PRIMORDIAL GERM-CELLS OF SYNAPTULA-HYDRIFORMIS (HOLOTHUROIDEA, ECHINODERMATA) ARE EPITHELIAL FLAGELLATED-COLLAR CELLS - THEIR APICAL-BASAL POLARITY BECOMES PRIMARY EGG POLARITY

The primordial germ cells (PGCs) of a recently metamorphosed juvenile Synaptula hydriformis occur with somatic cells in the germinal epithel ium of the gonad, As part of the epithelium, PGCs rest on a basal lami na, extend apically towards a lumen, are joined to other cells of the epithelium via apicolateral junctions, and express apical-basal polari ty. Each PGC has an apical flagellum that is surrounded by a collar of microvilli, The apicolateral junctions of PGCs consist of apical adhe ring and subapical septate junctions. Hemidesmosomes attach the PGCs t o the basal lamina, Although the somatic cells form an incomplete laye r over the PGCs, both the PGCs and somatic cells remain exposed to the apical lumen and retain contact with the basal lamina. The peritoneum is the outermost layer of the gonad and faces the perivisceral coelom . The epithelial-cell characteristics expressed by cells of the perito neum are identical to those of the germinal epithelium. PGCs of S. hyd riformis are epithelial flagellated-collar cells and express the apica l-basal polarity that is typical of epithelial cells. The apical-basal polarity of the oocyte, animal-vegetal axis of full-grown eggs, and a nterior-posterior axis of larvae and adults are all in correspondence. Thus the polarity of the germinal epithelium may determine the primar y body axis of the next generation.