Patterning of chick brain vesicles as revealed by peanut agglutinin and cholinesterases.

Differentiation of individual rhombomeres of the chicken hindbrain directly follows the emergence of primary brain vesicles. Immediately after the constriction of the prosencephalon at HH9, a series of vesicles of decreasing size is established almost simultaneously between HH9 and HH10, including mesencephalon, four preotic (R2-R5) and one postotic (R6/R7) rhombomeres. Thereby, the cranial neural tube is ventrally embedded in a mesodermal PNA-binding matrix that particularly accumulates underneath vesicular constriction sites, as demonstrated for the segregation of the prosencephalon at HH9 and the cerebellar rhombomere R1 from R2 at HH13. The subsequent period of hindbrain differentiation is analyzed by cholinesterase (AChE, BChE) and peanut lectin histochemistry, by the BrdU and the neurite-specific G4 antibodies. Preotically, differentiation of two pairs of rhombomeres (R4 + R5, R2 + R3) starts in R4, immediately followed by R2. The caudal rhombomeres of both pairs are delayed (R5, R3). Then the postotic rhombomere is subdivided, whereby R7 differentiates before R6. Thus, the development in the direct vicinity of the otic vesicle is delayed (R5, R6). R7 is the last rhombomere that is demarcated caudally. Based on these findings, we postulate two processes that may regulate rhombomere formation in the chicken embryo: (a) an early rostrocaudal wave establishing the major brain vesicles, (b) a superimposed pairwise segmentation emanating rostrally and caudally from the otic vesicle. The segregation of the cerebellar rhombomere is a late step.