Fibroblast

A fibroblast is a type of cell that synthesizes and maintains the extracellular matrix of many animal tissues. Fibroblasts provide a structural framework (stroma) for many tissues, and play a critical role in wound healing. They are the most common cells of connective tissue in animals.

The main function of fibroblasts is to maintain the structural integrity of connective tissue by continuously secreting precursors of the extracellular matrix. Fibroblasts secrete the precursors of all the components of the extracellular matrix, primarily the ground substance and a variety of fibers. The composition of the extracellular matrix determines the physical properties of connective tissues.

Fibroblasts are morphologically heterogeneous, with diverse appearances depending on their location and activity. Though morphologically inconspicuous, ectopically transplanted fibroblasts can often retain positional memory of the location and tissue context where they had previously resided, at least over a few generations.

Unlike the epithelial cells lining body structures, fibroblasts do not form flat monolayers and are not restricted by a polarizing attachment to a basal lamina on one side, though they may contribute to basal lamina components in some situation (eg subepithelial myofibroblsats in intestine may secrete the alpha2 chain carrying component of the laminin which is absent only in regions of follicle associated epithelia which lack the myofibroblast lining). Fibroblasts can also migrate slowly over substratum as individual cells, again in contrast to epithelial cells. While epithelial cells form the lining of body structures it is fibroblasts and related connective tissues which sculpt the "bulk" of an organism.

Embryologic Origin
Like other cells of connective tissue, fibroblasts are derived from primitive mesenchyme. They thus express the intermediate filament protein vimentin, a feature used as a marker to distinguish their mesodermal origin. However, this test is not specific as epithelial cells cultured in vitro on adherent substratum may also express vimentin after some time.

In certain situations epithelial cells can give rise to fibroblasts, a process called epithelial-mesenchymal transition (EMT).

Conversely, fibroblasts in some situations may give rise to epithelia by undergoing a mesenchymal to epithelial transition (MET) and organizing into a condensed, polarized, laterally connected true epithelial sheet. This process is seen in many developmental sitiation (eg. nephron and notocord development)

Structure and Function
Fibroblasts have a branched cytoplasm surrounding an elliptical, speckled nucleus having 1 or 2 nucleoli. Active fibroblasts can be recognized by their abundant rough endoplasmic reticulum. Inactive fibroblasts, which are also called fibrocytes, are smaller and spindle shaped. They have a reduced rough endoplasmic reticulum. Though disjointed and scattered when they have to cover a large space, fibroblasts when crowded often locally align parallelly in clusters.

Fibroblasts make collagens, glycosaminoglycans, reticular and elastic fibers, and glycoproteins found in the extracellular matrix. In growing individuals fibroblasts are dividing and synthesizing ground substance. Tissue damage stimulates fibrocytes and induces the mitosis of fibroblasts.

"Fibroblast-like cells proliferate, migrate, synthesize ECM macromolecules, secrete growth factors, and form cell projections of different morphologies that explore their environment and generate mechanical forces. None of these activities is unique to these cells: epithelial cells can engage in all of these behaviors. However, the difference between these two types of cells is a difference in scale. Fibroblast-like cells are “professionals” in movement, ECM production, and deposition, and in a multitude of filopodial activities that enable them to shape the ECM and pull other cells. All this they perform in a grand manner. Other cell types perform specific tasks (hormone production, contraction, production of electrical signals, etc.), and their morphology reflects their specific function. Fibroblast-like cells, in contrast, exhibit similar morphologies, even when they perform different behavioral activities in different tissues." -from The Sculpturing Role of Fibroblast-Like Cells in Morphogenesis

By Fanny Doljanski, in Perspectives in Biology and Medicine - Volume 47, Number 3, Summer 2004, pp. 339-356

Secondary actions
Fibroblasts can give rise to other cells, such as bone cells, fat cells, and smooth muscle cells. Note that all of these cells are of mesodermal origin.

The fact that fibroblasts easily proliferate makes them a popular cell type for cell cultures in biological research.

Notably, mouse embryonic fibroblasts (MEFs) are often used as "feeder cells" in human embryonic stem cell research. However, many researchers are gradually phasing out MEF's in favor of culture media with precisely defined ingredients of exclusively human derivation. Further, the difficulty of exclusively human derivation for media supplements is most often solved by the use of "defined media" where the supplements are synthetic and achieve the primary goal of eliminating the chance of contamination from derivative sources.