Internalization of exogenous fluoresceinated alpha-fetoprotein (FITC-AFP) was studied at different stages of development on embryonic chick neural retinas maintained, for short periods, in organ cultures. Cellular localization of endogenous, native AFP was carried out by immunohistoperoxidase methods. Cells which specifically internalized exogenous FITC-AFP (neurons and their processes) were precisely those showing positive immunolabeling for the endogenous, native protein. Such a result supports the hypothesis of a predominantly exogenous origin of intracellular neuroretinal AFP. A precise topography and temporal sequence of AFP labeling after internalization in retinal structures is given. AFP uptake was not displayed by undifferentiated cell precursors or germinal cell layers but was apparent in cells with phenotypic characteristics of maturing neurons. Nerve fibers and synaptic layers actively internalized FITC-AFP at specific stages of development. Fully differentiated neurons and processes did not internalize AFP. The possible role of AFP, a carrier of biologically active substances such as fatty acids, in neural retina differentiation is discussed.
Exogenous chicken alpha-Fetoprotein (AFP) was added to embryonic chick dorsal root ganglia plated on gelatin-coated tissue culture dishes at different stages during the differentiation process and its intracellular uptake demonstrated by immunocytochemical methods. Other embryonic serum proteins were added as a control. Morphologically well-differentiated neurons (ganglion cells) appeared positively labeled for AFP, contrasting with weakly stained other cell types: these included spindle-shaped cells identified as Schwann cells and larger, often closely packed cells, which could be less mature neurons. Fibroblasts were found negative or faintly stained. No AFP was noticeable in cultures grown in the absence of the protein. These results suggest that the presence of AFP in the developing CNS is for the most part, if not entirely, due to protein uptake as opposed to in situ synthesis.
