tailieunhanh - DEVELOPMENTAL NEUROBIOLOGY - PART 8

Hệ thần kinh trung ương các khớp thần kinh khác nhau đáng kể trong những chức năng và độ đặc hiệu, nhưng tương đối ít về kích thước và cấu trúc. | Synaptogenesis Chapter 10 299 and define its formation but myriad other signals undoubtedly contribute. Examples include growth factors and matrix proteases. Glial-derived neurotrophic factor GDNF perturbs NMJ formation when overexpressed in transgenic mice Nguyen et al. 1998 . Matrix metalloprotease 3 MMP3 is concentrated at synaptic sites where it is capable of releasing agrin from the synaptic basal lamina and could play a role in synaptic remodeling or the dispersal of uninnervated postsynaptic sites Vansaun and Werle 2000 . Despite years of study and real progress at the NMJ a great deal remains to be learned about the complex interactions of axon target and glial cell at this best understood synapse. CNS SYNAPSES Compared to the NMJ synapse formation in the CNS is poorly understood for several reasons. CNS synapses vary considerably in function and specificity but relatively little in size and structure. In addition the complex anatomical architecture of the brain has hindered the ability to identify either a single axon s presynaptic terminals or the postsynaptic specializations associated with a single dendrite. Even within topographically mapped populations there are numerous functional subtypes such as the On and Off retinal ganglion cells in the eye which so far lack molecular or anatomical features of distinction. Next it is hard to observe one CNS synapse even twice in search of changes that occur with development or use. Finally there has been no CNS ortholog of the Torpedo electroplaques that would allow the unique molecular signature of a specific type of CNS synapse to be identified by biochemical means. Perhaps it should not be surprising that no clear kingpin of CNS synapse formation has been identified. Nevertheless while the mechanisms of CNS synaptogenesis are relatively unknown there are many functional analogies and some direct commonalities between neuromuscular and central synapses. One emerging theme is that synapse formation in the CNS .