We develop connections between brain cells through what?

synapses

Explanation:

Brain cells have the efficiency to talk to its nearby cells and thus send or receive pre informed signals across connections. This process of intra brain cell communication is called as synapses

As we grow our brain organizes  the previously established synaptic pathways and shuts down less-active, inefficient synapses

Neurons (brain cells) preserve their most active connections with the nearby cells and thus lapse the remaining synapses

SIRP alpha protein (found on the surface of cells) helps in cementing the active synaptic connections between brain cells

the answer to your question is sperm

but there is no

i guess you forgot putting it.