Immunohistochemical localization of substance P, enkephalin and serotonin in the developing human retina

The localization of substance P (SP), enkephalin (ENK) and serotonin (5-HT) in the retinae of 12 human embryos/fetuses ranging in age from 6-30 weeks was determined immunohistochemically using the PAP method. At the 6 week stage [crown rump length (CRL) unknown due to incomplete specimen], the developing retina consisted of a single undifferentiated cell mass from which immunoreactive cells were absent. By 90 mm CRL (10th week of gestation), the retina was composed of an outer neuroblastic layer, an inner plexiform layer and an innermost layer of ganglion cells. At this stage, SP, ENK and 5-HT positive cells were detected solely in the outer neuroblastic layer. By 140 mm CRL (17 weeks), the retina consisted of cell layers similar in number and type to those of the adult retina. In specimens 140-216 mm CRL (gestation ages 17-24 weeks), SP, ENK and 5-HT neurons were present in the outer nuclear, inner nuclear and inner plexiform layers. In addition, 5-HT positive neurons and fibers were evident in the outer plexiform layer. By 285-295 mm CRL (26-30 weeks), neurons in the ganglion cell layer and the fovea were also SP and ENK positive. In earlier specimens, the cell bodies alone were immunopositive and not until 142 mm CRL (17 weeks) were positive processes observed. Finally, the presence of SP, ENK and 5-HT immunopositive structures occurred in a sequence from outer to inner layers of the developing human retina.