Immature thymocytes inhabit the outer layer of the thymus, called the cortex, while more mature thymocytes are found in the central medulla. How a thymocyte reacts to other cells as it wends its way through the thymus determines whether it matures into a helper or killer T cell, or undergoes programmed cell death. The signaling cues that guide this process remain obscure.
In their study, Robey and her colleagues took advantage of a recent technological innovation called two-photon microscopy to visualize the migration of thymocytes in intact thymuses extracted from mice. They found that after cells undergo positive selection - which seals their fate as either helper T or killer T cells - they make a beeline for the thymus interior (called the medulla).
Though it's been known that positively selected thymocytes migrate to the medulla, this study shows that migration follows a clear directional course, possibly guided by long-range signaling cues. Homing in on the source of these long-range signaling cues and characterizing the migratory patterns of these cells will go a long way toward understanding how the major components of immunity acquire their defensive chops.
Citation: Witt CM, Raychaudhuri S, Schaefer, Chakraborty AK, Robey EA (2005) Directed migration of positively selected thymocytes visualized in real time. PLoS Biol 3(6): e160.
PLOS Biology