N-Cadherin, a cell adhesion molecule involved in establishment of embryonic left-right asymmetry.

Within the bilaterally symmetric vertebrate body plan, many organs develop asymmetrically. Here, it is demonstrated that a cell adhesion molecule, N-cadherin, is one of the earliest proteins to be asymmetrically expressed in the chicken embryo and that its activity is required during gastrulation for proper establishment of the left-right axis. Blocking N-cadherin function randomizes heart looping and alters the expression of Snail and Pitx2, later components of the molecular cascade that regulate left-right asymmetry. However, the expression of other components of this cascade (Nodal and Lefty) was unchanged after blocking N-cadherin function, suggesting the existence of parallel pathways in the establishment of left-right morphogenesis. Here, the results suggest that N-cadherin-mediated cell adhesion events are required for establishment of left-right asymmetry.

Effects of Shh and Noggin on neural crest formation demonstrate that BMP is required in the neural tube but not ectoderm.

To define the timing of neural crest formation, we challenged the fate of presumptive neural crest cells by grafting notochords, Sonic Hedgehog- (Shh) or Noggin-secreting cells at different stages of neurulation in chick embryos. Notochords or Shh-secreting cells are able to prevent neural crest formation at open neural plate levels, as assayed by DiI-labeling and expression of the transcription factor, Slug, suggesting that neural crest cells are not committed to their fate at this time. In contrast, the BMP signaling antagonist, Noggin, does not repress neural crest formation at the open neural plate stage, but does so if injected into the lumen of the closing neural tube. The period of Noggin sensitivity corresponds to the time when BMPs are expressed in the dorsal neural tube but are down-regulated in the non-neural ectoderm. To confirm the timing of neural crest formation, Shh or Noggin were added to neural folds at defined times in culture. Shh inhibits neural crest production at early stages (0-5 hours in culture), whereas Noggin exerts an effect on neural crest production only later (5-10 hours in culture). Our results suggest three phases of neurulation that relate to neural crest formation: (1) an initial BMP-independent phase that can be prevented by Shh-mediated signals from the notochord; (2) an intermediate BMP-dependent phase around the time of neural tube closure, when BMP-4 is expressed in the dorsal neural tube; and (3) a later pre-migratory phase which is refractory to exogenous Shh and Noggin.

Interactions between germ cells and extracellular matrix glycoproteins during migration and gonad assembly in the mouse embryo.

Cells are known to bind to individual extracellular matrix glycoproteins in a complex and poorly understood way. Overall strength of adhesion is thought to be mediated by a combinatorial mechanism, involving adhesion of a cell to a variety of binding sites on the target glycoproteins. During migration in embryos, cells must alter their overall adhesiveness to the substrate to allow locomotion. The mechanism by which this is accomplished is not well understood. During early development, the cells destined to form the gametes, the primordial germ cells (PGCs), migrate from the developing hind gut to the site where the gonad will form. We have used whole-mount immunocytochemistry to study the changing distribution of three extracellular matrix glycoproteins, collagen IV, fibronectin, and laminin, during PGC migration and correlated this with quantitative assays of adhesiveness of PGCs to each of these. We show that PGCs change their strength of adhesion to each glycoprotein differentially during these stages. Furthermore, we show that PGCs interact with a discrete tract of laminin at the end of migration. Closer analysis of the adhesion of PGCs to laminin revealed that PGCs adhere particularly strongly to the E3 domain of laminin, and blocking experiments in vitro suggest that they adhere to this domain using a cell surface proteoglycan.