Fig. 3. Loss of either Slit2 or Robo1 leads to similarly abnormal EBs. (A-G) EBs immunostained with an antibody directed against SMA to identify cap cell layers. (A,E) Wild-type (+/+) EB morphology shows tight juxtaposition of cap and LEC layers. (B-D) Longitudinal sections through Slit2^-/- EBs. (B) Exaggerated space between the cap and LEC layers (double arrowhead), and dissociated cells detected in subcapsular space. (C) Loss of LECs underlying the cap cell layer (between arrows). (D) Complete disruption of EB morphology characterized by the infolding of cap cells into the LEC compartment (arrowheads) leading to lumen loss. (F,G) Longitudinal sections through Robo1^-/- EBs. (F) Exaggerated space between the cap and LEC layers (double arrowheads), dissociated cells in the subscapular space and infolding of the cap cell layer into the LEC compartment (arrowheads). (G) Complete disruption of EB morphology characterized by the infolding of cap cells into the LEC compartment (arrowheads) leading to lumen loss. (A-D) Slit2^-/- outgrowths were generated by transplantation with contralateral ^+/+ control outgrowths. (E-G) Robo1^-/- and ^+/+ mammary glands were from littermates. L, lumen. Scale bar: 20 µm.