CTCFL is important for spermatogenesis. A Testicular weight distribution. The testicular weight of Ctcfl heterozygous (Ctcfldel/+; diamonds) and homozygous (Ctcfldel/del; circles) mice was measured and plotted as a normalized probability distribution (i.e., the surface under the curve represents a total probability of 1). Testes of knockout mice are significantly smaller (p < 0.0005, t-test). White symbols represent infertile males, black symbols are fertile males, and grey symbols correspond to males not tested for fertility. B Ctcfl mutant mice display reduced fertility. Epididymal sperm count from Ctcfl heterozygous (Ctcfldel/+; black bar) and homozygous (Ctcfldel/del; grey bar) mice. Standard deviation is plotted (p = 0.0002, n = 4). C Testis histology. In the left three panels a timed series of HE-stained testicle sections is shown (postnatal day 21, 28, 90), while in the right hand panel an apoptosis assay (TUNEL staining) of testicle sections at day 90 is shown. Note that in CTCFL-deficient testes some seminiferous tubules appear normal, whereas others (which can be adjacent to the normal ones) have lost most of their meiotic and post-meiotic germ cells, leaving only mitotic spermatogonia (that stain positive for BrdU incorporation, see panel E) and Sertoli cells. Yet other tubules contain disorganized spermatocytes, and some of them even elongated sperm. Thus, there is no absolute block in differentiation or progression of germ cell development, but the incomplete penetrance of the infertility phenotype is however directly linked to the testicle weight (panel A) and to the degenerative level of the seminiferous tubules. D Apoptosis plot. Number of TUNEL-positive apoptotic cells per 100 seminiferous tubules counted at day 28 and day 90. Standard deviation of three animals per genotype and time point is indicated. E DNA synthesis marked by a 1-h pulse of BrdU in day 40 testicles reveals that mitotic spermatogonia are still present in degenerated tubules. Counterstaining with hematoxylin. F SCP3 staining in spermatocytes of day 90 testicles as a marker for tubule organization. G PRSS50 co-localizes only partially with STRA8. Immunofluorescence staining with a STRA8 antibody (top panel) or PRSS50 antibody (bottom panel) of adult testicle sections shows that PRSS50 and STRA8 expression overlaps only partially. Scale bars are 50 μm.