I. M. Sheiman

The nervous system of Tricladida. II. Neuroanatomy ofDugesia tigrina (Paludicola, Dugesiidae): An immunocytochemical study

The nervous system (NS) ofDugesia tigrina has been studied by immunocytochemical double-staining, using the authentic flatworm neuropeptide, neuropeptide F (NPF), and serotonin (5-HT) on cryosections. This technique provides a precise morphological (descriptive) account of the NS. The results show that the central nervous system is shaped like a horseshoe. The brain is composed of two lateral lobes connected by three commissures, one antero-dorsal in front of the cerebral eyes and two, more ventral, behind the eyes. The pair of main nerve cords extend from the lateral lobes of the brain to the tail end of the worm. Cross sections reveal a very close contact between lateral branches from the main cords and the submuscular plexus. Thin cord-like lateral nerves are formed by longitudinal plexal fibres. No dorsal cords were observed. The patterns of immunoreactivity to NPF and 5-HT differ from each other in several respects. In the walls of gut diverticula only NPF immunoreactive (IR) cells and fibres were observed. Only NPF-immunoreactive cells occur in the parenchyma along dorso-ventral nerve fibres connecting the dorsal and ventral parts of the submuscular plexus. The number of 5-HT-immunoreactive cells associated with the main nerve cords (MCs) is greater than that of the NPF-immunoreactive cells, and the spongy structure of the MCs is more apparent following immunostaining for 5-HT. Thin 5-HT-immunoreactive fibres were observed in the subepithelial plexus, penetrating the basal lamina and innervating a rhabdite-free ventro-lateral sensory area along the body periphery. The correspondence between MCs in the lower flatworms (Catenulida and Macrostomida) and the Seriata (Tricladida and Proseriata) confirms the status of the MCs in flatworms as the most important and stable neuronal characteristic, and constitutes support for the hypothesized common origin of the MCs in flatworms.

Development of the nervous system in Dugesia tigrina during regeneration after fission and decapitation

Summary In the present study of Dugesia tigrina, the development of the nervous system is followed and compared during regeneration after fission and after decapitation. Immunocytochemistry was used, with antisera raised against the biogenic mine, 5-hydroxyhyptamine (5-HT) and the two neuropeptides, neuropeptide F (NPF), and FMRFamide. The results indicate that two processes are involved in the formation of the new cerebral ganglion. First, new processes sprouting from the original main longitudinal nerve cords bend transversely, indicating the position of the developing horseshoe-shaped anterior cerebral commissure. Then new nerve cells in front of the commissure differentiate from neoblasts and their growth cones fasciculate with the fibres from the old main longitudinal nerve cords. In the cerebral ganglion, 5-HT-IR cells appear before NPF-IR cells, in contrast to the pharynx where NPF-IR cells differentiate before the 5-HT-IR cells. In the peripheral nervous system, NPF-IR fibres and cells appear at a...

Relationship between musculature and nervous system in the regenerating pharynx in Girardia tigrina (Plathelminthes)

Summary The study shows that the regenerating pharynx of Girardia tigrina forms a simple and valuable model system for studies of pattern formation in the nervous system and its relationship to the developing muscles. Immunocytochemistry was used, with antisera raised against 5-HT and the native planarian neuropeptide GYIRFamide. We studied the sequential development of the innervation in the regenerating pharynx, and using TRITC-labelled phalloidin we followed the corresponding differentiation and maturation of pharynx musculature. The peptidergic and aminergic neuronal cell types develop according to different time schedules and different modes. Throughout the process, the GYIRFarnide-IR elements are in contact with the old parts of the nervous system, while the apical 5-HT-IR elements develop de novo. In the regenerating pharynx the GYIRFamide-IR nerves develop in a proximodistal direction. The first 5-HT-IR cell bodies appear in the tip of the pharynx and are symmetrically placed. They have no contact to the rest of the nervous system. From these cell bodies processes grow disto-proximally and fasciculate with tibres from the proximal part. A striking parallelism between the appearance of GYIRFamide-IR nerves and muscle fibres stained with TRITC-phalloidin was observed. The GYIRFamide-IR nerves cling to the muscle fibres. These results suggest that the contact between muscle Iibres and GYIRF-IR nerves is essential for the function of pharynx. The delayed appearance of 5-HT-IR nervous elements is discussed in terms of the influence of 5-HT on sprouting of nerve fibres and synapse formation. The development of the pharynx in tail parts after fission is compared with that after pharynx amputation. The faster rate observed for pharynx regenerating after amputation in comparison with that in regenerating tail parts indicates the importance of the remains of the old nervous system in the pharynx structure.

Morphogenesis in Planarians Dugesia tigrina

We carried out computer morphometry in regenerates of planarians Dugesia tigrina. The blastema growth was analyzed in fragments of planarians after their fission and after transverse transection at different body levels. The blastema was growing at a higher rate on tail fragments than on the head fragments and the growth rate was the higher, the closer the transection was to the head end. After fission, the blastema was growing at a slower rate than after transection in the fission zone. The growth of adjacent blastemas formed on both sides after fission or transection proceeded at different rates as a function of new body polarity.

Regeneration of planarians: Experimental object

We discuss the expediency of using invertebrates, such as flatworms and planarians, as experimental objects. Free-living planarian flatworms (phylum Platyhelminthes, class Turbellaria) are invertebrate animals in which a bilateral symmetry appears for the first time in evolution and organs and tissues form. As the highest ecological link of the food chain—predators—these animals are characterized by a set of behavioral reactions controlled by a differentiated central nervous system. Planarians have unsurpassed ability to regenerate lost or damaged body parts. Owing to the ease of their cultivation and their convenience for manipulations, these animals are used to study the influence of chemical and physical factors on the processes of life, growth, and reproduction. Currently, planarians are recognized as a model for biological research in the field of regeneration, stem cell biology, study of their proliferation and differentiation, as well as the regulatory mechanisms of morphogenetic processes. The genome of the planarian Schmidtea mediterranea was fully sequenced, which opened up the opportunity to work with this object at the molecular biological level. Furthermore, planarians are used in neurobiological and toxicological studies, in studying the evolutionary aspects of centralization of the nervous system, mechanisms of muscle contraction, and in the development of new antiparasitic drugs. This review aims to demonstrate the relevance and diversity of research conducted on simple biological objects—planarians—to a wider audience to show the historical continuity of these studies and their wide geographical distribution and to focus on the studies carried out in Russia, which, as a rule, are not included in the foreign reviews on planarian regeneration.

Multimodal effects of nearly complete geomagnetic field deprivation on fission of the planarian Dugesia tigrina

The rate of asexual reproduction of planarians (fission) did not change upon total deprivation of the geomagnetic field (induction ±5 nT), but was 1.5–2-fold higher than in the control upon attenuation to 100–300 nT; the stimulatory effect disappeared at 400–600 nT, was almost restituted at 0.8–1.5 μT, and zeroed again at 3 μT.

A behavioral study of the beetle Tenebrio molitor infected with cysticercoids of the rat tapeworm Hymenolepis diminuta.

The host–parasite relationship, Tenebrio molitor–Hymenolepis diminuta, was analyzed. The learning behavior of infected and uninfected (control) beetles in a T-maze was compared. The infected beetles moved much slower in the T-maze than the controls. The infected beetles reached the same level of learning as the controls. However, they needed more trials than the controls. The effect of the infection was already distinct after the first week and even higher after the second week. This indicates that the initial phase of infection caused stress in the beetles. Longer infection did not worsen their ability to learn. Thus, the parasites clearly changed the behavior of their intermediate host and probably made them more susceptible to their final host, the rat.

Asexual Reproduction of Planarians: Metric Studies

A relationship was studied between fission and restoration of body and its individual parts under different experimental conditions in planarians of the Dugesia tigrina asexual race. The body and its fragments were studied morphomterically. After fission, the growth of planarians demonstrated topographic differences. The separated tail fragments and postpharyngeal area, in which the zone of fission is formed, were growing at the highest rate. More active growth was also noted over the long body axis. Fission and growth were more active in isolated planarians, as compared to those kept in groups.

Weak influences of physical and chemical factors on the morphogenetic process (in Invertebrates)

Analysis is presented of long-term data obtained in investigation of the effects of weak influences on morphogenetic processes in invertebrates (regeneration of planarians Dugesia (Girardia) tigrina and postembryonic development of insects, the grain beetle Tenebrio molitor). Weak physical and chemical factors were used: electromagnetic radiation, constant, alternating, and combined magnetic fields, and low concentrations of solutions of neuropeptides. It is shown that these influences are characterized by instability and opposite directions (from stimulatory to inhibitory). The dependence of the effects on external factors and the course of internal processes was established.

Regeneration processes in various species of planarians

Blastema growth and functional maturation of the pharynx during regeneration in various planarian species were compared. The intensity of blastema growth was highest in Polycelis tenuis; the lowest, in Schmidtea mediterranea. In the sexual and asexual races of Girardia tigrina blastema growth differed inconsiderably. The function of the pharynx during the regeneration of caudal fragments lacking pharynx was manifested in G. tigrina in the usual amount of time, while in the regeneration of head fragments lacking pharynx, this function occured earlier. In other planarian species of the other two typed, the times of pharynx regeneration had no regular character and took longer compared to the same process in G. tigrina.