Germ band formation and early neurogenesis of Leptodora kindti (Cladocera): first evidence for neuroblasts in the entomostracan crustaceans

Abstract

Summary Development of Leptodora kindti (Crustacea, Cladocera) was observed from the first cleavages to early segmentation and neurogenesis. The mode of cleavage is superficial. The first to seventh divisions are synchronous. After reaching the blastoderm stage at 128 cells, cells start to differ in timing and orientation of divisions. The prospective ectoderm cells divide superficially as before, prospective yolk cells stop dividing and prospective mesendoderm cells divide mostly perpendicular to the surface. After gastrulation, the germ band is elongated by equally distributed divisions and not by a budding zone like other entomostracan crustaceans nor by specialized cells like ectoteloblasts in the Malacostraca. After germ band elongation, a streak of median ectoderm cells differentiates into neuroectoderm. During thoracic segmentation, intersegmental furrows simultaneously divide ectoderm and neuroectoderm into segments. The neuroectoderm consists of neuroblasts and median cells. Divisions of the median cells are equal and tangential. The neuroblasts produce ganglion mother cells by highly unequal divisions perpendicular to the surface. The findings are discussed with regards to hypotheses on evolution of crustacean and mandibulate cleavage, segmentation and neural development.