Induction and differentiation of i3LMNs is nearly identical to the first 3 days of differentiation for i3Neurons (see Basic Protocol 5), including identical induction medium. Following replating, however, differences arise including the use of Motor Neuron Culture Medium (MM) for long-term culture (Table 5), additional reagents to reduce proliferative cells if necessary, and variable options for coating polymers.MM is sufficient to promote the maturation and long-term culture of i3LMNs. While a majority of these cells at Day 3 are committed to differentiation to post-mitotic neurons, a small subset may remain proliferative and can quickly overtake the culture. To compensate, a 1-day pulse of bromodeoxyuridine (BrdU) is recommended at the time of replating of Day 3 i3LMNs and has proven effective at impairing mitosis without causing neural toxicity. Following BrdU treatment, medium should be completely exchanged the following day. CultureOne is also effective at reducing levels of proliferative cells over time, and it may be included in MM medium with usually minimal effects on neural cell health. In general, one fourth to one half of the medium should be aspirated and replaced with fresh medium every 3 to 4 days.Neuron attachment and growth also requires a strongly adhesive substrate. Coating plates with synthetic polymers such as poly-L-ornithine (PLO), polyethyleneimine (PEI), or poly-D-lysine (PDL) is sufficient for cell attachment, and providing an optional additional coating of purified laminin improves i3LMN viability and neurite outgrowth. Laminincoated wells also support proliferative cells better than polymer without laminin, so BrdU is necessary in these conditions.While these substrates are stiffer than those under biological conditions, they reduce cell migration and clumping, facilitating imaging of individual cells. In our experience, PLO has produced the best neuronal morphology, but it is also the most sensitive to cell detachment resulting from medium exchanges. Detachment is a particular concern at high cell density and after extended time in culture, as the interconnected network of neural processes can cause entire wells to detach from the edges. PEI and PDL typically promote stronger adhesion, but PEI is toxic to cells if coating is not performed properly, and rapid degradation and batch-to-batch variability complicate the use of PDL. Cells are especially susceptible to detachment during the many washes required for immunocytochemistry, so these steps should be performed with extreme care. This protocol will assume use of PLO, although coating with PEI or PDL may be performed using an identical protocol except where noted.