Germ granule and germ cells in Chaetognaths

Formation of a large Vasa-positive Germ Granule and its inheritance by Germ Cells in the enigmatic Chaetognaths

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Danièle Carré, Chakib Djediat, Christian Sardet, Development, Development, 129, February 2002 .

Ancient histological observations dating from a century ago described the presence in eggs of a large granule presumed to be a germ plasm and its probable inheritance in 4 primary germ cells (PGCs). Using videomicroscopy, electron microscopy and immunocytochemistry (labelling with anti-Vasa antibodies) we have followed the cycle of aggregation and dispersion of germ plasm and nuage material in eggs, embryos, PGCs and oocytes in several species of benthic (Spadella) and planctonic (Sagitta) chaetognaths.
The germ plasm (green) is within and around the germinal vesicle (GV).During maturation it probably fragments into minute granules (1). At fertilization (2) and spawning (3) many small granules line the vegetal cortex (v) and then aggregate during amphimixy (4). They form a large granule at mitosis (see films)(5). This large granule is segregated into one of the first 2 blastomeres and continues to be inherited by only one blastomere until the 32 cell stage (7); it then splits into 2 at the 64 cell stage (8). The germ plasm is then found in four presumptive Primordial Germ Cells (pPGC's) at the tip of the archenteron in the gastrula (9). The four PGC's become the right and left male and female stem cells in the juvenile (10), which give rise to two ovaries and two testis in the adult.