Coexistence of α-fetoprotein, albumin and transferrin immunoreactivity in neurones of the developing mouse brain

Abstract

α-Fetoprotein (AFP), a major fetal plasma protein, is present at high levels (mg ml⁻¹) throughout development and at extremely low levels (ng ml⁻¹) in the normal adult (see refs 1,2). Unlike that of most other species, rat and mouse AFP bind oestrogens with a high affinity³. This aroused speculation regarding its involvement in the growth and differentiation of oestrogen target tissues⁴. Extracellular sequestration of maternal oestrogens through binding to AFP is generally assumed to protect the female rodent brain from excessive oestrogenization (masculinization) from the high oestrogen levels present during the critical period for sexual differentiation⁵. There is an intra-cellular pool of AFP of unknown function in brain cytosol of fetal and neonatal rats⁶ and mice⁷. Its intraneuronal localization has been confirmed within the central and peripheral nervous systems of the developing rat^8,9 and human fetus¹⁰ by immuno-peroxidase cytochemistry. Other major plasma binding proteins such as albumin^9,10 and prealbumin¹⁰ but not transf errin ^9,10 have also been localized within neurones of the developing rat and human brain. As part of a continuing ontogenetic study, I present here the first evidence, derived by single- and double-label immunofluorescence, for the intraneuronal localization and coexistence of immunoreactive AFP, albumin and trans-ferrin within the same neurones of the late fetal and postnatal mouse brain. These observations may be relevant to the process of sexual differentiation of the brain, and the mechanism of transport of hormones and other growth-promoting substances into neurones during development.