ROBO4 immunostaining and localisation across gestation. A, Using light-field microscopy, weak ROBO4 expression (positive brown DAB staining) was detected around the blood vessels of Stage 1 ovaries. B, At Stage 2 of gestation increased ROBO4 expression around the vasculature was observed. C, Positive brown DAB staining was also noted around oocytes of developing primordial follicles at Stage 2 of gestation. D, The oocytes of more mature primordial follicles and blood vessels contained intense ROBO4 expression (dark brown DAB staining) at Stage 3 of development. White arrows are pointing towards blood vessels. Oocytes are emphasised by black arrows. E, No DAB staining was observed in a negative control serial section. All the scale bars represent 100 μm.