How Acne Develops
Acne develops by a similar process in all the different clinical types of acne. The normal structure and function of human skin is described, followed by an explanation of the disruption which occurs in the skin of acne sufferers and the mechanism by which this causes spots to develop.
Function of Human Skin
To understand why spots develop in people with acne, you need first to know something of about the normal structure and function of human skin. There is a danger of this turning into a school biology lesson, but we'll try and keep it light and simple, leaving out the unnecessary bits. Human skin fulfils a number of different functions, four of which are important when thinking about acne. Skin acts as a:
- Physical Barrier
The outer layer of the skin consists of dead skin cells which are continually being shed from the surface and replaced from below. The dead skin cells form a protective layer which is usefully also impermeable to water. - Defence Against Infection
Human skin is colonised by numerous bacteria, but these do not result in an active infection if they are confined to the dead outer layer. In addition, the skin contains sebaceous glands which, after childhood, produce a natural fungistatic substance called sebum that helps protect the skin against fungal infections. - Temperature Control Mechanism
Human skin is covered with hair and contains numerous blood vessels and sweat glands. If the body is in danger of becoming too cold, the hairs stand on end to produce an insulating layer and the blood vessels narrow to help prevent heat escaping via the skin's surface. The mechanisms to prevent overheating include the blood vessels widening to encourage heat loss from the skin's surface and sweat production by the sweat glands, which causes cooling by evaporation. - Sensory Organ
The skin has nerve cells which respond to pressure, allowing you to touch and feel things, together with nerve cells which respond to temperature and others which respond to pain.
Structure of Human Skin
The skin consists of two layers, the outer dead layer is called the epidermis, below which lies the dermis, which is living tissue and rich with blood vessels. Hairs grow from individual hair follicles, which have their base deep in the dermis, but extend up through the epidermis so the hair protrudes above the skin's surface. Interspersed among the hair follicles are the openings of sweat glands, whilst buried beneath the epidermis are the various types of sensory nerve endings. Nestling at the base of each hair follicle is a knobbly, globular structure which produces sebum, the sebaceous gland.
The hair follicle and its associated sebaceous gland are together known as the pilo-sebaceous unit (pilus means hair in latin). Acne is a disorder of the pilo-sebaceous unit, in which the excessive production of sebum, known as seborrhoea, sets of a chain of events which results in the formation of a spot. The skin of the soles of the feet, the palms of the hands and the lips is non-hairy, i.e. it does not have pilo-sebaceous units, and so cannot be affected by acne and neither, for similar reasons, can the genital or oral mucosea. All other parts of the body are covered by hairy skin and could potentially develop acne spots.
The Role of Seborrhoea in Acne
One of the most potent factors in the development of acne is seborrhoea, the excessive production of sebum. The rate at which the sebaceous glands produce sebum is regulated by various of the body's hormones. The most potent stimulus for the production of sebum are androgens, a collective name for the male sex hormones. Other hormones such as growth hormone and thyroxin also stimulate sebum production, but with a much less powerful effect than androgens. Oestrogens, which are female sex hormones, tend to reduce the rate of sebum production, but a small amount of a male sex hormone such as testosterone will quickly overcome the inhibitory effect of even large amounts of oestrogen. The sebaceous glands are entirely inactive before puberty, but androgen stimulation during puberty triggers the sebaceous glands to start producing sebum in both boys and girls. Many pubescent acne patients have markedly greasy skin caused by seborrhoea. Indeed, a definite relationship has been demonstrated between the severity of acne and the rate of sebum production in such patients. However, seborrhoea cannot be the only factor implicated in the development of acne, since some of the people who develop seborrhoea do not develop acne.
Development of Acne Spots
Along with seborrhoea, another important factor in the development of acne is the colonisation of the pilo-sebaceous unit by a particular strain of bacteria, Propionibacterium acnes. Huge numbers of these bacteria are present in the hair follicles of people with seborrhoea. What is more, these bacteria break down sebum to produce free fatty acids, substances which has been shown experimentally to be capable of producing blackheads when applied to the skin of healthy individuals. The third factor involved in the development of acne spots is the formation of comedones, the result of a defect in the normal shedding of skin cells from the lining of the hair follicles. Instead of shed skin cells being carried to the surface of the hair follicle and dispersed, the cells stick together within the hair follicle and form a gunky mass - the comedo - made up of dead skin cells, bacteria and sebum. Comedones may lie dormant in the skin for months of even years, until either they are expressed from the skin's surface or provoke the body into an inflammatory response. The inflammatory reaction, which is the body's own defence mechanism busy trying to destroy the comedo, results in the formation on the surface of the skin over the comedo of a small, raised red spot, known as an acne papule. Papules are the smallest and least troublesome of the four characteristic types of spots found in acne.
