Answer:
Two signals that can lead to changes in gene expression in old age are the accumulation of cellular damage and changes in the levels of certain signaling molecules such as hormones or growth factors.
Accumulation of cellular damage: As cells age, they accumulate damage to their DNA, proteins, and other cellular structures. This damage can lead to changes in gene expression and can also trigger cellular stress responses.
Changes in signaling molecules: Hormones and growth factors play important roles in regulating gene expression. As people age, their levels of these signaling molecules can change, leading to changes in gene expression and cellular behavior. For example, levels of growth hormone and insulin-like growth factor 1 (IGF-1) decrease with age, which can lead to decreased muscle mass and increased fat storage.