Embryonic cell recognition: uncoupling tissue-specific affinities from cell-type specificities.

We have experimentally defined the two major aspects of embryonic cell recognition-adhesion (ReAd), tissue type-specific ReAd and cell type-specific ReAd; we showed that they arise consecutively during cell differentiation, and that the former can function in the absence of the latter. Embryonic chick cells (retina and chondroblasts) in which differentiation was arrested by BrdU at an early stage, failed to express cell-type ReAd, yet they continued to display tissue-type ReAd: they distinguished tissue-self from non-self and selectively cohered with self. Our results indicate that tissue-type and cell-type ReAd represent distinct, separately controlled mechanisms. BrdU appears to be useful as a probe for investigating the regulation of these mechanisms, and as an experimental effector of differentiation abnormalities associated with defects in cell recognition.