Fertilization occurs when the mature oocytes are released into the oviducts. A single sperm enters the egg cytoplasm through a special channel in the anterior region of the oocyte called the micropyle. Fertilization initiates the completion of meiosis I and II, producing two polar-body nuclei and the female pronucleus. After the haploid male and female pronuclei unite (syngamy), early embryogenesis takes place so rapidly there is no time for cell growth (Figure 4.4). Initial mitoses are atypical because the first nine divisions result in a syncytium containing approximately 512 nuclei that lack cellular membranes.
Figure 4.4. Early embryonic stages of Drosophila melanogaster from fertilization to just before gastrula-tion, showing the appearance of pole and somatic buds and cessation of division of yolk nuclei. Numbers indicate division cycles; each cycle begins with the start of interphase and ends at the conclusion of mitosis. Embryos are in longitudinal section without the vitelline membrane. All nuclei (black circles) are shown for cycles 1-5, and afterwards only some are shown. Stippled areas represent yolk, and open areas represent yolk-free cytoplasm. Yolk-free cytoplasm is found both at the periphery (= periplasm) and in islands around the nuclei. During cycles 1-7, nuclei multiply exponentially in the central region of the fertilized egg. Cycle 8 illustrates migration of the majority of the nuclei to the periphery, leaving the future yolk nuclei behind in the center. Yolk nuclei continue to divide in synchrony with other nuclei in cycles 8-10; they then cease dividing and become polyploid. Early in cycle 9, a few nuclei appear in the posterior periplasm and cause protrusions of the cytoplasm, called pole buds. During cycle 10, the remaining migrating nuclei enter the periplasmic region, forming somatic buds over the entire embryonic surface. During the 10th cycle, pole buds are pinched off to form pole cells. After this, synchrony between the pole cells and the syncytium is lost. The syncytial nuclei continue to divide synchronously. The periplasm begins to thicken in cycle 13. During cycle 14, the formation of a plasma membrane begins to separate cells over the entire surface of the embryo, with nuclei elongating to match elongated cells formed by late cycle 14A. During 14B, gastrulation movements begin with the infolding of the cephalic furrow (anterior) and posterior midgut furrow, and subsequently the cells no longer divide synchronously.
After seven nuclear divisions, and when there are 128 nuclei in the central region of the egg, most of the nuclei and their surrounding cytoplasm migrate outward as they continue to divide. A few nuclei are left behind, which divide once to become yolk nuclei that do not become incorporated in the embryo (Figure 4.3). After nine divisions, most of the nuclei have migrated to the egg surface. At this time, the soma and germ-line nuclei segregate when about 15 nuclei move to the posterior region of the egg, bud off, and eventually become the nuclei in the pole cells. These nuclei divide about twice more and become pole cells that will give rise to the germ-line tissues of the fly. Meanwhile, the other nuclei migrate to the surface of the egg and divide four times more in synchrony to produce a syncytial blastoderm.
Finally, the membrane covering the egg invaginates to enclose each nucleus in a separate membrane, to form a cellularized blastoderm (Figure 4.4). The blastoderm is the layer of cells in an insect embryo that completely surrounds an internal yolk mass. The cellular blastoderm develops from a syncytial blastoderm by partitioning the cleavage nuclei with membranes derived from infolding of the oolemma. During the cellular blastoderm stage, D. melanogaster exhibits the long germ band type of development in which the pattern of segmentation is established by the end of blastoderm. Some other insects exhibit the short germ band type of development in which all or most of the metameric pattern is completed by the sequential addition of segments during elongation of the caudal region of the embryo.
Prior to the cellularized blastoderm stage, the dividing nuclei are equivalent and totipo-tent, but after the cellularized blastoderm stage is reached, specific body segments have been determined. The cellularized blastoderm stage is a key transition point in embryogenesis in D. melanogaster because this is the period during which the products of maternal genes become less important. It is thought that only a few zygotic genes are active prior to cellularization. After the cellularized blastoderm stage, the genes in the zygote begin to dominate in directing the development of the embryo. After additional development, the insect embryo gives rise to a segmented larva with three major tagmata: the head, thorax, and abdomen.
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