A morphological and physiological comparison was made between embryonically and postnatally derived superior cervical ganglion neurons (SCGN) grown in dissociated cell culture. It was found that while morphologically distinct, the physiological properties of the postnatal neurons were the same as their embryonic counterparts. 2. Intracellular injection of horseradish peroxidase (HPR) demonstrated that SCGN from any age of animal elaborated two basic types of processes, although the pattern of process ramification was unique for each neuron. The two types of proceses were 1) the large, smooth, rapidly tapering; and 2) the thin, nontapering variety, which often contained varicosities along its length. It is suggested that the former are dendritic in function, while the latter act as axons. 3. A difference was noted in somal size and the number of primary processes extended by the embryonic and postnatal neurons, with the latter more closely resembling the in vivo morphology. 4. Resting potentials and action-potential amplitudes of postnatal SCGN were comparable to those found previously for embryonic SCGN in vitro. 5. Iontophoretic application of putative neurotransmitter substances revealed the presence of acetylcholine receptors (AChR) on both embryonic and postnatal SCGN. Picrotoxin-sensitive depolarizing responses to iontophoresed gamma-aminobutyric acid (GABA) was seen on a few embryonic neurons, but not on the older cells. No responses were detected when norepinephrine (NE), glutamate, cAMP, substance P, or dopamine were applied to the SCGN of either age group. 6. Synatpic interaction between postnatal SCGN were found at an earlier in vitro age (12 days) than was the case for embryonic neurons (20 days). 7. Synaptic transmission was found to be chemical in nature. This was shown by 1) a dependence on external Ca2+ concentrations; 2) steplike fluctuations in synpatic potential amplitude, and 3) a variation in potential amplitude with changes in membrane potential. 8. It is concluded that the postnatal SCGN are able to survive in culture even when taken from animals up to 12.5 wk old. The elaboration of processes is in many ways strikingly similar to sympathetic neurons in the animal, and they are able to form functional synaptic interactions.
