Fig. 5. Acidic phosphatase labeling of microglial cells in the developing E4 (B,C) and E6 (D,E) chick retina. The product of this enzymatic activity can be detected by a histochemical method that yields a red product. Intensely stained acidic phosphatase-positive cells are seen on the vitreal surface of anti-TGF-ß-treated (arrowheads in B,D) and untreated retinae (arrowheads in C,E). No enzyme activity was found in control sections incubated in medium without substrate (naphtol AS BI phosphate; A,C). Scale bar: 20 µm.