Teloblasts in Crustaceans

Gerhard Scholtz

CONTENTS

  • 6.1 Introduction..................................................................................................125
  • 6.2 Formation of the Germ Disc.........................................................................126
  • 6.3 Germ Band....................................................................................................128
  • 6.3.1 Shape.................................................................................................128
  • 6.3.2 Growth..............................................................................................130
  • 6.4 Evolution of the Stereotyped Division Pattern.............................................137
  • 6.5 Anomaly of Cell Shapes and Behavior.........................................................139
  • 6.6 Hierarchy of Germ Layers............................................................................141
  • 6.7 Segmentation................................................................................................141
  • 6.8 Short and Long Germ Development.............................................................142
  • 6.9 Segment Morphogenesis..............................................................................142
  • 6.10 Cell Lineage and Cell Fate...........................................................................143
  • 6.11 Homology Issues...........................................................................................145
  • 6.12 Perspectives..................................................................................................146

Acknowledgments..................................................................................................147

References..............................................................................................................147

Introduction

The great diversity of adult crustacean forms, body organizations, and life styles finds its correspondence in a comparable variety of ontogenetic pathways. Crustaceans are unmatched by other arthropod groups concerning their manifold cleavage and gastrulation patterns, segmentation processes, and larval types and development. This fascinating ontogenetic diversity is even greater if recent views on crustacean phylogeny are taken into account. According to these views, crustaceans are paraphyletic with Remipedia, either alone or together with Cephalocarida, as sister taxon to Hexapoda (Regier et al. 2010; von Reumont et al. 2012; Schwentner et al. 2017). Hence, insects are deeply nested within a group that was traditionally called Crustacea but now has been named Pancrustacea or Tetraconata (Zrzavy and Stys 1997; Dohle 2001).

Large cells that undergo a series of asymmetric divisions in one direction can be found in several taxa of Pancrustacea/Tetraconata including hexapods. These characteristic stem cells occur in different proliferation processes such as neurogenesis, then they are called neuroblasts, or the longitudinal growth of germ bands, then they are called teloblasts. In any case, neuroblasts and teloblasts are involved in the quick generation of competent cell material. Neuroblasts have been found in several crustacean groups, such as malacostracans, branchiopods, and copepods, and in hexapods (e.g., Brenneis et al. 2013; Hartenstein and Stollewerk 2015; Hein and Scholtz 2018). Neuroblasts are neuronal precursors and proliferate a set of other neuronal precursors (ganglion mother cells) that divide into neurons.

In contrast to neuroblasts, which might be an apomorphy of Pancrustacea/ Tetraconata (Ungerer and Scholtz 2008), the occurrence of teloblasts is restricted to malacostracan crustaceans (Dohle et al. 2004; Fischer et al. 2010). Malacostracan teloblasts are situated in the pre-anal region of the germ band and generate the ectodermal (ectoteloblasts) and mesodermal (mesoteloblasts) cellular material for segmentation. Hence, teloblasts are a special case of a growth zone.

Ecto- and mesoteloblasts have been first described in annelids, in particular in clitellates (oligochaetes and hirudineans) (see Anderson 1973; Dohle 1999), where they have a similar pre-anal position and function like those of malacostracans but show lower numbers (Shankland and Savage 1997).

The frequent occurrence of large stem cells with asymmetric divisions in different developmental processes and various taxa suggests that this cell type evolved several times independently. It seems an effective way for the proliferation of many cells in a longitudinal arrangement. However, the molecular mechanisms of asymmetric cell divisions are not yet completely understood (see Knoblich 2010).

 
Source
< Prev   CONTENTS   Source   Next >