The postnodal piece (PNP) of the chick embryo as a model system for studying organ differentiation

The postnodal piece of the chick embryo is prepared by cutting the blastoderm 0.6 mm behind Hensen's node at the primitive streak stage. The expiants with their area opaca can be maintained in flat culture (New's method) for 24–30 hr. With this method the polarity and the connections between various sheets are maintained, but the expiants are stretched and difficult to handle for histology and immunostaining. Several PNPs from which the area opaca had been trimmed were cultured on one vitelline membrane (Niu and Deshpande's method) for up to 4 days without stretching effects. The polarity and connections between the embryonic sheets are hard to recognize, but expiants can be easily processed for histology and immunofluorescence. In both culture types the expiants can be easily treated, even with high molecular weight substances. Although the flat culture was useful for the induction of somites and of neural plates, we describe the advantages of culture without the area opaca of neural plates induced by tubulin mRNA or by TPA, which can differentiate into neural tubes. We also demonstrated that TPA is a powerful neural inducer in the chick embryo and stimulates cell proliferation in ectoderm and endoderm.