The stroma is the supportive, non-functional connective tissue component found within organs and tissues, providing a structural framework and maintaining tissue organization. In connective tissues, the stroma consists of various cellular and non-cellular components that work together to provide mechanical support, maintain tissue integrity, and contribute to tissue homeostasis and repair.
The main components of stroma in connective tissues include:
- Extracellular matrix (ECM): The ECM is a complex network of proteins, carbohydrates, and other macromolecules that provide structural support and organization to the tissue. Key components of the ECM include fibrous proteins (e.g., collagen, elastin, and fibronectin) and proteoglycans. The ECM also contains various signaling molecules, such as growth factors and cytokines, that regulate cellular processes, including proliferation, differentiation, and survival.
- Fibroblasts: These are the primary cell type found in the stroma of connective tissues. Fibroblasts synthesize and secrete ECM components, such as collagen, elastin, and proteoglycans, and play a crucial role in ECM remodeling and tissue repair. Fibroblasts also produce signaling molecules that regulate inflammation, immune cell recruitment, and tissue homeostasis.
- Immune cells: The stroma can harbor various types of immune cells, such as macrophages, lymphocytes, and dendritic cells. These immune cells play essential roles in maintaining tissue homeostasis, defending against pathogens, and mediating tissue repair and inflammation.
- Blood vessels: The stroma contains a network of blood vessels that supply oxygen, nutrients, and signaling molecules to the tissue, and remove waste products and metabolic by-products. Blood vessels also facilitate the trafficking of immune cells and support angiogenesis during tissue repair and growth.
- Nerves: Nerve fibers can be found within the stroma, providing sensory and motor innervation to the tissue. Nerves also contribute to tissue homeostasis and repair by releasing signaling molecules, such as growth factors and neuropeptides, that regulate cellular processes and modulate immune responses.
The stroma plays a critical role in maintaining tissue integrity and function, and its components interact with the functional, parenchymal cells of the tissue to ensure proper tissue homeostasis. In pathological conditions, such as chronic inflammation, fibrosis, or cancer, the stroma can undergo significant changes, leading to altered ECM composition and organization, abnormal immune cell infiltration, and the activation of fibroblasts, which can contribute to disease progression and tissue dysfunction.