Distribution of Membrane-Associated Calcium in Fertilized Sea Urchin Eggs during Mitosis*

The distribution of membrane-associated calcium in dividing sea urchin eggs was examined with chlortetracycline as a fluorescent chelate probe. The fluorescence of bound chlortetracycline in fertilized eggs was initially evenly distributed, but began to gather around the nucleus in prophase, and formed a dumb-bell shaped condensation enclosing the mitotic apparatus by metaphase. During anaphase and telophase, the fluorescence was observed in kinetochore-to-pole regions of the spindle, with little fluorescence in the interkinetochore region. The astral regions showed intense fluorescence. The distribution of the chlortetracycline-fluorescence coincided with that of ER-like membranes seen in electron micrographs. The distribution of the fluorescence was obscure and the birefringence of spindles disappeared on perfusion on perfusion of the cells in metaphase with 1 mM tetracaine, which is known to displace membrane-bound calcium. These results suggest that intracellular free calcium ions are sequestered in the membrane system associated with the mitotic apparatus during mitosis.