What Type of Cleavage Is Illustrated in This Figure?

Embryonic cleavage is the process of rapid cell division that occurs in the early embryo after fertilization. These mitotic divisions are unique because they divide the large volume of the fertilized egg’s cytoplasm into numerous smaller cells, called blastomeres, without an overall increase in the embryo’s size. This series of divisions transforms the single-celled zygote into a multicellular structure. The process continues until a hollow sphere of cells, known as the blastula, is formed.

Yolk’s Influence on Cleavage

The pattern of embryonic cleavage is primarily determined by the amount and distribution of yolk within the egg. Yolk, a nutrient-rich substance, can physically impede the formation of cleavage furrows, thus dictating how the egg divides. Biologists classify eggs into several types based on their yolk content, which in turn predicts the style of cleavage the embryo will undergo.

Eggs with a scant and evenly distributed amount of yolk are called isolecithal. Mesolecithal eggs contain a moderate amount of yolk concentrated in one half of the egg, known as the vegetal pole. Both of these egg types undergo a complete form of division.

In contrast, telolecithal eggs have a very large, dense yolk mass, with the cytoplasm confined to a small area. Centrolecithal eggs have their yolk concentrated in the center. These yolk-rich eggs undergo an incomplete form of division.

The distinction based on yolk leads to two broad categories of cleavage. The first is holoblastic cleavage, where the entire egg is completely divided. The second is meroblastic cleavage, a partial division where furrows cannot penetrate the dense yolk.

Complete Cleavage: Holoblastic Patterns

Common in animals whose eggs have little to moderate amounts of yolk, holoblastic cleavage occurs in several distinct geometric patterns characteristic of different animal groups.

• Radial cleavage is characterized by division planes that are either parallel or perpendicular to the polar axis of the egg. This results in tiers of cells stacked directly on top of one another, forming a radially symmetrical arrangement. This pattern is typical of echinoderms, like sea urchins, and amphioxus.
• Spiral cleavage involves cleavage planes that are oblique to the animal-vegetal axis. This orientation causes the upper layer of blastomeres to be offset and sit in the furrows between the cells of the lower layer. This pattern is found in animals like annelids and most mollusks.
• Bilateral cleavage is defined by its first division plane, which establishes the axis of symmetry for the entire embryo. All subsequent cleavage divisions are organized in relation to this plane, resulting in two halves that are mirror images of each other. Tunicates provide a clear example of this pattern.
• Rotational cleavage is a pattern seen in mammals and nematodes. The first division is a standard meridional split, but the second division involves one blastomere dividing meridionally and the other dividing equatorially.

Incomplete Cleavage: Meroblastic Patterns

In this type of partial division, only a portion of the egg’s cytoplasm gets divided, while the yolk remains uncleaved. This results in distinct patterns based on where the yolk is concentrated.

Discoidal cleavage is characteristic of fish, reptiles, and birds. In these eggs, cell division is confined to a small disc of cytoplasm at the animal pole, called the blastodisc. The cleavage furrows divide this disc into a cap of cells that rests on top of the large, undivided yolk mass.

Superficial cleavage occurs in insects and other arthropods. In this process, initial nuclear divisions happen within the central yolk mass without forming cell membranes, creating a single, multinucleated cell called a syncytium. These nuclei then migrate to the outer edge of the egg, and cell membranes subsequently form around them, creating a layer of cells at the surface that encloses the central yolk.