Ontogenesis of the Nervous System in Cephalopods
Our knowledge on the ontogenisis of the nervous system in cephalopods is rather fragmentary. Many major questions such as the precise origin of the head ganglia from the (neur–)ectoderm in early embryogenesis, the chronology of ganglia “assemblage”, the developmental events in nerve cell differentiation and the “overlap” of simultaneous growth and functioning of the brain are essentially still open. However, thanks to histological studies made on developing embryos of a decapodan (Loligo vulgaris) and and octopodan species (Octopus vulgaris) the general course of the nervous system formation in cephalopods can be recognised. Occasional observations on ganglion formation are found dispersed in the literature. The differentiation of the giant fibre system has been studied also and the first experimental approach for studying neurogenesis has been made. So far, hardly any physiological work has been done on cephalopod embryos. Some insight into the postembryonic development of the cephalopod brain is also available. (In contrast to the few studies available on embryos, an important literature exists dealing with descriptive and experimental observations made on all parts of the nervous system of adults).
In a squid hatchling all the central ganglia (cerebral, pedal visceral, peduncular, olfactory, basal and optic ganglia) are present and form the supra- and suboesophageal ganglia mass (=head). Except for the optic ganglia, they are divided into varous subunits, the lobes. The giant fibre system is distinct by large nuclei and its differentiation state clearly precedes that of the other parts of the nervous system. Also, most of the peripheral ganglia (brachial, buccal, subradular, gastric and stellate) are present.
The morphological situation found in hatchlings (resembling essentially that of the adults) is reached during embryogenesis from about stage VII onwards. The period until stage XII is the phase of actual ganglion formation. From stage XIII until hatching stage XX, the final topology of the ganglia and their relations are established and the lobular and cellular differentiation progresses. Based on the different results given by various authors, on personal communications from colleagues and on personal observations, the crucial developmental steps in the formation of the nervous system in decapodan and octopodan cephalopods are reviewed.
KeywordsCerebral Ganglion Stellate Ganglion Giant Axon Optic Vesicle Pedal Ganglion
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