Stroma refers to the supportive framework of biological structures, acting as a scaffold that holds cells and tissues in place. This foundational component is present throughout biological systems, from animal organs to plant cell compartments, maintaining the form and function of living organisms.
Defining Stroma
Stroma is the non-parenchymal component of a tissue or organ, meaning it lacks the specific functions of that organ. It is largely composed of the extracellular matrix (ECM), a complex network of proteins and carbohydrates. Key ECM components include fibrous proteins like collagen, elastin, and fibronectin, along with proteoglycans and hyaluronic acid, which create a porous, hydrated gel. Various cell types are interspersed within this matrix, including fibroblasts that produce and maintain the ECM, endothelial cells forming blood vessels, and immune cells like macrophages and lymphocytes. This composition provides structural integrity and a dynamic environment for the specialized, functional cells of the tissue, known as parenchyma.
Essential Functions of Stroma
Stroma’s physiological roles extend beyond simple structural support. It provides the physical framework that anchors cells and tissues, maintaining their shape and organization, and facilitates the exchange of nutrients and waste products between blood vessels and parenchymal cells, ensuring tissue viability. Through signaling molecules and growth factors, stroma regulates cell behavior, influencing growth, differentiation, and migration. It acts as a scaffold for tissue repair and regeneration following injury, guiding cell reorganization and new matrix deposition. Stroma influences immune responses by modulating inflammation and supporting immune cell function and differentiation.
Stroma in Animal Tissues and Organs
Stroma is a component across animal tissues and organs, providing tailored support for specialized functions. Connective tissue stroma serves as a general supportive framework throughout the body, as seen in the skin where loose connective tissue stroma underlies epithelial membranes, attaching them to other tissues and supporting blood vessels and nerves. Dense irregular connective tissue stroma, found in tendons and ligaments, provides high tensile strength, binding tissues and converting tension.
In organs like the liver, kidney, and lung, stroma provides structural integrity and a unique microenvironment for parenchymal cells. The liver’s stroma supports hepatocytes, while in the kidney, it encases renal tubules. Breast stroma includes fibroblasts, adipocytes, vascular, and immune cells embedded in a matrix of collagen I, fibronectins, and proteoglycans, playing a role in mammary gland architecture.
Bone marrow stroma supports hematopoiesis, the process of blood cell formation. Mesenchymal stromal cells within the bone marrow provide physical support for differentiating hematopoietic cells and supply nutrients and growth factors. These stromal cells also express adhesion molecules that influence hematopoietic cell differentiation.
The corneal stroma, making up approximately 90% of the cornea’s thickness, is a highly organized connective tissue that contributes to the eye’s transparency and structural integrity. Its precise arrangement of collagen fibers and proteoglycans allows light to pass through unimpeded. The ovarian stroma, rich in connective tissue, blood and lymphatic vessels, and nerves, plays a role in follicle development and hormone production. It provides the microenvironment for ovarian follicles to mature and for endocrine cells to produce hormones like estrogen and progesterone.
Stroma in Plants
In plant biology, stroma refers to a compartment within chloroplasts. The chloroplast stroma is the fluid-filled region surrounding the thylakoid membranes. This internal environment is where the light-independent reactions of photosynthesis, also known as the Calvin cycle, occur.
During the Calvin cycle, carbon dioxide diffuses into the chloroplast stroma. Enzymes like RuBisCO catalyze the fixation of carbon dioxide into organic molecules, initiating sugar synthesis. Energy-carrying molecules (ATP and NADPH) produced during light-dependent reactions on the thylakoid membranes are then utilized in the stroma to power subsequent steps of the Calvin cycle, ultimately forming glucose and other carbohydrates.
Stroma’s Role in Health and Disease
Alterations in stroma can impact health and contribute to the progression of various diseases. Fibrosis, characterized by excessive accumulation of stromal components, particularly collagen, leads to tissue hardening and organ dysfunction. This pathological process is observed in conditions like liver cirrhosis and lung fibrosis, where normal tissue architecture is disrupted by an overgrowth of fibrous connective tissue, impairing organ function.
In cancer, the tumor microenvironment, where stroma is a component, plays a role in tumor growth, progression, and resistance to therapy. This altered stroma, often called desmoplasia, is characterized by dense fibrous tissue and disorganized blood vessels. Cancer-associated fibroblasts (CAFs) within the desmoplastic stroma contribute to tumor growth and invasion by remodeling the extracellular matrix and releasing factors that promote cancer cell survival and proliferation. The dense desmoplastic stroma can also hinder anti-cancer drug delivery, contributing to therapy resistance.