SEROTONIN NEURON STRUCTURES IN THE NERVOUS SYSTEM OF PLANARIANS SCHMIDTEA MEDITERRANEA (PLATYHELMINTHES)

Serotonin-immunopositive (IP) components were detected in the nervous system of planarians Schmidtea mediterranea using immunocytochemistry and fluorescence microscopy. Characteristics of serotonergic nervous system were detected by the immunocytochemical staining. The nervous system of the planarian S. mediterranea is represented by the arc-shaped head nerve ganglion located in the anterior region of the planarian; the inner part of the ganglion is formed by nerve cell processes; whereas neurocytons are situated on the ganglion surface. Well-stained ventral nerve cords connected by serotonin-IP commissures are extending from the ganglion caudally. A morphometric analysis of serotonergic nerve elements was performed on frozen serial sections of planarian tissues and showed that the thickness of the nerve ganglion stained by serotonin antibodies was about 110 µm, the size of serotonin neurons in the head ganglion varied from 9 to 26 µm, and the size of serotonergic neurons within the nerve plexus innervating the planarian body musculature was 7 to 13 µm. This information may be of evolutionary importance and will be useful for a comparative analysis of morphometric parameters of serotonergic neuron structures in free-living and parasitic flatworms.