The importance of teachers developing adaptive instructional practices consistent with metaphor of whole-class scaffolding has been well documented. However, teachers' development of such practices is currently not well understood. We draw on a 5-year professional development (PD) design experiment in which a group of middle school mathematics teachers developed aspects of such practices. We address questions of "how can teachers' development of instructional practices consistent with whole-class scaffolding be supported", and "what challenges should be anticipated when designing to support such development?" Findings indicate that leveraging teachers' existing practices and concerns was important in supporting them to focus on students' mathematical reasoning and develop adaptive practices. We discuss how the notion of whole-class scaffolding can orient the design of PD activities that remain grounded in classroom practice, and also consider how this notion can be further refined through analyses of teachers' learning in PD settings.