The term “stroma” describes a supporting framework or matrix within a biological structure, referring to its structural or connective components. Stroma is typically composed of non-functional elements like connective tissue, blood vessels, or ducts. It provides structural integrity and organization.
The Stroma in Chloroplasts
In plant cells, the stroma is the colorless fluid found within the inner membrane of the chloroplast. This fluid surrounds the thylakoids and grana, where light-dependent reactions occur. The stroma contains water, various enzymes, and minerals, forming a gel-like substance. It also holds the chloroplast’s genetic material, including circular DNA and ribosomes, allowing it to synthesize some proteins.
The stroma is the primary site for the light-independent reactions of photosynthesis, commonly known as the Calvin cycle. During this process, carbon dioxide from the atmosphere enters the chloroplast and diffuses into the stroma. Here, enzymes, including the highly abundant enzyme RuBisCO, facilitate the conversion of carbon dioxide and hydrogen-carrier compounds into glucose and other carbohydrates. This transformation allows plants to store and utilize energy captured from sunlight.
The Stroma in Animal Tissues and Organs
In animal biology, stroma refers to the cells and tissues that provide structural and nutritional support to the functional cells of an organ, known as the parenchyma. This supportive framework is largely composed of connective tissue, the extracellular matrix, blood vessels, and nerves. The stroma helps to secure the parenchymal tissue, supplying nutrients and removing waste products.
The corneal stroma, for example, makes up approximately 90% of the cornea and is primarily composed of collagen fibers. These fibers provide the eye’s transparent outer layer with its strength and shape. In the reproductive system, the ovarian stroma is a highly vascular connective tissue that supports ovarian follicles and regulates their growth. It also plays a role in hormone production, converting androgens into estrogen, which is important for the menstrual cycle and fertility.
Bone marrow stroma consists of mesenchymal stromal cells, which are important for hematopoiesis, the process of blood cell formation. These cells create a microenvironment that supports the development of various blood cell types. The tumor microenvironment also includes a significant stromal component. This tumor stroma comprises extracellular matrix, fibroblasts, and immune cells, and it actively influences tumor growth, progression, and resistance to therapies. Tumor cells can recruit specific immune cells to the stroma, which then suppress anti-tumor immune responses, further contributing to cancer progression.
Shared Characteristics and Fundamental Importance
Across both plant and animal systems, the stroma functions as a supportive framework, providing structural integrity and organization for specialized cells. This structural role extends beyond passive scaffolding, as the stroma is a dynamic and interactive environment.
It also facilitates signaling and communication. It facilitates cell-to-cell interactions and regulatory processes, often through its extracellular matrix and resident stromal cells. This dynamic interplay ensures the coordinated activity necessary for biological processes.
The stroma maintains biological balance and enables specific functions, such as photosynthesis in plants or proper organ functioning in animals. Its influence extends to disease progression.