Leanne Jones, Salk Institute Credit: Salk Institute

The fruit fly Drosophila is renowned for its giant sperm, produced in a well-characterized supportive niche in the fly's testes. The tail-flapping sperm are generated from germline stem cells that cluster around a well-defined 'hub' of non-dividing, non-germline cells that maintain germline stem cells and coordinate sperm development.

The question of how the niche itself is maintained has not been particularly pressing for developmental biologists. The ten or so cells that form the hub come together toward the end of embryonic development — about 17 hours after the egg is laid. Once in place, the hub cells were assumed to last the 40 days or so of the fly's life. A team led by Leanne Jones at the Salk Institute in La Jolla, California, has now made the surprising observation that this niche is being actively replenished by a population of non-germline stem cells. How stem cells create and maintain the niches that in turn sustain other stem cells is a pressing question for regenerative medicine; these new observations suggest that the fly testis will be a convenient system for studying this.

Some of Jones's team were already investigating why hub function declines as flies age, but the revelation that the hub is replenished with new cells came from a lab member studying quite a different question. Graduate student Justin Voog was looking at the function of a transcriptional repressor known as Escargot, homologs of which might encourage cancer metastasis in mammals by suppressing production of cadherin, a protein that helps cells stick to each other. Voog wanted to see if Escargot had a function in somatic stem cells (SSCs), the non-germline stem cells in the fly testes that produce cells that encapsulate the bundle of maturing sperm to form a cyst.

When he first told me SSCs were making cells that become hub cells in wild-type adult flies, I thought 'that would be cool, but we're bucking years of dogma'. Leanne Jones

“Justin likes to surprise me,” Jones says. Rather than popping into her office, he tends to wait for her to walk by as he's peering down a microscope, and then he whips interesting results out of the blue. This time, Voog was using a system designed to label dividing stem cells and their progeny. A few days after the labelling, he noticed some labelled cells appeared in the hub. As the hub cells themselves do not divide, this indicated that some of the progeny of the newly divided SSCs were entering the hub. Jones was sceptical: “When he first told me SSCs were making cells that become hub cells in wild-type adult flies, I thought 'that would be cool, but we're bucking years of dogma'.”

At first Jones suspected the result was an artefact of a leaky labelling system. But when Voog tried another, more reliable labelling technique and got the same results, she decided he must be on to something.

Next, Voog used a genetic strategy to knock out the function of Escargot in the SSCs. In a few days, hub cells that were mutant for Escargot were found in hubs that began to disintegrate. Altogether, these results indicated that some hub cells are derived from SSCs, and that Escargot function in SSCs is required for proper hub maintenance. “I saw that data and thought 'now I believe this; this all makes sense,'” Jones recalls.

These cell-labelling experiments revealed that the SSCs give rise to two lineages: sperm-encapsulating cyst cells and hub cells1. Ideally, Jones would have liked to use live imaging to watch SSC progeny moving either into the hub or into the sperm-encapsulating cyst, but currently testis tissue does not survive long enough in culture for this to be possible. Instead, following SSC fate is much more tedious. Voog had to fix and stain hundreds of testes at 5, 10 and 15 days after labelling. He then had to find and count dividing SSCs, their progeny and their precise locations. These dividing cells are rare, says Jones, and the counting process is laborious and frustrating.

The tedium has, however, raised a welter of new questions. In mammalian systems, most scientists focus on definitively identifying stem cells. The work at the Salk Institute points to a means of understanding the cells supporting the stem cells, and it also offers an accessible system for studying both the mechanisms that guide cells' transition between fates and how those mechanisms change with age.