Developing primary teachers' pedagogical content knowledge on fractions through transformative lesson study incorporating open approach model: a collaborative case study

Purpose This case study examines how primary teachers develop Pedagogical Content Knowledge (PCK) on fractions within the Transformative Lesson Study incorporating Open Approach (TLSOA) model. It identifies key PCK characteristics and how TLSOA model fosters them. Design/methodology/approach A Lesson Study team of six members (two student teachers, two in-service teachers, one researcher, and one expert) engaged in weekly cycles of collaborative planning, observation, and reflection across 10 fraction lessons in a Thai primary school. Data were collected through lesson planning sessions, classroom observations, reflections, and semi-structured interviews, and analyzed using thematic analysis. Findings Teachers' PCK was characterized and developed through designing “problem situations” centered on students' mathematical thinking as a PCK integration unit. The TLSOA model facilitated teachers' PCK integration, anchored collaboration in students’ ideas for design, observation and reflection. Teachers improved their anticipation of students’ thinking and enhanced classroom practice on part–whole relationships and fraction operations. Research limitations/implications The study examined only one year of fraction instruction, with teachers having limited experience. Participation across multiple years and varied professional learning communities (PLCs) with experienced reflection teams could enhance teachers' PCK awareness and development through broader collaborative experiences. Practical implications The use of the TLSOA model in developing teachers' PCK on fractions through student-centered collaborative lesson design strengthened the effectiveness of teacher professional development approaches in mathematics education. Originality/value This study demonstrates how collaborative PLCs foster dynamic integration of content and pedagogical knowledge in mathematics education, contributing to PCK literature by showing the TLSOA model's specific mechanisms for developing fraction instruction through student-centered collaborative practice.