Stroma refers to the supporting framework within biological systems. It is the non-functional part of an organ or tissue, providing structural support and enabling specialized cells to perform their roles. This supportive environment is dynamic, playing an active role in numerous cellular processes, including communication and regulation. Understanding this framework is fundamental to comprehending how biological systems function and respond to various conditions.
Stroma in Plant Cells
In plant cells, stroma refers to the colorless fluid that fills the inner space of chloroplasts. This fluid surrounds the stacked thylakoids. The stroma contains components like chloroplast DNA, ribosomes, starch granules, and all the enzymes necessary for the light-independent reactions of photosynthesis.
The stroma is the site where the Calvin cycle takes place. During this cycle, the chemical energy in ATP and NADPH, produced from the light-dependent reactions on the thylakoid membranes, is used to convert carbon dioxide into glucose. The enzyme RuBisCO, located in the stroma, catalyzes the initial step of carbon fixation, incorporating atmospheric carbon dioxide into organic molecules.
Stroma in Animal Tissues
In animals, stroma describes the connective tissue supporting organs and tissues. This framework is distinct from the parenchyma, comprising the organ’s functional cells. The stroma is a complex network composed of the extracellular matrix (ECM) and various cell types.
The extracellular matrix consists of proteins like collagen, elastin, fibronectin, and laminins, along with polysaccharides such as proteoglycans. These components provide mechanical strength, elasticity, and adhesion sites for cells. Embedded within this matrix are diverse cell populations, including fibroblasts, which are responsible for producing and maintaining the ECM, as well as immune cells, blood vessels, and nerve fibers.
Beyond physical support, stroma actively participates in cell communication, tissue development, and repair. Fibroblasts, for instance, adapt to their environment, sending and receiving signals that regulate cell behavior and tissue organization. This dynamic interplay is essential for maintaining tissue integrity and proper organ function.
Stroma’s Impact on Health and Disease
Alterations in stroma’s composition or cellular components can contribute to various diseases. In cancer, the tumor microenvironment, often termed the “cancer stroma,” plays a role in tumor initiation, growth, and spread. This altered stroma can promote cancer cell survival, proliferation, and invasion by secreting growth factors, cytokines, and chemokines.
Cancer-associated fibroblasts (CAFs), a cell type in the tumor stroma, actively remodel the extracellular matrix, making it denser and more rigid. This increased stiffness can enhance tumor cell migration and invasion, while also impeding the delivery and effectiveness of anti-cancer drugs. The stroma can also induce drug resistance, by secreting protective factors or metabolizing therapeutic agents.
Beyond cancer, changes in the stroma are implicated in fibrotic diseases. Fibrosis involves the excessive accumulation of extracellular matrix components, primarily collagen, leading to tissue scarring and impaired organ function. Chronic inflammation often precedes or accompanies fibrosis, with immune cells and activated fibroblasts interacting to sustain the fibrotic response.
The Broad Significance of Stroma
Stroma’s importance spans diverse biological fields. Its role as a supportive framework, whether in chloroplasts or animal tissues, highlights its importance in biological organization and function. Research into stroma provides insights into basic biological processes, from photosynthesis in plants to tissue development and repair in animals.
Stroma’s active participation in disease progression, particularly in cancer and fibrosis, has opened new avenues for therapeutic development. Targeting stromal components, rather than solely focusing on the diseased cells, offers strategies for improving treatment outcomes and overcoming drug resistance. Moreover, the study of stroma contributes to advancements in regenerative medicine, where engineering supportive stromal environments can facilitate tissue repair and regeneration.