Neurogenesis and apoptosis in the developmentally regulated loss of spinal cord regeneration
Unlike the adult mammal, the chick can successfully regenerate its spinal cord until embryonic day (E) 13. Multiple factors may contribute to the subsequent loss of regenerative capacity, although most research has concentrated on axonal re-growth inhibition as a key issue. The number of viable cells remaining in the spinal cord could also be important and may be affected by cell survival and cell replacement. In this thesis the early response of the chick spinal cord to injury has been investigated, focusing on cell death and the potential to replace lost cells by neurogenesis. Pharmacological reduction of haemorrhage after injury at El5 resulted in reduced apoptosis and cavitation, suggesting that blood-borne factors, such as the serine protease thrombin, may cause apoptosis. Endogenous thrombin expression and activity after injury was investigated. Thrombin was not up-regulated after injury at El5 however, evidence suggests that the activity of other serine proteases was increased. In parallel, in organotypic slice cultures, exogenous thrombin treatment did not increase apoptosis. These results provide new information about the contribution of serine proteases to apoptosis in the chick, suggesting that, although thrombin is not of primary importance, other serine proteases could play a greater role. Next, the contribution of neurogenesis to regeneration at Ell was examined. Changes in the expression and phosphorylation of the early neuronal marker, doublecortin, in response to injury were observed. Although increased proliferation in the grey matter was observed, no increase in neurogenesis after injury was detected. Surprisingly, ongoing neurogenesis was discovered in the normal spinal cord at Ell. These results challenge established views about the timing of neurogenesis in the chick spinal cord and suggest that ongoing proliferation and neurogenesis may contribute to the regenerative capacity at this stage. This thesis presents insights into factors involved in the early response of the chick spinal cord to injury, providing new information about the contribution of neurogenesis and cell survival to regenerative capacity.