Editing Retinol (section)

===Embryology===
Retinoic acid via the retinoic acid receptor influences the process of cell differentiation and, hence, the growth and development of embryos. During development, there is a concentration gradient of retinoic acid along the anterior-posterior (head-tail) axis. Cells in the embryo respond to retinoic acid differently depending on the amount present. For example, in vertebrates, the hindbrain transiently forms eight [[rhombomeres]] and each rhombomere has a specific pattern of genes being expressed. If retinoic acid is not present the last four rhombomeres do not develop. Instead, rhombomeres 1–4 grow to cover the same amount of space as all eight would normally occupy. Retinoic acid has its effects by turning on a differential pattern of Homeobox (Hox) genes that encode different homeodomain transcription factors which in turn can turn on cell type-specific genes.<ref name=gd>{{cite journal | vauthors = Duester G | title = Retinoic acid synthesis and signaling during early organogenesis | journal = Cell | volume = 134 | issue = 6 | pages = 921–931 | date = September 2008 | pmid = 18805086 | pmc = 2632951 | doi = 10.1016/j.cell.2008.09.002 }}</ref> Deletion of the Homeobox (Hox-1) gene from rhombomere 4 makes the neurons growing in that region behave like neurons from rhombomere 2.  Retinoic acid is not required for patterning of the retina as originally proposed, but retinoic acid synthesized in the retina is secreted into the surrounding [[mesenchyme]] where it is required to prevent overgrowth of perioptic mesenchyme which can cause microphthalmia, defects in the cornea and eyelid, and rotation of the optic cup.<ref name="Duester_2008"/>