is due to the difference in clearance rates of myelin in CNS and PNS. Regeneration Regeneration follows degeneration. Regeneration is rapid in PNS, allowing for rates of up to 1 millimeter a day of regrowth. Grafts may also be needed to allow for appropriate reinnervation. It is supported by Schwann cells through growth factors release. CNS regeneration is much slower, and is almost absent in most vertebrate species. The primary cause for this could be the delay in clearing up myelin debris. Myelin debris, present in CNS or PNS, contains several inhibitory factors. The prolonged presence of myelin debris in CNS could possibly hinder the regeneration. An experiment conducted on newts, animals that have fast CNS axon regeneration capabilities, found that Wallerian degeneration of an optic nerve injury took up to 10 to 14 days on average, further suggesting that slow clearance inhibits regeneration. Schwann cells and endoneural fibroblasts in PNS In healthy nerves, nerve growth factor (NGF) is produced in very small amounts. However, upon injury, NGF mRNA expression increases by five to seven-fold within a period of 14 days. Nerve fibroblasts and Schwann cells play an important role in increased expression of NGF mRNA. Macrophages also stimulate Schwann cells and fibroblasts to produce NGF via macrophage-derived interleukin-1. Other neurotrophic molecules produced by Schwann cells and fibroblasts together include brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, ciliary neurotrophic factor, leukemia inhibitory factor, insulin-like growth factor, and fibroblast growth factor. These factors together create a favorable environment for axonal growth and regeneration. Apart from growth factors, Schwann cells also provide structural guidance to further enhance regeneration. During their proliferation phase, Schwann cells begin to form a line of cells called Bands of Bungner within the basal laminar tube. Axons have been observed to regenerate in close association to these cells. Schwann cells upregulate the production of cell surface adhesion molecule ninjurin further promoting growth. These lines of cell guide the axon regeneration in proper direction. The possible source of error that could result from this is possible mismatching of the target cells as discussed earlier. Due to lack of such favorable promoting factors in CNS, regeneration is stunted in CNS. Wallerian degeneration slow Mice belonging to the strain C57BL/Wlds have delayed Wallerian degeneration, and, thus, allow for the study of the roles of various cell types and the underlying cellular and molecular processes. Current understanding of the process has