Existing studies suggest that HIPK3 plays an important role in FASmediated apoptosis. In the current study, FADD expression is upregulated in the 7-day and 21-day stress processes. The upregulation of FADD expression in the 21-day stress process is more significant than that in the 7-day stress process and is closely related to the up-regulation of HIPK3 expression. FADD is the main signal transduction protein for Fas/FasL-mediated apoptosis. FADD, Fas, and FasL form a trimer. The interaction of HIPK3FADD activates FADD and enhances the apoptosis signal transduction. Therefore, the 21-day stress breaks the balance in normal hippocampal cell growth and apoptosis, suppresses hippocampal cell growth, hastens apoptosis, and causes numerous hippocampal cell losses, thereby damaging the function of the hippocampus. The GO function analysis suggests that after the 21day stress, hippocampal functions are damaged. Hence, the regulation capacity of the hippocampus to stress reaction is very limited. The mechanism of chronic immobilization stress that results in a great amount of hippocampal cell loss is analyzed Dasatinib through signal pathway analysis. Signal pathway analysis shows that in the early stage of stress, the functions of multi-signal pathways are significantly activated. Among the 12 genes involved in the ECM receptor interaction pathway, 9 are up-regulated and 3 are down-regulated. This finding suggests that in the 7-day stress process, ECM synthesis and degradation 
of the hippocampal tissue are out of balance. ECM generation and degradation are increased and reduced, respectively, causing excessive deposition of ECM in the hippocampal tissue. The up-regulated expressions of the genes on the pathways are involved in integrin and its ligand ECM proteins, such as collagen, laminin, osteopontin, and heparan sulfateproteoglycans SDC3, and other components. Among them, collagen is the main component, which is involved in the activation of type I, III, and IV collagen, and down-regulated genes include integrin. Previous studies proved that activated integrin promotes axon growth and intracorporeal neural regeneration. Cells separated from adhesion matrix and lost intercellular linkage, which would cause anoikis. Integrin could promote cell movement to avoid anoikis. Both the degradation of laminin and its disturbance to the relationship between cells and laminin can cause nerve cell apoptosis. SDC3 is involved in CNS establishment, and SDC3 expression up-regulation possibly plays a role in repairing damaged hippocampus. Collagen, the main structural component of the ECM, is also essential in mitogen-stimulated cell cycle. However, some reports showed that collagen could suppress cell proliferation. In organ fibrosis, excessive deposition of fibrillar collagen, such as type I collagen, causes Nilotinib sclerosis of tissues and organs and finally leads to functional loss. The activation of type I collagen COL1A1 and COL1A of hippocampal tissues causes hippocampal sclerosis to a certain extent. Hippocampal sclerosis was firstly proposed by Falcomer et al. The main pathological manifestations of hippocampal sclerosis lay in hippocampal atrophy and neuron loss, as well as colloid proliferation in some regions of the hippocampal structure. Thus, the activation of hippocampal ECM in the 7-day stress clearly promotes the self-repair of damaged hippocampal cells, promoting hippocampal sclerosis and causing partial neuron loss.