G. Boyan et al., AXOGENESIS IN THE EMBRYONIC BRAIN OF THE GRASSHOPPER SCHISTOCERCA-GREGARIA - AN IDENTIFIED CELL ANALYSIS OF EARLY BRAIN-DEVELOPMENT, Development, 121(1), 1995, pp. 75-86
Axogenesis in the embryonic brain was studied at the single cell level
in the grasshopper Schistocerca gregaria. A small set of individually
identifiable pioneer neurons establishes a primary axon scaffold duri
ng early embryogenesis. At the beginning of scaffold formation, pionee
ring axons navigate along and between glial borders that surround clus
ters of proliferating neuroblasts, In each brain hemisphere, an axonal
outgrowth cascade involving a series of pioneer neurons establishes a
path way from the optic ganglia to the brain midline, At the midline
the primary preoral commissural interconnection in the embryonic brain
is pioneered by a pair of midline-derived pioneer neurons. A second p
reoral commissural connection is pioneered by two pairs of pars interc
erebralis pioneer neurons. Descending tracts are pioneered by the prog
eny of identified neuroblasts in the pars intercerebralis, deutocerebr
um and tritecerebrum; the postoral tritocerebral commissure is pioneer
ed by a pair of tritocerebral neurons. All of the pioneering brain neu
rons express the cell adhesion molecule fasciclin I during initial axo
n outgrowth and fasciculation. Once established, the primary axon scaf
fold of the brain is used for fasciculation by subsequently differenti
ating neurons and, by the 40% stage of embryogenesis, axonal projectio
ns that characterize the mature brain become evident. The single cell
analysis of grasshopper brain development presented here sets the stag
e for manipulative cell biological experiments and provides the basis
for comparative molecular genetic studies of embryonic brain developme
nt in Drosophila.