Tissue Engineering - Introduction to Biomedical Engineering - page 286

FIGURE 6.8 The epithelial-mesenchymal relationship.

epithelial cells require constant interaction with an appropriate mesenchymal partner or
with matrix and soluble signals mimicking that relationship. Communication between
and within the two cell layers coordinates local cell activities and is effected by soluble
signals (autocrine, paracrine, and endocrine factors) working synergistically with extracel-
lular matrix.

The Soluble Signals
Dissolved growth factors, hormones, and small molecules are often characterized in the
following way:

￿ Autocrine factors are factors produced by cells that then act on those same cells.

￿ Paracrine factors are factors produced by cells that then act on neighboring cells in the

same tissue.

￿ Endocrine factors are factors produced by cells that are then carried through

the blood or lymphatic fluid to target cells in a distant tissue.

or systemic

There are multiple large families of factors that operate as autocrine and paracrine
signals. All of them typically can act as mitogens (i.e., elicit growth responses from cells)
or can drive differentiation (i.e., induce expression of specialized tissue-specific functions),
depending on the characteristics of the extracellular matrix associated with the cells. Some
of the most well studied of these factors include the insulin-like growth factors (IGF),
epidermal growth factors (EGF), fibroblast growth factors (FGF), colony stimulating factors
(CSF), platelet derived growth factors (PDGF), transforming growth factors (TGF), and
cytokines such as interleukins. It should be noted that these molecules are highly pleio-
tropic and can produce multiple effects on many cell types. The name “growth factor”
comes from the fact that most were initially identified by assays in which a growth response
was being examined.