Molecular Principles of Hair Follicle Induction and Morphogenesis

Hair follicle (HF) development resulted from interactions between neuroectodermal and mesodermal tissues and could be divided into stages: induction, organogenesis, and cytodifferentiation. The process was regulated by gradients of inhibitors and activators from the skin epithelium and mesenchyme. For HF development, inhibitors had to be counteracted by specific antagonists or overridden by stimulators of hair placode formation. Once a mesenchymal condensate of inductive fibroblasts formed, it controlled subsequent HF organogenesis and epithelial stem cell differentiation. The review highlighted the morphological characteristics, principles, and molecular players in HF development, focusing on recent molecular insights and proposing a working hypothesis.