Cellular rejuvenation happens when cells take out the garbage

In systems where the stem cells are attached to a basement membrane, adult stem cells divide vertically. They keep the original stem cell attached to the basement membrane. The newly produced cell is stacked on top of the stem cell and is no longer attached. This  propels the unattached cell to a path of differentiation. (7, 8). This leaves the basement membrane with a population of stem cells that retain their "stemness" and ability to divide. But at the same time, the stem cells are regenerating a fresh supply of specialized tissue cells for that specific organ.

The image below shows several layers of corneal epithelium above the basement membrane: notice that the bottom layer of cells (the ones attached to the basement membrane) are larger with larger nuclei. As the cells move away from the basement membrane, they flatten out, the nucleus becomes smaller and loses its prominence. As the cells mature, they age. The key to maintaining asymmetric cell division in this process is the attachment of the stem cell to the basement membrane. Experimentally, when cells that lose that attachment, they start maturing into specialized cells and lose their capacity to divide.

In this way, stem cells that undergo asymmetric division stay young by transferring cellular trash or maturing proteins to daughter cells. The young cells get to stay young for another cycle, and maintain their ability to divide and thrive.

Cells work beautifully when they are young, removing damaged organelles, messed up molecules, and stray DNA strands from the cytoplasm. They do this through the 3 paths that I described:

• Asymmetric cell division, to partition out damaged material to one of the daughter cells
• Packaging cellular trash into garbage-bin type vesicles for disposal outside the cell
• Transferring cellular trash to protein compactors and incinerators; some of those incinerators have enzymes packaged into a highly acidic compartment (the lysosome)

With those three paths to cellular rejuvenation, it is fair to ask why our organs still age. Here's the thing; youthful systems beget youth and aging systems beget aging. 

Aging systems beget aging

The problem lies in the limits of cells to keep up with the spring cleaning as cells experience wear and tear over time. With aging, those processes described above become less functional and the cell moves towards the dark side of senescence. Cells divide less and less, they produce less vesicles, and their lysosomes become less acidic and display decreased incinerator effectiveness (5, 10, 11, 12,  13, 14). 

To add insult to injury, vesicles that are secreted  from the cell sends senescent proteins into the blood stream. This  creates its own set of problems, including perpetuating the aging cycle (5, 13, 14).

Youthful systems beget youth

On a positive note, researchers are looking into injecting vesicles derived from young individuals into older ones. Experimental data suggest that cellular bins/vesicles generated from young cells, particularly stem cells, have cellular rejuvenation activity (13, 14, 15).

Even better, researchers are looking at the promise of senolytics, drugs that kill senescent cells, breaking the aging feedback loop (14, 15) as well as drugs that target specific senescence-associated pathologies (14).