It really sucked to type this haha
Stem cells exist in adults, as they are the source of cells that need to regenerate. For example, after blood cells are lost, whether from a bleeding wound or just wearing out due to age they need to be replaced. The new blood cells arise from a pool of stem cells in the bone marrow.
Muscle cells are not lost and replaced like blood cells, but they depend upon satellite cells to fuse with mature myocytes for growth and maintenance. The supply of satellite cells is maintained somewhat by dividing to create more satellite cells before fusing with muscle cells. Additionally, new satellite cells are generated from stem cells residing alongside the muscle bundle.
Unfortunately for bodybuilders, these same stem cells can also become fat cells. Fat cells don’t fuse and they can grow without limit, as evidenced by the straining belt lines standing at the all you can eat buffets. Herein lie the wonders of testosterone. From stem cells to the mature myocyte, testosterone can increase muscle size (and strength) and decrease fat.
Stem cells lying alongside muscle cells include both pluripotent and medenchymal stem cells. Pluripotent stem cells can become nearly any other kind of cell in the body, whereas mesenchymal stem cells can only become certain types of cells, including fat cells and muscle cells. Mesenchymal stem cells have androgen receptors. When stimulated by testosterone (or possibly other androgens), mesenchymal stem cells are inhibited from converting into fat cells; instead, they are hormonally urged to become muscle cell precursors. This reduces the potential number of fat cells, while increasing the supply of satellite cells. This merely affects the potential of fat mass or muscle, but it’s an interesting control function of androgenic hormones.
Once stem cells have been dedicated to either the fat cell or muscle cell lineage, testosterone further influences the potential growth of both tissue types. Fat cells, called adipocytes, are generated from immature cells called pre-adipocytes. The conversion of pre-adipocytes to fully mature, functional adipocytes is inhibited by testosterone. Once, it was believed that the number of fat cells was constant throughout life. This data suggests fat cells must be replenished. Perhaps the loss of body fat documented during testosterone therapy is due in part to testosterones ability to reduce the formation of new fat cells.
Muscle cells arise from satellite cells. Under the influence of testosterone, satellite cells are stimulated to mature into myoblasts, which merge with myocytes to increase the number of nuclei in myocytes. The more nuclei present in a myocyte, the greater the size potential of the individual muscle cell. This is due to a restrictive regulatory mechanism referred to as the nuclear/cytoplasmic ratio.
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