Educational researchers widely acknowledge the promises and impediments of teachers' collaborative sensemaking, illuminating the need to recognize additional resources that are salient for teachers within professional interactions. In line with this overarching goal, this study explores the role of mathematics teachers' previous professional experiences in their collaborative sensemaking. Theoretically, it is rooted in ecological theories of learning, highlighting that learning is always shaped by an interconnected set of environments. Empirically, it builds on data from a research-practice partnership with a professional development (PD) organization, where we used classroom video to support secondary mathematics teachers' teams in improving their practice. The analysis first portrays two video-based conversations to illustrate the potential of inviting and building on teachers' previous experiences and resources to support their collaborative reasoning. Then, it looks across nine video-based conversations of the same two teams and systematically describes experiences and resources that teachers spontaneously reference through six categories: PD workshops, conferences, PD organizations, online resources, research and policy, and curricula. These categories provide a framework for designers and facilitators who want to take seriously the practice of acknowledging that teacher learning happens through a complex web of learning experiences. The study brings forth the affordances of taking a learning ecology perspective on teacher collaborative sensemaking as professional development (CSPD) and provides guidance for designing, facilitating, and analyzing CSPD conversations in ways that center on teachers' prior knowledge and experiences.