The discovery of the human chromosome number
The discovery, in 1956, by Joe-Hin Tjio and Albert Levan in Lund, Sweden that the correct human chromosome number is 46 was facilitated by technical advances, in vitro cell culture, colchicine and hypotonic pretreatment followed by squashing. Yet again, all these techniques had been used before; perhaps advantageous was the rapid divisions of foetal lung fibroblast, grown in bovine amniotic fluid, provided by Rune Grubb, these cells grown for a completely different purpose, namely to harbour viruses.
Previous attempts to count human chromosomes had mainly relied on spontaneous cell divisions, observed in testicular histological sections or in preparations of cancer cell populations. Publications on attempts to count human chromosomes were first published as early as 1892, by Hansemann and Bardeleben, only a year after the discovery of chromosomes per se. Both these and some early work to follow arrived at very low counts, in the order of 23-24. On the other hand, in 1912, Hans de Winiwarter, analysing Spermatogonial Metaphases, counted 47 chromosomes in 29, 46 chromosomes in two and 49 chromosomes in one. He proposed the correct male number to be 47, while at the same time dismissing the clear observation of 23 chromosomes in Metaphase II plates (!). Theophilus Shickel Painter, also using testicular biopsy material, concluded in his first paper, in 1921, that in his best Spermatogonial Metaphases the chromosome number was clearly 46, but in a paper to follow two years later, remarkably, he converted to 48. One problem here is that Spermatogonial Metaphases are not the ideal type of material for counting purposes. The heterochromatic blocks of chromosomes 1, 9, 16 and 20 may vary substantially between cells, often being extended and thin, leading to the false impression of the p and q arms as separate chromosomes.
The dogma of 48 human chromosomes was upheld during the following decades up until 1956, apparently leading many researchers in this interim period to jump to the wrong conclusion, even when it is in retrospect obvious they were on the right track. In this presentation I will provide some illustrative examples of what I think is a fascinating part of early human cytogenetics.