Localization of Retinoic Acid Receptors in Anterior Human Embryo
Retinoic acid receptors (RARs) are nuclear transcription factors that are activated by all-trans-retinoic acid or 9-cis-retinoic acid and are found in all tissues but predominantly in developing fetus, dividing tumor cells, and adult skin. Three forms of these receptors, α, β, and γ, have been described. In this paper we report the presence of RARα and β determined by hybridization with anti-sense messenger RNA, and histochemical localization of the three forms of RARs using monospecific polyclonal antibodies in various tissues of early human embryos. In a 54-day-old embryo, RARα was expressed primarily in the liver and the brain, with somewhat lesser expression in the intestine. RARβ was the highest in the brain, followed by a restricted expression in the intestine and the liver. Other organs, i.e., adrenal, kidney, and testis, did not show measurable amounts of RARβ. The immunohistochemical localization in anterior sections of a 43-day-old embryo indicated that RARα was present in the neuroepithelial cells and in cells lining the primitive pharyngeal sac, dorsal aorta, and pericardium. RARβ was somewhat less prevalent in same tissues, whereas the expression of RARγ was the lowest of the tree RARs in any tissues examined. Results indicated that RARα and β appear at early stages of human embryonic development and their expression is restricted to certain types of tissues.