Cell structure and microtubule organisation during gametogenesis of the green alga Ulva mutabilis was studied using light microscopy, transmission electron microscopy (TEM) and tubulin immunofluorescence. Micro-tubules in vegetative cells are organised in parallel bundles traversing the cortical cytoplasm. During gameto-genesis, induced blade cells are transformed to gametangia, depending on the maturity of the algae and the removal of regulatory sporulation inhibitors. This differentiation is accompanied by formation of a conical cell projection (papilla) towards the exterior of the thallus. Microtubules form a clear, basket-like configuration converging towards the conical tip, but not reaching it. The conical microtubule structure stops below the tip, leaving a circular "opening". Parallel to the above, the cell wall of the tip is differentiated, forming a "cap". Nuclear divisions start at this stage, finally forming the nuclei of future gametes. Cytokinesis takes place by membrane furrowing and vesicle fusion, giving rise to 16 oval-shaped gametes. The conical microtubule organisation is gradually depolymerised, and a cortical, intensely fluorescing micro-tubule bundle is formed in each gamete. At this stage, the cap at the conical cell wall projection is removed and the exit pore opens. The biflagellate gametes remain initially motionless, connected by thin cytoplasmic bridges. Finally, they are released to the environment upon additional removal of a swarming inhibitor accumulated in the growth medium during gametogenesis.
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