The substance that helps promote cell division in the epidermis of the skin is called Epidermal Growth Factor (EGF). It is a polypeptide growth factor that stimulates the growth and proliferation of keratinocytes, the most abundant cells in the epidermis. EGF binds to specific receptors on the surface of keratinocytes, which then triggers a series of intracellular signalling cascades that ultimately lead to DNA synthesis and cell division. EGF plays a crucial role in maintaining the structural integrity and barrier function of the skin, promoting wound healing, and regulating skin development and homeostasis.