毛囊在皮肤表面下方。每个头发都有自己的卵泡，由角质形成细胞衬里。角质形成细胞的寿命短。一旦死亡的角质形成细胞脱落，它们将被新的健康角质形成细胞所取代。每个毛囊都附着在称为皮脂腺的专门生产腺体。这些腺体产生一种称为皮脂的油。皮脂沿着头发的轴流，并通过卵泡的开口来到外面。当它流失时，它将死细胞从毛囊中移出。主要是遗传因素决定皮脂腺的大小。另外，皮脂腺产生的皮脂量通常取决于激素因子。 For example, during the puberty (which is caused by certain hormonal activities) sebaceous glands produce more sebum than ever before. Because of that more dead cells come out the follicle with sebum and they stick together to form a plug that block the opening of the pore. These pores lately develop as pimples. Usually in women, the follicles located in face, back and chest are filled with very tiny, light hairs called vellus hairs, which are invisible to the naked eye. However, in men, these areas are filled with dark, thick hairs known as terminal hairs.