Of Beans, Weeds, and Human Cytogenetics

Abstract

Cell culture proved to be a superior system for kary-ological investigations, but it required considerable experience and a sizable expenditure. It was not practical or even feasible for many laboratories to set up cell culture facilities just for studying chromosomes. Furthermore, it is extremely difficult to procure biopsy specimens from normal persons, except in surgery cases. Therefore, the lymphocyte culture method discovered by Peter Nowell (1960), as a byproduct of another research project, was one of the most timely and welcomed contributions to human cytogenetics. Only a small quantity of peripheral blood sufficed to yield a good number of mitoses for chromosome analysis. Peter, a pathologist teaching at the University of Pennsylvania, was studying leukocytes in culture. After several days of incubation of the cultures he obtained an unexpected result: a large number of mitotic cells. Leukocytes in the peripheral blood were regarded by most biologists as terminal points of cellular differentiation. Thus, they were not supposed to possess the ability to undergo mitosis. In Nowell’s cultures, however, many cells looked like lymphoblasts instead of mature lymphocytes. He thought that some factor in the tissue culture system caused this anomalous behavior. Therefore, he systematically tested the variables that he could think of, but initially failed to find the responsible factor. It turned out that tissue culture per se was not responsible for stimulating the lymphocytes to return to a more immature form. Nowell was using the popular method for separating erythrocytes and leukocytes prior to cell culture (a method used by Edwin Osgood for many years), agglutinating erythrocytes with phytohemagglutinin (PHA), a crude powder extracted from the common navy bean, Phaseolus vulgaris. Phytohemagglutinin not only agglutinates red cells to facilitate separation but also, it turned out, stimulates mature lymphocytes to return to a blastic state capable of cell division once again. When Nowell used other methods to separate red cells from leukocytes, no mitosis could be observed in his cultures.