Figure 1.

Characteristics of spheroids originating from human cancer cell lines: (A) SEM image of a colorectal cancer cell line (HT29) spheroid depicting the spherical shape as the main characteristic of spheroids. (B) Bright-field microscopy image of a pancreatic cancer cell line (HPAF II) spheroid confirming stability of the 3D cell assembly. (C) Like tumors in vivo, spheroids show physiological zoning, consisting of a rim of viable cells and, depending on their size, of hypoxic or necrotic core zone. Exemplarily, the hypoxic core of a melanoma cell line (A2508) spheroid with a diameter of approximately 0.55 mm is shown by pimonidazole staining. (D) Hematoxylin and eosin staining of a melanoma cell line (A2508) spheroid showing the heterogeneous composition that can vary in cell and extracellular matrix arrangement depending on cell line and treatment. (E) Distribution of viable cells and, as a further characteristic feature, the possible active migration of cells from the 3D cell assembly: here, exemplarily shown by fluorescence staining with DAPI (4′,6-diamidino-2-phenylindole) (blue) and calcein (green) in a 10-day-old co-cultivated spheroid of pancreatic carcinoma cell line (SW1990) and pancreatic stellate cells (HPaSteC) with an initial seeded cell number of 16,000 cells (ratio 1:3). (F) Spheroids are characterized by the formation of gradients, which, among other things, modify the transport of nutrients, metabolites, and drugs similar to the situation in vivo, as shown here in the passive transport of the fluorescent marker calcein in a 10-day-old pancreatic cancer cell line (PanC-1) spheroid with an initial seeded cell number of 16,000 cells.