These are cells in a tissue or tissues in an organ that are concerned with function. These are the characteristic cells or tissues that do the actual stuff. The importance to us is that parenchymal tissues expend much vital energy in their functions and are less tolerant of a degraded environment than the structural mesenchyme. A congested and impaired organ like the liver of a heavy drinker has so much regular dysfunction that eventually the more tolerant and metabolically less particular mesenchymal cells become more common, and the distressed, overworked, and metabolically compromised parenchymal cells become a minority. The structural cells can multiply with ease in a poor environment, the more delicate functional cells cannot-and you end up with the type of cirrhosis sometimes termed mesenchymal invasion disease.