Hierarchy of Germ Layers

There is a long debate as to whether the mesoderm determines the fate of the ectoderm or vice versa. Experimental approaches in insects revealed that the ectoderm induces segmental fates to the mesoderm cells (Bock 1942; Haget 1953). Since gas- trulation in pterygote insects follow's a largely different path than that of crustaceans (Anderson 1973), one could expect to find different hierarchic relationships between germ layers. Indeed, comparative data of malacostracans might at first sight argue for a leading role of the mesoderm in the differentiation and segment formation of the germ band (Scholtz 1990). Whereas ectoteloblasts underwent some evolutionary changes w'ith respect to differentiation, arrangement, and number (up to total loss), mesoteloblasts behave conservatively in this respect. There are always eight mesoteloblasts. Moreover, as far as is knowrn all mesoteloblasts are formed via a corresponding stereotyped cell division pattern (Dohle et al. 2004; Hunnekuhl and Wolff 2012) (Figure 6.4). However, a more detailed comparison of the segmentation in the ectoderm and mesoderm layers reveals that the differentiation of the ectoderm precedes that of the mesoderm. Hence, the ectoderm might have an impact on the mesoderm rather than the other way round (Scholtz 1990). This inference has been convincingly confirmed by experimental ablations of ectodermal cells, w'hich led to a distorted pattern of the mesoderm, including absence of segmental structures and gene expression (Hannibal et al. 2012). In contrast to this, the early formation and differentiation of the mesoteloblasts and their descendant row's are independent of ectodermal influences (Hannibal et al. 2012).

Segmentation

In most crustacean embryos, segment formation follows a more or less anteroposterior sequence with more anterior segments showing an advanced development. In these cases, the naupliar region comprising the eye region, the segments of the first and second antennae and of the mandibles develops first, more or less simultaneously. There is a distinct developmental gap between the naupliar segments and the post-naupliar region (Figure 6.1 B). This is true for species that hatch as nauplius larva and for most of those with a later hatchling stage such as a zoea larva or w'ith a direct development. In the latter two cases, one speaks of an egg nauplius (Scholtz

2000, but see Jirikowski et al. 2013) (Figure 6.1B). An egg nauplius is not formed in amphipod, tanaidacean, cumacean, and isopod embryos. In these cases the gradient between the naupliar and post-naupliar regions is smooth and a distinct gap between the development of the naupliar and the post-naupliar segments cannot be observed (Scholtz 2000) (Figure 6.1C,D).

 
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