Abstract
The taxonomic location of the Onychophora has been controversial because of their phenotypic and genotypic characteristics, related to both annelids and arthropods. We analyzed the ultrastructure of the neurons and their synapses in the cephalic ganglion of a poorly known invertebrate, the velvet worm Peripatus sedgwicki, from the mountainous region of El Valle, Mérida, Venezuela. Cephalic ganglia were dissected, fixed and processed for transmission electron microscopy. The animal has a high degree of neurobiological development, as evidenced by the presence of asymmetric (excitatory) and symmetric (inhibitory) synapses, as well as the existence of glial cell processes in a wide neuropile zone. The postsynaptic terminals were seen to contain subsynaptic cisterns formed by membranes of smooth endoplasmic reticulum beneath the postsynaptic density, whereas the presynaptic terminal showed numerous electron transparent synaptic vesicles. From the neurophylogenetic perspectives, the ultrastructural characteristics of the central nervous tissue of the Onychophora show important evolutionary acquirements, such as the presence of both excitatory and inhibitory synapses, indicating functional synaptic transmission, and the appearance of mature glial cells.
Highlights
An ongoing study of great interest in our laboratory is focused on the investigation of the nervous tissue natural history and, in particular, the occurrence of primitive morphological characteristics of nerve cells that contributed to the increasing complexity of nervous tissue in higher phyla of multicellular organisms
The taxonomic location of the phylum Onychophora, know as “velvet worms”, has been controversial because of its phenotypic and genotypic characteristics that are related to both annelids and arthropods, making it difficult to locate this phylum within the zoological scale (Hickman et al 1994, Strausfeld 1998, Schmidt-Rhaes et al 1998, Marotta and Ruhberg 2004)
The goal of the present study was to analyze the ultrastructure of the neurons and their synapses in the cephalic ganglion of this Venezuelan species in order to investigate the evolutionary acquisition of nervous tissue, differential characteristics indicative of their neurological development
Summary
An ongoing study of great interest in our laboratory is focused on the investigation of the nervous tissue natural history and, in particular, the occurrence of primitive morphological characteristics of nerve cells that contributed to the increasing complexity of nervous tissue in higher phyla of multicellular organisms. Onychophorans have a cylindrical and unsegmented body, with a pair of antennae on its anterior end, one eye at the base of each antenna, and a ventraly located mouth with a well-developed jaw, which represents a modification of the anterior region of the body This invertebrate shows non-articulated legs, varying in number from 14 to 43 pairs, which remind of the Polychaetus parapodia. Onychophorans are predators that eat smaller invertebrates Their circulatory system is similar to that of the arthropods, and their respiratory system is formed by windpipes and spiracles (Hickman et al 1994, Monge-Nájera and Lourenço 1995). The sexes are always separate; there are oviparous, ovoviviparous and viviparous (Walker and Campligia 1998)
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