Celina Bedini

The ultrastructure of ciliary sensory cells in two turbellaria acoela

Abstract. In Convoluta psammophila and Mecynostomum sp. it has been possible to distinguish two types of epithelial receptors. The first type possesses a dendrite protruding from the epithelial surface and is supplied with a long cilium and a large rootlet. The second type has a dendrite supplied with a central kinocilium surrounded by a crown of stereocilia. One particular structure, situated under the kinocilium and interpreted as a transformed rootlet, is connected to two or three stereocilia located inside the crown. The orientation of the inner stereocilia is similar to that of the pair of central tubules of the kinocilium. Morphological analogies and the functional role of these findings are discussed.

Fine structural observations on the ciliary receptors in the epidermis of three otoplanid species (Turbellaria proseriata)

In Notocaryoturbella bigermaria, Otoplana truncaspina and Paroto-planella heterorhabditica three types of epidermal receptors are recognized. Type I: with a single cilium running in a duct, piercing the distal dendrite process of the receptor. The internal wall of the dendrite process has eight ridges with longitudinal filaments lying inside them. The ciliary basal body lacks a longitudinal rootlet but is encircled by a thin annular formation. Type II: with a single (A) or several (B) cilia which protrude from the outer epithelial surface and are provided with a large and striped rootlet. Both types are considered as mechanoreceptors. Type III: with two or more short and stumpy cilia devoid of rootlets and displaying the usual 9 + 2 pattern in the proximal part only. They are considered as chemoreceptors.

The ultrastructure of the eyes in larval and adult polyclads (Turbellaria)

Examination of the larvae of Thysanozoon brocchii and Stylochus mediterraneus shows that they have both epidermal and cerebral eyes, while the young worms of Notoplana alcinoi have only cerebral eyes. A description is given of the ultrastructure of both kinds of eyes. The epidermal eye consists of one cup-shaped pigmented cell, whose cavity is filled with lamellae of ciliary origin. A small covering cell is located over the cup-opening. The cerebral eye is made up of three cells: one pigmented cell with ciliary projections and two rhabdomeric-type photoreceptor cells. The cerebral eye in the adult is formed of a pigmented cup without cilia and at least three rhabdomeric-type photoreceptors. A number of remarks of a morpho-onthological nature are presented.

Fine structural changes induced by circadian light—dark cycles in photoreceptors of Dalyelliidae (Turbellaria: Rhabdocoela)

The circadian light—dark cycles in the dalyelliid flatworm eyes induce two changes of particular interest. One involves the metabolic membrane system, its turnover, and the localization and appearance of mitochondria and glycogen; the other is connected with retinomotor response. In the second process the dark-adapted eye presents an empty eye cup. The microvillar border of the photoreceptors and the “lens” projections come out of the cup and the cell bodies are displaced laterally with respect to that cup. An attempt was made to elucidate the development within time of the retinomotor response. The plot of the ratio of empty area/microvillar area in median sections of several eyes shows a long-lasting dark-adaptation, completed in 6–9 hr and a short-term light-adaptation completed 5–10 min after light onset. Some hypotheses on the regulation of this response are discussed.

Electron microscopic study of larval eye development in Turbellaria Polycladida

The appearance and development of the embryonic and larval eyes of the polyclad turbellarian Stylochus mediterraneus were studied. In the embryo, the left epidermal eye appears first. Subsequently, the right epidermal eye appears, and within hours it sinks into the parenchyma and turns into a cerebral eye. Newly hatched Gottes larvae possess both the left epidermal and the right cerebral eye. Three days after hatching, an incomplete eye appears adjacent to the left epidermal eye. The left cerebral eye then originates from this incomplete eye as it sinks into the parenchyma. This third eye is believed to originate through a process of induction.

The ultrastructure of the sense organs of some turbellaria rhabdocoela. I. The eyes of Polycystis naegelii Klliker (Eukalyptorhynchia Polycystididae)

SummaryEach pigment-cup eye of Polycystis naegelii consists of two retinal clubs and a single pigmented cell. The latter is divided into two cavities by a septum. Under bright illumination the photoreceptor process appears as a disk containing membranous laminar whorls; under faint illumination the latter are replaced by numerous straight, closely packed, microvilli. This morphological variation is correlated with the intensity of the photoreceptors exposure to light. The lenticular structures described by previous light microscopists have not been observed.

Nuclear alterations in a baculovirus-like infection of midgut epithelial cells in the stick insect, Bacillus rossius.

Abstract In the epithelial cell nuclei of the posterior part of the midgut of the stick insect, Bacillus rossius, a symptomless virus infection is most commonly found. The virus particles consist of rod-shaped DNP nucleocapsids singly enveloped by a membrane and often packed in a pseudocristalline pattern along their major axis; occlusion bodies of any kind are not formed. On the basis of size, structural characteristics, and localization, the virus is thought to be a baculovirus-like virus. The viral infection, which is asynchronous in different nuclei, causes the formation of electron-light virogenic areas, the swelling of the nucleus, the nuclear chromatin lysis, the production of highly variable numbers of virus particles and, eventually, the extrusion of the whole infected nucleus into the gut lumen. Cellular DNA synthesis does not seem to be stimulated by the virus, RNA synthesis is apparently maintained as long as some cellular chromatin is present in the infected nucleus. Nucleolar segregation invariably occurs at late stages of infection, while major cytoplamic alterations have not been noticed until nuclear elimination and cell death occur.

Ultrastructural aspects of nervous-system and statocyst morphogenesis during embryonic development of Convoluta psammophila (Turbellaria, Acoela)

The notion that statocysts originated from an infolding of ectoderm lined by ciliated sensory cells has been challenged with evidence of ‘capsule’-limited, non-ciliary statocysts in several independent phyla. Statocysts in turbellarians primitively lack cilia and are embedded within or closely adjoined to the cerebral ganglion; they are likely to be derived from nervous tissue. We investigated the development of the simple statocyst in an acoel turbellarian, a statocyst consisting of three cells. Observations of serial TEM sections of embryos at different stages of development support the hypothesis of an inner (non-epithelial) origin of the statocyst. First, a three-cell complex is delimited by a basal lamina; it then undergoes cavitation by swelling, autophagy, and fluid secretion. The statocyst becomes discernible within the precursor ganglion cells while they still contain yolk inclusions. The two outer (parietal) cells, enclosed together by a 10-nm-thick basal lamina, arrange themselves in an ovoid of about 10 µm diameter and surround the inner statolith-forming cell. The statolith is formed later within vacuoles of the statolith-forming cell.

Fine structure and axonal organization in the buccal ganglia nerves ofAplysia (Mollusca, Gastropoda)

SummaryThe morphology of theAplysia buccal nerves and connectives has been studied by electron microscopic analysis. In these nerves the fine structure of the elements (connective sheath, glia, axons and their vesicular and cytoplasmatic content) is similar to that of other molluscan nerves. Some features seem to be comparable to other invertebrate groups such as Crustacea and Annelida. The axons have been divided into four classes on the basis of their calibre, and each type has been counted in all the nerves. The number of axons relating to identified buccal neurons is discussed. Finally, some speculations about relationships between buccal ganglia and peripheral regions connected by buccal nerves are proposed.

Is GABA present in the nervous system of acoel plathelminthes? An electron immunocytochemical study

Abstract Our immunocytochemical investigation positively identified for the first time at the ultrastructural level the presence of GABA‐ir neurones and terminals in the nervous system of a plathelminth, the acoel Symsagittifera psammophila. The gold particles are localized on clear vesicles (about 30–60 nm wide) of somata and terminals prevalently distributed in the neuropile area near the statocyst. GABA‐ir vesicles are also present at neuromuscular junctions. The significance of this finding in a lower invertebrate is discussed.