 Otherwise, expression levels of ApoD vary largely from organs to organs and species to species, with humans displaying the most diverse expression of ApoD, and mice and rats almost exclusively expressing ApoD in the nervous system (see Figure 2).ApoD concentration in human plasma varies between 5 and 23 mg/100 ml.In the nervous system, the ApoD mRNA is expressed by fibroblasts, astrocytes and oligodendrocytes As a glycoprotein with a peptide signal, ApoD is secreted.