Germ Band

After gastrulation and during the differentiation of the germ band, the embryos assume a characteristic shape that differs between taxa. The plesiomorphic condition within malacostracans is characterized by the formation of a transverse ventral groove, the caudal furrow (Figure 6.IB, C). With advanced development, this groove deepens and subdivides the embryo in an anterior yolky region and a posterior ventrally folded mostly yolk-free caudal papilla (Scholtz 2000) (Figures 6.1B and 6.2A). The latter starts as a dome-shaped bud in the posterior region of the embryo (Figure 6.IB). Concomitant with the budding of cells in the posterior growth zone, the caudal papilla elongates and becomes ventrally flexed (Figure 6.2A). Since the growth zone encircles the caudal papilla (Figure 6.2A, C, D), the ventral and dorsal parts of the embryo and the subsequent segments are formed. With hatching, the caudal papilla stretches backward and the egg envelopes rupture. The groove between the anterior region and the caudal papilla does not correspond to the boundary between thorax and pleon. Rather the caudal papilla comprises the anlagen of a varying number of thoracic, all pleonic segments, and the telson (Scholtz 2000) (Figure 6.2A). A caudal papilla of this sort has been found in embryos of Leptostraca, Stomatopoda, Decapoda, Syncarida, and Thermosbaenacea (Scholtz 2000). In the latter, the caudal papilla contains some yolk but the growth zone still encircles the papilla (Zilch 1974). Among peracarids, Mysidacea possess a caudal papilla, but the yolk content is relatively high and the growth of the germ band is restricted to the ventral side (Scholtz 1984). Amphipoda show a caudal groove, but the posterior body is almost as wide as the anterior part, and as in mysidaceans only the ventral side constitutes the germ band (Weygoldt 1958; Scholtz 1990; Ito et al. 2011) (Figure 6.1C). In contrast to this, the embryos of Isopoda, Tanaidacea and Cumacea, Spelaeogriphacea, and Mictacea show neither a caudal groove nor a caudal papilla (Scholtz 2000;

Teloblast patterns

FIGURE 6.2 Teloblast patterns. (A) The germ band with a ventrally folded caudal papilla of Astacus astacus (modified after Reichenbach 1886). The head lobes (hi), the labrum (lr), the first and second antennae (al, a2), the mandibles (md), the first maxillae (mxl), and the caudal papilla (cp) show an advanced development, when compared with Figure 6.IB. The proctodaeum (pr) is situated in the area of the telson (te). The ectoteloblasts (ET) are clearly recognizable based on their relatively large size; there are more than 19 in number (see D). Reichenbach’s publication is the first record of this type of stem cells. (B) The posterior region of the germ band of the isopod Cymothoa sp. (anterior on top) (modified after Patten 1890). This is the first record of mesoteloblasts (MT). The mesoteloblasts and their derivatives of the right animal’s side are highlighted (yellow). Note the regular row arrangement (compare with Figures 6.5C and 6.6) and the cytoplasmic longitudinal and horizontal connections of the mesoteloblasts and their derivatives. Furthermore, there is a certain distance between the median mesoteloblast and the more lateral mesoteloblasts, exemplified by the third mesoteloblast from the midline (MT,) (compare with C and D). (C) A cross section through the caudal papilla in the area of teloblasts of Homarus ameri- canus (modified after Dohle et al. 2004). An outer circle of 19 ectoteloblasts (ET) and an inner circle of 8 mesoteloblasts (MT) around the proctodaeum are visible. This is the ancestral condition within Malacostraca. (D) A similar section in the freshwater crayfish Cherax destructor. About 40 ectoteloblasts (ET) are combined with 8 mesoteloblasts (MT) both situated around the proctodaeum. (E) The growth zone of the peracarid Neomysis integer. The ectoteloblasts (ET) form a transverse row of about 15 cells anterior to the telson ectoderm (te). (F) The absence of ectoteloblasts in the germ band of the amphipod Cryptorchestia garbinii. The transverse ectoderm rows are formed by cell rearrangements in front of the telson ectoderm with the proctodaeum.

Olesen et al. 2014) (Figure 6.1 D). The germ band grows out in one plane and only later a dorsal furrow' forms that straightens with hatching. Accordingly, this condition has been considered as an apomorphy for the Mancoidea within the Peracarida (see Richter and Scholtz 2001).

< Prev   CONTENTS   Source   Next >