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. 2010 Apr 1;120(4):956–962. doi: 10.1172/JCI41303

Figure 4. The fate of vitamin A in the spermatogonia.

Figure 4

Retinol is delivered to germinal cells on the retinol-binding protein (RBP) and is internalized via the membrane receptor STRA6. It is also plausible that RA is delivered directly to spermatogonia either by Sertoli cells or from the serum. Inside the cell, retinol is converted to RA in a two-step process and can interact with RARs such as RARγ. The RA is bound up by an excess of cellular retinoic acid–binding protein (CRABP). The activated receptor can stimulate transcription of a number of genes including Stra8, which has been shown to be necessary for progression through meiosis. Excess RA can be metabolized by the enzyme CYP26B1 into 4-oxo and 4-hydroxy forms. These forms are then secreted from the cell.