Egg Cleavage

Development in nearly all animals involves a period in which the egg is subdivided into increasingly smaller cells. Compared with other animals, insect eggs undergo an unusual type of cleavage. In most animals, cleavage involves subdivision of both cytoplasm and nuclear material, to form individual cells called blastomeres. In contrast, the early cleavages of most insects involve only nuclear subdivisions (karyokinesis) and are not accompanied by cleavage of the cytoplasm (cytokinesis). This type of cleavage is called syncytial cleavage and results in the formation of a common compartment (syncytium), where up to several thousand nuclei reside (Fig. 1).

It is unclear how syncytial cleavage evolved in insects. The sister group to insects, the entognathans, has both total egg cleavage (Collembola) and syncytial cleavage (Diplura). Basal arthropods such as chelicerates, which include horseshoe crabs, spiders, and scorpions, have both syncytial and total cleavages. Even Onychophora, which are believed to be a sister group to Arthropoda, exhibit both syncytial cleavage (in oviparous species with yolky eggs) and total cleavage (in placental, viviparous species). However, it is reasonable to believe that syncytial cleavage in arthropods and insects evolved from total cleavage in the ancestor to the arthropods.

FIGURE 1 Diagram of the basic pattern of early insect embryogenesis: ventral views of eggs, anterior poles at top, are shown above cross sections at the levels indicated by bars in top row. (a) Syncytial cleavage. (b) Formation of the cellular blastoderm: arrows show that the lateral cells are coalescing toward the ventral surface to form the germ anlage. (c) Gastrulation. The prospective mesoderm begins invagination along the midline of the germ anlage. (d) Germ band after gastrulation, with segment borders (dotted) and amniotic folds forming: arrows indicate the movement of the serosal cells to enclose and cover the developing germ band. (e) Advanced germ band stage, with appendage buds, and transient coelomic sacs formed by the mesoderm. [Adapted from Sander, K., et al. (1985).]

FIGURE 1 Diagram of the basic pattern of early insect embryogenesis: ventral views of eggs, anterior poles at top, are shown above cross sections at the levels indicated by bars in top row. (a) Syncytial cleavage. (b) Formation of the cellular blastoderm: arrows show that the lateral cells are coalescing toward the ventral surface to form the germ anlage. (c) Gastrulation. The prospective mesoderm begins invagination along the midline of the germ anlage. (d) Germ band after gastrulation, with segment borders (dotted) and amniotic folds forming: arrows indicate the movement of the serosal cells to enclose and cover the developing germ band. (e) Advanced germ band stage, with appendage buds, and transient coelomic sacs formed by the mesoderm. [Adapted from Sander, K., et al. (1985).]

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