Topological degree theory Boundary conditions 2 Cell physiology 2 Complex model 2 Cytology 2 Delay differential equations 2 Differential equations 2 Existence of Solutions 2 Existence of positive periodic solutions 2 Existence results 2 Geometry 2 Leray-Schauder degree 2 Models with feedback 2 Nonlinear boundary conditions 2 Nonlinear elliptic systems 2 Periodic solution 2 Periodic solutions 2 Physiology 2 Problem solving 2 Regulatory mechanism 2 Second-order systemss 2 Self regulation 2 Systems of DDEs 2 Topological methods 2 Topology 2