Elena A. Kotikova

A commissural brain! The pattern of 5-HT immunoreactivity in Acoela (Plathelminthes)

Abstract In order to solve the question whether Plathelminthes belonging to the taxon Acoela have a brain and an orthogon of the common flatworm type, an immunocytochemical study of the pattern of serotonin (5-HT) in four species was performed. In all species the 5-HT immunoreactivity revealed no ganglionic cell mass typical for other Plathelminthes, only a symmetrical brain-like structure composed of commissural fibres associated with a few cell bodies. 5-HT immunoreactivity was detected in three to five pairs of longitudinal nerve cords, connected by an irregular network of immunoreactive transverse fibres. No regular orthogon was visualised. All the surface sensilla were strongly immunoreactive. The unique commissural brain and cordal nervous system found in Acoela support the view of a deep gap lying between Acoela and other Plathelminthes.

Rotifer nervous system visualized by FMRFamide and 5-HT immunocytochemistry and confocal laser scanning microscopy

We present the first results of immunocytochemical (ICC) observations on serotonin (5-HT) and FMRFamide (Phe-Met-Arg-Phe-NH2) immunoreactivity patterns in the rotifer nervous system investigated using a confocal laser scanning microscope (CLSM). Three species of rotifers are studied: Platyias patulus (Plationus patulus, Segers et al., 1993; Hydrobiologia 268: 1–8), Euchlanis dilatata; and Asplanchna herrickii. Independently from their systematic position, these species possess similar nerve structures. However, some differences were observed in the innervation of the corona, mastax, foot, and mostly in the pattern of the cerebral neurons. The general numbers of 5-HT-immunoreactive (IR) and FMRFamide-IR neurons are low (10–34), but constant for each species. The sizes of the neurons vary from 2, 5 to 10 µm. From 4 to 14 cerebral neurons lie at different levels and are arranged into an X- or a ring- or a curved arch shape. One or two pairs of neurons are localized along longitudinal nerve cords. Double staining of 5-HT and FMRFamide-IR elements shows no co-localization.

The nervous and muscular systems in the free-living flatworm Castrella truncata (Rhabdocoela): an immunocytochemical and phalloidin fluorescence study

In order to broaden the information about the organisation of the nervous system in the Plathelminthes, an immunocytochemical (ICC) study of the free-living flatworm Castrella truncata (Dalyellioida) has been performed. This is the first time a representative of the taxon Rhabdocoela has been studied with the ICC technique. Antibodies to 5-HT and FMRF-amide and confocal scanning laser microscope were used. 5-HT and FMRF-amide immunoreactivity was observed in the neuropile of the brain, in the cerebral neurones, in the three pairs of longitudinal nerve cords and the adjoining neurones, in the pharyngeal nerve ring and in twelve neurones associated with the pharynx. 5-HT and FMRF-amide immunoreactivity occurs in separate sets of neurones. Only FMRF-amide immunoreactivity was observed in a peculiar ellipse-shaped structure in the brain, and in the genital plexus. The type of orthogon is discussed. Staining of the muscular system with TRITC conjugated phalloidin revealed muscle patterns that have not been described previously.

Localization and neuroanatomy of catecholaminergic neurons in some rotifer species

We studied Dicranophorus sp., Platyias quadricornis (Ehrb.) and Rotaria tardigrada (Ehrb.). These rotifers, systematically distant from each other, show the same pattern of the catecholaminergic (CA-ergic) part of the nervous system. It is formed of a small (23–24), but steady number of neurons characteristic for each species. Three types of CA-ergic neurons are described. The sizes of neurons vary from to two to ten µm. The distribution of the brain neurons is correlated with body shape. Such a type of nervous system is topographically comparable to the concentrated orthogon of the flatworms.

Architectonics of the central nervous system of Acoela, Platyhelminthes, and Rotifera

Based on the literature and own data, consecutive stages of development of the central nervous system (CNS) in the lower Bilateria are considered-separation of brain from parenchyma, formation of its own envelopes, and development of the trunk and orthogonal nervous system. Results of histochemical (cholinergic and catecholaminergic) and immunocytochemical (5-HT-and FMRF-amid immunoreactive) studies of the CNS in representatives of Acoela, free living and parasitizing Platyhelminthes and Rotifera are considered. The comparative analysis makes it possible to describe development and complication of the initially primitive Bilateria pleux nervous system. A special attention will be paid to the Acoela phylogenesis, based on molecular-biology data and results of study of their nervous system.

Catecholaminergic neurons in the brain of rotifers

In 10 rotifer species from the subclasses Archeorotatoria (order Bdelloidea) and Eurotatoria (superorders Gnesiotrocha and Pseudotrocha) three patterns of catecholaminergic neurons are detected, namely: x-shaped, arch-shaped and ring-shaped. These brain complexes are developed independently and in a parallel fashion in different rotifer groups. The number of the brain catecholaminergic neurons varies from 6 to 11, constituting about 3–7% of the total number of the brain cells. The brain neuron pattern demonstrates a distinct bilateral symmetry.

Study of architectonics of rotifer musculature by the method of fluorescence with use of confocal microscopy

Musculature of two species of rotifers Testudinella patina (Testudinellidae) and Platyias patulus (Brachiomidae) was studied in confocal laser scanning microscope (CLSM) using fluorescent-labeled phalloidin. It includes cutaneous, visceral, and cutaneus-visceral musculature. The common pattern of structure of the cutaneous musculature is represented by postcoronal circular or transverse muscles and connected with them 2–3 pairs of retractors of the trunk, dorsolateral muscles (17-4), two pairs or bundles of lateral retractors of the corona, circular muscles of the foot, and 10-2 retractors of the foot. Visceral musculature includes muscles of the mastax of both kinds. Spiral-like muscle of cloaca of the T. patina and associated with it V-shaped one as well as strong dorsolateral retractors consisting of 6 longitudinal muscle bundles are typical of Testudinellidae only. Three pairs of cutaneus-visceral muscles bind the musculature of mastax with the body surface in T. patina. Differences in localization and thickness of some elements of musculature of these species are determined by morphological peculiarities of structure of the corona, mastax, and foot, as well as by the rotifer body shape.

Glyoxylic acid induced fluorescence in the nervous system of Gyratrix hermaphroditus (Kalyptorhynchia, Polycystididae)

Catecholamines (CAs) are found in the neuropile of the brain, in 3 pairs of longitudinal nerve cords, in the transverse ventral commissure, in anterior ventral and dorsal nerves, in two pharyngeal nerve rings and in 24 neurons in the nervous system of Gyratrix hermaphroditus. The CA distribution pattern is compared with those of other neuroactive substances. Homology of neurons in the family of Polycystididae and in Plathelminthes in general is discussed.