BLM-deficient cells undergo apoptosis during spermatogenesis. (A–D) H&E staining of histological sections from both Blm^flox/neo (A and C) and Blm^cko/neo (B and D) mice at either 5 (A and B) or 16 wk (C and D) of age. (A and B) Spermatids are present in both genotypes of mice at 5 wk, although fewer were in the Blm^cko/neo mice (arrowheads). (B) The seminiferous tubules are also less densely packed with cells in the Blm^cko/neo mice. (C and D) By 16 wk of age, sperm tails are evident in the lumen of both the Blm^flox/neo (C) and Blm^cko/neo (D; arrows) although, fewer were in the Blm^cko/neo sections. (E–G) TUNEL staining of testes sections from Blm^flox/neo (E) and Blm^cko/neo (F) mice reveals a significant increase in apoptotic cells in the Blm^cko/neo testes (G, gray bar) compared with Blm^flox/neo (8.6 and 2.4 apoptotic cells per tubule, respectively; *, P < 0.0001). Error bars show the SEM, and the asterisk indicates significance. (H) PCR analysis of several tissues (Te, testis; Sp, spermatozoa extruded from the epididymis; Ta, Tail; SV, seminal vesicle) from two Blm^cko/neo (CKO mouse ID no. 5439 and 3862) and two Blm^flox/neo (CKO mouse ID no. 4026 and 5443) mice. PCR amplification of the floxed allele produces two higher molecular mass bands (∼500 and 400 bp) present in all tissues from both genotypes, whereas the CKO (or deleted) allele is present almost exclusively in the testes of the Blm^cko/neo mice (small band at ∼350 bp). Some leaky expression of the Cre results in a faint cko band in seminal vesicle, tail, and occasionally in the cells of the epididymal extrusion (which contains some contaminating blood cells in addition to spermatozoa). Thus, the presence of the cko-deleted band in the spermatozoa lane of the second CKO mouse (ID no. 3862) may represent leaky Cre expression in blood cells or some persistent CKO spermatozoa (asterisks). Marker sizes are shown on the left.