Telomeres are repeated sequences that protect the ends of chromosomes and harbour DNA repair proteins. Telomeres shorten during each cell division in the absence of telomerase. When telomere length becomes critically short, cell senescence occurs. Telomere length therefore reflects both cellular ageing and capacity for division. We have measured telomere length in human germinal vesicle (GV) oocytes and preimplantation embryos, by quantitative fluorescence in situ hybridization (Q-FISH), providing baseline data towards our hypothesis that telomere length is a marker of embryo quality. The numbers of fluorescent foci suggest that extensive clustering of telomeres occurs in mature GV stage oocytes, and in preimplantation embryos. When calculating average telomere length by assuming that each signal presents one telomere, the calculated telomere length decreased from the oocyte to the cleavage stages, and increased between the cleavage stages and the blastocyst (11.12 versus 8.43 versus 12.22 kb, respectively, P < 0.001). Other methods of calculation, based upon expected maximum and minimum numbers of telomeres, confirm that telomere length in blastocysts is significantly longer than cleavage stages. Individual blastomeres within an embryo showed substantial variation in calculated average telomere length. This study implies that telomere length changes according to the stage of preimplantation embryo development.

Recent data show that the effects of ionising radiation are not restricted to the directly exposed parental germ cells, but can also manifest in their non-exposed offspring, resulting in elevated mutation rates and cancer predisposition. The mechanisms underlying these transgenerational changes remain poorly understood. One of the most important steps in elucidating these mechanisms is to investigate the initial cellular events that trigger genomic instability. Here we have analysed the effects of paternal treatment by ethylnitrosourea, an alkylating agent which is known to form specific types of DNA adducts, on the transgenerational effects in the first-generation (F 1 ) offspring of exposed CBA/Ca and BALB/c male mice. Mutation rates at two expanded simple tandem repeat loci were significantly elevated in the F 1 germline of both strains. Pre-and post-meiotic exposures resulted in similar increases in mutation rate in the F 1 germline. Within each strain mutation rates were equally elevated in the germline of male and female F 1 offspring of the directly exposed males. The results of our study suggest that transgenerational instability is not attributed to a specific sub-set of DNA lesions, such as double strand breaks, and is most probably triggered by a stress-like response to a generalised DNA damage.2

Telomeres are physical ends of mammalian chromosomes that dynamically change during the lifetime of a cell or organism. In order to understand mechanisms responsible for telomere dynamics, it is necessary to develop methods for accurate telomere length measurement. The most sensitive method for measuring telomere length in mouse chromosomes is quantitative fluorescence in situ hybridization (Q-FISH). The usual protocol for Q-FISH requires plasmids with variable numbers of telomeric repeats and fluorescence beads as calibration standards. Here, we describe a Q-FISH protocol in which two mouse lymphoma cell lines with well-defined telomere lengths are used as calibration standards. Using this protocol we demonstrate that reproducible results can be obtained in a set of four different mouse cell lines. This method can be adapted so that any pair of mammalian cell lines can serve as an internal calibration standard.

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Defective telomere function in 7926 correlates with chromosomal radiosensitivity. This implicates telomere function in addition to telomere length as a determinant of chromosomal radiosensitivity.