Structure and Functional Activity of Fibroblasts
Under a light microscope, fibroblasts appear as elongated, spindle-shaped cells with prominent cellular processes. When stained with iron hematoxylin, they acquire a light gray tint, and their nucleus is oval in shape.
The primary biological task of these cells is the production of extracellular matrix components. They continuously synthesize:
- Structural proteins (collagen and elastin).
- Amorphous ground substance components (proteoglycans and glycoproteins).
Cell ultrastructure directly reflects its function. A pale nucleus under light microscopy indicates a predominance of euchromatin (decondensed chromatin), which is direct evidence of active gene transcription. The cytoplasm contains a well-developed rough endoplasmic reticulum (RER), which ensures the massive synthesis of proteins destined for export outside the cell.
Lifecycle and Migration Mechanism
The cell lineage (diffjeron) begins with mesenchymal stem cells. These give rise to poorly specialized fibroblasts with high mitotic activity (ability to divide). Gradually, they transform into differentiated mature forms that have lost this mitotic capacity.
Mature cells are not static; they are capable of migrating along the fibrous structures of connective tissue. To prevent slipping, the cell uses a specific protein—fibronectin—as an "anchor" for firm adhesion to fibers.
The mechanism of movement is similar to that of leukocytes and consists of three stages:
- Formation of a pseudopodium (cellular extension).
- Flow of the cell cytoplasm forward.
- Retraction of the trailing edge.
The molecular basis of this movement is the interaction of microfilaments—the contractile proteins actin and myosin, which cyclically shorten and lengthen.
Fibrocytes — Senescent Forms
Fibrocytes represent the final stage of fibroblast development. Compared to their active precursors, their appearance and structure change significantly:
- Cells become narrow and very long.
- The number of cellular processes decreases significantly.
- The nucleus condenses and acquires a rod-like shape.
A dense, dark nucleus indicates profound heterochromatinization: intracellular macromolecule synthesis has virtually ceased. Like mature fibroblasts, fibrocytes completely lack the ability to divide.
Specialized Derivatives: Fibroclasts and Myofibroblasts
Under specific physiological conditions, fibroblasts can differentiate into highly specialized cells with novel functions.
Fibroclasts (Destructive Cells) Their primary function is the active resorption (breakdown) of the extracellular matrix. They appear en masse in tissues during organ involution (regression), such as in the uterus postpartum. Their mechanism combines phagocytosis and extracellular hydrolysis. Morphologically, they are large cells with pale oval nuclei (resembling fibroblasts), and their cytoplasm is rich in lysosomes and phagolysosomes, within which collagen fragments are digested.
Myofibroblasts (Repair Cells) They appear during tissue regeneration, particularly during wound healing. These cells perform a dual function:
- Synthetic: Due to a well-developed rough ER, they actively produce new matrix components to replace tissue defects.
- Contractile: The presence of numerous myofilaments in the cytoplasm allows them to physically approximate and contract wound edges.