Interaction with an epithelium is a prerequisite for avian cranial neural crest (NC) cells to differentiate into cartilage and bone (Bee & Thorogood, 1980). In order to investigate the causal mechanism we have selected one such interaction - that between mesencephalic NC and retinal pigmented epithelium (RPE) for further study. Premigratory NC cells were grown transfilter to RPE explants of different developmental ages and on Nuclepore filters of different pore size which either allowed or prevented penetration by cell processes. Initial scanning electron microscopy (SEM) observations established that pores of 0·8 μm allowed the passage of cell processes through the filter whereas 0·2 μm pores did not.
The transfilter experiments demonstrated that chondrogenic differentiation of NC cells will occur only if the Nuclepore filters have a pore size large enough to permit the passage of cell processes. Furthermore SEM observations established that cell processes do traverse the Nuclepore filter when NC and RPE are grown in transfilter combination. The results indicate that the mechanism is not mediated by diffusable factors but rather is mediated either by direct contact between NC cells and non-diffusable matrix closely associated with RPE or by direct plasmalemmal contact between RPE and NC cells through discontinuities in the basement membrane. The results of these experiments also demonstrate that younger (stage 17) RPE is more effective at eliciting chondrogenesis from premigratory NC cells than older (stage 24) RPE and that the interaction between RPE and NC cells is a prolonged one, taking place over days rather than within hours. Both of these in vitro observations are compatible with the timing of events leading to scleral cartilage formation in vivo.