Download Paracrines and Autocrines Are Chemical Signals Distribute by

Paracrines and Autocrines Are Chemical Signals Distribute by Diffusion
Several important groups of molecules act as paracrines. Neuromodulator are
paracrines and autocrines secreted by neurons. Cytokines are regulatory
peptides that usually act close to the site where they are secreted.
Cytokines act as both Local and Long-Distance Signals
Cytokines are the most recently identified family of communication molecules.
Initially the term cytokine referred only to proteins that modulate immune
response, but in the past few years it has been broadened to include a variety
of regulatory peptides. All nucleated cells synthesize and secrete cytokines in
response to stimuli. Cytokines control cell development, differentiation, and the
immune response. In development and differentiation, cytokines function as
autocrines or paracrines. In stress and inflammation, cytokines act on relatively
distant targets and are transported through the circulation just as hormones
are.
How do cytokines differ from hormones? In general, they act on a broader
spectrum of target cells. In addition, cytokines are not produced by specialized
glands and are made on demand. In contrast, most protein or peptide
hormones are made in advance and stored in hte endocrine cell until needed.
However, the distinction between cytokines and hormones is sometimes blurry.
For example, erythropoietin, the molecule that controls synthesis of red blood
cells, is by tradition considered a hormone but functionally fits the definition of
a cytokine.