ABSTRACT
This chapter reviews the structure and function of the pilosebaceous unit and the controlling influences on the pilosebaceous unit and sebum secretion. The chapter is divided into three sections. Section I gives an account of the structure and function of the normal pilosebaceous unit; Section II describes the biochemistry and regulation of pilosebaceous unit biology; and finally, Section III deals briefly with the biochemical changes occurring in the pilosebaceous duct in acne.
SECTION ONE: ANATOMY Structure of the Pilosebaceous Unit In humans, pilosebaceous units or pilosebaceous follicles are found on all skin surfaces, apart from the palms of the hands and soles of the feet. Essentially, they are invaginations of the epidermis into the dermis. Each comprises a duct, which ends in the dermal papilla, a hair fiber (or pilus) produced by the dermal papilla, a sebaceous gland and its associated sebaceous duct. The duct supports and protects the hair fiber and also drains sebum produced by the sebaceous gland and carries it to the skin surface. In addition, in split thickness wounds, the cells of the ductal epithelium are a source of proliferating keratinocytes, which migrate to re-epithelialize the wound (1). A specialized population of epithelial cells called stem cells, located in the bulge region situated below the sebaceous gland, are believed to be crucial for this (2). These cells are pluripotent and can also differentiate in some circumstances to produce ductal keratinocytes and sebocytes (3,4). Both the hair and sebum are products of pilosebaceous follicles, emerging onto the skin surface. Sebum is a holocrine secretion from the sebaceous gland cells or sebocytes, which means that the cells are destroyed when sebum is released. The function of sebum in humans is unclear, but as will be discussed later it may play a role in several skin functions (5,6).
