The ribonucleoprotein enzyme telomerase synthesizes telomeric DNA onto chromosome ends. Telomere length is maintained, by the presence of telomerase activity, in the vast majority of primary tumours and stem cells, suggesting that telomere maintenance is essential for cellular immortalization. Recently, the telomerase RNA component in human and mouse (TERC and Terc, respectively), a telomerase-associated protein TEP1/TLP1 (refs 6,7) and the human catalytic subunit protein TERT (refs 8,9) have been identified. To examine the role of telomerase in telomere maintenance and cellular viability, we established Terc-deficient embryonic stem (ES) cells. It is known that telomerase activity is absent in cells from Terc-knockout mice. Although the study showed that telomere shortening was observed in the Terc-deficient cells from first to six generation animals, whether telomerase-dependent telomere maintenance was essential for cellular viability remained to be elucidated. To address this issue, we examined Terc-deficient ES cells under long-term culture conditions. Accompanying the continual telomere shortening, the growth rate of Terc-deficient ES cells was gradually reduced after more than 300 divisions. An impaired growth rate was maintained to approximately 450 divisions, and then cell growth virtually stopped. These data clearly show that telomerase-dependent telomere maintenance is critical for the growth of mammalian cells.

A member of the mitogen-activated protein kinase superfamily, MAK, has been proposed to have an important role in spermatogenesis, since Mak gene expression is highly restricted to testicular germ cells. To assess the biological function of MAK, we have established MAK-deficient (Mak ؊/؊ ) mice. Mak ؊/؊ mice developed normally, and no gross abnormalities were observed. Spermatogenesis of the Mak ؊/؊ mice was also intact, and most of the mice were fertile. However, Mak ؊/؊ male-derived litter sizes and their sperm motility in vitro were mildly reduced. These data show that function of MAK is not essential for spermatogenesis and male fertility. Spermatogenesis consists of three major stages: (i) a selfrenewing stage of spermatogonia (stem cells) by mitosis, (ii) a meiotic division stage of spermatocytes, and (iii) a morphological maturation stage during which haploid spermatids become mature spermatozoa. To understand the molecular mechanisms of mammalian spermatogenesis, research approaches using other cellular systems or experimental results from other species (for example, invertebrate systems) sometimes provide useful information. Since stem cells from other types of tissues also self renew and proliferate like spermatogonia, it is quite possible that similar mechanisms control the cellular events of both reproductive and other stem cells. Indeed, similar molecular properties have been described for different stem cell systems. For example, both steel factor and its receptor, c-kit, is crucial for hematopoiesis and spermatogenesis (9). Furthermore, hematopoietic stem cells and spermatogonial stem cells express integrin molecules on the cell surface and can be isolated based on expression of specific integrins (12,19,22). Meiosis is a process unique to the germ lineage cell; however, all eukaryotes undergo meiotic cell division in special circumstances. It has been shown that molecules critical for meiotic recombination in yeast also exist in mammals and have similar functions (reviewed in reference 6).Identification and characterization of specific molecules expressed within the testis is another approach to understanding spermatogenesis at a molecular level. Many testicular proteins have been identified and partially characterized (reviewed in reference 7). A serine-threonine kinase, MAK (male germ cell-associated kinase), is one such molecule. It was originally identified by weak cross-hybridization with a tyrosine kinase gene, v-ros (16). Since the expression of MAK protein was shown to be highly restricted in testicular germ cells at and after meiosis, it has been strongly speculated that MAK plays an important role(s) in cellular processes of spermatogenesis (10,13,16).To assess the function of MAK in spermatogenesis and male reproductive physiology, we generated Mak Ϫ/Ϫ mice by homologous recombination in embryonic stem (ES) cells and characterized their reproductive processes, including spermatogenesis and fertility. MATERIALS AND METHODS Establishment of Mak knockout ES cells and mice.A DNA fragment c...

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The sex of the Japanese eel could be controlled by the oral administration of DES-Na (sodium diethylstilbestrol) which also promoted growth. Elvers were reared from glass eels for more than 500 days with a commercial feed which contained DES-Na (0 , 0.5, 0.75, 1.0 ppm).All the eels were dissected at the end of the experiment to check the sex by both external appearance and histological examinations of gonads.The sex groups were divided into male, male?, female, female? and unknown by their external appearance, and into male, female, hermaphrodite, and non-sex by histological examination.The largest discrepancy (27.5%) in the two examinations was found in the male: the histological examination was found to be essential.The female ratio increased along with an increase in DES-Na.It was 7.7% in the control and 23 .2-35.4% in DES-Na groups. The male ratio was 64.1% in the control and 32.0-40.4% in DES-Na groups.The hermaphrodite ratio was 1.1-4.6%, and the non-sexual ratio was 23.8-38.0%.Females were the largest of all sex groups. The growth promoting effect was found to vary with the growth stage.

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