Identification of Inducing, Responding, and Suppressing Regions in an Experimental Model of Notochord Formation in Avian Embryos

Abstract

The notochord normally arises from committed cells in the rostral tip of the primitive streak. After removal of these cells from the avian gastrula, embryos with notochords nevertheless develop in the majority of cases. A region required for the formation of this reconstituted notochord lies lateral to the primitive streak. In the present study we have determined that this region acts as an inducer for more lateral cells in the epiblast, which actually give rise to the reconstituted notochord. The strongest inducing region lies between 0-250 micrometer lateral to the streak and 500-750 micrometer caudal to the rostral end of the streak and chiefly contains cells normally fated to form lateral plate and somitic mesoderm. The responding region is located 250-500 micrometer lateral to the streak and 0-750 micrometer caudal to the rostral end of the streak. This area chiefly contains cells normally fated to form neural ectoderm, although cells normally fated to form lateral plate and somitic mesoderm are also within this area. The inducing and responding areas interact to form reconstituted notochord either when the primitive streak, including its rostral end (Hensen's node), is removed from the cultured blastoderm or when the inducer and responder are grafted together into an ectopic site. Grafting Hensen's node into isolates containing both inducer and responder blocks formation of reconstituted notochord, suggesting that Hensen's node suppresses formation of lateral notochords during normal development. These findings increase our understanding of the early interactions between mesoderm and ectoderm and provide a novel model system that is well defined and accessible for studying inductive events in higher vertebrates.

Anterior identity is established in chick epiblast by hypoblast and anterior definitive endoderm

“…Neural specification in RBIs depends on the presence of a population of central epiblast (CE) cells with 'organising' ability, acting either as an organiser or inducer of an organiser (Darnell et al, 1999). After removal of Hensen's node in whole embryos and tissue isolates, the organiser reconstitutes (Joubin and Stern, 2001;Psychoyos and Stern, 1996;Yuan et al, 1995a;Yuan et al, 1995b;Yuan and Schoenwolf, 1998;Yuan and Schoenwolf, 1999). We have not determined whether the same mechanism operates in the rostral blastoderm isolates, although markers of notochord (Not1), node (Shh) and primitive streak (Brachyury/T) were detected in RBIs, suggesting that the organiser is reconstituted (Darnell et al, 1999).…”

“…We used the transection assay ( Fig. 2) (Darnell et al, 1999;Healy et al, 2001;Schoenwolf et al, 1989b;Yuan et al, 1995a) to address what the role for these lower layer tissues is, and determined that anterior positional identity seems to be separable from neural specification. Isolating rostral epiblast (prospective anterior neural plate) from the influences of Hensen's node and ingressing axial mesoderm was crucial, because ingressing axial mesoderm from stage 4+ has a role in the induction of Ganf, the earliest specific maker of anterior neural plate (Knoetgen et al, 1999).…”

The compulsion of chirality: toward an understanding of left-right asymmetry

“…When the chick node is ablated prior to stage 4, it is regenerated such that the embryo develops with normal AP and LR asymmetry (Yuan et al 1995;Psychoyos and Stern 1996). In addition to defining a window of developmental plasticity, this experiment also suggests that the node might be sensitive to signals from surrounding tissue.…”

Cellular dynamics and molecular control of the development of organizer-derived cells in quail-chick chimeras

“…When these experiments are performed at 5-6-ss, Hensen's node does not yield midline structures. The neural tube and the somites proliferation and cell migration and gives rise to normal embryonic development (Waddington, 1932;Abercrombie and Bellairs, 1954;Yuan et al, 1995;Psychoyos and Stern, 1996). A recent reinvestigation of the node regeneration process in the chick embryo suggested that early Hensen's node is a site where precursor cells are recruited and acquire specific molecular characteristics conferring them an organizer status (Joubin and Stern, 1999).…”