The multidrug-resistance P-glycoprotein (Pgp, MDR1) is an early marker of blood-brain barrier development in the microvessels of the developing human brain.

The multidrug-resistance P-glycoprotein (Pgp) was initially identified as an energy-dependent proton pump, which transports a variety of non-related compounds out of chemotherapy-resistant cancer cells. Molecular biological investigations using knockout mice for the mouse homologue of the human Pgp showed that these mice partially lack a functioning blood-brain barrier, indicating that Pgp has an important role in the blood-brain barrier as its normal function. The presence of Pgp expression in formalin-fixed and wax-processed tissue sections can be assessed using the monoclonal antibody, JSB-1. Since no data on the developmental expression of Pgp are available, we stained a developmental series of human brain sections with JSB-1. Our results indicate that Pgp expression in endothelia of brain microvessels occurs regularly in embryos of about 30-mm crown-rump length (CRL). Strong reactivity is seen in blood vessels of fetuses from 123-mm CRL. There is also reactivity in pial blood vessels but not in choroid plexus blood vessels known to be without a blood-brain barrier. Pgp expression is therefore an early marker of the blood-brain barrier in the developing human brain.