Granular Extensions of the Nucleoli in Giant Neurons of Aplysia Californica

The nervous systems of many gastropods contain unusually large neurons . The cell bodies of these giant neurons can be more than 1 mm in diameter, as in the case of the colossal cells of Aplysia (Coggeshall, 1967) . The nucleus represents approximately 30 per cent of the volume of the giant cell body, and in Aplysia californica the amount of nuclear DNA is many thousands of times greater than the haploid amount . It has been suggested that the entire genome in these cells undergoes repeated endoreduplication, resulting in a high level of polyploidy (Coggeshall et al ., 1970 ; Lasek and Dower, 1971) . The possibility that the entire genome is replicated synchronously many times is supported by the finding that the number of nucleoli in the giant cells of Aplysia californica increase in a fixed ratio to the DNA content of the nucleus (Garner and Lasek, unpublished observation). As many as 16,000 nucleoli have been estimated in cells which contain 0.067 μg of nuclear DNA . An unusual structural element is found to extend from the nucleoli of the giant cells . The prominence of this nucleolus-associated structure and its general distribution in giant neurons of Aplysia californica led us to make the following descriptive report .