% TikZ chains with labeled edges
% Author: Stefan Kottwitz , http://texblog.net
documentclass[a4paper,10pt]{article}
usepackage{tikz}
usetikzlibrary{arrows,chains,matrix,positioning,scopes,calc,fit,shapes.geometric}
%
makeatletter
tikzset{join/.code=tikzset{after node path={%
ifxtikzchainpreviouspgfutil@emptyelse(tikzchainprevious)%
edge[every join]#1(tikzchaincurrent)fi}}}
makeatother
%
tikzset{>=stealth',every on chain/.append style={join},
         every join/.style={->}}
tikzset{labeled/.style={execute at begin node=$scriptstyle,
   execute at end node=$}}
%
begin{document}
begin{tikzpicture}
  matrix (m) [matrix of math nodes, row sep=3em, column sep=3em]
    { 0 & A  & B  & C  & 0 \
      0 & A' & B' & C' & 0 \ };
  { [start chain] chainin (m-1-1);
    chainin (m-1-2);
    { [start branch=A] chainin (m-2-2)
        [join={node[right,labeled] {eta_1}}];}
    chainin (m-1-3) [join={node[above,labeled] {varphi}}];
    { [start branch=B] chainin (m-2-3)
        [join={node[right,labeled] {eta_2}}];}
    chainin (m-1-4) [join={node[above,labeled] {psi}}];
    { [start branch=C] chainin (m-2-4)
        [join={node[right,labeled] {eta_3}}];}
    chainin (m-1-5); }
  { [start chain] chainin (m-2-1);
    chainin (m-2-2);
    chainin (m-2-3) [join={node[above,labeled] {varphi'}}];
    chainin (m-2-4) [join={node[above,labeled] {psi'}}];
    chainin (m-2-5); }
path let p1=(m-1-1), p2=(m-2-2), n1={atan2(y2-y1,x2-x1)} in
node[draw,dashed,ellipse,inner sep=0pt,rotate fit=n1,fit=(m-1-1) (m-2-2)]{}; %added
end{tikzpicture}
end{document}