Immunocytochemical localisation of alpha-fetoprotein (AFP) and serum albumin (ALB) in ecto-, meso- and endodermal tissue derivatives of the developing rat.

Recent work has provided evidence of the intracellular presence of AFP and serum albumin (ALB) in the central nervous system of rats, mice and primates. We report here the immunocytochemical localization of these proteins in tissue derivatives of the three germ layers (with the exclusion of nervous system components) during rat fetal and neonatal development. Positive labelling for AFP and ALB was transitorily observed in all tissue derivatives examined. For example, labelling for AFP in the epidermis started around the 16th day of fetal life and localized in some cell aggregations of the peridermis. Between the 17th and the 19th day AFP positive cells were observed in the basal layer of epidermis, while the stratum germinativum was always negative. The maximum intensity of labelling was at birth. At this time, AFP positive cells were visible in the strata spinosum, granulosum and corneum. On day 6 to 8 after birth, the labelling become negative. The localisation of ALB showed a similar pattern with the only difference of a short (2 to 5 days) displacement in time. It can be concluded that the intracellular uptake of AFP in the structures studied is transitory and follows the stage of cell and tissue differentiation. Relative to AFP, labelling for ALB seems to correspond to a more advanced stage of morphologic development.