Kidney-like structure regenerated from a single cell

Japanese researchers have successfully generated a kidney-like structure from just a single cell, thus making a huge leap forward in organ regeneration research. The team comprises members from Okayama University Graduate School of Medicine and Kyorin University School of Medicine.

Writing in the journal Stem Cells, the researchers said, “It has been predicted that the kidney will be among the last organs successfully regenerated in vitro due to its complex structure and multiple functions.” They explained that in embryos, kidney development requires two types of ‘primordial’ cells (cells at the earliest stage of development) - the metanephric mesenchyme and the ureteric bud - to eventually differentiate into more than a dozen distinct types of kidney cell.

“The metanephric mesenchyme develops into the kidney nephron, the minimal functional unit of the kidney,” the researchers said. A nephron filters the blood to regulate the concentration of water and soluble substances, such as sodium salts, and comprises several segments: the glomerulus, the proximal tubule, the loop of Henle, the distal tube and the collecting duct.

But the researchers managed to produce kidney cells that could differentiate into a kidney-like structure without these primordial cell types, suggesting they are adult kidney stem cells. The team micro-dissected adult rat kidneys into segments from the glomeruli, proximal convoluted tubule, proximal straight tubule, medullary thick ascending limb of Henle’s loop and the collecting duct, then grew the stem cells on mouse mesenchymal cells. The cell cluster was then suspended in an extracellular matrix gel - which provides a complex environment similar to that found in many tissues - and cultured in the presence of several growth factors.

A kidney stem/progenitor cell (KS cell) cluster formed a kidney-like structure (KLS) after four weeks of incubation. Glomerulus-like structures were formed at the tips of the tubular structures, proximal-like tubules, distal-like tubules, collecting duct-like tubules and renal pelvis-like structures. G - glomerulus-like structure; Pr - proximal tubule-like structure; H - loop of Henle-like structure; D/C - distal tubule-like structure or collecting duct-like structure; Pe - renal pelvis-like structure.

Kidney-like structures could form from these cell clusters so long as the initial cluster was large enough (ideally 100,000 cells or more), although the minimum cluster size required might suggest that not all the kidney stem cells have stem-cell characteristics. Therefore, the researchers cloned kidney stem cells and confirmed that kidney-like structures still formed from the clusters of clone cells after a few weeks.

“Morphological analyses revealed that these kidney-like structures contained every substructure of the kidney, including glomeruli, proximal tubules, the loop of Henle, distal tubules and collecting ducts, but no vasculature,” the researchers said. This means that the structures did not show the formation of blood vessels or make urine.

The relationship between cell number in the cluster and the ability to reconstitute a kidney-like structure. Red bar: cyst formation; blue bar: long distinct tubules formation; green bar: tubules with ball-like structures at the tip. The horizontal axis represents KS cell number (6.25-200 × 10³ cells)/cluster. The KS cell clusters were cultured for three weeks. Representative photomicrographs were from three independent experiments. Scale bar = 100 μm.

Nevertheless, the researchers confirmed that “a cluster of tissue stem/progenitor cells has the ability to reconstitute the minimum unit of its organ of origin by differentiating into specialised cells in the correct location” in a process which differs from embryonic kidney development”.

“Although the physiological roles of such cells are currently unclear, analogous cells in the adult human kidney would be a valuable resource for the regeneration of kidneys in vitro,” they added.

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