Stra8 expression is elevated in the absence of CYP26B1.

<p>Reverse transcription- PCR was performed with RNA collected from E15.5 and E16.5 <i>Cyp26b1^SC+/SC−</i> and <i>Cyp26b1^SC−/SC−</i> testes. <i>Stra8</i> is only detected in RNA from E16.5 <i>Cyp26b1^SC−/SC−</i> testes. <i>Mvh</i> expression was analyzed as a positive control for RNA integrity.</p

Re-entry into mitotic cell cycle in embryonic <i>Cyp26b1^SC−/SC−</i> male germ cells.

<p>Sections of testes from <i>Cyp26b1^SC+/SC+</i> (A, B, C, G, H, I) and <i>Cyp26b1^SC−/SC−</i> (D, E, F, J, K, L) littermates at E15.5 (A–F) and E16.5 (G–L) stained for the mitotic marker Ki67 (B, E, H, K, red) and the germ cell marker MVH (A, D, G, J, green). Overlays of images show Ki67 expressing germ cells are observed only in <i>Cyp26b1^SC−/SC−</i> fetuses (F, arrowheads). Bar, 20 µm.</p

Proposed model for the role of CYP26B1 in maintaining male germ cells in an undifferentiated state during embryogenesis.

<p>In wild-type gonads, germ cells exhibit sex-specific divergence during embryogenesis as male germ cells enter mitotic arrest, while female germ cells enter mitosis followed by meiosis. However, in <i>Cyp26b1^SC−/SC−</i> fetuses, Cyp26b1 activity is inactivated after E15.5, thus elevating levels of retinoic acid within the testes. As a result, male germ cells exit from G0 to re-enter the cell cycle and initiate meiotic prophase, which subsequently culminates in loss of male germ cells.</p

<i>Cyp26b1^SC−/SC−</i> male germ cells enter meiosis prematurely.

<p>Sections of testes from <i>Cyp26b1^SC+/SC+</i> (A) and <i>Cyp26b1^SC−/SC−</i> (B) littermates at E16.5 stained for the meiotic marker SCP3 (green). Sections were counterstained with DAPI. Bar, 20 µm.</p

Generation of Sertoli cell-specific <i>Cyp26b1</i> knockout mice (<i>Cyp26b1^SC−/SC−</i>).

<p>(A) Floxed <i>Cyp26b1</i> locus showing position of primers (P1, P2, P3) used for genotyping. LoxP sites are indicated by triangles, and the exons of <i>Cyp26b1</i> are numbered. In Sertoli cells, exons 3–6 will be excised <i>(Cyp26b1^SC−/SC−)</i>, thus allowing for PCR amplification of a 364 bp product using P1 and P3. (B) PCR genotyping using P1, P2 and P3 showing detection of an excised allele (364 bp) only in the testes of a <i>Cyp26b1^fl/fl</i> mouse also expressing Cre (mouse #1).</p

Neonatal loss of germ cells in the absence of CYP26B1.

<p>Postnatal day 0 (P0) testes stained for TRA98 show a loss of germ cells in <i>Cyp26b1^SC−/SC−</i> mice. Bar, 20 µm.</p

qPCR primers.

<p>qPCR primers.</p

Dose response for 2MbisP, CAGE-3 and atRA on utricle area and epidermal thickness.

<p>Utricle area and epidermal thickness were measured in H&E stained tissue sections after the application of 7 doses of vehicle or varying doses of 2MbisP (A, B), CAGE-3 (C, D), or atRA (E, F). Significant differences from the vehicle group at each respective dose are indicated by an asterisk, *<i>P</i>≤0.05 (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0188887#pone.0188887.s002" target="_blank">S2 Table</a>).</p

Structure of 1α,25(OH)₂D₃ compared to the 2-methylene-19-nor analogs, 2MbisP and CAGE-3.

<p>Structure of 1α,25(OH)₂D₃ compared to the 2-methylene-19-nor analogs, 2MbisP and CAGE-3.</p

EGFR ligand mRNA after 7 topical doses of 2MbisP, CAGE-3 or atRA.

<p>EGFR ligand mRNAs were analyzed by RT-PCR in skin taken 4 h after receiving the final dose of vehicle, 2MbisP (690 nmol/kg), CAGE-3 (0.25 nmol/kg), or atRA (224 nmol/kg) and the data are expressed relative to the respective vehicle-treated group (treatment/vehicle).</p