Cellto Cell Signalling During Sea Urchin Development

The early development of sea urchins has been thoroughly studied since the beginning of the 20th century. The great challenge of understanding development has been achieved thanks to the particular characteristics of the model involving cell signalling: echinoderm eggs are about 80-100 |m in diameter, and may be observed with the aid of a simple stereomicroscope. Each stage of the development, including fertilisation, occurs outside the maternal body, the eggs and embryos are transparent and all the cell dynamics can be followed in vivo. During fertilisation and egg activation and the first developmental events, the cortical reaction responsible for the block to polyspermy is revealed by a dramatic elevation of the envelope, called the fertilisation layer (Fig. 1; see Giudice 1973,1986 for reviews). Moreover, a large number of eggs are released for each spawning, and development proceeds synchronously up to metamorphosis.

For this reason, sea urchin development represents one of the few models selected by developmental biologists for both basic and applied research. The advantage of this model is its simple organisation, making it easy to follow the complex cell-to-cell interactions that lead to development with non-invasive methods which in other animal models are possible only in cultured cell lines.

The development of echinoderms, and in particular of sea urchins, has been studied at each stage, including fertilisation. The most significant dis-

Fig. 1. Paracentrotus lividus unfertilised and fertilised eggs. Fertilised eggs show the elevated fertilisation layer (arrows)

coveries on this subject have been possible thanks to this model: for instance, most of the electric events responsible for fertilisation were first studied during early sea urchin development (see Epel 1975,1980 for reviews). In a recent paper, Quiao et al. (2003) have shown that sea urchin developmental events are exportable to mammalian development.

In recent years, there has been increasing awareness concerning the prominent role of cell-to-cell communication in developmental events: in particular, neurotransmitter system molecules were found to have a relevant role in modulating cell-to-cell interaction mediated by ion fluxes or ion intracellular changes. Buznikov and colleagues (since the 1970s: Buznikov et al. 1972,1996, 2001a; Buznikov 1980, 1990; Quiao et al. 2003) have detailed the history of classic neurotransmitter involvement (biogenic amines, acetylcholine and GABA) in early developmental events from segmentation to the formation of highly specialised structures, and have adapted the discoveries obtained from sea urchin embryos to about every other animal model (Buznikov 1990; Buznikov et al. 1996; Quiao et al. 2003). Ryberg and colleagues (Gustafson et al. 1972; Ryberg 1973,1974,1979) used the localisation of acetylcholinesterase activity (AChE, E.C. 3.1.1.7., the lytic enzyme of the neurotransmitter acetyl-choline) as a marker to investigate cholinergic neurodevelopment, and found a diffuse net of neuro-epithelial cells, not organised in a real nervous system, while Nakajima (1986, 1988; Nakajima et al. 1993) identified a system of organised catecholaminergic ganglia. The presence and role of a serotonergic system were studied by Thorndyke and colleagues (Thorndyke et al. 1992; Beer et al. 2001). Professor Drews' group (Drews 1975) localised cholin-esterase (ChE) in sea urchin mesenchyme cells undergoing morphogenetic movements, and compared this function with the one found in high-vertebrate developing structures. Following his discoveries, some authors interpreted the presence of AChE in cells and tissues as a "symptom" of cell migration (Weinberger et al. 1984).

Professor Minganti's group focused attention on the role of cholinergic neurotransmitter system molecules in cell-to-cell communication mediated by ion fluxes, and confirmed the exportability of the results to other animal models as well, including chordates and high vertebrates (Minganti et al. 1981). We have followed Minganti's line of research with studies on the development of the Mediterranean sea urchin, Paracentrotus lividus, from fertilisation up to metamorphosis. Cholinergic signalling system molecules have been found at each developmental stage, playing different roles according to the temporal windows and the degree of differentiation, including the neuromuscular function in the rudiment and in the juvenile (Vidal et al. 1993; Falugi et al. 1999; 2002), localised in radial muscles, in the tube feet and at the basis of the spines (Fig. 2).

Here, we present a summary of studies on fertilisation and early cell cycles, some conducted by our group, along with information from some of the outstanding papers available in the literature.

Fig. 2A-E. Paracentrotus lividus larva 18 days old, competent for metamorphosis. A Section of the larva containing a sagittal section of the rudiment. B Section of the larva containing an oblique section of the rudiment. Histochemical reaction to acetylcholinesterase, revealed by dark precipitation. C, D Co-localisation of formaldehyde-induced fluorescence suggesting presence of biogenic amines and AChE (dark staining) at basis of forming spines. E AChE reaction in the radial mouth muscles. musc Muscles under the forming spines of the rudiment (rudim); mrm mouth radial muscle; Sp b spine bases. Resin section, 5 |m thick; bar 100 |m. (Karnovsky and Roots 1964)

Fig. 2A-E. Paracentrotus lividus larva 18 days old, competent for metamorphosis. A Section of the larva containing a sagittal section of the rudiment. B Section of the larva containing an oblique section of the rudiment. Histochemical reaction to acetylcholinesterase, revealed by dark precipitation. C, D Co-localisation of formaldehyde-induced fluorescence suggesting presence of biogenic amines and AChE (dark staining) at basis of forming spines. E AChE reaction in the radial mouth muscles. musc Muscles under the forming spines of the rudiment (rudim); mrm mouth radial muscle; Sp b spine bases. Resin section, 5 |m thick; bar 100 |m. (Karnovsky and Roots 1964)

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