Segment‐related, mosaic neurogenetic pattern in the forebrain and mesencephalon of early chick embryos: I. Topography of ache‐positive neuroblasts up to stage HH18

Abstract

Histochemical mapping of AChE-positive neuroblasts in sectioned and whole-mounted preparations of the chick embryo mesencephalon and prosencephalon allows a correlation of early neural tube morphogenesis (segmentation, longitudinal compartmentation) with the heterochronic pattern of neurogenesis. One significant finding is that the initial appearance of neuroblasts in the forebrain does not follow neuromeric segmentation, but evolves in parallel with it. Early neuroblasts appear as separate, distinct groups within specific matrix territories at the center of the transverse neuromeric segments. Neighbouring segments display different spatiotemporal patterns of neurogenesis. Overall gradients of differentiation in the rostrocaudal and ventrodorsal directions are absent, whereas a clear-cut segment-related, mosaic pattern becomes evident. Notwithstanding this, gross regularities of heterochrony in the neurogenetic behavior of the different segments lead to a definition of elemental longitudinal compartments of the forebrain and mesencephalon (floor, paramedian, basal, and alar regions) on the basis of precocious differentiation of the basal region and retarded differentiation of the paramedian and alar regions.

Protooncogene expression identifies a transient columnar organization of the forebrain within the late embryonic ventricular zone.

“…However, there are two previous reports that butyrylcholinesterase (24) and acetylcholinesterase (25) are distributed heterogeneously in the embryonic ventricular zone. The present description of a columnar organization of the ventricular zone revealed by protooncogene expression (as well as by vimentin expression and cellular proliferation) suggests a heterogeneity in the differentiation of subpopulations of ventricular zone cells at E18.…”

Expression analysis of Sulf1 in the chick forebrain at early and late stages of development

“…The diencephalic domain is subdivided into three segments or neuromeres, known as prosomeres 1, 2, and 3 (Puelles et al, 1987Puelles and Rubenstein, 2003). In prosomere 1 and 2 regions, we observed intense Sulf1 expression in alar and basal territories.…”

“…The hypothalamus forms the nontelencephalic region of the secondary prosencephalon (Puelles et al, 1987(Puelles et al, , 2004Puelles, 1995Puelles, , 2001aPuelles and Rubenstein, 2003). Most of the caudal hypothalamic ventricular epithelium strongly expressed Sulf1, with the exception of the floor plate that appeared negative except for the mammilar region (MM).…”

“…The diencephalic region develops three segments or neuromeres, known as prosomeres 1-3 (p1 lying caudally, adjacent to the midbrain, and p3 rostrally, contacting the hypothalamus and the telencephalon; Puelles et al, 1987;Puelles and Rubenstein, 2003). Each prosomere displays characteristic dorsal and ventral domains, namely, the dorsalized roof and alar plates and the ventralized basal and floor plates.…”

“…Rubenstein, and coworkers (Bulfone et al, 1993;Puelles and Rubentein, 1993;Puelles, 1995;Rubenstein et al, 1998;Puelles and Rubenstein, 2003). In this model, the anteroposterior (AP) regionalization first causes the forebrain to become subdivided into rostral secondary prosencephalon (hypothalamo-telencephalic complex) and caudal diencephalon (Puelles et al, 1987(Puelles et al, , 2004Rubenstein, 1993, 2003;Puelles, 1995). The diencephalic region next develops three prosomeric transverse units, known as prosomeres 1-3 (p1-p3; Puelles and Rubenstein, 2003).…”