Nucleolar growth and fusion during cellular differentiation

Recent reviews of the nucleolus emphasize the increasing interest in nucleolar function during the last 20 years (Gates, '42; Vincent, '55; Stich, '56; Bogoroch and Siegel, '61). Such interest may be traced to the investigations of Brachet ('42, '57) and of Caspersson ('47, '50; Caspersson et al., '41), implicating the nucleolus in ribonucleic acid metabolism and, perhaps as a consequence of that, in cytoplasmic protein synthesis. Nucleoli are believed to be particularly active in the processes of cellular differentiation and growth, and in those cells whose function involves the production of considerable amounts of protein. Following the rapid turnover of nucleolar material, this deduction is largely based on the reasonable supposition that nucleolar size and thus the quantity of nucleolar material are directly related to the metabolic activity of the nucleolus. The evidence for nucleolar participation in cellular differentiation, for instance, includes the appearance of nucleoli only shortly before this stage in amphibian development, their increased size during differentiation with an attendant increase in cytoplasmic ribonucleic acid, and perhaps the defective differentiation of embryos which lack nucleoli (Beermann, '60; Wallace, '60, '62a). There is still a clear need for quantitative data on nucleolar size during the processes which involve protein synthesis. Most quantitative observations concern material which may be considered atypical or which does not pennit the distinction between growth, differentiation and function, that would be necessary in order to associate nucleolar enlargement with any one such process. Observations on growing oocytes and the salivary glands of DTosophila are open to both of these objections; they are discussed later with other examples whose validity is more certain. Cellular differentiation is frequently accompanied by cellular division and growth, and usually merges into or overlaps with cellular function. Consequently, there are no accurate or accepted criteria of cellular differentiation (cf. Grobstein, '59), and it is particularly difficult to identify nucleolar enlargement or activity with this process. The investigation described here traces two aspects of nucleolar development, their growth and fusion, during the differentiation of several typical somatic tissues. As the degree of cellular growth and the kind of cellular function differ from one tissue to another, any nucleolar changes common to all the tissues studied may be related to their differentiation rather than to other processes involving protein synthesis.