Osmoregulatory function of large vacuoles found in notochordal cells of the intervertebral disc running title: an osmoregulatory vacuole.

The nucleus pulposi of many species contain residual cells from the embryonic notochord, which exhibit a very unusual appearance (large vacuoles occupying approximately 80% of the cell volume, surrounded by an actin cytoskeleton). While the vacuoles have been qualitatively described, their composition and function has remained elusive. Given that these cells are believed to generate and experience significant osmotic pressures in both the notochord and intervertebral disc, we hypothesized that the vacuoles may serve as osmoregulatory organelles. Using both experimental and theoretical means, we demonstrated that the vacuoles contain a low-osmolality solution, generated via ion pumps on the vacuolar membrane. During hypotonic stress the vacuoles release their contents into the cytoplasm, diluting the cytoplasm and restoring the osmotic balance across the cell membrane. Thus the vacuoles function to regulate the cell volume and tonicity during rapid osmotic stress, protecting the cells from potentially damaging swelling pressures.