Anomaly of Cell Shapes and Behavior
The shape and behavior of the ectoderm cells in the malacostracan germ band are also interesting from a cytological perspective. The normal appearance of cells in a cell layer is a hexagonal shape (e.g., Irvine and Wieschaus 1994: Stollewerk et al. 2001). This shape allows the greatest density of cells, comparable to the arrangement of wax and paper combs in social hymenopterans or the facets of most arthropod compound eyes. Accordingly, the cells in the germ band of most arthropod embryos are hexagonal. In contrast to this, the cells of the malacostracan germ band are more or less squared and arranged in regular rows (Figure 6.10). The reason for this is unknown. In those species that differentiate ectoteloblasts, the explanation can be seen in the regular gridlike arrangement of the ectoteloblasts, which is passed on to
FIGURE 6.10 Cell shape in the malacostracan germ band. The post-naupliar germ band of the mysidacean Neomysis integer depicting rectangular cell shapes, exemplified in the first thoracic (thl) and more posterior segments. This stands in contrast to the naupliar region with the first and second antennae (al, a2) and the mandibles (md) that do not show the regular cell arrangement as the post-naupliar region and that represents the ancestral condition of germ bands of arthropods in general (compare with Figures 6.5A and 6.8A). The dark cells show expression of the Engrailed protein (compare with Figures 6.11 and 6.12)
their progeny. However, this assumption is falsified by the row formation process in amphipods. Here cell rows are formed by migrating cells, which nevertheless form squares.
The netlike cytoplasmic connections between the derivatives of the mesotelo- blasts are another unusual feature (Patten 1890; Scholtz 1990) (Figure 6.2). As yet, it is not clear whether these connections are indicative of a syncytial post-naupliar mesoderm. Nonetheless, this appears likely and a syncytium might be sensible if the cells are as distantly arranged as the mesodermal segment precursors. Currently, this is only a speculation.
Consecutive division planes in animal cells are mostly oriented perpendicularly to each other (Strome 1993). The movements of the centrosomes serve as explanation for this. Prior to each mitosis, the centrosome duplicates, and the two daughter centrosomes separate to opposite sides of the nucleus. Here they serve as the microtubule-organizing centers of the mitotic spindle. The division plane is perpendicular to the orientation of the spindles and each daughter cell gains one centrosome. Due to their position, they divide at right angles to the previous centrosome division. Hence, deviations from this alternating 90° pattern require different or additional movements of the centrosomes. The cells of the regular ectoderm rows in the germ band of malacostracans divide twice in the same direction w'ith spindles oriented parallel to the longitudinal axis of the embryo. During the subsequent differential cleavages several of these cells continue w ith this spindle direction, w'hereas most others show all various spindle orientations, w'hich are nevertheless stereotyped (see earlier). The mechanisms for this unusual cell behavior are not clear. Extrinsic signals or intrinsic cues as well as the cell shape are possible explanations (Gillies and Cabernard 2011). However, this requires further studies.