Every so often I run into an especially intriguing paper, and a recent open access work on the interaction between stem cells and other cells is a good example of the type. If you've been following stem cell research over the past decade, you'll know that one of the important questions has been how stem cell transplants cause regeneration. At present, the best answer appears to be some combination of creating new cells and issuing chemical signals, with much more of the latter taking place in most of the earlier therapies, such as stem cell transplants for heart disease.
This paper looks at another mechanism that has been known for a while, but is far less well explored: a stem cell can form a thin connecting tunnel to a damaged cell and use that tunnel to donate some portion of its internal machinery. Cells are equipped with roving, self-replicating herds of organelles, including the recycling machines called lysosomes and energy generating machines called mitochondria - and sending some through the tunnel can help to restore proper functioning to some degree in the recipient cell. From the paper:
Although therapeutic effect of adoptive transfer of endothelial progenitor cells (EPC) has been well-substantiated, the actual engraftment is relatively low [usually averaging 1-2%] compared to a robust functional improvement of [blood vessel function]. ... This discrepancy explains an intense search for indirect mechanism(s) of vascular repair by EPC that could reconcile the scarcity of engrafted cells with the notable functional response.
Tunneling nanotubes (TNT) formation between cultured cells has been described and proved to be a viable mechanism of organellar exchange between the partners. This mechanism has been shown to account for mitochondrial transfer between adult stem cells and somatic cells and rescue their respiration. This mechanism is believed to play a significant role in intercellular communication, although it remains technically difficult to morphologically document it.
The researcher show that stem cells are very selective in which cells they choose to connect to, and that transfer of lysosomes appears to be just as effective in rescuing cellular recycling functions as mitochondrial transfer is in restoring energy generation. That suggests that this is a significant mechanism in the operation of stem cell therapies. Be sure to take a look at the photographs of cells donating components, which are quite striking.
The whole process well illustrates the truth that biology is always more complex than we'd like it to be. Damage to both mitochondria and lysosomes are strongly implicated as causes of degenerative aging, and it makes theorizing, design of therapies, and evaluation of potential therapies that much harder if cells are willing to promiscuously swap these components. You'll find background on mitochondrial damage and lysosomal decline as it relates to aging back in the archives: