Rhombomere-specific patterns of apoptosis in the tree shrew Tupaia belangeri.

Whether rhombomere-specific patterns of apoptosis exist in the developing hindbrain of vertebrates is under debate. We have investigated the sequence of apoptotic events in three-dimensionally reconstructed hindbrains of Tupaia belangeri (8- to 19-somite embryos). Apoptotic cells were identified by structural criteria and by applying an in situ tailing technique to visualize DNA fragmentation. Seven rhombomeres originated from three pro-rhombomeres. Among pre-migratory neural crest cells in the dorsal thirds of the neural folds, the earliest apoptotic concentrations appeared in the developing third rhombomere (r3). Dorsal apoptotic maxima then persisted in r3, extended from r3 to r2, and also arose in r5. Transverse apoptotic bands increased the total amount of apoptotic cells in odd-numbered rhombomeres first in r3 and, with a delay, also in r5. This sequence of apoptotic events was paralleled by an approximate rostrocaudal sequence of neural crest cell delamination from the even-numbered rhombomeres. Thus, large-scale apoptosis in r3 and r5 helped to establish crest-free zones that segregated streams of migrating neural crest cells adjacent to r2, r4, and r6. The sequence of apoptotic events observed in the dorsal thirds of rhombomeres matches that reported for the chick embryo. Other shared features are apoptotic peaks in the position of a circumscribed ventricular protrusion of fusing parts of the neural folds in r1 and r2, and Y-shaped apoptotic patterns composed of apoptotic maxima in the dorsal and lateral thirds of r1, r2, and r3. These rhombomere-specific patterns of apoptosis may therefore represent a conserved character, at least in amniotes.