The skin epidermis and its appendages undergo continuous renewal through a process called homeostasis. Stem cells in the epidermis maintain tissue homeostasis by replacing cells lost during turnover or injury. Different skin stem cell pools contribute to the maintenance and repair of epidermal tissues, including the interfollicular epidermis, hair follicles, and sebaceous glands. The mechanisms and signaling pathways that regulate epithelial morphogenesis in the skin are reused during adult life to maintain skin homeostasis. The skin epidermis, though thin, protects against environmental stresses and provides a surface for appendages like hair, nails, and glands. These appendages aid in thermal regulation, protection, and social interactions. The skin barrier is essential for survival and is established before birth. During embryonic development, the epidermis originates from the ectoderm and undergoes stratification, with mesenchymal cells guiding this process. The epidermis forms a basement membrane that supports its structure and functions as a boundary between the epithelium and dermis. During stratification, cells transition from basal to suprabasal layers, undergoing distinct transcriptional stages to form the stratum corneum. The skin barrier is continuously renewed through tissue homeostasis, with the number of epidermal cells remaining constant. Mammalian skin contains pilosebaceous units, including hair follicles and sebaceous glands, maintained by stem cells. Hair follicles have a specialized niche called the bulge, where stem cells reside and differentiate into matrix cells that form the hair shaft. During the hair cycle, stem cells in the bulge generate new hair follicles and maintain the skin barrier. Signaling pathways such as Wnt and BMP regulate hair follicle development and homeostasis. Wnt signaling is crucial for specifying hair follicle stem cells and their differentiation, while BMP signaling maintains stem cell quiescence. These pathways interact with transcription factors like p63, Notch, and LEF1 to regulate cell fate and differentiation. MicroRNAs like miR-203 also play a role in controlling stem cell differentiation and proliferation. Asymmetric cell division is essential for maintaining stem cell pools, with stem cells differentiating into terminal cells while retaining their self-renewal capacity. The bulge region of the hair follicle contains stem cells that can regenerate hair follicles and maintain skin homeostasis. These stem cells are crucial for wound healing and tissue repair, and their function is regulated by various signaling pathways and transcription factors. Skin cancers may arise from mutations in stem cells, particularly those involving the Wnt/β-catenin pathway. The balance between stem cell quiescence and activation is critical for maintaining skin homeostasis, and disruptions in this balance can lead to cancer. Understanding the molecular mechanisms that regulate stem cell behavior and homeostasis is essential for developing therapeutic strategies for skin diseases and cancers.The skin epidermis and its appendages undergo continuous renewal through a process called homeostasis. Stem cells in the epidermis maintain tissue homeostasis by replacing cells lost during turnover or injury. Different skin stem cell pools contribute to the maintenance and repair of epidermal tissues, including the interfollicular epidermis, hair follicles, and sebaceous glands. The mechanisms and signaling pathways that regulate epithelial morphogenesis in the skin are reused during adult life to maintain skin homeostasis. The skin epidermis, though thin, protects against environmental stresses and provides a surface for appendages like hair, nails, and glands. These appendages aid in thermal regulation, protection, and social interactions. The skin barrier is essential for survival and is established before birth. During embryonic development, the epidermis originates from the ectoderm and undergoes stratification, with mesenchymal cells guiding this process. The epidermis forms a basement membrane that supports its structure and functions as a boundary between the epithelium and dermis. During stratification, cells transition from basal to suprabasal layers, undergoing distinct transcriptional stages to form the stratum corneum. The skin barrier is continuously renewed through tissue homeostasis, with the number of epidermal cells remaining constant. Mammalian skin contains pilosebaceous units, including hair follicles and sebaceous glands, maintained by stem cells. Hair follicles have a specialized niche called the bulge, where stem cells reside and differentiate into matrix cells that form the hair shaft. During the hair cycle, stem cells in the bulge generate new hair follicles and maintain the skin barrier. Signaling pathways such as Wnt and BMP regulate hair follicle development and homeostasis. Wnt signaling is crucial for specifying hair follicle stem cells and their differentiation, while BMP signaling maintains stem cell quiescence. These pathways interact with transcription factors like p63, Notch, and LEF1 to regulate cell fate and differentiation. MicroRNAs like miR-203 also play a role in controlling stem cell differentiation and proliferation. Asymmetric cell division is essential for maintaining stem cell pools, with stem cells differentiating into terminal cells while retaining their self-renewal capacity. The bulge region of the hair follicle contains stem cells that can regenerate hair follicles and maintain skin homeostasis. These stem cells are crucial for wound healing and tissue repair, and their function is regulated by various signaling pathways and transcription factors. Skin cancers may arise from mutations in stem cells, particularly those involving the Wnt/β-catenin pathway. The balance between stem cell quiescence and activation is critical for maintaining skin homeostasis, and disruptions in this balance can lead to cancer. Understanding the molecular mechanisms that regulate stem cell behavior and homeostasis is essential for developing therapeutic strategies for skin diseases and cancers.