def A_cells_um(nc, rho, A_c_rho1=800):
    """
    Returns the area of `nc` cells at density `rho` in 
    micrometers^2.
    `A_c_rho1` is the area of each cell at `rho=1` in
    micrometers^2.
    """
    return nc * A_c_rho1 / rho