Despite its four (or five) layers, the epidermis is actually relatively thin compared to the dermis, which makes up the majority of the skin. Within the dermis are blood vessels, connective tissues, lymph vessels, elastin and collagen fibres, and living skin cells. Nerve cells, hair follicles, sebaceous glands and sweat glands are also present in the dermis, with pores opening out onto the skin’s surface to control secretions.
With the exception of the palms, soles and tops of the feet, sebaceous glands are found all over the body, varying in size depending on their location. Sebaceous glands on the face and chest are bigger, for example, than those on the arms and legs, but they all open out into hair follicles or directly onto the skin. The purpose of the sebaceous glands is to produce sebum, an oily substance that lubricates the hair and prevents excess water-loss to keep the skin naturally healthy.
Sebum also has the effect of keeping the skin clear of some kinds of bacteria, although it is implicated in the build-up of other kinds of bacteria. The rate of production of sebum is hormone-dependent, which is why the hormonal upheaval experienced in puberty can lead to acne as sebum may clog pores and promote infection. Blackheads, a familiar sight in teenage skin, are a result of pores blocked by sebum containing the pigment melanin.