Cell culture research. Researcher using a light microscope to analyse cultured cells growing in a petri dish. These cells are growing in basal medium, an agar-based substance that contains nutrients, growth factors and hormones. The cells are to be used in cytogenetic research, the study of how chromosomes function and what their abnormalities are. Chromosomes are composed of the genetic material, DNA (deoxyribonucleic acid) and proteins. Photographed at Leiden University Medical Centre, Leiden, The Netherlands.