Biology Reference
In-Depth Information
INTRODUCTION
Why would anyone interested in the nervous system of nematodes
work on Ascaris? There are a number of disadvantages that could
undermine its selection as a research model: (1) due to the success of
anthelmintic treatment of pigs, Ascaris suum, the most common species in
domestic animals, is increasingly hard to find in host small intestines at
slaughterhouses; (2) it is difficult to maintain for more than a few days in
good physiological condition in the laboratory; (3) it does not provide
a tractable genetic system; (4) it smells bad; and (5) it exudes a powerful
allergen that can lead to debilitating asthma. The property that trumps all
of these disadvantages is size. Adult female A. suum can be up to 35 cm in
length. Correspondingly, the neurons are also large, with cell bodies
ranging from 5 to 60
m
m in diameter, features that have made possible the
electrophysiological study of neurons and their synapses by using sharp
microelectrodes. In addition, large neuronal size means that the dissection
of single identified neurons is relatively easy, such that the biochemistry
of single neurons is possible.
At the turn of the 19th/20th centuries, Richard Hesse
1
and Richard
Goldschmidt
2
used light microscopy to investigate and to generate
anatomical reconstructions of the neuronal tissue of the parasitic nema-
tode Ascaris (as was standard practice until the mid-1970s, these worms
were designated Ascaris lumbricoides even though they were recovered
from host pigs
they should now be referred to as Ascaris suum) (see
Chapter 10) providing the earliest data on its organization in nematodes.
Even then, the relative simplicity and consistent arrangement of the
nematode nervous system between individual specimens of Ascaris was
noted. Around 100 years later White et al. published their transmission
electron microscopy-based reconstructions of the ventral nerve cord
3
and
entire nervous system
4
of Caenorhabditis elegans, providing the first
opportunities for meaningful comparisons between distinct nematode
species and elevating the appeal of this free-living nematode as a model
organism for research.
The technical advantages offered by the size of Ascaris meant that much
of the early electrophysiological and pharmacological understanding of
nematode nerve-muscle function was derived from research on this
genus. Classical work on the electrophysiology of Ascaris muscle was
pioneered by del Castillo and colleagues.
5,6
Importantly, Chitwood
and Chitwood
7
noted the structural conservation in the nervous systems
of many nematodes, including Ascaris. White et al.
3
noted the close
similarities in the structure of the muscles, and of the nerves that
innervate them, between both C. elegans and Ascaris. Further work on
the nervous system of A. suum provided more detailed data on its struc-
ture and organization and facilitated more meaningful comparisons.
8,9
e
