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Caenorhabditis elegans is a combination of Greek and Latin words meaning Caeno, recent; rhabditis, rod-like; elegans, elegant. C.elegans is a small rounded, unsegmented worm. Maupas (1900) was the first scientist to describe the worm from decomposing soil collected from Algeria (Fatt, 1961).The first classification was published by Osche (1952) and after by Dougherty (1955). They placed the worm in the subgenus Caenorhabditis, and generic status respectively. Recent systematics based on morphometric and molecular data placed the worm in the class of Scernentea, order of Rabditida, family Rabditidae (Kiotke et al. 2004, De ley & Blaxter, 2002.). There are two commonly wild type strains used in scientific community: Bergerac and Bristol strains. The Bergerac was originally isolated from soil sampled in Lion, France by Victor Nigon and L.N. Staniland collected the other, Bristol, from mushroom compost near Bristol, England. C. elegans is among eighteen Caenorhabditis species fully described in terms of their ecology (Kiontke et al. 2005).The worm was found as soil inhabitant in decomposing niches that are rich in nutrient where it feeds on bacteria and probably other microorganisms. In laboratory maintained environment , C.elegans can be grown on a number of bacteria species but Escherichia coli was chosen as standard culture food in petri dish due to its efficacy. The worms can also be grown in liquid cultures namely monoxenic and axenic culture. For laboratory efficacy and efficiency, worm is kept in freezing liquid nitrogen at -196 0c (Brenner, 1974). In natural ecosystem, C. elegans lives in association with other soil invertebrates with which it has different types of relation defined as phoresy and necronemy (Caswell-Chen et al., 2005; Kiontke and Sudhaus, 2006).
Life cycle. The two sexes are present in C.elegans: hermaphrodite worms that reproduce by either automic form or amphimixis and males. The sexes can be distinguished by morphological, anatomical and behavioural tests. The hermaphrodite animals are obviously bigger and female reproductive organs well developed exteriorly (vulva slit), and the tapered tail shape. The male body is slender with sex specific characteristics such copulatory organs, the sensory rays and fan. In addition, males show characteristic behaviour with its fast twisting movements. Despite these differences, hermaphrodite and male share some tissues, pharynx, excretory system and main body muscles (Nguyen et al., 1999, Emmons, 2005).
Hermaphrodite produces sperms that mature before the eggs, so that self-fertilization occurs in the early procreation. In this case, the number of fertilized eggs produced by hermaphrodite is about 300egges and then after the fertilization occurs by cross-fertization with males to produce up to 1000 oocytes. It was noticed that males are very rare in C.elegans population by spontaneous non-disjunction in the hermaphrodite germ line and the proportion increase up to 50% through mating.
C. elegans life cycle is temperature dependent. At optimal conditions 25oC, the complete life cycle is completeted in three days (Ledoux, M. 2005). The worm undergoes a development that takes four larval stages (L1, L2, and L3, L4 larvae) to reach the adulthood. The end of each stage is marked by a molt, but the normal development process may be interrupted under harsh environmental conditions resulting from high temperature, overcrowding and food shortage. In this case L2 larvae molts to produce a dauer, an alternative resistant L3 larvae form that resume life cycle when conditions favourable. Dauer is a non-feeding, has a thickened and modified body cuticle, the intestine is reduced, secretory-excretory gland is inactive and there is modification of sensory organs. Interestingly, dauers can remain viable for three months.
Anatomy. C.elegans has a transparent body 1.2mm length and 50Âµm width .The body shows a simplified shape that refers to all nematodes structure (Bird and Bird 1991). The body construction consists of two tubes separated by a fluid cavity containing functional organs. The outer tube was described as protective walls strengthened externally by a collagenous extracellular matrix (James M. et al. 2007). The inner one is an alimentary channel, from anterior to posterior, a sequence of parts: pharynx, intestine, and rectum (Fukushige T et al 1996). C.elegans ingests food (microorganism) by pumping using the pharyngeal muscles made up by two lobes and joining part to the intestine (pharyngeal-intestinal valve) that control the entree of food into the intestine (Susan E. Mango et al. 2007). The apical surfaces of the intestinal are mounted with brush structure -like cells, microvilli, that are believed to play a role in digestion (Leung et al. 1999). The intestinal contents are excreted to the outside via a rectal valve that connects the gut to the rectum and anus (McGhee, 2007, Sewell, S.T. et al. 2003).
Nervous system .In adult hermaphrodite C. elegans, a high proportion of its somatic nuclei build up its complex nervous system. In a total number of 959 cells, 302 have neuronal function and 56 neuronal support cells (Hobert, 2005). C.elegans nervous system comprises two subsystems: the somatic system with 282 neurons inserted in between hypodermis and the body wall muscles, the other, pharyngeal nervous system that count 20 nuclei and all in tied connection with their pharyngeal muscles. Pair of interneurons type connects the two neuronal systems (White et al., 1986). Neuronal classification based on specific anatomical features, defined 118 neuron classes (Hobert 2005). Based on the functional grouping, C.elegans neurons are assembled into four groups: motor neurons, sensory neurons, and interneuron. The two sexes have sensory dimorphism that is of two types in male: male-specific and sexually dimorphic. In male C.elegans, the sensory organs have no homology in the hermaphrodite and are formed by cell types generated only in the male. Sexual dimorphic sense organs contain at least some cells that have the same lineal origin in both sexes. They express distinct differentiated characteristics in the both sexes, reflected in cell morphology, and organization ((Emmons, 2005).
Reproduction. The reproductive system presents a sexual dimorphism between hermaphrodite worm and male. Hermaphrodite reproductive system is composed of somatic gonad, the germ line and the egg-laying apparatus. It consists of two symmetric branches folded into two U-shaped gonad arms fused into a common uterus by mean of spermtheca. Gonads develop from germ cells and the general organization is completed at L4 larvae stage (Newman et al., 1996). The male reproductive system arises from three types of cells: germ line, somatic cells and neuronal cells. The male gonad has three components: testis, seminal vesicle and vas deferens.
C.elegans as model organism. Brenner (1974) was the first scientist to suggest C.elegans as model organism for the study of genetics. He cited numerous technical advantages of working with C.elegans (small, simple, easy to grow, short life cycle, transparency of the body,). Considerable number of researchers followed the idea by using the worm in different research area. The completion of, C. elegans has opened many doors to scientists. C. elegans genome sequence revealed high similarities with other eukaryotes. It shares most of the biological systems, feeding, nervous system, muscle, reproductive, with advanced organism. It is easily handled in laboratory and it reproduce rapidly. Its transparent body makes every cell to be studied in living animal under microscope. This characteristic made the C. elegans an organism easy the study of gene expression patterns using the reporter genes green fluorescent protein (GFP) and ï¢GAL (Boulin, T. et al., 2006). It is easily transformed with additional and removal of genes (RNAi). At the end of its life, it shows signs of ageing and then be a good candidate for the study of ageing process in other animals. However, the nematode model is not without its experimental challenges. C. elegans cells and organs are small when compared to their mammalian counterparts and they are enclosed within a pressurized and relatively impermeable cuticle. This is quite challenging for in vivo electrophysiological analysis of channel function, particularly in neurons (Bianchi, L. and Driscoll, M., 2006).ï€ ï€®