The skin and its appendages that derive from the epidermis (hair follicles, sweat glands, sebaceous glands, nails, and mammary glands) establish the integumentary system. Histologically, skin has two main layers—the epidermis and the dermis—with a subcutaneous fascia called the hypodermis, which lies deep in the dermis. The epidermis is formed of four to five layers of cells made mostly out of keratinocytes, along with three other different and less abundant cells. The dermis underlies the epidermis. The hypodermis is a looser connective tissue that is located beneath the dermis. It blends to the dermis with an unclear boundary.
Skin has eccrine sweat glands, apocrine sweat glands, and sebaceous glands. Mammary glands are specialized sweat glands. Vellus hair, terminal hair, and lanugo hair are the three types of hair of human skin. Skin receives a wide-ranging amount of stimuli from the environment and has appropriately responding capsulated or unencapsulated receptors that are distributed in various levels of its layers. The dermis hosts an extensive amount of vascularization to circulate blood and lymph.
Facial skin has common functions, to protect and serve. It protects body integrity, prevents protein and fluid loss, repairs injuries, and regulates body temperature.