Embryonic, human spinal ganglion explants were plated at 5-12 weeks postconceptional age and cultured for 5-50 days on a semisynthetic substrate in a serum-containing culture medium without addition of antibiotics or preconditioned medium. The growth pattern in vitro was found to be age dependent. Five- to 6-week ganglia showed a characteristic semicircular growth pattern with bidirectional extension of neurites on top of a monolayer of supportive cells. Explanted 9- to 10-week ganglia showed an extensive, multidirectional neurite outgrowth with less pronounced proliferation of nonneuronal cells. Neurite extension, fasciculation, cell migration and morphology were studied immunohistochemically with antibodies to neurofilament (NF), S-100, and the Thy-1 glycoprotein. Both NF and S-100 were expressed at 5 weeks gestational age in ganglionic neurons and in proliferating Schwann cells in contact with axonal processes, respectively. NF was homogeneously distributed in both cell somata and neurites, whereas S-100 immunoreactivity showed an intense nuclear and a weaker cytoplasmic distribution in spindle-shaped, bipolar Schwann cells. This staining pattern was conserved during differentiation in long-term culture. Thy-1 was expressed on ganglionic neurites forming fascicles by the third week in culture. However, Thy-1 was never expressed until the total age of 10 weeks. In addition, Thy-1 was found on fibroblasts from the first week in culture. The distribution of Thy-1 on the cytoplasmic membrane was similar in both cell types, showing a coarsely granulated membrane staining. The temporal as well as the spatial expression of differentiation antigens in tissue sections of early embryonic spinal cord and spinal ganglia were very similar to what was observed in vitro.