A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the
structural framework for animal tissues, and plays a critical role in wound healing.
Fibroblasts are the most common cells of connective tissue in animals.
Background information Fibroblasts and fibrocytes are two states
of the same cells, the former being the activated state, the latter the less active state, concerned
with maintenance and tissue metabolism.
Currently, there is a tendency to call both forms fibroblasts.
The suffix "blast" is used in cellular biology to denote a stem cell or a cell in an activated
state of metabolism.
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.
This remarkable behavior may lead to discomfort in the rare event that they stagnate there
excessively.
Embryologic origin The main function of fibroblasts is to maintain
the structural integrity of connective tissues 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.
Like other cells of connective tissue, fibroblasts are derived from primitive mesenchyme.
Thus they 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.
Conversely, fibroblasts in some situations may give rise to epithelia by undergoing a
mesenchymal to epithelial transition and organizing into a condensed, polarized, laterally connected
true epithelial sheet.
This process is seen in many developmental situations, as well as in wound healing and
tumorigenesis.
Structure and function
Fibroblasts have a branched cytoplasm surrounding an elliptical, speckled nucleus having two
or more 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.
Although disjointed and scattered when they have to cover a large space, fibroblasts,
when crowded, often locally align in parallel clusters.
Fibroblasts make collagens, glycosaminoglycans, reticular and elastic fibers, glycoproteins
found in the extracellular matrix, and cytokine TSLP.
Growing individuals' fibroblasts are dividing and synthesizing ground substance.
Tissue damage stimulates fibrocytes and induces the mitosis of fibroblasts.
Unlike the epithelial cells lining the body structures, fibroblasts do not form flat monolayers
and are not restricted by a polarizing attachment to a basal lamina on one side, although they
may contribute to basal lamina components in some situations.
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.
The life span of a fibroblast, as measured in chick embryos, is 57 ± 3 days.
Secondary actions Mouse embryonic fibroblasts are often used
as "feeder cells" in human embryonic stem cell research.
However, many researchers are gradually phasing out MEFs in favor of culture media with precisely
defined ingredients of exclusively human derivation.
Further, the difficulty of exclusively using 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.
See also 3T3 cells
Fibrocartilage callus Fibrous connective tissue
References
External links UIUC Histology Subject 240
MedEd at Loyola Histoctproper/hp3-15.html