Purinergic signaling in embryonic and stem cell development. 
Nucleotides are of crucial importance as carriers of energy in all organisms.  However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted.  Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals.  Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells.  Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system.  In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells.  Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors.  Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.