Table 2.

Age-related changes in cellular structure of bone cells.

Cellular structure of bone cells	Age-related changes
Cell membrane components	The composition of the cell membrane was changed, the content of PC was significantly up-regulated, and the content of PE was significantly down-regulated. Moreover, in senescent cells, most phospholipids, including phosphatidylcholine, phosphatidylglycolamine, phosphatidylglycerol, and sphingolipids, are significantly increased, while phosphatidyl acid, phosphatidylinositol, and phosphatidylserine are decreased.
Cell cytoplasm	The contents of multiple miRNAs in the cytoplasm were changed, with increased expression of miR-195 and decreased expression of miR-543, miR-590-3p and miR-219a-5p. In addition, the expression of a variety of substances was changed. For example, the expression of Sirt3, RANKL, M-CSF, CD73, CD90, malondialdehyde, P21, P53, p16, and SASP increased, while the expression of DDR2 FOXP1, CD34, CD19, CD14, OPG, 2-ketoglutarate, and Alkbh1 decreased.
Cell nucleus	Chromosome telomeres were shortened, heterochromatin regions appeared, nuclear morphology changed to an enlarged nucleolus, and DNA damage markers increased.
Mitochondria	DNA mutations in mitochondria gradually increased, and morphological and functional changes were shown by disruption of mitochondrial membrane integrity, reduced biogenesis, mitochondrial dysfunction (including mitochondrial respiration, ATP reduction, and metabolite changes), and bioenergetic exhaustion.
Cell lysosomes	The activity of lysosome was changed, which was reflected in the activity of SA-β-gal and TRAP.
Cell exosomes	Both the number of exosomes secreted and the type of exosomes were changed in aging.