PROTRUDING INTRANUCLEAR CRYSTALLOIDS IN THE CHOANOCYTES OF SPONGES

The ultrastructure of intranuclear crystalloids protruding outward was investigated in the choanocytes of the sponge, Haliclona sp. In the y oung sponges, almost all (> 90%) choanocytes have one crystalloid in t he nucleus, but nor any other cells. It is located only in the nucleus , and not in the cytoplasm. The crystalloids are straight rods usually smaller than about 2.5 mu m in length and 0.15 mu m in thickness, and have a hexagonal appearance in the cross section. The intranuclear cr ystalloids protrude toward the outside of a cell, but do not break thr ough the nuclear envelope and cell membrane thus deforming the nucleus and cell body. The deformation results necessarily from the rapid gro wth of the long crystalloid because it is formed de novo in the nucleu s after choanocyte development.