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Retinoic acid (RA), the active metabolite of vitamin A, has been demonstrated to be important for growth and branching morphogenesis of mammalian embryonic salivary gland epithelium. However, it is not known whether RA functions directly within epithelial cells or in associated tissues that influence morphogenesis of salivary epithelium. Moreover, downstream targets of RA regulation have not been identified.Here, we show that canonical RA signaling occurs in multiple tissues of embryonic mouse salivary glands, including epithelium, associated parasympathetic ganglion neurons, and nonneuronal mesenchyme. By culturing epithelium explants in isolation from other tissues, we demonstrate that RA influences epithelium morphogenesis by direct action in that tissue. Moreover, we demonstrate that inhibition of RA signaling represses cell proliferation and expression of FGF10 signaling targets, and upregulates expression of basal epithelial keratins Krt5 and Krt14. Importantly, we show that the stem cell gene Kit is regulated inversely from Krt5/Krt14 by RA signaling.RA regulates Krt5 and Krt14 expression independently of stem cell character in developing salivary epithelium. RA, or chemical inhibitors of RA signaling, could potentially be used for modulating growth and differentiation of epithelial stem cells for the purpose of re-populating damaged glands or generating bioengineered organs. Developmental Dynamics 246:135-147, 2017. © 2016 Wiley Periodicals, Inc.

Retinoic acid (RA) is purported to be required for expression of genes controlling proximodistal (Meis2) or anteroposterior (Shh) limb patterning. Embryos lacking RDH10, the primary enzyme synthesizing retinaldehyde during mouse development, survive until E14.5 with stunted forelimbs but apparently normal hindlimbs. Using embryos carrying the RARE-lacZ RA-reporter transgene, we show that endogenous RA activity in Rdh10trex/trex mutants is detected in neuroectoderm but not limbs during initiation and patterning. Treatment of Rdh10 mutants with 25 nM RA restores RARE-lacZ activity to limb mesoderm, validating RARE-lacZ and verifying that RA is absent in mutant limbs. In Rdh10 mutants, hindlimbs exhibit normal Meis2/Shh expression and skeletal patterning, and although forelimbs are growth-retarded their Meis2 expression remains normal. Later in development, Rdh10 mutants lack interdigital RA activity and accordingly fail to exhibit normal loss of interdigital mesenchyme. These findings demonstrate that RA is unnecessary for limb patterning but required later for interdigital tissue loss. Developmental Dynamics 240:1142–1150, 2011. © 2011 Wiley-Liss, Inc.