Stroma

Stroma is a term used in biology to describe the supportive, nonfunctional framework of an organ, tissue, or cell, in contrast to the functional tissue called parenchyma. In plants, the stroma is the fluid-filled matrix inside chloroplasts that surrounds the thylakoid membranes. It contains enzymes of the Calvin cycle, as well as chloroplast DNA and ribosomes, soluble sugars, and starch. The stroma also includes stromal lamellae that connect grana and is the site of carbon fixation during photosynthesis.

In animals and humans, stroma refers to the connective tissue component of an organ, providing structural support and housing blood vessels, nerves, and resident immune and other nonparenchymal cells. It comprises the extracellular matrix, fibroblasts, adipocytes, and various other cell types and components that create the microenvironment for parenchymal cells. In pathology, distinguishing stroma from parenchyma helps describe tumors and diseased tissue, and the stromal environment can influence growth, invasion, and response to therapy.

Several tissues feature prominent stroma. The cornea, for example, contains a corneal stroma that is a dense, arranged collagen layer accounting for most of the corneal thickness and contributing to transparency. Other examples include uterine stroma and dental or gingival connective tissue stroma, which provide the supportive framework around functional cells.