This mouse hippocampal pyramidal neuron growing in a tissue culture dish is studded with thousands of synaptic connections, which are marked in yellow. These connections are rich in glutamate receptors (green) and synaptic scaffolding proteins (red). Such in vitro systems are an ideal place to study the questions Ramon y Cajal posed in 1937: "What mysterious forces precede the appearance of the processes, promote their growth and ramification, stimulate the corresponding migration of the cells and fibers in predetermined directions, as if in obedience to a skillfully arranged architectural plan, and finally establish those protoplasmic kisses which seem to constitute the final ecstasy of an epic love story?"