Although teachers' instruction has been extensively studied, there is a dearth of research investigating how teachers design lessons and integrate new information into their existing lessons. This study utilized a case study approach to examine how teachers engaged in a nanoscale science professional development experience integrated new science content into their lesson plans. Five experienced science teachers were tracked as they conducted science research and developed new lessons that integrated their research into their science instruction. The findings showed that these teachers had to cross borders between science and education to design lessons, and three priorities framed their lesson planning processes: understandability, engaging students, and relevant real-world applications. For teachers whose students were assessed with end-of-grade high-takes testing, fitting the new information into the tested material was essential. The study provides insight into this border-crossing process and has implications for the design of programs that implement research experiences for teachers.