Neurogenesis

C. elegans nervous system development.

C. elegans nervous system. Adapted from: Simon Fraser University, 2008. — Fig 1. *C. elegan*s nervous system. Pharynx (green), nerve ring (orange) and nervous system (pink, includes the nerve ring). Adapted from: Simon Fraser University, 2008.

Adult hermaphrodite C.elegans have 302 neurons, 50 glial from neuronal epithelial progenitors and 6 glial cells mesodermally derived. These numbers vary from the human brain, which consists of roughly equal numbers of neurons and glial cells (approximately 100 billion of each) (Oikonomou et al., 2011; Azevedo et al., 2009). The majority of neurons in C. elegans are localized in the head, where they are organized in the head ganglia, a cluster of neurons, to form the brain of the animal (Simon Fraser University 2008).

C. elegans neurogenesis during embryogenesis.

The nervous system begins at the 2-cell stage of C.elegans development, which comprises of two founder cells: AB and P1. The AB cell is the first cell that will be fated to produce neurons. Additionally at the 8-cell stage neurons are produced from the original P1 founder cell (Figure 2; Eisenmann, 2005; Altun, 2009). The ABp and ABa give rise to neurons and hypodermal cells, and neurons, hypodermis and anterior pharynx cells, respectively (Gilbert, 2000). This is strikingly different from human brain development, where the brain begins from the tip of a 3mm neural tube to eventually become a tissue of more than 100 billion neurons (Ackerman, 1992). 3 to 4-weeks post conception, the neural tube begins to close and the three brain regions (hindbrain, midbrain and forebrain) begins to take shape (Ackerman, 1992). C. elegans neurons develop later than in humans, when normalized to their gestation periods.

A great benefit of using C. elegans to study neurogenesis is the ability to directly trace the cell lineage of all cells, in particular of neurons. Neurons in C. elegans arise in a predetermined numbers and in a predetermined fashion. This differs from vertebrates where neurons arise in excess and then undergo a subsequent selection process before ending with the mature neurons found in adults (Fig 4).

Cell divisions and cell fates in early embryogenesis of C. elegans. Adapted from: Alberts et al., 2002. — Fig 2. Cell divisions and cell fates in early embryogenesis of *C. elegans.* Adapted from: Alberts et al., 2002.

C. elegans vs vertebrate neuron cell fate. Adapted from: Oikonomou et al., 2012. — Fig 4. *C. elegans* vs vertebrate neuron cell fate. Adapted from: Oikonomou et al., 2012.