[Phylogeny and evolution of hormone systems].
Classically hormones are defined as molecules that are secreted by endocrine glandular or neurosecretory cells into the blood stream and transported to their target tissue where they induce physiological processes at very low concentrations. Studies on the potential origin and the evolution of cell-to-cell communication systems suggest that exocrine pheromones (food signals and toxins) might have been the primitive bioregulatory molecules of unicellular organisms for chemical communication with each other and with the biosphere. The broad distribution and the structural diversity of pheromones suggests that these molecules and their receptors were predecessor modules of cell communication systems in metazoa. Neurosecretory cells, as we find them in Cnidarians, possibly served as basic modules for the evolution of neurohormonal systems of higher animals. Studies on genetic model organisms, such as Drosophila or the mouse, have demonstrated that chemical communication between neighbouring or more distant cells does not just involve endocrine and neurosecretory cells, but also unexpectedly tissues and organs such as the heart or the adipose tissue (e. g. the leptin signalling pathway). Comparative endocrinology could show that molecular components of hormonal systems represent signalling networks that are generally used during cellular communication processes and the differentiation of cell types during ontogenesis. Some of their functions are evolutionarily conserved, others not, as disscussion on steroid hormones and the prolactin signalling pathway will demonstrate.