The Central Nervous System

Neural development begins early in the embryonic stage but is last to complete, it makes the most complex structures that are in the embryo and take a long time to do so. The central nervous system starts as a simple neural plate which folds to make a groove then a tube which opens at each end. Within the tube stem, cells make two major classes of cells which the nervous system is made up of, neurons and glia. These two classes separate into more types because they have different functions and shapes (Hill, 2013). Neurons develop polarity by generating an axon and many dendrites. Retinal pigment epithelium (RPE) cells are seen in a cross section of the retina and the cells are separated into apical and basal. The apical membrane faces the photoreceptors of the neural retina and the basal membrane faces the choroid. The basal membrane has several folds to increase the surface area for absorption and secretion of molecules (Marmor & Wolfensberger, 1998). The two membranes have a concentration gradient between them due to the epithelial monolayer cell which produce tight junctions and provide a barrier (Rahner et al., 2004). Neurons have to undergo morphogenesis and make connection in the neuronal network to survive. The retina contains 6 different types of neurons which is why it is a great model. One particular cell type of importance is the retinal ganglion cells, their axons extend out to the brain and target the centers of the brain which perceive vision (optic tract).

Peculiar molecules that have been drawn in developmental events such as cell migration, differentiation, survival and death are known as Fibroblast growth factors (FGFs) (Bottcher et al 2005). The FGF molecules contain 22 FGF and 4 receptors (FGFRs). It is suggested that FGF signaling is required for RGC morphogenesis.

Danio rerio is an ideal model system to study eye development. Zebrafish adapt well to a laboratory setting and reach sexual maturity 3-4 months past fertilization. The embryos are easily manipulated and a pair of breeding fish can produce up to 200 fertilized eggs each week. The high sequence homology between humans and fish provides further evidence that zebrafish are a suitable organism to model human disease (Fadool and Dowling 2008). Egg and Embryo transparency in the Zebrafish allow for visual identification of morphogenic movements. Their major organ systems develop within 24 hours after fertilization (Kimmel et al.