Involution

Involution is the reverse development or atrophy of an organ or tissue, in which it shrinks to its previous, smaller size. Unlike pathological atrophy caused by disease, involution is a programmed and controlled physiological process.

The main mechanism of involution is apoptosis, i.e., programmed cell death, which allows tissue to decrease in volume without a significant inflammatory response.

Physiological examples

Involution is a normal process at different stages of a body’s life.

• Involution of the uterus. Classical example. After giving birth, the significantly enlarged uterus returns to its normal pre-pregnancy size within a few weeks.
• Age-related thymus involution. The thymus gland, which is very active in childhood, begins to gradually shrink after puberty and is replaced by fatty tissue.
• Mammary gland involution. After breastfeeding ends, the glandular tissue of the mammary glands undergoes reverse development.
• Involution of adenoids. This is a natural process of reduction in the size of the nasopharyngeal tonsil, which occurs with age, beginning at 7 years old and ending at puberty (around 14 to 15 years old), when most of the lymphoid tissue is replaced by connective tissue.

Clinical Significance

Involution is an important physiological mechanism. Disruption of this process may indicate pathology. For example, subinvolution of the uterus after childbirth, when it does not shrink quickly enough, can lead to postpartum bleeding and infections.

Age-related organ involution (e.g., of the thymus or ovaries during menopause) is a natural and integral part of the body’s ageing process.