Atrophy/Hypotrophy

Cells respond when asked to function more, however, they can also find themselves in a situation where they are "forced" to function less. In this case, the cells try to adapt and the condition is called: Atrophy/Hypotrophy.

"Atrophic" means without nourishment, but it should be called hypotrophy since the former term would indicate that the cell is no longer there.

What are the characteristics of atrophy? 

Atrophy is a regressive process in which there is a reduction in the volume of a tissue or organ that has already reached its normal volume. So an organ develops, reaches its normal volume and then regresses by decreasing its volume and therefore also its functionality.

The fact that this organ has already reached its normal volume and then regresses means that atrophy can be described as an acquired defect, that is, one that arises at a later time. 

The decrease in volume of an organ/tissue can occur for two reasons:

• decreases the volume of individual cells
• loss of cells through apoptosis

In atrophy, normally, it is the parenchymal cells that decrease in volume. Instead, in this situation, the stromal component of the cells (the external one) persists more.

The ultimate goal of atrophy is to ensure the survival of the cell even if in conditions of minimal functional activity. A cell to become atrophic on the one hand increases protein catabolism and on the other decreases synthesis. 

Physiologic atrophy: 

• Uterus and mammary gland at termination of pregnancy
• Organs during old age: uterus, ovaries, testes, and even the brain
• Structures during fetal development when they are no longer needed
• Thymus: organ in which T lymphocytes mature, it becomes atrophic at the end of adolescence in all subjects

Pathologic atrophy:

Atrophy is pathological if functional demand decreases and therefore the organ/tissue must function less:

• Skeletal muscle tissue when immobilized (e.g., during a period of cast). In this case, atrophy also depends on the amount of time this remains stationary. 
• Skeletal muscle tissue when there is a lack of innervation which may be due to either gene defects or traumatic events.

Reduced blood perfusion can cause atrophy in any tissue: there has been a decrease in blood flow in a particular district. This can be the result of atherosclerosis phenomena, a pathology that develops at the level of the vessel walls in which the lumen is reduced. As the lumen is reduced, all tissues downstream will receive less blood. An organ can also receive less blood if a neoplasm (tumor mass) compresses the part of the circulatory system that supplies it. 

Tissues become atrophic if there is inadequate nutrition. This may be associated with different geographic contexts (e.g., fasting). Nutrition may be inadequate in both protein and calorie intake. In a diet low in protein and calories, protein synthesis at the tissue level is deficient and tissue will tend to become atrophic. A form of malnutrition involves only protein intake and, again, tissues will become atrophic and the tissue most targeted will be muscle tissue.

The last condition is cachexia. In this situation the subject is subject to a significant loss of weight as a result of a loss of lean mass (muscle) which may or may not be associated with a loss of fat mass. This situation is associated with the most advanced stages of the path of carcinogenesis, in a subject carrying neoplasms, or even in subjects with AIDS in the terminal stages. In these cases the subject is emaciated, no longer has muscle mass and has little fat mass. Therefore cachexia is a condition of pathological atrophy associated with neoplasms, AIDS or chronic inflammatory diseases that can lead to organ failure.