Fig. 6. Spinal interneurons undergo a normal developmental pattern of apoptosis that proceeds in a ventral-to-dorsal temporal gradient. (A-C) Multiple molecularly defined interneuron populations were quantified in wild-type (WT) mouse spinal cords at E14, E17, P0, P2 and P5 (at least six sections per marker per animal, three sets of animals). Means are graphed in A and B as a percentage of population size at E14. Ventral interneuron populations are lost, to differing extents, primarily between E14 and P0 (B), whereas dorsal interneuron populations are lost primarily after P0 (A). The increased apoptosis observed in Pcdh- null mutant mice is likely to reflect an exacerbation of this normal pattern, as there is a near-perfect correlation between the extent of cell loss in each population during WT development (y-axis in C) and the extent to which apoptosis is increased in that population in mutants as compared with controls (x-axis in C).