Growth factors are present in the follicular bulge area, where stem cells are found, and they interact with cells of the matrix, thus activating the proliferative phase of the hair. Stem cells are more primitive and of ectodermal origin; they give origin to the epidermal cells and the sebaceous glands. Cells of the dermal papilla, which are found at the capillary base, are of mesenchymal origin. Both cells are reliant on each other, and thus, when they interact through the stimulation of various growth factors, they give rise to the future follicular unit. While some growth factors are pro inflammatory and have negative effects on hair, other growth factors such as Platelet Derived Growth Factor (PDGF) and Vascular Endothelial Growth Factor (VEGF) are important for hair formation and follicle size. PDGF signals are involved in both epidermis-follicle interaction and the dermal mesenchyme interaction required for: hair canal formation and the growth of dermal mesenchyme. In 2001, McElwee was the first to perform functional assays on hair follicle growth under the influence of VEGF. This growth factor has an important role in controlling hair biology and hair follicle size is partly dependant on VEGF induced angiogenesis. Other studies by Yano provided the first direct evidence that VEGF improved follicle vascularisation, promoted hair growth and increased follicle and hair size and studies by Rinaldi, demonstrate that VEGF accelerated revascularization and increased the size of the follicular unit.