Cleavage Repackaging the Cytoplasm

The transformation of the diploid zygote into a mass of cells occurs through a rapid series of cell divisions, called cleavage. Because the cytoplasm of the zygote is not homogeneous, these first cell divisions result in the differential distribution of nutrients and cytoplasmic determinants among the cells of the early embryo. In most animals, cleavage proceeds with rapid DNA replication and mitosis, but no cell growth and little gene expression. The embryo becomes a solid ball of smaller and smaller cells, called a morula (from the Latin word for "mulberry"). Eventually, this ball forms a central fluid-filled cavity called a blastocoel, at which point the embryo is called a blastula. Its individual cells are called blastomeres.

The pattern of cleavage, and therefore the form of the blastula, is influenced by two major factors. First, the amount of nutrient material, or yolk, stored in the egg differs among species. Yolk influences the pattern of cell divisions by impeding the pinching in of the plasma membrane to form a cleavage furrow between the daugh-

Plasma Membrane
(c) Dorsal enrichment inhibitor

(d) Dorsal inhibition of GSK-3

(e) Dorsal enrichment of ß-catenin

Frog Development Zygote

20.3 Cytoplasmic Factors Set Up Signaling Cascades

Cytoplasmic movement changes the distributions of critical developmental signals. In the frog zygote, the interaction of the protein kinase GSK-3, its inhibitor, and the protein ß-catenin are crucial in specifying the dorsal-ventral (back-belly) axis of the embryo.

20.3 Cytoplasmic Factors Set Up Signaling Cascades

Cytoplasmic movement changes the distributions of critical developmental signals. In the frog zygote, the interaction of the protein kinase GSK-3, its inhibitor, and the protein ß-catenin are crucial in specifying the dorsal-ventral (back-belly) axis of the embryo.

ter cells. Second, cytoplasmic determinants stored in the egg by the mother guide the formation of mitotic spindles and the timing of cell divisions.

The amount of yolk influences cleavage

In embryos with little or no yolk, there is little interference with cleavage furrow formation, and all the daughter cells are of similar size; the sea urchin egg provides an example (Figure 20.4a). More yolk means more resistance to cleavage furrow formation; therefore, cell divisions progress more rapidly in the animal hemisphere than in the vegetal hemisphere, where the yolk is concentrated. As a result, the cells derived from the vegetal hemisphere are fewer and larger; the frog egg provides an example of this pattern (Figure 20.4b).

In spite of this difference between sea urchin and frog eggs, the cleavage furrows completely divide the egg mass in both cases; thus these animals are said to have complete cleavage. In contrast, in eggs that contain a lot of yolk, such as the chicken egg, the cleavage furrows do not penetrate the yolk. As a result, cleavage is incomplete, and the embryo forms as a disc of cells, called a blastodisc, on top of the yolk mass (Figure 20.4c). This type of incomplete cleavage, called discoidal cleavage, is common in fishes, reptiles, and birds.

Another type of incomplete cleavage, called superficial cleavage, occurs in insects such as the fruit fly (Drosophila). In the insect egg, the mass of yolk is centrally located (Figure 20.4d). Early in development, cycles of mitosis occur without cytokinesis. Eventually the resulting nuclei migrate to the periphery of the egg, and after several more mitotic cycles, the plasma membrane of the egg grows inward, partitioning the nuclei into individual cells.

The orientation of mitotic spindles influences the pattern of cleavage

The positions of the mitotic spindles during cleavage are not random; rather, they are defined by cytoplasmic determinants that were produced from the maternal genome and stored in the egg. The orientation of the mitotic spindles determines the planes of cleavage and, therefore, the arrangement of the daughter cells.

If the mitotic spindles of successive cell divisions form parallel or perpendicular to the animal-vegetal axis of the zygote, the cleavage pattern is radial, as in the sea urchin and the frog. In these organisms, the first two cell divisions are parallel to the animal-vegetal axis and the third is perpendicular to it (Figure 20.4a,b). Another cleavage pattern, spiral cleavage, results when the mitotic spindles are at oblique angles to the animal-vegetal axis. Mollusks have spiral cleavage, and a visible expression of this is the coiling of snail shells.

(a) Sea urchin

(lateral view)

Yolk platelets are evenly distributed. j~

Complete cleavage

Yolk is concentrated L at the vegetal pole. j~

(view from top)

Incomplete cleavage

(d) Drosophila (lateral section)

Superficial cleavage

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Responses

  • matthias
    How is the orientation of mitotic spindle influenced by factors present in egg cytoplasm?
    8 years ago
  • ERMIAS
    What influences patterns of cleavage?
    8 years ago
  • Angelika Frankfurter
    How is orientation of mitotic spindle influenced by cytoplasmic factors?
    8 years ago
  • bo
    How mitotic spindle orientation influences radial cleavage?
    8 years ago
  • tanja
    What is cytoplasmic repackaging?
    8 years ago
  • ilta
    What does repackaging the cytoplasm mean?
    8 years ago

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