\begin{tikzpicture}[node distance=1cm,>=stealth',scale=1,transform shape, on grid, auto]%>=latex 
  \tikzstyle{every label}=[black]
  \begin{scope}

    \node (MMS) {$MM_s$};
    \node (ms) [below of=MMS, yshift=-0.4cm] {$m_s$};
    %\path[->] (ms) edge [dash pattern=on 2pt off 2pt] node {$conforme à$} (MMS);
    \path[->] (ms) edge [dash pattern=on 2pt off 2pt] (MMS);
    \node (ts) [above of=ms, xshift=-0.1cm, yshift=-0.2cm]
    {\scriptsize{conforme à}};

    \node (MMT) [right of=MMS, xshift=0.8cm] {$MM_t$};
    \node (mt) [below of=MMT, yshift=-0.4cm] {$m_t$};
    %\path[->] (mt) edge [dash pattern=on 2pt off 2pt] node {$conforme à$} (MMT);
    \path[->] (mt) edge [dash pattern=on 2pt off 2pt] (MMT);
    \node (tt) [above of=mt, xshift=-0.1cm, yshift=-0.2cm] {\scriptsize{conforme à}};

    \path[->,thick] (ms) edge node [below,yshift=-0.2cm] {\scriptsize{Transformation exogène}} (mt);


  \end{scope}
\end{tikzpicture}