Cell signaling occurs between cells of a single organism. However, cell signaling
may also occur between cells of two different organisms. The ability of cells to
perceive and correctly respond to their microenvironment is the basis of
development, tissue repair, and immunity as well as normal tissue homeostasis.
It is a type of cell-cell or cell-extracellular matrix signaling in multicellular
organisms that requires close contact. There are three types of contact-
- A membrane ligand (protein, oligosaccharide, lipid) and a membrane protein of
two adjacent cells interact
- A communicating junction links the intracellular compartments of two adjacent
cells, allowing transit of relatively small molecules
- An extracellular matrix glycoprotein and a membrane protein interact
In cell-cell signaling, a cell places a specific ligand on the surface of its membrane,
and subsequently another cell can bind it with an appropriate cell surface
receptor or cell adhesion molecule. The notch signaling mechanism is an
example of this, in which two adjacent cells must make physical contact in order
to communicate. This requirement for direct contact allows for very precise
control of cell differentiation during embryonic development.
The extracellular matrix is composed of glycoproteins (proteins and
glycosaminoglycan) produced by the organism's cells. They are secreted not only
to build a supportive structure but also to provide critical information on the
immediate environment to nearby cells. Indeed, the cells can themselves interact
by contact with extracellular matrix molecules and as such, this can be
considered an indirect cell / cell communication.Cells use mainly the receptor
integrin to interact with ECM proteins. This signaling can influence the cell cycle
and cellular differentiation.
Many cell signals are carried by molecules that are released by one cell and move
to make contact with another cell. Endocrine signals are called hormones.
Hormones are produced by endocrine cells and they travel through the blood to
reach all parts of the body. Specificity of signaling can be controlled if only some
cells can respond to a particular hormone.
Synaptic signaling is similar to paracrine signaling but there is a special structure
called the synapse between the cell originating and the cell receiving the signal.
Synaptic signaling only occurs between cells with the synapse; for example
between a neuron and the muscle that is controlled by neural activity. Paracrine Signaling:
Paracrine signaling is a form of cell-cell communication in which a cell produces
a signal to induce changes in nearby cells, altering the behavior or differentiation
of those cells. Signaling molecules known as paracrine factors diffuse over a
relatively short distance (local distance), as opposed to endocrine factors
(hormones which travel longer distances via the circulatory system), and
juxtacrine interactions (autocrine signaling). Cells that produce paracrine factors
secrete them into the immediate extracellular environment. Factors then travel
to nearby cells in which the gradient of factor received determines the outcome.
In order for paracrine factors to successfully induce a response in the receiving
cell, that cell must have the appropriate receptors available on the cell
membrane to receive the signals, also known as being competent. Additionally,
the responding cell must also have the ability to be mechanistically induced.
The highly conserved receptors and pathw