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:
- The position of the zygote nucleus and mitotic spindle at the point of sperm entry
- The amount and distribution of yolk
- Cytoplasmic rotation
The first cleavage in a frog’s egg was observed by Swammerdam in 1738.
Planes of Cleavage
| Plane | Description | Example |
|---|---|---|
| Meridional | Furrow passes through the center of the egg from animal pole to vegetal pole, dividing it into two equal halves | 1st and 2nd cleavage in frog; 1st cleavage in chick |
| Vertical | Furrow runs parallel to the meridional plane but slightly away from the egg’s center | 3rd cleavage in chick eggs |
| Equatorial | Furrow forms at a right angle to the meridional plane along the equator of the egg | 3rd cleavage in sea urchin |
| Latitudinal | Similar to equatorial, but the furrow passes through the egg on either side of the equator along its latitude | 8th cleavage in frog |
Types of Cleavage
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. The blastomeres formed during early embryonic cleavage cannot develop into a complete organism if separated.
Many protostomes such as annelids, mollusks, and nematodes exhibit this type.
Indeterminate Cleavage
Indeterminate cleavage is a type of cleavage where blastomeres are qualitatively equal and capable of developing into a complete embryo if isolated. Such blastomeres retain the potential to develop into an independent organism, with all animal features intact.
Deuterostomes typically exhibit this type. Found in the eggs of echinoderms and vertebrates — such eggs are termed regulative eggs.
| Feature | Determinate (Mosaic) | Indeterminate (Regulative) |
|---|---|---|
| Blastomere fate | Fixed and predetermined | Flexible, equal potential |
| Can isolated blastomere form complete embryo? | No | Yes |
| Animal group | Protostomes | Deuterostomes |
| Examples | Annelids, mollusks, nematodes | Echinoderms, vertebrates |
Cleavage Patterns Based on Amount and Distribution of Yolk
Holoblastic Cleavage
(Greek: holos = complete)
In holoblastic cleavage, each cleavage furrow completely divides the entire egg. This occurs in eggs with 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 Occurs in mesolecithal and moderately telolecithal eggs (e.g., some bony fishes and amphibians). Due to the increased concentration of yolk in the vegetal hemisphere, blastomeres of unequal size are produced:
- Smaller blastomeres → micromeres
- Largest, yolk-laden blastomeres → macromeres
Meroblastic Cleavage
(Greek: meros = part)
Meroblastic cleavage occurs where the furrow does not divide the egg completely, being restricted to a small cytoplasmic part. This happens in eggs with large amounts of yolk (telolecithal and centrolecithal eggs).
Discoidal Meroblastic Occurs in macrolecithal and highly telolecithal eggs (fish, reptiles, birds, 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.
| Type | Subtype | Yolk Amount | Blastomere Size | Examples |
|---|---|---|---|---|
| Holoblastic | Equal | Alecithal / Microlecithal | Equal | Sea urchin, Amphioxus, mammals |
| Unequal | Mesolecithal | Unequal (micro + macromeres) | Frog, some bony fishes | |
| Meroblastic | Discoidal | Macrolecithal / Telolecithal | Disc-shaped region only | Fish, reptiles, birds, monotremes |
| Superficial | Centrolecithal | Peripheral cytoplasm only | Insects |
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 Patterns
The pattern of cleavage in an egg is determined by factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation.
| Pattern | Description | Examples |
|---|---|---|
| Radial | Blastomeres lie exactly on blastomeres of the lower tier, symmetrically arranged around the polar axis | Echinoderms, Amphioxus, amphibians |
| Bilateral | Blastomeres arranged radially but of different sizes; cleavage does not divide from center, giving bilateral symmetry | Tunicates |
| Biradial | First two planes are meridional; third is vertical — embryo can be cut into equal halves from any plane | Ctenophores |
| Spiral | Upper tier of cells is displaced clockwise (dextral) or anticlockwise (sinistral) relative to the lower tier, due to an oblique mitotic spindle | Nematodes, annelids, molluscs (except cephalopods) |
| Rotational | First division is meridional; second is rotational — one blastomere divides meridionally, the other equatorially. Followed by compaction after the 3rd cleavage | Mammals |
Laws of Cleavage
| Law | Statement |
|---|---|
| Sachs’s Law | Cells divide into two equal halves and each new plane intersects the preceding one at right angles |
| Hertwig’s Law | The mitotic spindle lies in the longest axis of the protoplasmic mass and its divisions cut the protoplasmic mass at right angles |
| Balfour’s Law | Rate of cleavage is inversely proportional to the amount of yolk present |
| Pflüger’s Law | The 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. 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 enters the morula phase (16 cells). Cavitation then takes place as the outer layer of cells becomes looser. When the number of cells reaches 40–50, a central fluid-filled cavity called the blastocoel is formed.
The zona pellucida begins to deteriorate, allowing the embryo to increase its volume. This phase is reached after 4–6 days in the blastocyst (related to the blastula phase) and lasts until implantation in the uterus.
Types of Blastulae
| Type | Description | Examples |
|---|---|---|
| Coeloblastula | Hollow blastula with a large spacious blastocoel, typically filled with liquid containing mucopolysaccharides. Two subtypes: equal coeloblastula (from holoblastic equal cleavage) and unequal coeloblastula (from holoblastic unequal cleavage, e.g. frogs — blastocoel displaced toward animal pole) | Equal: echinoderms, Amphioxus Unequal: frogs |
| Stereoblastula | Solid blastula with no blastocoel, consisting of unequal-sized blastomeres (micromeres and macromeres) | Annelids, molluscs, nematodes, planaria |
| Periblastula | Blastomeres form towards the periphery of the cytoplasm without forming a blastocoel. A one-cell-thick blastoderm layer encloses a yolk-filled central cavity | Insects (centrolecithal eggs) |
| Discoblastula | Multilayered disc of cells separated from the yolk by the subgerminal cavity (not comparable to the blastocoel). A true blastocoel appears later below the epiblast and hypoblast in birds | Fish, reptiles, birds, prototherian mammals |
| Blastocyst | The blastula stage in eutherian mammals. Consists of a single trophoblast layer enclosing a large fluid-filled blastocoel and an inner cell mass (ICM). Trophoblast gives rise to extra-embryonic membranes and placenta; ICM forms the embryo proper | Eutherian mammals (humans, dogs, etc.) |
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
- A clump of cells attached to its interior — the inner cell mass (ICM), also called formative cells because they form the embryo proper
The cells of the trophoblast give rise to extra-embryonic membranes and placenta.
Embryo + embryo’s membrane layers = conceptus
