Developmental stages of cultured hippocampal neurons imaged in phase contrast. (A) Cells in stage 1 are round and are encircled by a prominent lamella. (B) By stage 2, short neurites have formed. (C) In stage 3, the symmetry of the short neurites has been broken; one neurite has grown longer than the rest and has become specified as the axon. (D) In stage 4, the remaining minor neurites begin to grow and to acquire the tapered morphology characteristic of dendrites. This image represents an unusual case in which the somatodendritic domain of the neuron has not been contacted by axonal branches of any neighboring cell. (E) By stage 5, the network has become so dense that individual neurites are difficult to follow. The proximal portions of the dendrites that arise from each cell are obvious, but it is not possible to identify their axons without some additional method for selectively labeling them (see Kaech and Banker 2006, for further details). Scale bars, 25 µm in A–C, 50 µm in D. (Reprinted, with permission, from Kaech and Banker 2006.)