Define Role in growth and cellular Differentiation?

The growth and differentiation of epithelial cells throughout the body are especially affected by vitamin A deficiency.  In addition, goblet cell numbers we reduced in epithelial tissues and as a consequence, mucous secretions diminish. Cells lining protective tissue surfaces fail to regenerate and differentiate; hence they flatten and accumulate keratin.  Classical symptoms of xerosis (drying or non-wet ability) and desquamation of  dead surface cells as seen in ocular tissue (i.e. xerophthalmia) are the external  evidence of the changes also occurring to various degrees in internal epithelial  tissues. Current understanding of the mechanism of vitamin A action within cells outside the visual cycle is that cellular functions are mediated through specific nuclear receptors. Binding with specific isomers of retinoic acid (i.e.all Tran- and 9-cis-retinoic acid) activates these receptors. Activated receptors bind to DNA response elements located upstream of specific genes to regulate the level of expression of those genes. These retinoid-activated genes regulate the synthesis of a large number of proteins vital to maintaining normal physiologic functions. There may, however be other mechanisms of action that are as yet undiscovered.