Structure and functional activity of fibroblasts
Under a light microscope, fibroblasts have an elongated, spindle-like shape with distinct processes. When stained with iron hematoxylin, they acquire a light-gray hue, and their nucleus is oval in shape.
The main biological task of these cells is to produce components of the extracellular matrix. They continuously synthesize:
- Proteins for building fibers (collagen and elastin).
- Components of the amorphous ground substance (proteoglycans and glycoproteins).
The ultrastructure of the cell fully reflects its function. A light nucleus at the light-microscopic level indicates the predominance of euchromatin (decondensed chromatin), which is direct evidence of active gene transcription. The cytoplasm features a strongly developed rough endoplasmic reticulum (RER), which ensures the massive synthesis of proteins destined for export outside the cell.
Life cycle and migration mechanism
The differon (life cycle) begins with stem cells of mesenchymal origin. These give rise to poorly specialized fibroblasts with high mitotic activity (the capacity for division). Gradually, they transform into differentiated mature forms that have lost this capacity.
Mature cells are not static: they are capable of migrating along the fibrous structures of connective tissue. To avoid "slipping," the cell uses a specialized protein—fibronectin—as an "anchor" for firm adhesion to the fibers.
The movement mechanism itself is similar to that of leukocytes and consists of three stages:
- Formation of a pseudopodium (protrusion).
- Flowing of the cell contents forward.
- Retraction of the rear end.
The molecular basis of this movement is the interaction of microfilaments—the contractile proteins actin and myosin, which cyclically shorten and lengthen.
Fibrocytes: aging forms
Fibrocytes represent the final stage of fibroblast development. Compared to their active precursors, their appearance and structure change significantly:
- The cells become narrow and very long.
- The number of processes decreases significantly.
- The nucleus condenses and becomes rod-shaped.
A dense, dark nucleus indicates profound heterochromatinization: intracellular macromolecule synthesis has practically ceased. Fibrocytes completely lack the ability to divide, just like mature fibroblasts.
Specialized derivatives: fibroclasts and myofibroblasts
Under special physiological conditions, fibroblasts can differentiate into highly specialized cells with new functions.
Fibroclasts (destructor cells) Their main function is the active resorption (destruction) of the intercellular substance. They appear en masse in tissues during organ involution (reverse development), for example, in the postpartum uterus. Their mechanism of action combines phagocytosis and extracellular hydrolysis. Externally, they are large cells with light oval nuclei (similar to 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 in the process of wound healing. These cells perform a dual function:
- Synthetic: due to a well-developed rough ER, they actively produce components of the new matrix to replace the defect.
- Contractile: the presence of a large number of microfilaments in the cytoplasm allows them to physically approximate and pull together the wound edges.
Briefly: answers to topical questions
Which organelles are most developed in the cytoplasm of an active fibroblast?
The rough endoplasmic reticulum (RER) is the most powerfully developed organelle in the cytoplasm of an active fibroblast. This organelle ensures the synthesis of exported proteins, such as extracellular matrix components (collagen, elastin, proteoglycans, glycoproteins).
What specific enzymes do fibroclasts secrete for the extracellular cleavage of fibers?
Fibroclasts perform active resorption of the intercellular substance through a combination of phagocytosis and extracellular hydrolysis. The cytoplasm of these cells is rich in lysosomes (phagolysosomes) containing collagen fragments. However, the exact names of the specific enzymes secreted by fibroclasts for extracellular cleavage are not specified in the sources.
What surface receptors are localized on the plasmalemma of fibroblasts?
The sources explicitly state that fibroblasts use fibronectin to adhere to fibers during migration. It is also indicated that fibronectin interacts with the cell surface via integrin receptors and ensures cell attachment to extracellular matrix components. A separate list of surface receptors specific to fibroblasts is not provided in the sources.
FibroBLAST — builds the matrix (nucleus is light, ER is robust), FibroCYTE — rests (nucleus is dense, few processes), FibroCLAST — destroys (many lysosomes), MyofibroBLAST — pulls the wound edges together.
Class and exam questions
Why does a fibroblast nucleus appear light and a fibrocyte nucleus dark under the microscope?
The pale nucleus of a fibroblast is due to the predominance of euchromatin, indicating active gene transcription for protein synthesis. In a fibrocyte, synthesis has nearly ceased and heterochromatinization occurs, making the nucleus dense and dark.
Are mature fibroblasts and fibrocytes capable of mitosis?
No, the capacity for division is lost in both differentiated (mature) fibroblasts and fibrocytes. Only poorly specialized precursors possess high mitotic activity.
How do fibroblasts move within tissue?
They use amoeboid movement similar to leukocytes: they extend a pseudopodium, flow forward, and retract the rear end. The protein fibronectin is used to adhere to fibers, and the contraction of actin and myosin drives the movement itself.
What is the dual function of myofibroblasts?
During wound healing, they act both as builders—producing new matrix components via a well-developed ER—and as muscle cells, contracting microfilaments to bring wound edges closer together.
What else to study on this topic
- Ultrastructure and biochemistry of collagen synthesis in the rough ER of fibroblasts.
- Molecular interaction of actin and myosin during pseudopodium formation.
- The role of fibronectin in cell adhesion to extracellular matrix components.
- Mechanisms of extracellular hydrolysis and phagocytosis in fibroclasts during uterine involution.
- The process of nuclear heterochromatinization during the differentiation of a fibroblast into a fibrocyte.
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