The lack of a structured blood-brain barrier in the onychophoran Peripatus acacioi |
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Authors: | Nancy J Lane and Sylvia S Campiglia |
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Institution: | (1) AFRC Unit of Insect Neurophysiology and Pharmacology, Department of Zoology, University of Cambridge, Downing Street, CB2 3EJ Cambridge, UK;(2) Present address: Dept. de Fisiologia, Institute de Biociências, Universidade de S?o Paulo, Caixa Postal 11461, S?o Paulo, SP, Brazil |
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Abstract: | Summary Onychophorans are ‘living fossils’ frequently purported to have evolved from the same ancestor as the arthropods and annelids.
In the CNS ofPeripatus acacioi, beneath an outer acellular neural lamella, glial cells ensheath the cerebral ganglion and the nerve cords. These glial cells
are, however, attenuated and rather few in number and, although they interdigitate with one another, they seem to lack intercellular
junctions. Exogenous tracers penetrate between them and into the underlying neuropile, suggesting that there is no structural
blood-brain barrier. Throughout the nervous tissue, extracellular spaces occur which contain banded collagen fibrils embedded
in a matrix material. Thin glial cell processes, characterized by dense filaments, surround these regions and frequently form
hemi-desmosomes with the extracellular matrix. The peripheral nerve cell bodies have a range of diameters; some have the characteristics
of neurosecretory neurons. Granules in such neurons are produced by the Golgi saccules and associated fenestrated membranes
which also possess many coated vesicles. Comparable granules are also found in axonal tracts, but no distinct peripheral neurohaemal
areas have been found. Lysosomes are common in the nerve cell bodies and are frequently in the form of multivesicular bodies
or large phagocytic vacuoles. Beneath the outer nerve cells lie many tracheae, arranged as a ring around the central neuropile
which consists of glial processes, extracellular matrix, axons and nerve terminals. These nerve terminals occur throughout
the central neuropile and are characterized by dense pyramidal presynaptic specializations and postsynaptic subsurface cisternae.
The nervous system ofPeripatus is relatively simple in its organization, in the lack of glial intercellular junctions and in the ready accessibility of
substances from the external milieu. |
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