The translocation of phosphatidylserine from the cytosol to the external surface 
of the plasma membrane has been documented as a characteristic feature of 
apoptosis in a number of cell types. Annexin V is a calcium-dependent 
phospholipid binding protein that has high affinity for phosphatidylserine. To 
investigate whether Annexin V provides a marker of apoptosis in the central 
nervous system we carried out histochemical analysis of its binding in the 
21-day-old rat brain at various time-points following a moderate unilateral 
hypoxic-ischemic (HI) insult. The CA1 pyramidal neurons, which are selectively 
vulnerable to HI injury and that die by an apoptotic mechanism showed an 
increase in Annexin V binding 48-168 hours post-insult.