In this article we analyze the relations between academic mathematical knowledge and the mathematical knowledge associated with issues mathematics school teachers face in practice, according to the specialized literature, and restricted to the theme "number systems". We present examples that illustrate some areas of conflict between those forms of knowledge. We point out some implications of our study for teacher education, such as: 1) the importance of making conflicts explicit and of discussing them with prospective teachers in order to develop a professionally relevant perception of academic mathematics; 2) the relevance of further research in order to better understand the extent of those conflicts and their effects on the process of integrating, in a body of professional knowledge, the different kinds of mathematical knowledge presented to prospective teachers.