Fig. 5. (A,B) Apoptosis in Ercc1-deficient testes and (B,C) pattern of expression of the Ercc1 protein. (A) 7-week wild type and (B) 7-week transgene-positive Ercc1 deficient testes. Note that apoptosis, visualised by the Apotag assay, was detected in only a few cells in each sample. Apoptotic cells in the wild type (arrows) were stage dependent (stages I and XII), whereas in Ercc1-deficient testis some apoptotic cells were observed at all ages and often occurred in clusters (arrows in B). (C) 9-week wild-type and (D) 9-week transgene-positive Ercc1-deficient testes. Non-specific staining of sperm tails was seen in both samples. Weak immunopositive staining was detected in cell nuclei of Leydig and Sertoli cells from wild-type testis. Examination of specific staining of wild-type germ cells (C) revealed that some immunopositive reaction was present in pre-meiotic germ cells and that the most intense immunopositive reaction was localised to late pachytene spermatocytes (stages IX-XI) and round spermatids (stages I-VII, arrowheads). Very faint specific nuclear staining was detected in a few round spermatids from transgene-positive Ercc1-deficient testis (arrowheads in D) (^*, SCO). (E) A summary diagram based on the stages of the spermatogenic cycle (adapted from Oakberg, 1956) showing germ-cell-specific staining for Ercc1. As germ cells proceed through meiosis from pre-meiotic spermatogonia (bottom left) to mature spermatozoa (top right) they are arranged vertically in the seminiferous tubules in characteristic associations (numbered stages) as indicated. The intensity of the immunopositive staining observed is indicated by the intensity of shading: the heaviest shading denotes the intense immunopositive reaction from late pachytene spermatocytes (stages IX-XI) to round spermatids (stages I-VII); the lighter shading denotes the faint staining in pre-meiotic germ cells through to mid pachytene spermatocytes.