Brain Development

C. elegans head ganglia (brain).

Neurons are the basic unit of the nervous system, consisting of a cell body (soma), axon and dendrite, which relay chemical or electrical signals throughout the body. Unlike vertebrates, the structure of neurons in C. elegans is very simple as C. elegans neuron structure is mainly unbranched, in a unipolar and bipolar fashion, whereas vertebrates have highly branched, complex neurons (Fig 1).

Types of neurons. Adapted from: OpenStax Biology. — Fig 1. Types of neurons. *C.elegans* have mainly unipolar and bipolar neurons, unlike vertebrates which have mostly multipolar, highly branched neurons and a few pseudounipolar neurons. Adapted from: OpenStax Biology.

The head ganglia consist of the cell bodies of neurons responsible for sensing the external environment and relaying those signals through the rest of the body. These neurons have a single dendrite, which extends to the anterior tip of C. elegans nose, and an axon that extends to the ventral cord (not shown) and loops around in a half circle around the pharynx with a large axon bundle, known as the nerve ring (Fig 2; Simon Fraser University, 2008). The nerve ring houses the neuronal processes of the head ganglia neurons, which form synapses as they run alongside each other. Interestingly, neurons in the nerve ring have limited axon regeneration, whereas neurons outside of the nerve are able to regenerate post injury (Chiu et al., 2011).

C. elegans nervous system and zoomed in head ganglia (brain). Adapted from: Simon Fraser University, 2008 and Oikonomou et al., 2011. — Fig 2. C. elegans head ganglia (brain) region of the nervous system. A single neuron (orange) is depicted with surrounding glia cells (turquoise and purple). Adapted from: Simon Fraser University, 2008 and Oikonomou et al., 2011.

The neurons in the head of C. elegans accompanied by glia, which are neuronal supporting cells that aid neurons in processing and relaying signals. Similar to vertebrates, glia in C. elegans help organize and direct axon growth, and help form the nerve ring (Fig 2; Oikonomou et al., 2011). The sheath glia interact with the receptive endings of neurons and the nerve ring, and contribute more support near the posterior regions of the head ganglia. The socket glia have been less studied, but are also important in ensuring properly functioning neurons in the head ganglia.

Fig 3. Amphid neurons in C. elegans. Adapted from: Ortis et al., 2006.

Amphid neurons are the major sensing neurons of C. elegans. In the head ganglia, there are 12 types of amphid neurons that are chemosensory, thermosensory, odorsensory or polymodal (Fig 3). This set of dozen neurons has a matching set on the right side of the head ganglia as well (Fig 4).

Fig 4. Dorsoventral view of the amphid neurons in the head ganglia of C. elegans. Adapted from: Ortis, et al., 2006. — Fig 4. Dorsoventral view of the amphid neurons in the head ganglia of *C. elegans*. Adapted from: Ortis, et al., 2006.