A possible cause of termination of cell growth in the cellular slime mold Dictyostelium discoideum.

The gill ctenidium growth tip of the lamellibranch mollusc Aequipecten irradians recapitulates the temporal development of ciliated gill filaments and related structures in a spatial fashion. This “meristematic” relationship has allowed a study of basal body formation and ciliogenesis in adjacent cells of gill filament papillae at stages of progressively more advanced relative development. Basal bodies appear to originate quite rapidly, subsequent to the appearance of a complex of dense granules, quite reminiscent of the “condensation forms” or “procentriole precursors” typically seen in vertebrate ciliogenesis. Unlike basal body generation in higher forms, that in Aequipecten shows no obvious organized intermediate stages. During ciliation, randomly-oriented, nearly complete procentrioles are found concomitantly with actively-functioning basal bodies. Cilia formation in more advanced, already-ciliated cells is again preceded by the presence of granular complexes. Ciliogenesis in this mollusc thus shares with certain lower forms the property of very rapid basal body formation but, like many higher forms, it is preceded by the formation of a granular precursor complex, presumably consisting of particulate microtubule protein.