Figure 1 | Image of the LaAlO3/SrTiO3 structure. A transparent (clear) gas of electrons is formed at the atomically abrupt interface between two oxide insulators — LaAlO3 and SrTiO3 — laid on top of each other. Caviglia et al.3 use an electric field to control the electron density at this interface and thereby study the emergence of superconductivity in the electron gas. (Image courtesy J. Mannhart.)

systems, huge strains can be imparted on the constituent monolayers assembled on top of one another. This could enable electronic-reconfiguration and orbital-ordering effects to enhance the superconductivity, which could be turned on and off simply by using an electric field to modulate the charge-carrier density. Engineered oxide films, in particu-lar, offer a new means of searching for super-conductivity that traditional methods lacked. Several decades ago, it was proposed that interfaces could be used to boost the tempera-ture at which superconductivity can exist8,9, but it was also noted that this would be a chal-lenge to achieve experimentally 9. Studies such as that of Caviglia et al., together with advances in theory10, now show that this idea is becoming a reality. ■

Darrell G. Schlom is in the Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853-1501, USA. Charles H. Ahn is in the Departments of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520-8284, USA.e-mails: charles.ahn@yale.edu; schlom@cornell.edu

1. Ohtomo, A. & Hwang, H. Y. Nature 427, 423–426 (2004).2. Reyren, N. et al. Science 317, 1196–1199 (2007).3. Caviglia, A. D. et al. Nature 456, 624–627 (2008).4. Eckstein, J. N. Nature Mater. 6, 473–474 (2007).5. Brinkman, A. et al. Nature Mater. 6, 493–496 (2007).6. Parendo, K. A. et al. Phys. Rev. Lett. 94, 197004 (2005).7. Ahn, C. H., Triscone, J.-M. & Mannhart, J. Nature 424,

1015–1018 (2003).8. Ginzburg, V. L. Phys. Lett. 13, 101–102 (1964).9. Allender, D. et al. Phys. Rev. B 7, 1020–1029 (1973).10. Chaloupka, J. & Khaliullin, G. Phys. Rev. Lett. 100, 016404

(2008).

STEM CELLS

Makeshift sperm productionAllan C. Spradling

Early middle age is a difficult time, not least for male fruitflies when sperm production falls. The unexpected reason for this decline seems to be that, as tissues age, maintaining functional stem cells becomes difficult.

Stem cells arose early in evolution and underlie the body plans of most multicellular animals. So we tend to picture stem cells and their niches — the microenvironments in which they operate — as finely tuned systems with near perfect, almost magical, capabilities. A report by Yamashita and colleagues (Cheng et al., page 599 of this issue)1 reminds us,

however, that stem cells are no exception to François Jacob’s description2 of evolution as “tinkering”.

The subject of the Yamashita group’s studies is the germline stem cells (GSCs) in the testis of adult male fruitflies (Drosophila melanogaster). The authors discovered that by day 20 of adult-hood — early middle age for a fruitfly — a

significant fraction of GSCs had become arrested; that is, they had ceased the cell-cycle activity and asymmetric cell division that lead to each GSC producing one self-renewing cell and eventually one differentiated sperm cell. This slows sperm production without changing stem-cell number.

Normally, GSCs align one of their two cen-trosomes — key organelles controlling the direction of division — with the niche, which in this case is the periphery of a cluster of sup-port cells called the hub (Fig. 1a, overleaf). This arrangement ensures that, on division, one daughter remains a stem cell while the other one differentiates. Cheng et al.1 found that centrosomes in the arrested stem cells had become misaligned. Remarkably, it seems that centrosome misalignment is a by-product of the de differentiation of older germ cells that enter the niche to replace GSCs that have turned over; dedifferentiation randomizes centrosome ori-entation. These observations provide a much clearer picture of the ageing Drosophila testis, and hint at pro cesses that may limit cell produc-tion in many other tissues: stem-cell homeo-static mechanisms are not perfect, and cannot maintain full activity even into middle age.

A case in point is the signature ability of stem cells to divide asymmetrically — to self-renew. Centrosome alignment seems to be a highly adaptive mechanism that acts to ensure self-renewal by limiting stem-cell loss or over-proliferation3. Keeping one centrosome, the maternal centrosome4, permanently local-ized within the cell cytoplasm adjacent to the boundary between the GSCs and the niche-generating hub ensures that divisions will be oriented perpendicular to the niche (Fig. 1a). Because the niche microenvironment that maintains the stem-cell state exists only in a one-cell-wide strip around the hub, such an orientation guarantees that just one GSC and one differentiating germ cell will be produced by each division (Fig. 1b). This elegant system, or a variant of it, seems to be used in other stem cells, such as Drosophila neuroblasts5,6. It may have evolved from a fundamental molecular asymmetry in all dividing cells — the non-equivalence of newly duplicated centrosomes — and hence may be an ancient stem-cell mechanism.

We must now ponder the fact that, despite the operation of this system, many GSCs are lost with age anyway and have to be replaced. Moreover, replacement cells seem to have dif-ficulty realigning with the niche, and during realignment they cannot function as GSCs (Fig. 1c). However, it is still too early to blame these problems on centrosome misalignment as such. Age-related changes in niche adhesion or signal reception could underlie stem-cell loss, and dedifferentiation itself might be the slow step in replacement and simply be a pre-requisite to alignment. Nonetheless, the system as a whole comes under stress by early middle age, and problems only get worse in older flies, in which niche signals begin to fail as well7.

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A major contribution of this work1 is the finding that new GSCs are regularly produced by dedifferentiation of downstream germ cells (Fig. 1d). The GSC daughter that leaves the niche divides four times, but incompletely, to generate 2-, 4-, 8- and 16-cell intercon-nected germline ‘cysts’. It was already known from lineage marking that male GSCs turn over and are replaced at a moderate rate8, but the replacement of stem cells by the dediffer-entiation of germ cells up to the 8-cell stage had been observed only under experimental conditions9,10.

The data of Cheng et al. suggest that, in an old male, all the stem cells that started out in the niche are likely to have undergone replacement by a dedifferentiated cell at least once. Consequently, we now know that GSC activity in male Drosophila resides not just in the GSCs themselves, but also in the pool of 2-, 4- and 8-cell germline cysts. This is very similar to a long-standing but controver-sial model of germ-cell production in the mammalian testes. According to this model, sperm production can be sustained by both individual stem cells and a pool of ‘poten-tial stem cells’ that correspond to the 2-, 4-, 8- and 16-cell germline cysts, which in mice are called A-type spermato gonia11,12. The discovery that dedifferentiation and GSC replacement occur regularly in the normal Drosophila testis opens these fascinating pro-cesses up to detailed mechanistic study in an accessible system.

How general are these discoveries likely to be? We already know that many other stem cells depend on niches, undergo turn over and replacement, and decline with age13. But do they do so in the context of a grow-ing pool of arrested stem cells? Similar cells have not been described during ageing in the Drosophila female germ line14. In the ovary, separate small niches, each supporting just two or three GSCs, are isolated within each of 16 ovarioles. This cellular organization prob-ably reduces the opportunities for contact

between niches and downstream germ cells. In addition, replacement seems to rely on chang-ing the direction of GSC division, to keep both daughters in the niche, rather than on dedifferentiation15.

But there are many other systems that are compatible with the possibility of such changes. One attractive idea, particularly in large, long-lived animals such as mammals, is that a decline in stem-cell activity with age is actively programmed in order to contain the escalating probability of tumour initiation13. It is hard to see how this can be a significant factor in Drosophila, however, which is too small and short-lived to suffer significantly from terminal cancer. Short of finding some currently unknown genetic or environmental factors that put Yamashita and colleagues’ flies1 under stress, we are left with the possibility that stem-cell maintenance is simply less perfect than we might have imagined. Why entrust something as essential as sperm production to a makeshift system that breaks down regu-larly? “Why indeed?” we hear Jacob asking us. “It works.” ■

Allan C. Spradling is in the Department of Embryology, Carnegie Institution of Washington, Baltimore, Maryland 21218, USA. e-mail: spradling@ciwemb.edu

1. Cheng, J. et al. Nature 456, 599–604 (2008). 2. Jacob, F. Science 196, 1161–1166 (1977).3. Yamashita, Y. M., Jones, D. L. & Fuller, M. T. Science 301,

1547–1550 (2003).4. Yamashita, Y. M., Mahowald, A. P., Perlin, J. R. & Fuller,

M. T. Science 315, 518–521 (2007).5. Rebollo, E. et al. Dev. Cell 12, 467–474 (2007).6. Rusan, N. M. & Peifer, M. J. Cell Biol. 177, 13–20 (2007).7. Boyle, M., Wong, C., Rocha, M. & Jones, D. L. Cell Stem Cell

1, 470–478 (2007).8. Wallenfang, M. R., Nayak, R. & DiNardo, S. Aging Cell 5,

297–304 (2006).9. Kai, T. & Spradling, A. Nature 428, 564–569 (2004).10. Brawley, C. & Matunis, E. Science 304, 1331–1334 (2004). 11. Clermont, Y. & Bustos-Obregon, E. Am. J. Anat. 122,

237–247 (1968).12. Nakagawa, T., Nabeshima, Y.-i. & Yoshida, S. Dev. Cell 12,

195–206 (2007). 13. Morrison, S. J. & Spradling, A. C. Cell 132, 598–611 (2008).14. Pan, L. et al. Cell Stem Cell 1, 458–469 (2007).15. Xie, T. & Spradling, A. C. Science 290, 328–330 (2000).

Figure 1 | Centrosome alignment and germline stem-cell (GSC) maintenance in the Drosophila testis niche1. a, Normally, GSCs keep one centrosome (purple circle) aligned with the support cells of the hub, so that upon division (b) one daughter will remain in the niche while the other will exit and differentiate. c, With increasing age, more and more GSCs have misaligned centrosomes — that is, neither is adjacent to the hub — and so do not divide. d, Some of these GSCs arise from dedifferentiating older germ cells that re-enter the niche with randomly positioned centrosomes.

Hub

Alignedcentrosomes

Cell division Misaligned centrosomes Dedifferentiation

a

b c d

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50 YEARS AGODuring the latter half of October 1957, following the Windscale reactor accident, we observed that the gamma-ray activity of certain members of the staff ... was higher than expected ... The source of the greater part of this increased activity was identified as iodine-131 ... We decided, therefore, to make some measurements of the iodine-131 in human thyroids ... Eighteen persons were measured, the majority of whom were resident either in, or on the fringe of, the Greater London area ... Since the main route of entry to the human thyroid was via milk, then as the cloud with its iodine-131 from Windscale passed over Britain on October 11, we may expect that the first day on which the milk contained this isotope was October 12 ... If we assume the same pattern of uptake throughout the group, then it is evident that the average person received about 0.04 rad. The magnitude of the radiation dose to adult human thyroids in the people measured was therefore comparable with that expected each year from natural background radiation. From Nature 6 December 1958.

100 YEARS AGOA Bill for putting in force the decisions of the Berlin Wireless Telegraphy Conference of November, 1906, as embodied in an international convention, has been laid before the French Chamber ... The conference has fixed wave-lengths, one of 300 metres, the other of 600 metres, for the transmission of public messages by the wireless current ... All stations must be able to produce one, at all events, of these two wave-lengths ... Stations on board ship must use the 300-metre wave-length. They are permitted, however, to use other wave-lengths as well, provided that these are under 600 metres. Ships of small tonnage will be allowed to use a wave-length below 300 metres.From Nature 3 December 1908.

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