Cleavage

Most insect eggs undergo superficial cleavage, wherein a large mass of centrally located yolk confines cleavage to the cytoplasmic rim of the egg. One of the fascinating features of this cleavage type is that cells do not form until after the nuclei have divided. Cleavage in a Drosophila egg is shown in Figure 9.1. The zygote nucleus undergoes several mitotic divisions within the central portion of the egg. In Drosophila, 256 nuclei are produced by a series of eight nuclear divisions averaging 8 minutes each. The nuclei then migrate to the periphery of the egg, where the mitoses continue, albeit at a progressively slower rate. During the ninth division cycle, about five nuclei reach the surface of the posterior pole of the embryo. These nuclei become enclosed by cell membranes and generate the pole cells that give rise to the gametes of the adult. Most of the other nuclei arrive at the periphery of the embryo at cycle 10 and then undergo four more divisions at progressively slower rates. During these stages of nuclear division, the embryo is called a syncytial blastoderm, meaning that all the cleavage nuclei are contained within a common cytoplasm. No cell membranes exist other than that of the egg itself.

Figure 9.1

Superficial cleavage in a Drosophila embryo. The early divisions occur centrally. The numbers refer to the cell cycle. At the tenth cell cycle (512-nucleus stage 2 hours after fertilization), the pole cells form in the posterior, and the nuclei and their (more...)

Although the nuclei divide within a common cytoplasm, this does not mean that the cytoplasm is itself uniform. Karr and Alberts (1986) have shown that each nucleus within the syncytial blastoderm is contained within its own little territory of cytoskeletal proteins. When the nuclei reach the periphery of the egg during the tenth cleavage cycle, each nucleus becomes surrounded by microtubules and microfilaments. The nuclei and their associated cytoplasmic islands are called energids. Figure 9.2 shows the nuclei and their essential microfilament and microtubule domains in prophase of the twelfth mitotic division.

Figure 9.2

Localization of the cytoskeleton around nuclei in the syncytial blastoderm of Drosophila. A Drosophila embryo entering the mitotic prophase of its twelfth division was sectioned and triple-stained. (A) The nuclei were localized by a dye that binds to (more...)

Following cycle 13, the oocyte plasma membrane folds inward between the nuclei, eventually partitioning off each somatic nucleus into a single cell (Figure 9.3). This process creates the cellular blastoderm, in which all the cells are arranged in a single-layered jacket around the yolky core of the egg (Turner and Mahowald 1977; Foe and Alberts 1983). Like any other cell formation, the formation of the cellular blastoderm involves a delicate interplay between microtubules and microfilaments. The first phase of blastoderm cellularization is characterized by the invagination of cell membranes and their underlying actin microfilament network into the regions between the nuclei to form furrow canals. This process can be inhibited by drugs that block microtubules. After the furrow canals have passed the level of the nuclei, the second phase of cellularization occurs. Here, the rate of invagination increases, and the actin-membrane complex begins to constrict at what will be the basal end of the cell (Schejter and Wieschaus 1993; Foe et al. 1993). In Drosophila, the cellular blastoderm consists of approximately 6000 cells and is formed within 4 hours of fertilization.

Figure 9.3

Formation of the cellular blastoderm in Drosophila. (A) Developmental series showing the progressive cellularization. (B) Confocal fluorescence photomicrographs of nuclei dividing during the cellularization of the blastoderm. While there are no cell boundaries, (more...)

The midblastula transition

After the nuclei reach the periphery, the time required to complete each of the next four divisions becomes progressively longer. While cycles 1–10 are each 8 minutes long, cycle 13, the last cycle in the syncytial blastoderm, takes 25 minutes to complete. Cycle 14, in which the Drosophila embryo forms cells (i.e., after 13 divisions), is asynchronous. Some groups of cells complete this cycle in 75 minutes, whereas other groups of cells take 175 minutes (Figure 9.4; Foe 1989). Transcription from the nuclei (which begins around the eleventh cycle) is greatly enhanced at this stage. This slowdown of nuclear division and the concomitant increase in RNA transcription is often referred to as the midblastula transition (see Chapter 8). Such a transition is also seen in the embryos of numerous vertebrate and invertebrate phyla. The control of this mitotic slowdown (in Xenopus, sea urchin, starfish, and Drosophila embryos) appears to be effected by the ratio of chromatin to cytoplasm (Newport and Kirschner 1982; Edgar et al. 1986a). Edgar and his colleagues compared the early development of wild-type Drosophila embryos with that of a haploid mutant. These haploid Drosophila embryos have half the wild-type quantity of chromatin at each cell division. Hence a haploid embryo at the eighth cell cycle has the same amount of chromatin that a wild-type embryo has at cell cycle 7. The investigators found that whereas wild-type embryos formed their cellular blastoderm immediately after the thirteenth division, the haploid embryos underwent an extra, fourteenth, division before cellularization. Moreover, the lengths of cycles 11–14 in wild-type embryos corresponded to those of cycles 12–15 in the haploid embryos. Thus, the haploid embryos follow a pattern similar to that of the wild-type embryos—only they lag by one cell division.

Figure 9.4

Differences in regional rates of cell division in Drosophila embryos. (A) Expression of the string gene correlates with cell division. In this example, a late stage 14 embryo is stained with a radioactive nucleotide sequence that specifically recognizes (more...)

WEBSITE

9.2 The regulation ofDrosophila cleavage. The control of the cell cycle in Drosophila is a story of how the zygote nucleus gradually takes control from the mRNAs and proteins stored in the oocyte cytoplasm. http://www.devbio.com/chap09/link0902.shtml

WEBSITE

9.3 The early development of other insects. Drosophila is a highly derived species. There are other insect species that develop in ways very different from the “standard” fruit fly. http://www.devbio.com/chap09/link0903.shtml

Gastrulation

At the time of midblastula transition, gastrulation begins. The first movements of Drosophila gastrulation segregate the presumptive mesoderm, endoderm, and ectoderm. The prospective mesoderm—about 1000 cells constituting the ventral midline of the embryo—folds inward to produce the ventral furrow (Figure 9.5). This furrow eventually pinches off from the surface to become a ventral tube within the embryo. It then flattens to form a layer of mesodermal tissue beneath the ventral ectoderm. The prospective endoderm invaginates as two pockets at the anterior and posterior ends of the ventral furrow. The pole cells are internalized along with the endoderm. At this time, the embryo bends to form the cephalic furrow.

Figure 9.5

Gastrulation in Drosophila. (A) Ventral furrow beginning to form as cells flanking the ventral midline invaginate. (B) Closing of ventral furrow, with mesodermal cells placed internally and surface ectoderm flanking the ventral midline. (C) Dorsal view (more...)

The ectodermal cells on the surface and the mesoderm undergo convergence and extension, migrating toward the ventral midline to form the germ band, a collection of cells along the ventral midline that includes all the cells that will form the trunk of the embryo. The germ band extends posteriorly and, perhaps because of the egg case, wraps around the top (dorsal) surface of the embryo (Figure 9.5D). Thus, at the end of germ band formation, the cells destined to form the most posterior larval structures are located immediately behind the future head region. At this time, the body segments begin to appear, dividing the ectoderm and mesoderm. The germ band then retracts, placing the presumptive posterior segments into the posterior tip of the embryo (Figure 9.5E).

While the germ band is in its extended position, several key morphogenetic processes occur: organogenesis, segmentation, and the segregation of the imaginal discs^* (Figure 9.5e). In addition, the nervous system forms from two regions of ventral ectoderm. As described in Chapter 6, neuroblasts differentiate from this neurogenic ectoderm within each segment (and also from the nonsegmented region of the head ectoderm). Therefore, in insects like Drosophila, the nervous system is located ventrally, rather than being derived from a dorsal neural tube as in vertebrates.

The general body plan of Drosophila is the same in the embryo, the larva, and the adult, each of which has a distinct head end and a distinct tail end, between which are repeating segmental units (Figure 9.7). Three of these segments form the thorax, while another eight segments form the abdomen. Each segment of the adult fly has its own identity. The first thoracic segment, for example, has only legs; the second thoracic segment has legs and wings; and the third thoracic segment has legs and halteres (balancers). Thoracic and abdominal segments can also be distinguished from each other by differences in the cuticle. How does this pattern arise? During the past decade, the combined approaches of molecular biology, genetics, and embryology have led to a detailed model describing how a segmented pattern is generated along the anterior-posterior axis and how each segment is differentiated from the others.

Figure 9.7

Comparison of larval and adult segmentation in Drosophila. The three thoracic segments can be distinguished by their appendages: T1 (prothorax) has legs only; T2 (mesothorax) has wings and legs; T3 (metathorax) has halteres and legs.

The anterior-posterior and dorsal-ventral axes of Drosophila form at right angles to one another, and they are both determined by the position of the oocyte within the follicle cells of the ovary. The rest of this chapter is divided into three main parts. The first part concerns how the anterior-posterior axis is specified and how it determines the identity of each segment. The second part concerns how the dorsal-ventral axis is specified by the interactions between the oocyte and its surrounding follicle cells. The third part concerns how embryonic tissues are specified to become particular organs by their placement along these two axes.

Figure 9.6

Schematic representation of gastrulation in Drosophila. (A) and (B) are surface and cut-away views showing the fates of the tissues immediately prior to gastrulation. (C) shows the beginning of gastrulation as the ventral mesoderm invaginates into the (more...)

Footnotes

*

Imaginal discs are those cells set aside to produce the adult structures. The details of imaginal disc differentiation will be discussed in Chapter 18. For more information on Drosophila developmental anatomy, see Bate and Martinez-Arias 1993; Tyler and Schetzer 1996; and Schwalm 1997.