Localization of the major sites of rabbit apolipoprotein D gene transcription by in situ hybridization.

We have identified the sites of transcription of the apolipoprotein D (apoD) gene in the rabbit by in situ hybridization. We show here for the first time that 1) apoD mRNA production does not correlate with the sites of steroid hormone production in adrenal glands nor male genital tissues; and 2) the expression of the apoD gene is clearly higher in white than in gray matter throughout the central nervous system (CNS). Specifically, apoD mRNA was abundant near blood vessels and was expressed mostly in fibroblast-like cells, in particular in the testis, the efferent ducts, the ductus epididymis, the lung, and the subarachnoid space of the CNS. Other positive cell types were endothelial cells of adrenal sinusoidal capillaries and glial cells of the CNS. We detected apoD mRNA in both the adrenal cortex and medulla. White but not gray matter showed high levels of apoD mRNA throughout both the rabbit CNS and in human brain. The red pulp of spleen showed a strong hybridization. In prepubertal rabbits apoD mRNA levels were moderate in both testis and epididymis. Epididymal but not testicular expression increased with the onset of puberty and epididymal levels always exceeded those of the testes in animals showing spermatogenesis. Thus, the variation in levels of apoD mRNA among organs in vivo, that we and others have previously reported, can be explained by transcription being not only characteristic of cell type, with a few common cell types producing in each organ, but transcription also varied among cells of the same lineage.