The paper presents data on the effect of biogenic amine, serotonin, on morphogenetic processes in planarians Schmidtea mediterranea and Girardia tigrina(Turbellaria, Platyhelminthes). For the study, cut-off median and tail fragments of the planarian body were used, in which eye regeneration was observed. Photoreceptor recovery occurred from the 3rd to the 6th day of regeneration. In experimental specimens exposed to serotonin at a concentration of 0.1–1 μM, acceleration of the photoreceptor regeneration process was observed. The stimulating effect was observed at 4–5 days after surgery. Planaria (Turbellaria, Platyhelminthes) are free-living flatworms related to parasitic trematodes, cestodes and monogenges. In addition to the nervous, muscular, digestive, excretory and reproductive systems, they have a pair of simple eyes (photoreceptors), so they can distinguish the intensity and direction of the light flux and are oriented in space [1]. Planarium eyes are located on the dorsal surface of the head part of the body and consist of photoreceptor and pigment cells that form an "eye cup". Planaria can regenerate the whole organism from a small fragment. In the process of regeneration, the planarium can completely restore the head ganglion (brain), as well as other organs, including the eyes.Our task was to study the dynamics of photoreceptor differentiation in planarians Schmidtea mediterranea, and to investigate the possible morphogenetic function of serotonin in S. mediterranea and G. tigrina

Serotonin has been identified in all classes of parasitic and free-living Platyhelminthes. Its function in the body of Platyhelminthes remains poorly studied. In present work, the serotonin effect on morphogenetic processes in planarian Schmidtea mediterranea was studied for the first time. Pharyngeal regeneration was studied in Schmidtea mediterranea planaria. Pharynx is a relatively autonomous organ with well differentiated morphological structure and specialized function. The dissected planarian body fragments were able to regenerate the new pharynx and restore its function – food uptake and feeding. It was observed that in group of animals, the pharynx regeneration occurred from day 5 to day 10 after amputation. The regeneration time varied in different experimental series and seems to depend on the season of the experiment. The maximum number of animals restored their feeding response for 6–8 days after the intervention. Under the serotonin treatments (10–0.001 µМ), the acceleration of the restoration of a new pharynx function has been observed as compared to the control animals. The preliminary results exhibit the stimulatory action of serotonin on pharyngeal regeneration in tail fragments of S. mediterranea, thus, indicating the morphogenetic properties of serotonin.

Free-living flatworms – planarians posses fascinated regenerative ability amongst metazoan. They can regenerate whole organism from small body piece. Precise mechanisms of its regeneration remain poorly investigated. The results of the experiments on head blastema growth, which was formed in regenerating planarians Schmidtea mediterranea and Girardia tigrina after decapitation, have been presented. Planarians were operated with thin scalpel under dissecting microscope and left for regeneration. Animals from control group were kept in normal water. And to the experimental group of animals the solutions of the serotonin in concentration of 0.1 and 1 μM have been added. The experiments were repeated several times. With the methods of vital computer morphomethry the areas of blastemas were measured in regenerating control and experimental (treated with serotonin) planarians. The results were analyzed by t-Student statistical method. The observation indicated the acceleration of blastema growth under serotonin treatment (0.1–1 μM) in two planarian species – S. mediterraneaand G. tigrina. Stimulating effect of serotonin has attained the level of significance on day 2 or day 3 after the operation. Serotonin is known as regulator of neural development in vertebrate organisms. In present work the morphogenetic action of biogenic amine, serotonin, in lower metazoan is postulated.