole.

A good example is an amphibian egg (mesolecithal), where smaller blastomeres (micromeres) are formed in the animal pole, and larger blastomeres (macromeres) are formed in the vegetal pole.

Mammalian eggs, which are alecithal (lack yolk), produce blastomeres of equal size during cleavage.

Cleavage Pattern

The pattern of cleavage in an egg is also determined by the factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation. Different types of cleavage patterns are recognized based on the formation of cleavage furrows, the position of the mitotic spindle, and arrangement of blastomeres in the tiers.

Basic Patterns

Radial

Blastomeres lie exactly on blastomeres of the lower tier, symmetrically arranged around the polar axis.

Example: Echinoderms, Amphioxus, and amphibians.

Bilateral

The blastomeres are arranged radially and are of different sizes.

The cleavage doesn’t divide the egg from the center, thus forming large and small blastomeres, giving bilateral symmetry to the embryo.

Example: Tunicates.

Biradial

First two cleavage planes are meridional, but the third division is vertical, so the embryo can be cut into two equal halves from any plane, as in ctenophores.

Spiral

The upper tier of cells is slightly displaced in a clockwise (dextral) or anticlockwise (sinistral) direction with respect to the lower tier.

This is because of the oblique position of the mitotic spindle, as seen in nematodes, annelids, and molluscs except cephalopods.

Rotational

This cleavage is characteristic of mammalian eggs.

The first division is normal meridional, but the second cleavage is rotational, in which one of the two blastomeres divides meridionally and the other divides equatorially.

Following the third cleavage, the blastomeres show the phenomena of compaction.

Laws of Cleavage

The cleavage follows different laws which are as follows:

Sach’s LawCells divide into two equal halves and each new plane intersects the preceding one at right angles.Hertwig’s LawThe mitotic spindle lies in the longest axis of protoplasmic mass and its divisions cut the protoplasmic mass at right angles.Balfour’s LawRate of cleavage is inversely proportional to the amount of yolk present.Pflüger’s LawThe spindle axis elongates in the direction of least resistance.

Blastulae

Many embryos are transformed from a solid round of cells into a hollow ball of cells called a blastula.

From One Cell to Blastocyst

An organism develops from a single cell called zygote which arises from an ovum being fertilized by a single spermatozoan (sperm).

The cell is surrounded by a solid membrane layer of glycoproteins called the zona pellucida, which the successful sperm has managed to pass through.

The zygote undergoes cleavage, increasing the number of cells within the zona pellucida.

After the 8-cell stage, embryos undergo compaction, where the cells bond tightly to each other forming a compact ball.

After compaction, the embryo is in the morula phase (16 cells). Cavitation takes place after the outer layer of cells becomes looser into the morula. When the number of cells reaches 40-50 cells, a central fluid-filled cavity (Blastocoel) has been formed.

The zona pellucida begins to deteriorate, allowing the embryo to increase its volume. This phase in the developing embryo is reached after 4-6 days in the blastocyst (related to the blastula phase) and lasts until implantation in the uterus.

Types of Blastulae

CoeloblastulaIt is a hollow blastula having a huge spacious blastocoel. Typically, the blastocoel is full of liquid consisting of mucopolysaccharides.Equal CoeloblastulaThe blastula resulting from holoblastic equal cleavage is called equal coeloblastula. Ex – echinoderms and amphioxus.Unequal CoeloblastulaHoloblastic unequal cleavage (like in frogs) causes an unequal coeloblastula. It has a blastocoel displaced in the direction of the animal pole and multilayered blastoderm.StereoblastulaSolid blastula with no blastocoel, consisting of unequal-sized blastomeres called micromere and macromere.

Example – annelids, molluscs, nematodes, and planaria.

PeriblastulaEggs of insects undergoing superficial cleavage produce blastomeres towards the periphery of the cytoplasm without forming a blastocoel. In insects, there is a one-cell-thick layer of blastoderm enclosing a cavity filled with yolk. This is called Periblastula.DiscoblastulaIt is a type of blastula with a multilayered disc of cells separated from the yolk by the subgerminal cavity which is not comparable to the blastocoel.

This type is seen in the meroblastic eggs of fishes, reptiles, birds, and prototherian mammals.

In birds, a true blastocoel will appear later below the epiblast and hypoblast.

Blastocyst Differentiation

In eutherian mammals, the blastula stage is called blastocyst. It consists of a single layer of cells called trophectoderm or trophoblast, enclosing a large, fluid-filled blastocoel, and a clump of cells attached to its interior, the inner cell mass (ICM).

These cells are also referred to as formative cells because they form the embryo proper.

The cells of trophoblast give rise to extra-embryonic membranes and placenta.

The embryo + embryo’s membrane layers = conceptus.